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2017


(217) SAYES, M.; CHARETTE, A. B.
Diphenylsilane as a Coupling Reagent for Amide Bond Formation

Green Chem. 2017, 19, Just Accepted.

A simple procedure for amide bond formation using diphenylsilane as a coupling reagent is described. This methodology enables the direct coupling of carboxylic acids with primary and secondary amines, releasing only hydrogen and a siloxane as by-products. Only one equivalent of each partner is needed, providing a more sustainable amidation method producing minimal wastes. This methodology was also extended to the synthesis of peptides and lactams by addition of Hünig's base (DIPEA) and 4-dimethylaminopyridine (DMAP).

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(216) RACHEL, N. M.; QUAGLIA, D.; LÉVESQUE, É.; CHARETTE, A. B., PELLETIER, J. N.
Engineered, Highly Reactive Substrates of Microbial Transglutaminase Enable Protein Labeling Within Various Secondary Structure Elements

Protein Sci. 2017, 26, Just Accepted.

Microbial transglutaminase (MTG) is a practical tool to enzymatically form isopeptide bonds between peptide or protein substrates. This natural approach to crosslinking the sidechains of reactive glutamine and lysine residues is solidly rooted in food and textile processing. More recently, MTGs tolerance for various primary amines in lieu of lysine have revealed its potential for site-specific protein labeling with aminated compounds, including fluorophores. Importantly, MTG can label glutamines at accessible positions in the body of a target protein, setting it apart from most labeling enzymes that react exclusively at protein termini. To expand its applicability as a labeling tool, we engineered the B1 domain of Protein G (GB1) to probe the selectivity and enhance the reactivity of MTG towards its glutamine substrate. We built a GB1 library where each variant contained a single glutamine at positions covering all secondary structure elements. The most reactive and selective variants displayed a >100-fold increase in incorporation of a recently developed aminated benzo[a]imidazo[2,1,5-cd]indolizine-type fluorophore, relative to native GB1. None of the variants were destabilized. Our results demonstrate that MTG can react readily with glutamines in ?-helical, ?-sheet, and unstructured loop elements and does not favor one type of secondary structure. Introducing point mutations within MTGs active site further increased reactivity towards the most reactive substrate variant, I6Q-GB1, enhancing MTGs capacity to fluorescently label an engineered, highly reactive glutamine substrate. This work demonstrates that MTG-reactive glutamines can be readily introduced into a protein domain for fluorescent labeling.

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(215) BOS, M.; HUANG, W.-S.; POISSON, T.; PANNECOUCKE, X; CHARETTE, A. B., JUBAULT, P.
Catalytic Enantioselective Synthesis of Highly Functionalized Difluoromethylated Cyclopropanes

Angew. Chem. Int. Ed. 2017, 56, 13319-13323.

The first catalytic asymmetric synthesis of highly functionalized difluoromethylated cyclopropanes is described. The method, based on a Rh-catalyzed cyclopropanation of difluoromethylated olefins, gave access to a broad range of cyclopropanes bearing ester, ketone or nitro functional groups. By using Rh2((S)-BTPCP)4 as a catalyst, the corresponding products were obtained in high yields and high diastereo- and enantioselectivities (up 20:1 dr and 99% ee). This methodology allowed the preparation of enantioenriched difluoromethylcyclopropanes for the first time.

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(214) ALLOUCHE, E. M. D.; TAILLEMAUD, S.; CHARETTE, A. B.
Spectroscopic Characterization of (Diiodomethyl)zinc Iodide: Application to the Stereoselective Synthesis and Functionalization of Iodocyclopropanes

Chem. Commun. 2017, 53, 9606-9609.

Herein is described an improved synthetic route to enantio- and diastereoenriched iodocyclopropylmethanols using (diiodomethyl)zinc iodide etherate as the active species. The products obtained by this methodology were successfully functionalized by Suzuki-Miyaura cross-coupling reactions. A Buchwald-type palladium precatalyst allowed access to highly substituted and stereoenriched cyclopropanes without requiring a protecting group.

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(213) LÉVESQUE, É.; BECHARA, W. S.; CONSTANTINEAU-FORGET, L.; PELLETIER, G.; RACHEL, N. M.; PELLETIER, J. N.; CHARETTE, A. B.
General C-H Arylation Strategy for the Synthesis of Tunable Visible Light Emitting Benzo[a]imidazo[2,1,5-c,d]indolizine Fluorophores

J. Org. Chem. 2017, 82, 5046-5067.

Herein we report the discovery of the benzo-[a]imidazo[2,1,5-c,d]indolizine motif displaying tunable emission covering most of the visible spectrum. The polycyclic core is obtained from readily available amides via a chemoselective process involving Tf2O-mediated amide cyclodehydration, followed by intramolecular C-H arylation. Additionally, these fluorescent heterocycles are easily functionalized using electrophilic reagents, enabling divergent access to varied substitution. The effects of said substitution on the compounds' photophysical properties were rationalized by density functional theory calculations. For some compounds, emission wavelengths are directly correlated to the substituent's Hammett constants. Easily introduced nonconjugated reactive functional groups allow the labeling of biomolecules without modification of emissive properties. This work provides a straightforward platform for the synthesis of new moderately bright fluorescent dyes remarkable for their chemical stability, predictability, and unusually high excitation-emission differential.

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(212) BENOIT, G.; CHARETTE, A. B.
Diastereoselective Borocyclopropanation of Allylic Ethers Using a Boromethylzinc Carbenoid

J. Am. Chem. Soc. 2017, 139, 1364-1367.

A borocyclopropanation of (E)- and (Z)-allylic ethers and styrene derivatives via the Simmons-Smith reaction using a novel boromethylzinc carbenoid is described. The carbenoid precursor is prepared via a 3-step sequence from inexpensive and commercially available starting materials. This methodology allows for the preparation of 1,2,3-substituted borocyclopropanes in high yields and diastereoselectivities. Several post-functionalization reactions were also performed to illustrate the versatility of these building blocks.

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(211) BOS, M.; POISSON, T.; PANNECOUCKE, X; CHARETTE, A. B.; JUBAULT, P.
Recent Progress Toward the Synthesis of Trifluoromethyl and Difluoromethyl-Substituted Cyclopropanes

Chem. Eur. J. 2016, 23, 4950-4961.

This Minireview describes recent advances toward the synthesis of trifluoromethylated and difluoromethylated-substituted cyclopropanes. Synthetic methodologies using [2+1] cycloaddition, ring contraction and ring closure cyclopropanation since 2010 are reported.

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(210) LÉVESQUE, É.; LAPORTE, S. T.; CHARETTE, A. B.
Continuous Flow Synthesis and Purification of Aryldiazomethanes via Hydrazone Fragmentation

Angew. Chem. Int. Ed. 2017, 56, 837-841.

Electron-rich diazo compounds, such as aryldiazomethanes, are powerful reagents for the synthesis of complex structures, but the risks associated with their toxicity and instability often limit their use. Flow chemistry techniques make these issues avoidable, as the hazardous intermediate can be used as it is produced, avoiding accumulation and handling. Unfortunately, the produced stream is often contaminated with other reagents and by-products, making it incompatible with many applications, especially in catalysis. Herein is reported a metal-free continuous flow method for the production of aryldiazomethane solutions in a non-coordinating solvent from easily prepared, bench-stable sulfonylhydrazones. All by-products are removed by an in-line aqueous wash, leaving a clean, base-free diazo stream. Three successful sensitive metal-catalyzed transformations demonstrated the value of the method.

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2016


(209) LADD, C. L.; CHARETTE, A. B.
Access to Cyclopropyl-Fused Azacycle via a Palladium-Catalyzed Direct Alkenylation Strategy

Org. Lett. 2016, 18, 6046-6049.

Palladium-catalyzed direct alkenylation of cyclopropyl C-H bonds proceeds in high efficiency. This transformation provides access to novel cyclopropyl-fused azacycles. Ligand studies suggest that bisphosphine monoxide analogues of dppf and rac -BINAP are the active ligand species. Preliminary results support that both BozPhos and IPrMonophos ligands can achieve high enantioinduction for this novel direct alkenylation reaction. To date, this represents the first example of enantioselective C-H functionalization employing a bisphosphine monoxide ligand.

>> publié gratuitement dans la section ACS Editor's Choice
>> souligné dans Synfacts par la rédaction pour avancées importantes

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(208) ASPIN, S. J.; TAILLEMAUD, S.; CYR, P.; CHARETTE, A. B.
9-Silafluorenyl Dichlorides as Chemically Ligating Coupling Agents and Their Application in Peptide Synthesis

Angew. Chem. Int. Ed. 2016, 55, 13833-13837.

A fundamentally simple, mild, and practical procedure for peptide bond formation is reported that employs a stoichiometric amount of easy-to-access 9-silafluorenyl dichlorides as the coupling agent. Without initial preactivation or elaboration of the carboxylic acid or amine termini of the amino acids, the developed reagent is proposed to act through an unprecedented chemical ligation mechanism, bringing the two coupling partners together before being subsequently eliminated. The desired amides or peptide bonds are thus furnished in good yields and with low to no epimerization.

>> sélectionné comme Hot Paper par la rédaction
>> souligné dans ChemistryViews.org par la rédaction

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(207) RÉGNIER, S.; BECHARA, W. S.; CHARETTE, A. B.
Synthesis of 3-Aminoimidazo[1,2-a]pyridines from α-Aminopyridinyl Amides

J. Org. Chem. 2016, 81, 10348-10356.

3-Aminoimidazo[1,2-a]pyridines are rapidly synthesized via a facile and mild cyclodehydration-aromatization reaction starting from readily available amides. The cyclodehydration step is mediated by the activation of N-Boc-protected 2-aminopyridine-containing amides by triflic anhydride (Tf2O) in the presence of 2-methoxypyridine (2-MeO-Py). Subsequently, the addition of K2CO3 in THF ensured a clean deprotection-aromatization sequence to afford the desired heterocycle. A wide variety of functional groups and substitution patterns were tolerated under the optimized procedure, and good to excellent yields were obtained for the fused bicyclic 3-azaheterocycles. In addition, the reaction was found to be scalable to gram-scale and could be performed with unprotected acyclic amide precursors. We also found that the resulting products were valuable intermediates for both Pd- and Ru-catalyzed C-H arylation reactions, allowing for the elaboration to diversely functionalized building blocks.

>> sur invitation dans un numéro spécial de Heterocycles

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(206) PONS, A.; POISSON, T.; PANNECOUCKE, X.; CHARETTE, A. B.; JUBAULT, P.
Synthesis and Applications of Fluorocyclopropanes (Short Review)

Synthesis 2016, 4060-4071.

The combination of a fluorine atom and a cyclopropane ring, which both possess unique structural and chemical features, can generate new relevant scaffolds with potential interest for the synthesis of new bioactive compounds. In this review, we report the impressive progress recently devoted to the synthesis of fluorocyclopropanes especially using asymmetric methods and highlight some recent reported applications.

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(205) RULLIÈRE, P.; CYR, P.; CHARETTE, A. B.
Difluorocarbene Addition to Alkenes and Alkynes in Continuous Flow

Org. Lett. 2016, 18, 1988-1991.

The first in-flow difluorocarbene generation and addition to alkenes and alkynes is reported. The application of continuous fl ow technology allowed for the controlled generation of difluorocarbene from TMSCF3 and a catalytic quantity of NaI. The in situ generated electrophilic carbene reacts smoothly with a broad range of alkenes and alkynes, allowing the synthesis of the corresponding difluorocyclopropanes and difluorocyclopropenes. The reaction is complete within a 10-min residence time at high reaction concentrations. With a production flow rate of 1 mmol/min, continuous flow chemistry enables scale up of thisprocess in a green, atom-economic, and safe manner.

>> souligné dans ChemInform par la rédaction en septembre 2016

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(204) AUSTAD, B. C.; HAGUE, A. B.; WHITE, P.; PELUSO, S.; NAIR, S. J.; DEPEW, K. M.; GROGAN, M. J.; CHARETTE, A. B.; YU, L.-C.; LORY, C. D.; GRENIER, L.; LESCARBEAU, A.; LANE, B. S.; LOMBARDY, R.; BEHNKE, M. L.; KONEY, N.; PORTER, J. R.; CAMPBELL, M. J.; SHAFFER, J.; XIONG, J.; HELBLE, J. C.; FOLEY, M. A.; ADAMS, J.; CASTRO, A. C.; TREMBLAY, M. R.
Development of a Multi Kilogram-Scale, Tandem Cyclopropanation Ring-Expansion Reaction en Route to Hedgehog Antagonist IPI-926

Org. Process Res. Dev. 2016, 20, 786-798.

The formation of the D-homocyclopamine ring system in IPI-926 is the key step in its semisynthesis and proceeds via a chemoselective cyclopropanation followed by a stereoselective acid-catalyzed carbocation rearrangement. In order to perform large-scale cyclopropanation reactions, we developed new iodomethylzinc bis(aryl)phosphate reagents that were found to be both eff ective and safe. These soluble reagents can be prepared under mild conditions and are stable during the course of the reaction. Importantly, they have favorable energetics relative to other cyclopropanating agents such as EtZnCH2I. Herein, we describe the process optimization studies that led to successful large-scale production of the D-homocyclopamine core necessary for IPI-926.

>> sélectionné comme Synfact of the Month par la rédaction en juillet 2016

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(203) PONS, A.; IVASHKIN, P.; POISSON, T.; CHARETTE, A. B.; PANNECOUCKE, X.; JUBAULT, P.
Catalytic Enantioselective Synthesis of Halo-Cyclopropanes

Chem. Eur. J. 2016, 22, 6239-6242.

A catalytic asymmetric synthesis of halo-cyclopropanes is described. The developed method is based on a carbenoid cyclopropanation of 2-haloalkenes with tert-butyl α-cyano-α-diazoacetate using a chiral rhodium catalyst, permitted access to a broad range of highly functionalized chiral halo-cyclopropanes (F, Cl, Br and I) in good yields, moderate diastereoselectivity and excellent enantiomeric ratios. The reported methodology represents the first general catalytic enantioselective approach to halo-cyclopropanes.

>> souligné dans Synfacts par la rédaction pour avancées importantes

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(202) LADD, C. L.; BELOUIN, A. V.; CHARETTE, A. B.
Intramolecular sp3 Functionalization of Cyclopropyl α-Amino Acid-Derived Benzamides

J. Org. Chem. 2016, 81, 256-264.

Intramolecular Pd-catalyzed functionalization of cyclopropyl α-amino acid-derived benzamides proceeds without silver or pivalate additives. Both electronically- and sterically-diverse ethyl 1,2,3,4-tetrahydroisoquinolone-3-carboxylates were accessible in good to excellent yields.

>> souligné dans ChemInform par la rédaction
>> souligné dans Org. Process Res. Dev.

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2015


(201) TAILLEMAUD, S.; DIERCXSENS, N.; GAGNON, A.; CHARETTE, A. B.
Mechanism-Driven Elaboration of an Enantioselective Bromocyclopropanation Reaction of Allylic Alcohols

Angew. Chem. Int. Ed. 2015, 54, 14108-14112.

A stereoselective bromocyclopropanation of allylic alcohols using dibromomethylzinc bromide is described. Spectroscopic studies to monitor the formation of transient intermediates not only led to the development of a more-atomeconomical halocyclopropanation reaction, but also highlighted the unique role of ether additives in the process. The desired bromo-substituted cyclopropanes were isolated in high yields and excellent diastereo- as well as enantioselectivities using readily available reagents.

>> sélectionné comme Synfact of the Month par la rédaction en décembre 2015

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(200) NAVULURI, C.; CHARETTE, A. B.
Diastereoselective Fluorocyclopropanation of Chiral Allylic Alcohols Using an α-Fluoroiodomethylzinc Carbenoid

Org. Lett. 2015, 17, 4288-4291.

Chiral fluorocyclopropyl carbinols were synthesized in high diastereoselectivities via a zinc mediated cyclopropanation reaction, using sec-allylic alcohols as simple building blocks. An enantioselective version of this transformation was achieved through in situ formation of chiral allylic zinc sec-alkoxides from the requisite aldehydes using Walsh's protocol.

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(199) CYR, P.; RÉGNIER, S.; BECHARA, W. S.; CHARETTE, A. B.
Rapid Access to 3-Aminoindazoles from Tertiary Amides

Org. Lett. 2015, 17, 3386-3389.

A two-step synthesis of structurally diverse 3-aminoindazoles from readily available starting materials was developed. This sequence includes a one-pot synthesis of aminohydrazones through chemoselective Tf2O-mediated activation of tertiary amides and subsequent addition of nucleophilic hydrazides. These precursors then participate in an intramolecular ligand-free Pd-catalyzed C-H amination reaction. The azaheterocycles synthesized via this approach were further diversifi ed through subsequent deprotection/functionalization reactions.

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(198) ZIMBRON, J. M.; DAUPHINAIS, M.; CHARETTE, A. B.
Noyori-Ikariya Catalyst Supported on Tetraarylphosphonium Salt for Asymmetric Transfer Hydrogenation in Water

Green Chem. 2015, 17, 3255-3259.

A straightforward synthesis of a tetraarylphosphonium (TAP)-supported Noyori-Ikariya catalyst is described. The TAP-supported ruthenium precatalyst provided good conversions and high enantioselectivities for the asymmetric transfer hydrogenation of ketones in water. In addition the catalyst was easily recovered and used in multiple catalytic cycles.

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(197) PONS, A.; BEUCHER, H.; IVASHKIN, P.; LEMONNIER, G.; POISSON, T.; CHARETTE, A. B.; JUBAULT, P.; PANNECOUCKE, X.
Rhodium-Catalyzed Cyclopropanation of Fluorinated Olefins: A Straightforward Route to Highly Functionalized Fluorocyclopropanes

Org. Lett. 2015, 17, 1790-1793.

An efficient access to highly functionalized monofluorocyclopropanes is described. The developed methodology allowed straightforward access to a large panel of polysubstituted fluorinated cyclopropanes in good to excellent yields and good diastereoselectivities. The Rh-catalyzed cyclopropanation proved to be efficient on several fluorinated olefins and several diazo compounds. This method represents the first general route to complex fluorinated cyclopropanes.

>> travaux soulignés dans Chem. Eng. News par la rédaction

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(196) BECHARA, W. S.; KHAZHIEVA, I. S.; RODRIGUEZ, E.; CHARETTE, A. B.
One-Pot Synthesis of 2,4,5,-Trisubstituted 1,2,4-Triazoles via the Addition of Hydrazides to Activated Secondary Amides

Org. Lett. 2015, 17, 1184-1187.

A general approach has been developed for the one-pot synthesis of 3,4,5-trisubstituted 1,2,4-triazoles from secondary amides and hydrazides via triflic anhydride activation followed by microwave-induced cyclodehydration. In addition, the 1,2,4-triazole moiety is shown to be a useful directing group for Ru-catalyzed C-H arylation. Access to 1,2,4- triazolophenanthridine can be achieved from the reaction products using a Pd-catalyzed intramolecular C-H functionalization reaction.

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(195) SUSTAC ROMAN, D.; POIRET, V.; PELLETIER, G.; CHARETTE, A. B.
Directed Arylation of Imidazo[1,5-a]azines Through Ruthenium and Palladium Catalysis

Eur. J. Org. Chem. 2015, 2015, 67-71.

The regioselective RuII-catalyzed direct ortho-arylation of C- 3 aryl-substituted imidazo[1,5-a]azines with (hetero)aryl hal- ides was investigated. The employment of RuII and PdII catalysts in the same flask resulted in two sequential and distinct C-H bond arylations, which allowed the rapid synthesis of highly conjugated compounds.

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2014


(194) PELLETIER, G.; CONSTANTINEAU-FORGET, L.; CHARETTE, A. B.
Directed Functionalization of 1,2-Dihydropyridines: Stereoselective Synthesis of 2,6-Disubstituted Piperidines

Chem. Commun. 2014, 50, 6883-6885.

A practical and highly stereoselective approach to access 2,6-disubstituted piperidines using an amidine auxiliary is reported. Following the diastereoselective addition of Grignard reagents at the 2-position of an activated pyridinium salt, the amidine group directs a regioselective metalation at the 6-position, enabling further functionalization. A subsequent electrophilic quench or a Negishi cross-coupling could be performed, resulting in 2,6-disubstituted dihydropyridines. These were reduced to the saturated piperidine rings with high diastereoselectivity.

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(193) CYR, P.; CHARETTE, A. B.
Continuous-Flow Hydroxylation of Aryl Iodides Promoted by Copper Tubing

Synlett 2014, 1409-1412.

A simple and ligand-free synthesis of phenols from the corresponding aryl iodides in a continuous-flow system is described. The reaction is complete in only 4 to 20 minutes when heated between 150 and 165 °C in a reactor consisting of a commercially available copper coil. An example of trapping of the phenoxide in situ is also shown.

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(192) LÉVESQUE, É.; GOUDREAU, S. R.; CHARETTE, A. B.
Improved Zinc-Catalyzed Simmons-Smith Reaction: Access to Various 1,2,3-Trisubstituted Cyclopropanes

Org. Lett. 2014, 16, 1490-1493.

The Simmons-Smith reaction of zinc carbenoids with alkenes is a powerful method to access cyclopropanes containing various substitution patterns. This work exploits the high reactivity of aryldiazomethanes toward zinc halides to generate aryl-substituted carbenoids catalytically. These carbenoids are able to cyclopropanate various alkenes diastereoselectively, including unfunctionalized substrates such as styrenes. The zinc catalyst can be modified to tolerate the use of free allylic alcohols.

>> souligné dans Synfacts par la rédaction

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2013


(191) SUSTAC ROMAN, D.; CHARETTE, A. B.
C-H Functionalization of Cyclopropanes: A Practical Approach Employing a Picolinamide Auxiliary

Org. Lett. 2013, 15, 4394-4397.

A Pd-catalyzed picolinamide-enabled, and efficient C-H arylation of cyclopropanes is described. The reaction can be promoted by either a silver additive or catalytic pivalic acid in the presence of a carbonate base. Various aryl iodides can be employed as coupling partners, providing exclusively cis-substituted cyclopropylpicolinamides.

>> classé dans le top 20 des articles OL les plus lus en septembre 2013

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(190) BEAULIEU, L.-P. B.; SCHNEIDER, J. F.; CHARETTE, A. B.
Highly Enantioselective Simmons-Smith Fluorocyclopropanation of Allylic Alcohols via the Halogen Scrambling Strategy of Zinc Carbenoids

J. Am. Chem. Soc. 2013, 135, 7819-7822.

Highly enantio- and diastereoenriched monofluorocyclopropanes were accessed via the Simmons-Smith fluorocyclopropanation of allylic alcohols using difluoroiodomethane and ethylzinc iodide as the substituted carbenoid precursors. The scrambling of halogens at the zinc carbenoid led to the formation of the fluorocyclopropanating agent (fluoroiodomethyl)zinc-(II) fluoride. This strategy circumvented the ongoing limitation in Simmons-Smith fluorocyclopropanations relying on the use of the relatively inaccessible and expensive carbenoid precursor fluorodiiodomethane.

>> souligné dans Synfacts par la rédaction

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(189) PELLETIER, G.; CHARETTE, A. B.
Triflic Anhydride Mediated Synthesis of Imidazo[1,5-a]azines

Org. Lett. 2013, 15, 2290-2293.

Imidazo[1,5-a]azines are synthesized in moderate to excellent yields using a mild cyclodehydration/aromatization reaction triggered by the use of triflic anhydride (Tf2O) and 2-methoxypyridine (2-MeOPyr). Various substitution patterns and functional groups were found to be compatible under the optimized conditions. In addition, a 5-bromo-3-aryl derivative was also shown to be active in a Sonogashira cross-coupling and direct arylation reactions. A tertiary amide was compatible as a substrate leading to the synthesis of an imidazo[1,5-a]pyridinium triflate.

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(188) LADD, C.; SUSTAC ROMAN, D.; CHARETTE, A. B.
Palladium-Catalyzed Ring-Opening of Cyclopropyl Benzamides: Synthesis of Benzo[c]azepine-1-ones via C(sp3)-H Functionalization

Tetrahedron 2013, 69, 4479-4487.

A variety of difficult to obtain benzo[c]azepine-1-ones are synthesized via a novel palladium-catalyzed, silver-promoted intramolecular cyclization of cyclopropyl benzamides. This biologically important class of molecules is prepared in an efficient and high-yielding manner from easily accessible starting materials. Both aryl bromides and iodides are effective substrates for the transformation. Mechanistic studies indicate that the reaction proceeds through a cyclopropyl C(sp3)-H cleavage step, followed by ring-opening, deprotonation, and reductive elimination.

>> souligné dans ChemInform par la rédaction
>> sur invitation pour le numéro de Tetrahedron Symposium-In-Print portant sur le thème de Metal-Catalyzed C-H Bond Functionalization

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(187) LADD, C.; SUSTAC ROMAN, D.; CHARETTE, A. B.
Silver-Promoted, Palladium-Catalyzed Direct Arylation of Cyclopropanes: Facile Access to Spiro 3,3'-Cyclopropyl Oxindoles

Org. Lett. 2013, 15, 1350-1353.

The Pd-catalyzed, Ag(I)-mediated intramolecular direct arylation of cyclopropane C-H bonds is described. Various spiro 3,3'-cyclopropyl oxindoles can be obtained in good to excellent yields from easily accessible 2-bromoanilides. The kinetic isotope effect was determined and epimerization studies were conducted, suggesting that the formation of a putative Pd-enolate is not operative and that the reaction proceeds via a C-H arylation pathway.

>> souligné dans Synfacts par la rédaction
>> souligné dans ChemInform par la rédaction

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(186) MOUSSEAU, J. J.; CHARETTE, A. B.
Direct Functionalization Processes: A Journey from Palladium to Copper to Iron to Nickel to Metal-Free Coupling Reactions

Acc. Chem. Res. 2013, 46, 412-424.

The possibility of finding novel disconnections for the efficient synthesis of organic molecules has driven the interest in developing technologies to directly functionalize C-H bonds. The ubiquity of these bonds makes such transformations attractive, while also posing several challenges. The first, and perhaps most important, is the selective functionalization of one C-H bond over another. Another key problem is inducing reactivity at sites that have been historically unreactive and difficult to access without prior inefficient prefunctionalization. Although remarkable advances have been made over the past decade toward solving these and other problems, several difficult tasks remain as researchers attempt to bring C-H functionalization reactions into common use. The functionalization of sp3 centers continues to be challenging relative to their sp and sp2 counterparts. Directing groups are often needed to increase the effective concentration of the catalyst at the targeted reaction site, forming thermodynamically stable coordination complexes. As such, the development of removable or convertible directing groups is desirable. Finally, the replacement of expensive rare earth reagents with less expensive and more sustainable catalysts or abandoning the use of catalysts entirely is essential for future practicality. This Account describes our efforts toward solving some of these quandaries. We began our work in this area with the direct arylation of N-iminopyridinium ylides as a universal means to derivatize the germane six-membered heterocycle. We found that the Lewis basic benzoyl group of the pyridinium ylide could direct a palladium catalyst toward insertion at the 2-position of the pyridinium ring, forming a thermodynamically stable six-membered metallocycle. Subsequently we discovered the arylation of the benzylic site of 2-picolonium ylides. The same N-benzoyl group could direct a number of inexpensive copper salts to the 2-position of the pyridinium ylide, which led to the first description of a direct copper-catalyzed alkenylation onto an electron-deficient arene. This particular directing group offers two advantages: (1) it can be easily appended and removed to reveal the desired pyridine target, and (2) it can be incorporated in a cascade process in the preparation of pharmacologically relevant 2-pyrazolo[1,5-a]pyridines. This work has solved some of the challenges in the direct arylation of nonheterocyclic arenes, including reversing the reactivity often observed with such transformations. Readily convertible directing groups were applied to facilitate the transformation. We also demonstrated that iron can promote intermolecular arylations effectively and that the omission of any metal still permits intramolecular arylation reactions. Lastly, we recently discovered a nickel-catalyzed intramolecular arylation of sp3 C-H bonds. Our mechanistic investigations of these processes have elucidated radical pathways, opening new avenues in future direct C-H functionalization reactions.

>> classé dans le top 5 des articles ACR les plus lus en novembre-décembre 2012

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(185) LINDSAY, V. N. G.; FISET, D.; GRITSCH, P. J.; AZZI, S; CHARETTE, A. B.
Stereoselective Rh2(S-IBAZ)4-Catalyzed Cyclopropanation of Alkenes, Alkynes and Allenes: Asymmetric Synthesis of Diacceptor Cyclopropylphosphonates and Alkylidenecyclo-propanes

J. Am. Chem. Soc. 2013, 135, 1463-1470.

A mild and highly stereoselective rhodium(II)-catalyzed cyclopropanation of alkenes, alkynes and allenes with diacceptor diazo compounds is reported. Using the phosphonate moiety as an efficient trans-directing group, the first catalytic asymmetric route to diacceptor cycloprop(en)ylphosphonates was developed by employing an α-cyano diazophosphonate and Rh2(S-IBAZ)4 as chiral catalyst. The isosteric character of phosphonic and carboxylic acid derivatives allowed the alternative use of an α-cyano diazoester in the process, leading to α-cyano cycloprop(en)ylcarboxylates in high yields and stereoselectivities. Taking advantage of the particular reactivity of the cyanocarbene intermediates involved in this system, the scope of compatible substrates could be extended to substituted allenes, leading to the development of the first catalytic enantioselective method for the synthesis of diacceptor alkylidenecyclopropanes.

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2012


(184) BEAULIEU, L.-P. B.; ZIMMER, L. E.; GAGNON, A.; CHARETTE, A. B.
Highly Enantioselective Synthesis of 1,2,3-Substituted Cyclopropanes by Using α-Iodo- and α-Chloromethylzinc Carbenoids

Chem. Eur. J. 2012, 18, 14784-14791.

Herein, we report the enantio- and diastereoselective formation of trans-iodo- and trans-chlorocyclopropanesfrom α-iodo- and α-chlorozinc carbenoids by using a dioxaborolane-derived chiral ligand. The synthetically useful iodocyclopropane building blocks were derivatized by an electrophilic trapping of the corresponding cyclopropyl lithium species or a Negishi coupling to give access to a variety of enantioenriched 1,2,3-substituted cyclopropanes. The synthetic utility of this method was demonstrated by the formal synthesis of an HIV-1 protease inhibitor. In addition, the related stereoselective bromocyclopropanation was also investigated. New insights about the relative electrophilicity of haloiodomethylzinc carbenoids are alsopresented.

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(183) PELLETIER, G.; LIE, S.; MOUSSEAU, J. J.; CHARETTE, A. B.
One-Pot Synthesis of 1-Iodoalkynes and Trisubstituted Alkenes from Benzylic and Allylic Bromides

Org. Lett. 2012, 14, 5464-5467.

1-Iodoalkynes are formed in moderate to high yields from readily accessible benzylic and allylic alkyl bromides by a one-pot homologation/double elimination procedure with iodoform (CHI3). The developed conditions include facile purification and avoid the use of an excess of triphenylphosphine (PPh3), as described in classical Corey-Fuchs iodoalkynylation conditions. Replacing CHI3 with CHI2Cl allows the isolation of the corresponding gem-(Z)-chloro-(E)-iodoalkene in good yield and stereoselectivity. Moreover, the use of benzhydryl bromides as nucleophiles enables the synthesis of trisubstituted alkenes under similar reaction conditions.

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(182) BEAULIEU, L.-P. B.; SUSTAC ROMAN, D.; VALLÉE, F.; CHARETTE, A. B.
Nickel(0)/NaHMDS Adduct-Mediated Intramolecular Alkylation of Unactivated Arenes Via a Homolytic Aromatic Substitution Mechanism

Chem. Commun. 2012, 48, 8249-8251.

A variety of polycycles can be synthesized via an intramolecular alkylation cyclization promoted by Ni(PPh3)4 and NaHMDS. Mechanistic investigations support the catalytic nature of Ni0 in the course of TEMPO scavenging experiments and its association with the substrate and NaHMDS to form an adduct by DOSY NMR.

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(181) LEMONNIER, G.; CHARETTE, A. B.
Grob Fragmentation of 2-Azabicyclo[2.2.2]oct-7-ene: Tool for the Stereoselective Synthesis of Polysubstituted Piperidines

J. Org. Chem. 2012, 77, 5832-5837.

The Grob fragmentation of azabicyclo[2.2.2]octene leads to a dihydropyridinium intermediate. This highly reactive species reacts with a variety of organocuprates and other soft nucleophiles in a regioselective manner, allowing for the rapid and stereoselective synthesis of 2,3,4-trisubstituted 1,2,3,4-tetrahydropyridines. The resulting products were either reduced in situ to the corresponding piperidine or used to achieve the stereoselective construction of various nitrogen heterocycles.

>> classé dans le top 20 des articles JOC les plus lus en juin 2012
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(180) FISET, D.; CHARETTE, A. B.
Microwave-Assisted Hydrolysis: Efficient Synthesis of α-Substituted Cysteines on Multi-gram Scale

RSC Adv. 2012, 2, 5502-5505.

A microwave-assisted hydrolysis of thiazolidines was developed for the synthesis of enantioenriched α-alkylcysteines. Under optimized conditions, the efficiency of the reaction can be considerably improved and the scope significantly broadened as compared to previously described processes. This approach is shown to be applicable to multi-gram scale using a high-throughput microwave system.

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(179) LINDSAY, V. N. G.; CHARETTE, A. B.
Design and Synthesis of Chiral Heteroleptic Rh(II)-Carboxylate Catalysts. Experimental Investigations of the Halogen Bond Rigidification Effects in Asymmetric Cyclopropanation Reactions

ACS Catal. 2012, 2, 1221-1225.

A general method for the synthesis of chiral heteroleptic Rh(II) tetracarboxylate catalysts is reported. The chlorinated TCPT unit was found to be an efficient polarity-control group, allowing the isolation of each complex from a mixture of six possible products. This approach contributes to enlarge the scope of accessible chiral Rh(II) catalysts, and allowed to further study the halogen bond rigidification effect observed in chlorinated complexes.

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(178) BULL, J. A.; MOUSSEAU, J. J.; PELLETIER, G.; CHARETTE, A. B.
Synthesis of Pyridine and Dihydropyridine Derivatives by Regio- and Stereoselective Addition to N-Activated Pyridines

Chem. Rev. 2012, 112, 2642-2713.

Pyridines are among the most prevalent heterocyclic structural units in pharmaceutical and agrochemical targets, as well as in materials science. Pyridines also provide convenient synthetic precursors to chiral dihydro- and tetrahydropyridines, as well as piperidines, which continue to be of interest as intermediates in alkaloid synthesis, in NADH models, and as important biologically active structures. Dihydropyridines in turn may also be converted to substituted pyridines through oxidation. Similarly, pyridines, dihydropyridines, and tetrahydropyridines are important building blocks in the preparation of piperidine scaffolds via reduction or nucleophilic additions. The chemistry to directly functionalize pyridine remains a significant challenge due to poor chemoselectivity and the lower energy of the π-system relative to benzene. As a result, electrophilic aromatic substitution is not effective without the inclusion of substituents to activate the pyridine ring. This review examines the functionalization of N-activated pyridinium species by the addition of nucleophiles and electrophiles, as well as by transition metal-mediated functionalization. The issues of regioselectivity, stereoselectivity, and the mechanisms of the additions will be addressed. Sections 2-4 are concerned with nucleophilic additions to pyridinium species forming dihydropyridines, which may be progressed to substituted pyridines or piperidines. The following two sections are concerned with the synthesis of substituted pyridines without the disruption of the aromaticity, via pyridinyl metal species. Section 5 examines electrophilic addition to N-heteroatom pyridinium species following deprotonation adjacent to the nitrogen to afford substituted pyridines. The last section is concerned with transition metal-mediated C-H functionalization of pyridinium species. This review covers a substantial time period of the literature, from the 1940s to the current day, and state-of-the-art methodologies, and as such reflects some of the changing approaches and challenges in organic chemistry. The chemistry of benzopyridine derivatives (such as quinolines), intramolecular rearrangement, cycloaddition reactions, reductions (including addition of hydride), electrophilic aromatic substitution, and radical reactions, though important, will not be considered in this review.

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(177) BECHARA, W. S.; PELLETIER, G.; CHARETTE, A. B.
Chemoselective Synthesis of Ketones and Ketimines by Addition of Organometallic Reagents to Secondary Amides

Nature Chem. 2012, 4, 228-234.

The development of efficient and selective transformations is crucial in synthetic chemistry as it opens new possibilities in the total synthesis of complex molecules. Applying such reactions to the synthesis of ketones is of great importance, as this motif serves as a synthetic handle for the elaboration of numerous organic functionalities. In this context, we report a general and chemoselective method based on an activation/addition sequence on secondary amides allowing the controlled isolation of structurally diverse ketones and ketimines. The generation of a highly electrophilic imidoyl triflate intermediate was found to be pivotal in the observed exceptional functional group tolerance, allowing the facile addition of readily available Grignard and diorganozinc reagents to amides, and avoiding commonly observed over-addition or reduction side reactions. The methodology has been applied to the formal synthesis of analogues of the antineoplastic agent Bexarotene and to the rapid and efficient synthesis of unsymmetrical diketones in a one-pot procedure.

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>> classé parmi les 50 articles les plus lus dans Nature Chem. au début de 2012

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(176) MOREAU, B.; ALBERICO, D.; LINDSAY, V. N. G.; CHARETTE, A. B.
Catalytic Asymmetric Synthesis of Nitrocyclopropane Carboxylates

Tetrahedron 2012, 68, 3487-3496.

A Cu(I)-catalyzed asymmetric cyclopropanation of alkenes with an iodonium ylide has been developed. The copper source, hypervalent iodine source, solvent, and additives all have a significant effect on the yields and enantioselectivities. High enantioselectivity (up to 99:1 er) and diastereoselectivity (95:5 dr trans/cis) were achieved for a wide range of alkenes. Conditions were developed to convert the trans products to the cis isomers. In addition, 1-nitrocyclopropyl carboxylates were transformed into the corresponding substituted cyclopropane amino acids and aminocyclopropanes. Moreover, a comparative study between Zn- and In-mediated reduction reactions of the nitro group in these compounds with regards to the er erosion in the process is also documented.

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2011


(175) MOUSSEAU, J. J.; BULL, J. A.; LADD, C. L.; FORTIER, A.; SUSTAC ROMAN, D.; CHARETTE, A. B.
Synthesis of 2- and 2,3-Substituted Pyrazolo[1,5-a]pyridines: Scope and Mechanistic Considerations of a Domino Direct Alkynylation and Cyclization of N-Iminopyridinium Ylides Using Alkenyl Bromides, Alkenyl Iodides, and Alkynes

J. Org. Chem. 2011, 76, 8243-8261.

Direct functionalization and tandem processes have both received considerable recent interest due to their cost and time efficiency. Herein we report the synthesis of difficult to obtain 2-substituted pyrazolo[1,5-a]pyridines through a tandem palladium-catalyzed/silver-mediated elimination/direct functionalization/cyclization reaction involving N-benzoyliminopyridinium ylides. As such, these biologically important molecules are prepared in an efficient, high yielding manner, only requiring a two-step sequence from pyridine. Aryl substituted alkenyl bromides and iodides are effective ylide coupling partners. Mechanistic studies led to the use of terminal alkynes, which extended the scope of the reaction to include alkyl substitution on the unsaturated reactive site. The optimization, scope, and mechanistic considerations of the process are discussed.

>> classé dans le top 20 des articles JOC les plus lus en octobre 2011

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(174) JARVIS, S. B. D.; CHARETTE, A. B.
Synthesis of Enantiopure Substituted Piperidines via an Aziridinium Ring Expansion

Org. Lett. 2011, 13, 3830-3833.

Herein we report a novel methodology for the asymmetric synthesis of 3-substituted piperidines from readily available chiral building blocks. This method, which features a novel irreversible dihydropyrole-tetrahydropyridine ring expansion, allows the introduction of a large variety of substituents at the 3-position and permits substitution at the 2- and 6-position giving mono-, di-, or trisubstituted piperidines with high diastereocontrol.

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(173) LEBEL, H.; DAVI, M.; ROY, M.-N.; ZEGHIDA, W.; CHARETTE, A. B.
Olefination Reactions Using Tetraarylphosphonium (TAP)-Supported Phosphorus Ylide

Synthesis 2011, 2275-2280.

Tetraarylphosphonium (TAP)-supported phosphorus ylides were prepared and used in copper-catalyzed olefination reactions with diazo compounds to produce conjugated esters, amides, and phosphonates. The TAP-phosphine oxie can easily be separated from the alkene product, recycled and reused.

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(172) BARBE, G.; FISET, D.; CHARETTE, A. B.
Asymmetric Total Synthesis of (+)-Luciduline: Toward a General Approach to Related Lycophodium Alkaloids

J. Org. Chem. 2011, 76, 5354-5362.

As part of a research program directed toward the synthesis of Lycopodium alkaloids, a multigram scale asymmetric synthesis of intermediate 11 was achieved in 11 steps from pyridine (17). In addition to our alkene metathesis strategy, a key feature of this synthetic approach consists of a Fukuyama's Diels-Alder cycloaddition between 1,2-dihydropyridine and acrolein using MacMillan's catalyst (18) on a 50-g scale. This led to a 12-step catalytic asymmetric synthesis of (+)-luciduline (1). A broader subset of Lycopodium alkaloids could also be obtained, as demonstrated by the derivatization of 11 into advanced intermediates for the synthesis of some of these natural products.

>> classé dans le top 5 des articles JOC les plus lus en septembre 2011

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(171) LINDSAY, V. N. G.; NICOLAS, C.; CHARETTE, A. B.
Asymmetric Rh(II)-Catalyzed Cyclopropanation of Alkenes with Diacceptor Diazo Compounds: the p-Methoxyphenyl Ketone as a General Stereoselectivity Controlling Group

J. Am. Chem. Soc. 2011, 133, 8972-8981.

Different diacceptor diazo compounds bearing an α-PMP-ketone group were found to be effective carbene precursors for the highly stereoselective Rh2(S-TCPTTL)4- catalyzed cyclopropanation of alkenes (EWG = NO2, CN, CO2Me). The resulting products were readily transformed into a variety of biologically relevant enantiopure molecules, such as cyclopropane α- and β-amino acid derivatives. Different mechanistic studies carried out led to a rationale for the high diastereo- and enantioselectivity obtained, where the PMP-ketone moiety was found to play a critical role in the stereoinduction process. Additionally, the use of catalytic amounts of achiral Lewis bases to influence the enantioinduction of the reactions developed is documented.

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(170) SUSTAC ROMAN, D.; TAKAHASHI, Y.; CHARETTE, A. B.
Potassium tert-Butoxide Promoted Intramolecular Arylation via a Radical Pathway

Org. Lett. 2011, 13, 3242-3245.

Potassium tert-butoxide mediated intramolecular cyclization of aryl ethers, amines, and amides was efficiently performed under microwave irradiation to provide the corresponding products in high regioisomeric ratios. The reaction proceeds via single-electron transfer to initiate the formation of an aryl radical, followed by a kinetically favored 5-exo-trig and subsequent ring expansion.

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2010


(169) LEMONNIER, G.; CHARETTE, A. B.
Stereoselective Synthesis of 2,3,6-Trisubstituted Tetrahydropyridines via Tf2O-Mediated Grob Fragmentation: Access to Indolizidines (-)-209I and (-)-223J

J. Org. Chem. 2010, 75, 7465-7467.

Herein we describe the γ-amino hydroxide Grob fragmentation of the aza-bicyclo[2.2.2]octene 1 using triflic anhydride as the activating agent. The resulting dihydropyridinium ion can react with a wide variety of Grignard reagents, giving access to 2,3,6-trisubstituted tetrahydropyridines (2) with high regio- and stereoselectivities. This methodology has been applied to the short synthesis of natural indolizidines (-)-209I (3) and (-)-223J (4).

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(168) MOUSSEAU, J. J.; VALLÉE, F.; LORION, M.; CHARETTE, A. B.
Umpolung Direct Arylation Reactions: Facile Process Requiring Only Catalytic Palladium and Substoichiometric Amount of Silver Salts

J. Am. Chem. Soc. 2010, 132, 14412-14414.

An umpolung direct arylation process is described. The reaction requires only a catalytic amount of Pd(OAc)2 and a substoichiometric amount of silver salts, without any external base or ligand to proceed. The directed oxidative insertion of the transition metal followed by the coupling into the C-H bond of an unactivated arene has surprisingly not yet been reported, despite the clear advantages in the ease of starting material synthesis. The reaction is regioselective with regards to the arene partner, and the role of the acetate and carbonate groups has been elucidated. This methodology adds to the very few examples of benzene coupling without the inclusion of electron-withdrawing groups to increase acidity.

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(167) PELLETIER, G.; BECHARA, W. S.; CHARETTE, A. B.
Controlled and Chemoselective Reduction of Secondary Amides

J. Am. Chem. Soc. 2010, 132, 12817-12819.

This communication describes a metal-free methodology involving an efficient and controlled reduction of secondary amides to imines, aldehydes, and amines in good to excellent yields under ambient pressure and temperature. The process includes a chemoselective activation of a secondary amide with triflic anhydride in the presence of 2-fluoropyridine. The electrophilic activated amide can then be reduced to the corresponding iminium using triethylsilane, a cheap, rather inert, and commercially available reagent. Imines can be isolated after a basic work-up or readily transformed to the aldehydes following an acidic work-up. The amine moiety can be accessed via a sequential reductive amination by the addition of silane and Hantzsch ester hydride (HEH) as a one-pot reaction. Moreover, this reduction tolerates various functional groups, which are usually reactive under reductive conditions, and is very selective to secondary amides.

>> classé dans le top 20 des articles JACS les plus lus en 2010-2011

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(166) BEAULIEU, L,-P. B.; DELVOS, L. B., CHARETTE, A. B.
Dual Role of Silanol Groups in Cyclopropanation and Hiyama-Denmark Cross-Coupling Reactions

Org. Lett. 2010, 12, 1348-1351.

Di-tert-butoxy(alkenyl)silanols serve as substrates in the Simmons-Smith cyclopropanation reaction furnishing the corresponding di-tert-butoxy(cyclopropyl)silanols, which may be included in a Hiyama-Denmark cross-coupling reaction. The silanol group bears two distinct roles as it provides a directing group during the cyclopropanation and mediates the transmetallation event in the cross-coupling. The nature of the ligands on the silicon atom had a profound effect on reactivity in the cross-coupling, whereby substituting the alkoxide groups for fluorides allowed for efficient cross-coupling.

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(165) LEMIRE, A.; CHARETTE, A. B.
Stereoselective Syntheses of L-Pipecolic Acid and (2S,3S)-3-Hydroxypipecolic Acid from a Chiral N-Imino-2-phenyl-1,2-dihydropyridine Intermediate

J. Org. Chem. 2010, 75, 2077-2080.

Stereoselective syntheses of L-pipecolic acid and (2S,3S)-3-hydroxypipecolic acid were achieved from a chiral N-imino-2-phenyl-1,2-dihydropyridine intermediate. The 3-hydroxysubstituent of the latter amino acid was introduced by hetero-Diels-Alder of singlet oxygen with the 1,2-dihydropyridine.

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(164) BULL, J. A.; MOUSSEAU, J. J.; CHARETTE, A. B.
Preparation of (E)-(2-Iodovinyl)Benzene from Benzyl Bromide and Diiodomethane [(E)-β-styryl Iodide]

Org. Synth. 2010, 87, 170-177.

Abstract not available.

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(163) GOUDREAU, S. R.; MARCOUX, D.; CHARETTE, A. B.
Synthesis of Dimethyl 2-Phenylcyclopropane-1,1-Dicarboxylate Using an Iodonium Ylide Derived from Dimethyl Malonate

Org. Synth. 2010, 87, 115-125.

Abstract not available.

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(162) VOITURIEZ, A.; ZIMMER, L. E.; CHARETTE, A. B.
Preparation of a Storable Zinc Carbenoid Species and its Application in Cyclopropanation, Chain Extension, and [2,3]-Sigmatropic Rearrangement Reactions

J. Org. Chem. 2010, 75, 1244-1250.

The formation of a new phospate carbenoid (n-BuO)2P(O)OZnCH2I and its application in organozinc-mediated reactions is described. This carbenoid undergoes ver slow degardation in solution and can be stored for several weeks at -20 °C. Its reactivity was tested with many representative alkenes and was determined to be a powerful cyclopropanating reagent, giving the corresponding cyclopropanes in 72-99% yield. The use of this carbenoid in the chain extension of 1,3-diketones and [2,3]-sigmatropic rearrangement reactions is also described.

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(161) BULL, J. A.; CHARETTE, A. B.
Intramolecular Simmons-Smith Cyclopropanation. Studies into the Reactivity of Alkyl-Substituted Zinc Carbenoids, Effect of Directing Groups and Synthesis of Bicyclo[n.1.0]alkanes

J. Am. Chem. Soc. 2010, 132, 1895-1902.

An intramolecular Simmons-Smith (IMSS) cyclopropanation has been developed, providing a novel method for the construction of substituted bicycloalkanes. Firstly, functionalized gem-diiodoalkanes containing allylic alcohols were prepared in high yield. Then the intramolecular cyclization to form different ring sizes was investigated, and proved to be successful for the synthesis of bicyclo[3.1.0]hexanes and bicyclo[4.1.0]heptanes. Larger chain lengths led to terminal alkene containing products. Analysis of the product distribution for the different ring sizes and under various reaction conditions provided insight into the reactivity of substituted zinc carbenoids, and by the appropriate choice of conditions cyclopropanation could be promoted over alternative reaction pathways. Next the ability of allylic groups to promote the IMSS reaction by directing the zinc carbenoid was examined for the formation of bicycloheptanes. A scale of 'directing-ability' for these allylic groups has been rationalized, with an OMOM directing group providing the greatest enhancement in formation of the bicycle. Finally, the scope of the cyclization in forming substituted bicyclo[3.1.0]hexanes was explored. Substitution on the alkene and at the allylic position was well tolerated providing the bicyclic products in high yields. Additionally, the IMSS reaction allowed a highly diastereoselective synthesis of a 5-3-5 fused tricycloalkane. These studies will have implications for the use of substituted carbenoids in cyclopropanation reactions and for directed cyclopropanation reactions as well as in the synthesis of substituted bicycloalkanes.

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(160) VALLÉE, F.; MOUSSEAU, J, J.; CHARETTE, A. B.
Iron-Catalyzed Direct Arylation Through an Aryl Radical Transfer Pathway

J. Am. Chem. Soc. 2010, 132, 1514-1516.

A general and efficient iron-catalyzed direct arylation of benzene and hetereoaryl derivatives using a cost-effective and environmentally benign catalyst is described. The reaction is performed under neat conditions and can proceed at room temperature.

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(159) MARCOUX, D.; LINDSAY, V. N. G.; CHARETTE, A. B.
Use of Achiral Additives to Increase the Stereoselectivity in Rh(II)-Catalyzed Cyclopropanations

Chem. Commun. 2010, 46, 910-912.

We describe our studies on the effect of various Lewis bases and Brønsted acids as achiral additives on the stereoselectivity of some Rh(II)-catalyzed cyclopropanations.

>> sur invitation dans un numéro spécial en ligne portant sur le thème de Selective Catalysis for Organic Synthesis

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(158) VANIER, S. F.; LAROUCHE, G.; WURZ, R. P.; CHARETTE, A. B.
Formal Synthesis of Belactosin A and Hormaomycin via a Diastereoselective Intramolecular Cyclopropanation of an α-Nitro Diazoester

Org. Lett. 2010, 12, 672-675.

An efficient and convenient methodology for the synthesis of the 3-(trans-2-nitrocyclopropyl) alanine moiety found in the core of belactosin A and hormaomycin is reported. By using an enantioenriched substituted α-nitro diazoester in a diastereoselective intramolecular cyclopropanation reaction, the trans-nitrocyclopropyl alanine moiety can be obtained efficiently in five steps from the initial α-nitrocyclopropyl lactone unit, thus achieving the synthesis of the cyclopropane core of the two natural products.

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(157) CÉRAT, P.; GRITSCH, P. J.; GOUDREAU, S. R.; CHARETTE, A. B.
Synthesis of Enantioenriched Allenes from 1,1-Cyclopropanediesters

Org. Lett. 2010, 12, 564-567.

Highly substituted allenes were obtained by the SN2' addition of organocuprate reagents on 2-propargyl-1,1-cyclopropanediesters. This new methodology permits the synthesis of highly enantioenriched allenes as the reaction proceeds with retention of the enantiomeric purity of the starting cyclopropane. The use of higher order cuprates was instrumental in obtaining the reported results.

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(156) MOUSSEAU, J. J.; FORTIER, A.; CHARETTE, A. B.
Synthesis of 2-Substituted Pyrazolo[1,5-a]Pyridines Through Cascade Direct Alkenylation/Cyclization Reactions

Org. Lett. 2010, 12, 516-519.

The synthesis of 2-substituted pyrazolo[1,5-a]pyridines from N-iminopyridinium ylides is described. These medicinally interesting compounds are formed through a cascade process involving a palladium-catalyzed direct alkenylation reaction followed by silver mediated cyclization. The reaction can be performed with a wide range of electron-poor and electron-rich alkenyl iodides in good yields. This work represents perhaps the most direct route for the preparation of these compounds.

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(155) MOUSSEAU, J. J.; BULL, J. A.; CHARETTE, A. B.
Copper-Catalyzed Direct Alkenylation of N-Iminopyridinium Ylides

Angew. Chem. Int. Ed. 2010, 49, 1115-1118.

A versatile Cu-catalyzed direct C-H alkenylation of N-iminopyridinium ylides, compatible with several different copper sources (including a penny), provides a powerful and inexpensive method for the synthesis of functionalized pyridine derivatives. Chemoselective functionalization of halides-containing compounds allows the synthesis of alkenyl pyridines containing reactive tethers for further functionalization.

>> à la mémoire du Professeur Keith Fagnou

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(154) GOUDREAU, S. R.; CHARETTE, A. B.
Defying Ring Dtrain: New Approaches to Cyclopropanes

Angew. Chem. Int. Ed. 2010, 49, 486-488. (Highlight)

Beating ring strain: two novel cyclopropanation reactions (see scheme; LA = Lewis acid, Nu = nucleophile) achieved a great level of selectivity using surprisingly mild conditions. The first used epoxides as a source of methylene in the synthesis of enantioenriched cyclopropanes. The second pushed the limit of the well-established allylic alkylation chemistry to the synthesis of three-membered rings with excellent ee values.

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2009


(153) LEMIRE, A.; CÔTÉ, A.; JANES, M. K.; CHARETTE, A. B.
Synthesis and Application of Diorganozinc Reagents: Beyond Diethylzinc

Aldrichim. Acta 2009, 42, 71-83.

Diorganozinc reagents are versatile nucleophiles that accommodate either Lewis base or metal catalysis in enantioselective additions to electrophiles. They also display a ver high functional-group tolerance. While a large number of diorganozinc reagents are now readily available via simple procedures, and despite tremendous improvements in accessing functionalized diorganozincs, as pioneered by Knochel, Seebach, Bolm, and others, there is still a strong need to increase their availability, which should significantly enhance their uselfulness in synthesis.

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(152) MARCOUX, D.; GOUDREAU, S. R.; CHARETTE, A. B.
trans-Directing Ability of the Amide Group: Enabling the Enantiocontrol in the Synthesis of 1,1-Dicarboxy Cyclopropanes. Reaction Development, Scope, and Synthetic Applications

J. Org. Chem. 2009, 74, 8939-8955.

In this article, we describe our efforts toward the enantioselective formation of 1,1-cyclopropane diesters via the metal catalyzed cyclopropanation of olefins. The strategies envisioned to achieve such a goal are discussed as well as the results that led us to the discovery of the powerful trans-directing ability of the amide group in Rh(II)-catalyzed cyclopropanation reactions. We show how this feature enables a solution for the stereoselective synthesis of 1,1-dicarboxy cyclopropane derivatives. The scope and limitations are discussed as well as the demonstration that these newly formed cyclopropanes display similar reactivity as 1,1-cyclopropane diesters. Conversely, 1,1-cyclopropane diesters could be accessed in two steps from commercially available alkenes. The potential utility of this methodology is illustrated by several functional group transformations and its use in the expedient stereoselective formal synthesis of (S)-(+)-curcumene, (S)-(+)-nuciferal, (S)-(+)-nuciferol, (+)-erogorgiaene, (±)-xanthorrhizol, and (±)-2-hydroxycalamenene.

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(151) LINDSAY, V. N. G.; LIN, W.; CHARETTE, A. B.
Experimental Evidence for the All-Up Reactive Conformation of Chiral Rhodium(II) Carboxylate Catalysts: Enantioselective Synthesis of cis-Cyclopropane α-Amino Acids

J. Am. Chem. Soc. 2009, 131, 16383-16385.

Useful empirical insights onto the enantioinduction process of chiral Rh(II)-carboxylate catalysts are described in the first catalytic asymmetric cyclopropanation of alkenes with α-nitro diazoacetophenones. X-Ray, solution NMR and reactivity studies made on these complexes suggest that the level of asymmetric induction strongly depends on their active symmetry, which in turn relies on the nature of the chiral ligands' substituents. The catalyst's 'All Up' reactive conformation turned out to be necessary to obtain good stereoselectivity, and the resulting products are shown to be key intermediates in a concise synthesis of highly enantioenriched cis-cyclopropane α-amino acids..

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(150) ROY, M.-N.; POUPON, J.-C.; CHARETTE, A. B.
Tetraarylphosphonium Salts as Soluble Supports for Oxidative Catalysts and Reagents

J. Org. Chem. 2009, 74, 8510-8515.

Tetraarylphosphonium (TAP)-supported DMSO, TEMPO and DIB reagents were synthesized and respectively used for the oxidation of alcohols, including Swern oxidation and for the α-acetoxylation of ketones. By taking advantage of the predictable solubility properties of the TAP unit, simple precipitation and filtration of the phosphonium moiety permits complete separation of the desired oxidation products. This paper describes the preparation of these three TAP-supported oxidative reagents and their activity in the aforementioned oxidative transformations. Furthermore, we have demonstrated that these reagents can be recycled directly when used in catalytic processes and following regeneration when used in stoichiometric processes.

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(149) BEAULIEU, L.-P. B.; ZIMMER, L. E.; CHARETTE, A. B.
Enantio- and Diastereoselective Iodocyclopropanation of Allylic Alcohols Using a Substituted Zinc Carbenoid

Chem. Eur. J. 2009, 15, 11829-11832.

(Zinc)redible! 2,3-Disubstituted and 2,3-trisubstituted iodocyclopropanes can be accessed in enantiomerically enriched form via the enantioselective Simmons-Smith iodocyclopropanation of allylic alcohols in presence of the stoichiometric dioxaborolane chiral ligand. These iodocyclopropanes serve as versatile chiral building blocks towards highly functionalized 1,2,3-trisubstituted cyclopropanes.

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(148) GOUDREAU, S. R.; CHARETTE, A. B.
In situ Generation of Zinc Carbenoids from Diazo Compounds and Zinc Salts: Asymmetric Synthesis of 1,2,3-Substituted Cyclopropanes

J. Am. Chem. Soc. 2009, 131, 15633-15635.

The first enantioselective cyclopropanation of alkenes using zinc carbenoids generated in situ from diazo compounds and zinc salts is reported. This new method allows the highly enantio- and diastereoselective synthesis of 1,2,3-substituted cyclopropanes via aryl-substituted carbenoids. The first Simmons-Smith reaction using a catalytic amount of zinc to generate enantioenriched cyclopropane is also reported.

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(147) ZIMMER, L. E.; CHARETTE, A. B.
Enantioselective Synthesis of 1,2,3-Trisubstituted Cyclopropanes Using gem-Dizinc Reagents

J. Am. Chem. Soc. 2009, 131, 15624-15626.

The first asymmetric cyclopropanation of allylic alcohols using gem-dizinc carbenoids that allows the synthesis of 1,2,3-substituted cyclopropane derivatives in high yields and excellent enantio- and diastereoselectivities is reported. The initially formed cyclopropyl zinc undergoes an in situ B/Zn exchange with the stoichiometric chiral ligand to generate a cyclopropyl borinate that can be directly engaged in a Suzuki-Miyaura cross-coupling.

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(146) BARBE, G.; PELLETIER, G.; CHARETTE, A. B.
Intramolecular Pyridine Activation-Dearomatization Reaction: Highly Stereoselective Synthesis of Polysubstituted Indolizidines and Quinolizidines

Org. Lett. 2009, 11, 3398-3401.

An unprecedented intramolecular pyridine activation-asymmetric dearomatization reaction is described. This process produces 5-substituted indolizidines and 6-substituted quinolizidines in excellent yields and in a highly regio- and diastereoselective fashion. Formal syntheses of trans-indolizidine alkaloids are presented along with some preliminary results in the formation C-5 quaternary centers.

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(145) BONNAVENTURE, I.; CHARETTE, A. B.
Stereoselective Synthesis of N-Heterocycles: Application of the Asymmetric Cu-Catalyzed Addition of Et2Zn to Functionalized Alkyl and Aryl Imines

Tetrahedron 2009, 65, 4968-4976.

The preparation of N-heterocycles from alkyl- and aryl-substituted imines is described. The key step involves a copper-catalyzed addition of diethylzinc to functionalized alkyl-substituted imines that were generated in situ from the sulfinic acid adducts. The nucleophilic addition products were then converted to 2-substituted pyrrolidines and piperidines. Aryl-substituted imines were transformed into enantioenriched 1- and 1,4-substituted tetrahydroisoquinolines.

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(144) MARCOUX, D.; AZZI, S.; CHARETTE, A. B.
TfNH2 as Achiral Hydrogen-Bond Donor Additive to Enhance the Selectivity of a Transition Metal-Catalyzed Reaction. Highly Enantio- and Diastereoselective Rhodium-Catalyzed Cyclopropanation of Alkenes Using α-Cyano Diazoacetamide

J. Am. Chem. Soc. 2009, 131, 6970-6972.

A highly stereoselective (up to 98% ee and 99:1 dr) Rh(II)-catalyzed formation of nitrile-substituted cyclopropanes is described. α-Cyano diazoacetamide reagents react with a variety of mono- and disubstituted olefins in good yields and excellent enantio- and diastereocontrol. Less reactive substrates, such as aliphatic olefins, also undergo the reaction. This new methodology features the unprecedented use of an achiral hydrogen-bond donor additive to enhance the selectivity and exploits the powerful trans-directing ability of amides solving the diastereocontrol issue of the formation of substituted 1-cyanocyclopropane-1-carboxy derivatives.

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(143) GOUDREAU, S. R.; MARCOUX, D.; CHARETTE, A. B.
General Method for the Synthesis of Phenyliodonium Ylides from Malonate Esters: Easy Access to 1,1-Cyclopropane Diesters

J. Org. Chem. 2009, 74, 470-473.

A general method to access phenyliodonium ylides from malonates has been developed. These ylides provide easy access to a variety of useful 1,1-cyclopropane diesters using rhodium or copper catalysis. Moreover, the iodonium ylide of dimethyl malonate was obtained in 78% yield using improved conditions that involve a simple filtration step to isolate the desired product. This ylide was shown to be a safer and convenient alternative to the corresponding diazo compound and a very efficient way to 1,1-cyclopropane diesters when used with a catalytic amount of Rh2(esp)2.

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2008


(142) MARCOUX, D.; CHARETTE, A. B.
Nickel-Catalyzed Synthesis of Phosphonium Salts from Aryl Halides and Triphenylphosphine

Adv. Synth. Catal. 2008, 350, 2967-2974.

An efficient method to synthesize functionalized tetraarylphosphonium salts is described. The nickel-catalyzed coupling reaction between aryl iodides, bromides, chlorides, or triflates and triphenylphosphine generates tetraarylphosphonium salts in high yields. The coupling is wide in scope and tolerates a variety of functional groups such as alcohols, amides, ketones, aldehydes, phenols, phosphines and amines.

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(141) MARCOUX, D.; CHARETTE, A. B.
trans-Directing Ability of Amide Groups in Cyclopropanation: Application to the Asymmetric Cyclopropanation of Alkenes with Diazo Reagents Bearing Two Carboxy Groups

Angew. Chem. Int. Ed. 2008, 47, 10155-10158.

A highly enantioselective (up to 97% ee) and diastereoselective (> 30:1 dr) Rh(II)-catalyzed cyclopropanation of alkenes using a diazo reagent bearing two carboxyl groups is described. This new methodology exploits the powerful trans-directing ability of amides to improve enantiocontrol. Mono- and disubstituted olefins are cyclopropanated in good yields. These newly formed cyclopropanes could be converted into a wide range of synthetically useful chiral building blocks.

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(140) BARBE, G.; ST-ONGE, M.; CHARETTE, A. B.
Silver Ion-Induced Grob Fragmentation of γ-Amino Iodides: Highly Stereoselective Synthesis of Polysubstituted Piperidines

Org. Lett. 2008, 10, 5497-5499.

A new concerted silver ion-mediated Grob fragmentation process is described in which a 1,2-dihydropyridinium ion is formed and trapped in situ with Grignard reagents in a highly regio- and diastereoselective fashion. Using this methodology, 2,3,6-trisubstituted piperidines were synthesized in good yields and further derivatized to polysubstituted indolizidine.

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(139) BULL, J. A.; MOUSSEAU, J. J.; CHARETTE A. B.
Convenient One-Pot Synthesis of (E)-β-Aryl Vinyl Halides from Benzyl Bromides and Dihalomethanes

Org. Lett. 2008, 10, 5485-5488.

(E)-β-aryl vinyl iodides are synthesized in high yield with excellent stereoselectivity from benzyl bromides by a one-pot homologation/stereoselective elimination procedure. Convenient conditions involving the anion of diiodomethane and an excess of base provide complete consumption of the benzyl bromide and elimination from a diiodoalkane intermediate. Similar coniditons provide (E)-β-aryl vinyl chlorides and bromides by employing the anions of ICH2Cl or CH2Br2. The functional group tolerance and facile purification allows rapid access to a wide range of functional vinyl halides.

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(138) CHARETTE, A. B.
Short Cuts to Complexity

Nature 2008, 456, 451-453.

The credit crunch is forcing people to tighten their belts, but chemists have long known the benefits of being economical with atoms. The latest synthesis of an anticancer agent shows how effective parsimony can be.

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(137) PELLETIER, G.; LARIVÉE, A.; CHARETTE, A. B.
Highly Regioselective Intermolecular Arylation of 1,2,3,4-Tetrahydropyridines

Org. Lett. 2008, 10, 4791-4794.

Using a catalytic amount of PdCl2(dppf)·CH2Cl2 in combination with Ag3PO4 and NaOAc, a range of arylated 1,2,3,4-tetrahydropyridines are synthesized in good yields and with complete selectivity at the β-position. The reaction is compatible with a variety of electron-donating and electron-withdrawing aryl iodides as well as with heteroaryl iodides. The application of these tetrahydropyridines toward the synthesis of polysubstituted piperidines is also demonstrated.

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(136) BARBE, G.; CHARETTE, A. B.
Total Synthesis of (+)-Lepadin B: Stereoselective Synthesis of Nonracemic Polysubstituted Hydroquinolines Using an RC-ROM Process.

J. Am. Chem. Soc. 2008, 130, 13873-13875.

This communication describes a new tandem metathesis reaction for which a RC-ROM mechanism was experimentally supported. This process was successfully applied to the synthesis of cis-fused polyhydroquinolines enabling a short stereoselective total synthesis of ent-lepadin B.

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(135) BULL, J. A.; CHARETTE, A. B.
Improved Procedure for the Synthesis of gem-Diiodoalkanes by the Alkylation of Diiodomethane. Scope and Limitations

J. Org. Chem. 2008, 73, 8097-8100.

Functionalized gem-diiodoalkanes are obtained in good to excellent yields by employing a convenient modified procedure for the alkylation of NaCHI2 with alkyl iodides. Complete conversion to the diiodide is reliably obtained avoiding a problematic separation of any remaining iodide starting material. Functional group tolerance towards olefins, acetals, ethers and silyl ethers, carbamates and hindered esters is demonstrated. The use of an excess of LiCHI2 allows complete conversion of allyl and benzyl bromides with minimal elimination from the diiodide product.

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(134) LEGAULT, C. Y.; CHARETTE, A. B. ; COZZI, P. G.
Iridium Catalyzed Enantioselective Hydrogenation of N-Iminopyridinium Ylides: Mechanistic Insights

Heterocycles 2008, 76, 1271-1283.

Mechanistic insights of the efficient iridium catalyzed asymmetric hydrogenation of N-iminopyridinium ylides are reported. Using NMR and mass spectrometry, the nature of the pre-catalyst and role of iodine was studied. The effect of solvent as well as an extensive ligand screening is presented. Deuteration experiments also show that C-H(D) insertion pathways occur in competition with the hydrogenation process.

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(133) LIFCHITS, O.; ALBERICO, D.; ZAKHARIAN, I.; CHARETTE. A. B.
Nucleophilic Addition of Phenol Derivatives to Methyl 1-Nitrocyclopropyl Carboxylates

J. Org. Chem. 2008, 73, 6838-6840.

Nucleophilic ring opening of methyl 1-nitrocyclopropane-carboxylates by phenol derivatives in the presence of Cs2CO3 is described. The reaction tolerates a variety of substituents on both the aromatic alcohol and the cyclopropane, and affords the products in good yields (53-84%) and with complete preservation of the enantiomeric excess at C-4. The methodology was applied in an enantioselective synthesis of the norepinephrine reuptake inhibitor atomoxetine (StratteraTM).

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(132) BONNAVENTURE, I.; CHARETTE, A. B.
Probing the Importance of the Hemilabile Site of Bis(phosphine) Monoxide Ligands in the Copper-Catalyzed Addition of Diethylzinc to N-Phosphinoylimines: Discovery of New Effective Chiral Ligands

J. Org. Chem. 2008, 73, 6330-6340.

The hemilabile ligand Me-DuPHOS(O) 2 has proven to be a successful ligand for the copper-catalyzed addition of diethylzinc to N-phosphinoylimines. The corresponding α-chiral amines were obtained in high yields (80-98%) and enantiomeric ratios (19.0:1 to 99.0:1 er). Furthermore, this Cu·2 catalytic system has been shown to be effective in the addition of diethylzinc to nitroalkenes and in the reduction of β,β-disubstituted vinyl phenyl sulfones. This paper describes a general structure/selectivity study in which the three ligand subunits (chiral phospholane-linker-labile coordinating group (Z)) are systematically modified and tested in the copper-catalyzed addition of diethylzinc to the N-phosphinoylimine 1 derived from benzaldehyde. This study led to the discovery of a new class of effective chiral ligands that combine a chiral phospholane unit and an achiral phosphine oxide.

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(131) LIFCHITS, O.; CHARETTE, A. B.
A Mild Procedure for the Lewis Acid-Catalyzed Ring-Opening of Activated Cyclopropanes with Amine Nucleophiles

Org. Lett. 2008, 10, 2809-2812.

The Lewis acid-catalyzed ring-opening of methyl 1-nitrocyclopropanecarboxylates with amine nucleophiles is described. The reaction proceeds at room temperature and with complete preservation of the enantiomeric purity from the electrophilic center of the cyclopropane to the acyclic product. The methodology was applied in an enantioselective synthesis of the dual serotonin/norepinephrine ruptake inhibitor (3R)-3-(1H-indol-1-yl)-N-methyl-3-phenylpropan-1-amine.

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(130) DESROSIERS, J.-N.; BECHARA, W. S.: CHARETTE, A. B.
Catalytic Enantioselective Addition of Diorganozinc Reagents to Vinyl Sulfones

Org. Lett. 2008, 10, 2315-2318.

NB : l'auteur de la correspondance a été corrigé dans Org. Lett. 2008, 10, 4709-4709 (Additions/Corrections OL802157Q).

An efficient method for the catalytic asymmetric conjugate addition of diorganozinc reagents to vinyl sulfones is reported. Using a Binap·Cu complex, enantioselectivities up to 98% ee and high yields can be attained for a variety of substrates. Several dialkylzinc reagents are also compatible with this procedure.

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(129) CHARETTE, A. B.; CÔTÉ, A.; DESROSIERS, J.-N.; BONNAVENTURE, I.; LINDSAY, V. N. G.; LAUZON, C.; TANNOUS, J.; BOEZIO, A. A.
New Methods in Asymmetric Catalysis Based on New Hemi-Labile Bidentate Ligands

Pure Appl. Chem. 2008, 80, 881-890.

Chiral bidentate hemi-labile bis(phosphine) monoxide ligands were shown to be quite effective in various copper-catalyzed transformations. Among them, the nucleophilic addition to imines, the conjugate addition to α,β-unsaturated nitro derivatives, and the conjugate reduction of α,β-unsaturated sulfones generally gave good to excelletn yields and high enantiomeric excesses.

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(128) MOUSSEAU, J. J.; LARIVÉE, A.; CHARETTE, A. B.
Palladium-Catalyzed Benzylic C-H Insertion of 2-Substituted N-Iminopyridinium Ylides

Org. Lett. 2008, 10, 1641-1643.

Palladium-catalyzed direct benzylic C-H arylation of 2-alkyl substituted N-iminopyridinium ylides is described. The insertion can be conducted with several electron-poor and electron-rich aryl chlorides in good yields. This work adds to the few examples of sp3 C-H insertions that have been reported so far.

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(127) GINISTY, M.; ROY, M.-N.; POUPON, J.-C.; PICARD, A.; CHARETTE, A. B.
Tetraarylphosphonium-Supported Carbodiimide Reagents: Synthesis, Structure Optimization and Applications

J. Org. Chem. 2008, 73, 2542-2547.

NB : Deux nouveaux auteurs se sont ajoutés dans J. Org. Chem. 2008, 73, 6946-6946 (Additions/Corrections JO801495B).

New tetraarylphosphonium (TAP)-supported alkyl- and arylcarbodiimides were synthesized and used as coupling reagents for esterification reactions, amidation reactions and dehydration reactions of hydroxyesters. Taking advantage of the solubility properties imparted by the tetraarylphosphonium unit, a simple precipitation and filtration allowed complete separation of the urea by-products. This paper describes the structure optimization study of the various TAP-supported carbodiimide reagents to obtain the desired reactivity and solubility profile. Furthermore, we have demonstrated that the diimide reagent can be regenerated from the urea to recycle the reagents.

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(126) PERREAULT, C.; GOUDREAU, S. R.; ZIMMER, L. E.; CHARETTE, A. B.
Cycloadditions of Aromatic Azomethine Imines with 1,1-Cyclopropane Diesters

Org. Lett. 2008, 10, 689-692.

The cycloaddition of aromatic azomethine imines to 1,1-cyclopropane diesters was achieved using Ni(ClO4)2 as catalyst. The methodology gives access to unique tricyclic dihydroquinoline derivatives with dr up to 6.6:1. A non-concerted mechanism is proposed on the basis of stereochemical analysis of the reaction.

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(125) CÔTÉ, A.; CHARETTE, A. B.
General Method for the Expedient Synthesis of Salt-Free Diorganozinc Reagents Using Zinc Methoxide

J. Am. Chem. Soc. 2008, 130, 2771-2773.

The effect of counterions has a great impact on the solubility magnesium salts in Et2O. By reacting Zn(OMe)2 with readily available Grignard reagents, it was possible to induce the complete precipitation of magnesium salts and then obtain salt-free diorganozinc reagents after centrifugation/filtration. This practical method to generate dialkylzinc reagents as well as mixed diorganozinc reagents was successfully tested in various catalytic enantioselective reactions, proving the purity of the product and its synthetic usefulness.

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(124) MARCOUX, D.; CHARETTE, A. B.
Palladium-Catalyzed Synthesis of Functionalized Tetraarylphosphonium Salts

J. Org. Chem. 2008, 73, 590-593.

An efficient method to synthesize functionalized tetraarylphosphonium salts is described. This palladium-catalyzed coupling reaction between aryl iodides, bromides, or triflates and triphenylphosphine generates phosphonium salts in high yields. The coupling is compatible with a variety of functional groups such as alcohols, ketones, aldehydes, phenols and amides.

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(123) LARIVÉE, A.; MOUSSEAU, J. J.; CHARETTE, A. B.
Palladium-Catalyzed Direct C-H Arylation of N-Iminopyridinium Ylides: Application to the Synthesis of (±)-Anabasine

J. Am. Chem. Soc. 2008, 130, 52-54.

Palladium-catalyzed direct C-H arylation of N-iminopyridinium ylides provides a powerful and versatile method for the synthesis of functionalized piperidines in good yields. Chemoselective functionalization of the pyridinium ring in the presence of a pyridine substituent is possible, as exemplified by the expedient synthesis of anabasine in 61% overall yield over 3 steps.

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(122) BARBE, G.; CHARETTE, A. B.
Highly Chemoselective Metal-Free Reduction of Tertiary Amides

J. Am. Chem. Soc. 2008, 130, 18-19.

This communication describes the chemoselective metal-free reduction of tertiary amides to the corresponding amines. Hantzsch ester is used as a mild reducing agent for the reduction of trifluoromethanesulfonic anhydride activated amides providing the tertiary amines with high functional group tolerance.

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2007


(121) POUPON, J.-C.; MARCOUX, D.; CLOAREC, J.-M.; CHARETTE, A. B.
Removal, Recovery, and Recycling of Triarylphosphonium-Supported Tin Reagents for Various Organic Transformations

Org. Lett. 2007, 9, 3591-3594.

Phosphonium-supported tin reagents and catalysts were prepared and were shown to be effective in Stille couplings, radical dehalogenations, radical cyclizations, and carbonyl allylations. Not only could the tin residues be removed from the crude reaction mixture through a phase separation process but also they could be recovered and recycled.

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(120) DESROSIERS, J.-N.; CHARETTE, A. B.
Catalytic Enantioselective Reduction of β,β-Disubstituted Vinyl Phenyl Sulfones by Using Bisphosphine Monoxide Ligands

Angew. Chem. Int. Ed. 2007, 46, 5955-5957.

Synthesis of chiral sulfones: a copper-phosphine complex efficiently provided optically active alkyl phenyl sulfones by hydrosilylation at room temperature. The reduction of β,μ-disubstituted vinyl sulfones in the presence of the Me-DuPhos monoxide ligand leads to excellent enantiomeric excesses and high yields. The prepared chiral sulfones were desulfonylated and subjected to Julia olefination conditions.

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(119) STAZI, F.; MARCOUX, D.; POUPON, J.-C.; LATASSA, D.; CHARETTE, A. B.
Tetraarylphosphonium Salts as Soluble Supports for the Synthesis of Small Molecules

Angew. Chem. Int. Ed. 2007, 46, 5011-5014.

Phosphonium salts can be used as solubility-control groups (SCGs) in small-molecule and peptide synthesis. The concept, which is a hybrid approach between solution- and solid-phase syntheses, is illustrated by the preparation of 2-substituted piperidine derivatives and a variety of small peptides with high yields and purities. In some cases, recycling of the support is possible.

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(118)MOREAU, B.; GINISTY, M.; ALBERICO, D.; CHARETTE, A. B.
Expedient Stereoselective Synthesis of Coronafacic Acid Through Intramolecular Diels-Alder Cyclization

J. Org. Chem. 2007, 72, 1235-1240.

A stereoselective synthesis of coronafacic acid, a natural component of the phytotoxin coronatin, was achieved using an intramolecular Diels-Alder reaction as the key step. The triene precursor bearing a substituted diene and a vinylketone as dienophile was synthesized and then tested in the thermal intramolecular cyclization. We have devised a new strategy to assemble the E,Z-diene through the stereoselective aldol reaction of an ester enolate followed by a stereoselective dehydration. Following the thermal cyclization, the corresponding hydrindanone thereby obtained with the desired relative stereochemistry could easily be converted into the natural product. The synthesis of coronafacic acid was accomplished in six steps in 29% yield.

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(117) CÔTÉ, A.; LINDSAY, V. N. G.; CHARETTE, A. B.
Application of the Chiral Bis(phosphine) Monoxide Ligand to Catalytic Enantioselective Addition of Dialkylzinc Reagents to β-Nitroalkenes

Org. Lett. 2007, 9, 85-87.

Me-DuPHOS monoxide is shown to be a very effective ligand in the enantioselective copper-catalyzed addition of dialkylzinc reagents to β-nitroalkenes providing access to chiral nitroalkanes. The major advantages of this process are high yields, broad and complementary substrate scope, and high enantioselectivities. The effect of achiral dummy ligands as an additive has also been documented.

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2006


(116) VOITURIEZ, A.; CHARETTE, A. B.
Enantioselective Cyclopropanation with TADDOL-Derived Phosphate Ligands

Adv. Synth. Catal. 2006, 348, 2363-2370.

The synthesis of new chiral TADDOL-derived phosphate ligands is described. The ligands were efficiently synthesized on a multi-gram scale in three steps starting from the readily available corresponding TADDOL and were fully characterized. An X-ray structure was obtained and compared to known BINOL-phosphates. Their use in the asymmetric Simmons-Smith cyclopropanation of both functionalized and unfunctionalized olefins gave the desired cyclopropanes in good yields and good to moderate enantioselectivities.

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(115) ZOUTE, L.; LACOMBE, C.; QUIRION, J.-C.; CHARETTE, A. B.; JUBAULT, P.
Phosphonium Supported Triphenylphosphine Reagent: an Improved Access to α-Fluoro-α,β-Unsaturated Esters

Tetrahedron Lett. 2006, 47, 7931-7933.

α-Fluoro-α,β-unsaturated esters 2 were efficiently synthesized via diethylzinc-promoted Wittig reaction using a phosphonium-supported triphenylphosphine SCG-PPh3 1, which possesses similar reactivity as its parent analog triphenylphosphine. The main advantage of this system is the use of a novel low-molecular-weight support that is soluble in solvents of medium polarities for the attachment of reagents and insoluble in solvents of low polarities.

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(114) LARIVÉE, A.; CHARETTE, A. B.
New Methodology Towards Chiral, Non-Racemic 2,5-cis-Substituted Piperidines via Suzuki Cross-Coupling

Org. Lett. 2006, 8, 3955-3957.

1,2,3,4-Tetrahydropyridines were halogenated upon treatment with iodine to obtain the desired cross-coupling precursors. Diastereoselective hydrogenation of Suzuki cross-coupling adducts allowed the facile asymmetric synthesis of 2,5-cis-substituted piperidines in five steps from readily available pyridine.

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(113) FOCKEN, T.; CHARETTE, A. B.
Stereoselective Synthesis of Pyridinones: Application to the Synthesis of (-)-Barrenanzines

Org. Lett. 2006, 8, 2985-2988.

The stereoselective synthesis of pyridinones was accomplished by the nucleophilic addition of Grignard reagents to a chiral pyridinium salt derived from 4-methoxypyridine. This methodology was applied to an expedient synthesis of (-)-barrenazine A and B. After N-functionalization and 1,4-reduction of the pyridinone system, the corresponding α-amino piperidinones readily undergo dimerization to give the hexahydrodipyridinopyrazine skeleton of the barrenazines alkaloids.

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(112) LAUZON, C.; CHARETTE, A. B.
Catalytic Asymmetric Synthesis of α,α,α-Trifluoromethylamines by the Copper-Catalyzed Nucleophilic Addition of Diorganozinc Reagents to Imines

Org. Lett. 2006, 8, 2743-2745.

A copper-catalyzed asymmetric addition of diorganozinc reagents to N-phosphinoylimines has been developed for the synthesis of chiral α,α,α-trifluoromethylamines. The trifluoromethyl ketimines, generated in situ from the corresponding hemiaminals, led to the chiral amides in high yields (71-89%) and excellent enantiocontrol (91-99% ee).

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(111) POUPON, J.-C.; BOEZIO, A. A.; CHARETTE, A. B.
Tetraarylphosphonium Salts as Solubility-Control Groups: Phosphonium-Supported Triphenylphosphine and Azodicarboxylate Reagents

Angew. Chem. Int. Ed. 2006, 45, 1415-1420.

Reagents and reaction by-products supported on the tetraarylphosphonium group have predictable solubilities. The reagents and their by-products can easily be removed, recovered and recycled after a reaction by a precipitation/filtration sequence that is initiated by the addition of a low polarity solvent such as diethyl ether. Furthermore, the solubility control group (SCG) supported reagents are robust and they possess similar reactivities as their parent analog.

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(110) DESROSIERS, J.-N.; CÔTÉ, A.; BOEZIO, A. A.; CHARETTE, A. B.
Preparation of Enantiomerically Enriched (1S)-1-Phenylpropan-1-Amine Hydrochloride by a Catalytic Addition of Diorganozinc Reagents to Imines

Org. Synth. 2006, 83, 5-17.

Abstract not available

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(109) CÔTÉ, A.; DESROSIERS, J.-N.; BOEZIO, A. A.; CHARETTE, A. B.
Preparation of Enantiomerically Pure (R,R)-BozPHOS

Org. Synth. 2006, 83, 1-4.

Abstract not available.

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2005


(108) MOREAU, B.; CHARETTE, A. B.
Expedient Synthesis of Cyclopropane α-Amino Acids by the Catalytic Asymmetric Cyclopropanation of Alkenes Using Iodonium Ylides Derived from Methyl Nitroacetate

J. Am. Chem. Soc. 2005, 127, 18014-18015.

A highly enantioselective (up to 97.5% ee) and diastereoselective (95:5 dr trans/cis) Cu(I)-catalyzed cyclopropanation of alkenes using phenyliodonium ylide generated in situ from iodosobenzene and methyl nitroacetate is reported. The cyclopropanation took place with high enantioselectivity for a wide range of alkenes and reaction was performed at room temperature. 1-Nitrocyclopropyl esters are versatile building blocks to access the corresponding cyclopropane amino esters and aminocyclopropanes in two and three steps respectively, from commercially available products.

>> souligné dans Synfacts par la rédaction

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(107) SALES, M.; CHARETTE, A. B.
A Diels-Alder Approach to the Stereoselective Synthesis of 2,3,5,6-Tetra- and 2,3,4,5,6-Pentasubstituted Piperidines

Org. Lett. 2005, 7, 5773-5776.

A stereoselective synthesis of 2,3,5,6-tetra- and 2,3,4,5,6-pentasubstituted piperidines was achieved from oxidative cleavage of 2-aza-bicyclo-[2.2.2]octene Diels-Alder adducts derived from N-protected 2-methyl-1,2-dihydropyridine. A chiral auxiliary mediated asymmetric synthesis of the pentasubstituted piperidine is also demonstrated. This methodology incorporates orthogonal protecting groups, thus providing a piperidine scaffold with easily modified points fo diversity.

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(106) CÔTÉ, A.; CHARETTE, A. B.
Multicomponent One-Pot Procedure for the Synthesis of Free α-Chiral Amines from Aldehydes

J. Org. Chem. 2005, 70, 10864-10867.

The synthesis of free α-chiral amines by a one-pot multicomponent procedure from commercially available starting materials is described. This enantioselective reaction involves a catalytic asymmetric addition of dialkylzinc reagents to N-diphenylphosphinoylimines with use of an air-stable precatalyst complex 1. The α-chiral amines are prepared with a one-pot procedure from alkyl and aryl akdehydes in good yield (41-90%) and with excellent enantioselectivity.

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(105) LAUZON, C.; DESROSIERS, J.-N.; CHARETTE, A. B.
Synthesis of N-Diphenylphosphinoyiimines Using the Kresze Reaction

J. Org. Chem. 2005, 70, 10579-10580.

The synthesis of N-diphenylphosphinoylimines involving the treatment of aldehydes with P,P-diphenyl N-sulfinylphosphoramidate (Ph2P(O)NSO) is described. the reagent is prepared from P,P-diphenylphosphinic amide, thiomyl chloride, and imidazole.

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(104) CHARETTE, A. B.; MATHIEU, S.; MARTEL, J.
Electrophilic Activation of Lactams with Tf2O and Pyridine: Expedient Synthesis of (±)-Tetraponerine T4

Org. Lett. 2005, 7, 5401-5404.

The electrophilic activation of lactams with triflic anhydride in the presence of pyridine was investigated by NMR. It was found that 2-pyrrolidone led to the clean formation of the corresponding pyridinium imidate in 89% isolated yield. The subsequent nucleophilic addition of organometallic reagents led to 2-substituted dihyropyridines. A synthesis of (±)-tetraponerine T4 with three simple building blocks was accomplished in 3 steps (38% yield).

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(103) LIN, W.; CHARETTE, A. B.
Rhodium-Catalyzed Asymmetric Intramolecular Cyclopropanation of Substituted Allylic Cyanodiazoacetates

Adv. Synth. Catal. 2005, 347, 1547-1552.

A chiral rhodium catalyst, Rh2[4S-(4')-FBNAZ]4, was synthesized and was shown to catalyze the intramolecular cyclopropanation of substituted allylic cyanodiazoacetates. Alkenes bearing an electron-deficient substituent including carbonyl and halogens are converted into the corresponding cyclopropanes in high yields with both chiral and achiral rhodium catalysts. The cyclopropane derivatives were generated with an enantioselectivity of up to 91% ee. For the asymmetric intramolecular cyclopropanation, cis-halogen-substituted substrates afforded cyclopropanes with higher enantioselectivites then the corresponding trans diastereomers. However, only moderate enantioselectivities were observed with alkenes bearing electron-donating groups such as alkyls. The cyclopropanation of 3-substituted 2-propenyl cyanodiazoacetates was strongly influenced by steric and electronic factors arising from substituent on the alkenes. For the first time, we demonstrated that the intramolecular reaction of gem-dihaloallylic cyanodiazoacetate afforded highly functionalized gem-dihalocyclopropanes. This reaction is an appealing alternative to the addition of dihalocarbenes to alkenes for the formation of gem-dihalocyclopropanes.

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(102) FOURNIER, J.-F.; MATHIEU, S.; CHARETTE, A. B.
Diastereoselective Zinco-Cyclopropanation of Chiral Allylic Alcohols with gem-Dizinc Carbenoids

J. Am. Chem. Soc. 2005, 127, 13140-13141.

The highly diastereoselective zinco-cyclopropanation of chiral allylic alcohols using gem-dizinc carbenoids is described. The reaction produces three contiguous stereogenic centers, and the resulting chiral cyclopropylzinc derivatives can be trapped with electrophiles with retention of configuration. Simple functional group manipulations lead to the efficient synthesis of orthogonally protected 1,2,3-substituted cyclopropane derivatives.

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(101) LACASSE, M.-C.; POULARD, C.; CHARETTE, A. B.
Iodomethylzinc Phosphates: Powerful Reagents for the Cyclopropanation of Alkenes

J. Am. Chem. Soc. 2005,127, 12440-12441.

A new family of zinc carbenoids derived from phosphoric acids was developed and used in the cyclopropanation of allylic alcohols and ehters and also of unfunctionalized olefins. The use of the chiral phosphoric acid of a 3,3'-disubstituted BINOL leads to efficient stereocontrol, affording the cyclopropanes of allylic and homoallylic ethers with complete conversions and up to 93% ee. A catalytic version of this reaction using 10 mol% of the chiral phospate reagent is also disclosed.

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(100) LEGAULT, C. Y.; KENDALL, C; CHARETTE, A. B.
Structure and Reactivity of a New Anionic N-Heterocyclic Varbene Silver(I) Complex

Chem. Commun. 2005, 3826-3828.

The efficient synthesis of a N-benzoyliminoimidazolium ylide provides access to a new type of N-hetereocyclic anionic carbene, from which air stable Ag(I) and Cu(II) complexes and a catalytically active Rh(I) complex were formed.

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(99) CHARETTE, A. B.; MATHIEU, S.; FOURNIER, J.-F.
Diastereoselective Synthesis of 1,2,3-Substituted Potassium Cycloproplyl Trifluoroborates via an Unusual Zinc-Boron Exchange

Synlett 2005, 1779-1782.

Diastereoselective cyclopropanation of an allylic alcohol with a gem-dizinc carbenoid followed by an unusual zinc-boron exchange and further treatment with excess KHF2 afforded 1,2,3-syn-cis-substituted cyclopropyl trifluoroborates in 58-63% overall yields. The potassium cyclopropyl trifluoroborates underwent Suzuki-Miyaura cross-coupling reactions to give 1,2,3-functionalized cyclopropanes in good yields. Finally, an oxidation-epimerization sequence gave access to 1,2,3-trans-substituted cyclopropyl trifluoroborate.

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(98) LEGAULT, C. Y.; CHARETTE, A. B.
Catalytic Asymmetric Hydrogenation of N-Iminopyridinium Ylides: Expedient Approach to Enantioenriched Substituted Piperidine Derivatives

J. Am. Chem. Soc. 2005, 127, 8966-8967.

We have developed an efficient catalytic enantioselective hydrogenation of pyridine derivatives. Enhanced reactivity was possible by an optimization of the electronic properties of the catalyst through ligand modification. This methodology shows the particular reactivity of N-iminopyridinium ylides that provides access to substituted piperidines in good enantiomeric excesses.

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(97) LEMIRE, A.; CHARETTE, A. B.
Stereoselective Synthesis of 2,6-Disubstituted 3-Piperidinols: Application to the Rxpedient Synthesis of (+)-Julifloridine

Org. Lett. 2005, 7, 2747-2750.

The asymmetric synthesis of 2,6-disubstituted 3-piperidinols having a 2,3-cis and 2,6-trans relative stereochemistry was accomplished in three steps using the following sequence: stereocontrolled nucleophilic addition an organomagnesium reagent to a chiral pyridinium salts; monohydrogenation of the resulting 2-substituted 1,2-dihydropyridine; and a one-pot, highly diastereoselective epoxidation-nucleophilic addition with a heteroatom nucleophile or an organometallic reagent. This methodology was applied to the expedient asymmetric synthesis of (+)-julifloridine in four steps.

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(96) DESROSIERS, J.-N.; CÔTÉ, A.; CHARETTE, A. B.
Enantioselective Synthesis of β-Amino Alcohols and α-Amino Acids via a Copper Catalyzed Addition of Diorganozinc Reagents to N-Phosphinoylimines

Tetrahedron 2005, 61, 6186-6192.

Enantioenriched β-amino alcohols were prepared via an asymmetric addition of diethylzinc, catalyzed by the BozPHOS·Cu(I) complex, on in situ formed N-phosphinoylimines. the nature of the hydroxyl protecting groups was found to affect the enantioselectivities. Subsequent deprotection and oxidation of N-phosphinoyl β-amino alcohols afforded optically active α-amino acids (97% ee).

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(95) WURZ, R. P.; CHARETTE, A. B.
Doubly Activated Cyclopropanes as Synthetic Precursors for the Preparation of 4-Nitro- and 4-Cyano-Dihydropyrroles and Pyrroles

Org. Lett. 2005, 7, 2313-2316.

1-Nitro-and 1-cyano-cyclopropyl ketones have been prepared in an expedient manner from cyclopropanation reactions of alkenes by diazo compounds or in situ-generated phenyliodonium ylides catalyzed by Rh(II) carboxylates. The doubly activated cyclopropanes were used as synthetic precursors for the regiospecific synthesis of 4-nitro- and 4-cyano-dihydropyrroles upon treatment with primary amines. oxidation of the dihydropyrroles with DDQ allows rapid access to densely functionalized pyrroles.

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(94) CHARETTE, A. B.; BOEZIO, A. A.; CÔTÉ, A.; MOREAU, E.; PYTKOWICZ, J.; DESROSIERS, J.-N.; LEGAULT, C.
Asymmetric Catalytic Addition of Diorganozinc Reagents to Imines: Scope and Applications

Pure Appl. Chem. 2005, 77, 1259-1267.

The copper-catalyzed diorganozinc addition to N-diphenylphosphinoylimines was shown to proceed with a very high degree of enantiocontrol if the reaction was run in the presence of Me-DuPHOS monoxide ligand (BozPHOS). The scope of the reaction is described as well as our efforts to identify the nature of the enantioactive metal complex responsible for the high asymmetric induction.

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(93) CHARETTE, A. B.; BOUCHARD, J.-E.
Catalytic Asymmetric Synthesis of Cyclopropylphosphonates - Catalysts' Scope and Reactivity

Can. J. Chem. 2005, 83, 533-542.

The transition metal-catalyzed cyclopropanation of alkenes using α-diazomethylphosphonates leads to cyclopropylphosphonate derivatives in high yields. The reaction proceeds well with copper, rhodium, and ruthenium catalysts. The best catalyts for the enantioselective version are either Evans' Cu·bis(oxazoline) or Nishiyama's Ru·pybox.

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(92) LEMIRE, A.; BEAUDOIN, D.; GRENON, M.; CHARETTE, A. B.
[4+2] Cycloaddition of 2-Substituted 1,2-Dihydropyridines with Nitrosobenzene: Asymmetric Synthesis of trans-2-Substituted 3-Amino-1,2,3,6-Tetrahydropyridines

J. Org. Chem. 2005, 70, 2368-2371.

A new methodology for the stereoselective synthesis of trans-2-substituted 3-amino-1,2,3,6-tetrahydropyridines is reported. The preparation of these 3-aminopiperidines is achieved by cycloaddition of nitrosobenzene with 2-substituted 1,2-dihydropyridines followed by chemoselective reduction of the cycloadducts. Enantioenriched 1,2-dihydropyridine derivatives are easily prepared from pyridine and a chiral amide following a previous report from our laboratories. Moreover, the in situ hydrogenation of these cycloadducts over palladium in a solution of hydrogen chloride in methanol led to tetrahydropyrroloimidazoles.

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2004


(91) CHARETTE, A. B.; CLOAREC, J.-M.
Highly Efficient Two-Step Synthesis of C-sp3-Centered Geminal Diiodides

Org. Lett. 2004, 6, 4731-4734.

Trisubstituted gem-diiodoalkenes of functionalized chains are efficiently reduced to the corresponding terminal geminal diiodides in high yields upon treatment with the diazene precursor, diethyl 4-(hydrazinosulfonyl)-benzyl phosphonate.

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(90) CÔTÉ, A.; BOEZIO, A. A.; CHARETTE, A. B.
Evidence for the Structure of the Enantioactive Ligand in the Phosphine·Copper Catalyzed Addition of Diorganozinc Reagents to Imines

Angew. Chem., Int. Ed. 2004, 43, 6525-6528.

The highly enantioselective Me-DuPHOS·cu(II)-catalyzed nucleophilic addition of diorganozinc reagents to N-diphenylphosphinoylimines involves initial oxidation of the bis(phosphine) ligand into the hemilabile monophosphine oxide. The oxidation readily occurs when the bis(phosphine) ligand is mixed with either copper(II) or copper(I) triflate. The analogous reaction using Me-DuPHOS under non-oxidizing conditions leads to a much lower reaction rate and no enantioselectivity. Furthermore, the bis(phosphine) monoxide·cu complex displays first-order kinetics in catalyst. This observation is consistent with the conclusion that the bis(phosphine) monoxide ligand is the enantioactive ligand in this reaction.

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(89) CHARETTE, A. B.; LEGAULT, C.; BÉLANGER-GARIÉPY, F.
3,3-Pentamethylenediaziridine

Act. Cryst. 2004, E60, o1921-o1922.

The title compound, C6H12N2, is the first free N,N-unsubstituted 3,3-dialyldiaziridine to be structurally characterized. The crystal structure indicates the presence of intermolecular N--H···N Hydrogen Bonds, resulting in a three-dimensional network.

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(88) LEMIRE, A.; GRENON, M.; POURASHRAF, M.; CHARETTE, A. B.
Nucleophilic Addition to 3-Substituted Pyridinium Salts: Expedient Syntheses of (-)-L-733,061 and (-)-CP-99,994

Org. Lett. 2004, 6, 3517-3520.

The addition of nucleophiles to 3-substituted pyridinium salts prepared from N-methylbenzamide and various pyridines has been investigated. Good to excellent regioselectivities favoring the 2,3-disubstituted 1,2-dihydropyridines were observed. The resulting 1,2-dihydropyridines led to the corresponding 2,3-disubstituted pyridines upon treatment with Mn(OAc)3/NaIO4. This methodology was also successfully applied to the enantioselective syntheses of (-)-L-733,061 and (-)-CP-99,994, two members of a new class of highly potent, nonpeptide, Substance P antagonists.

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(87) CHARETTE, A. B.; CÔTÉ, A.
Asymmetric Catalytic Addition of Diorganozinc Reagents to Imines

Latvian J. Chem 2004, 2, 147-153.

The catalytic enantioselective addition of nucleophiles to imines is a potentially very important process due to the predominant role that α-chiral amines play in biologically important systems. Although these subunits are widely found in pharmaceutically important compounds, it is only recently that methods based on asymmetric catalysis involving nucleophilic addition of organometallic reagents to imines have appeared. The various state-of-the-art methodologies that became available in recent years along with the advantages and specific limitations of each is presented in this article.

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(86) LEBEL, H.; JANES, M. J.; CHARETTE, A. B.; NOLAN, S. P.
Structure and Reactivity of 'Unusual' N-Heterocyclic Carbene (NHC) Palladium Complexes Synthesized from Imidazolium Salts

J. Am. Chem. Soc. 2004, 126, 5046-5047.

The reaction between palladium acetate and IMES·HCl leads to the formation of a novel palladium complex. The X-ray crystal structure analysis reveals that the palladium is C(2) bound to one NHC ligand (the normal binding mode), whereas the second ligand is attached through the C(5) carbon of the second imidazolium. The metalation site on the imidazolium salt is strongly influenced by the presence of base. Furthermore, the binding mode of the NHC to Pd is shown to substantially affect the catalytic behavior of the palladium complexes in cross-coupling reactions.

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(85) CÔTÉ, A.; BOEZIO, A. A.; CHARETTE, A. B.
Asymmetric Catalytic Addition of Diorganozinc Reagents to N-Phosphinoylalkylimines

Proc. Natl Acad. Sci. 2004, 101, 5405-5410.

The Synthesis of α-chiral amines bearing two alkyl groups has been hampered by the accessibility and stability of the alkylimine precursor. Herein, we report an efficient strategy to generate the alkyl-substituted imine in situ that is compatible with the Me-DuPHOS monoxide-Cu(I) catalyzed addition of diorganozinc reagents. The sulfinic acid adduct of the imine is readily prepared by mixing diphenylphosphinic amide, the aldehyde, and sulfinic acid. The sulfinic acid adduct is generally isolated by filtration. The addition of diorganozinc reagents in the presence of Me-DuPHOS monoxide-Cu(I) and the in situ-generated imines affords the corresponding α-chiral amines in high yields and enantiomeric excesses.

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(84) FOURNIER, J.-F.; CHARETTE, A. B.
Improved Protocol for the Diastereoselective Cyclopropanation of Alkenes Using Geminal Dizinc Carbenoids: A Study on the Effect of Zinc Iodide

Eur. J. Org. Chem 2004, 1401-1404.

A mixture of ZnI2, EtZnI·2OEt2 and CHI3 produces a gem-dizinc carbenoid that is an efficient cyclopropanating reagent. The presence of ZnI2 allows for shorter reaction times and cleaner reactions, particularly with less reactive substrates. This modification improves the scope of the reaction and it raises important mechanistic issues about this reaction.

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(83) WURZ, R. P.; CHARETTE, A. B.
An Expedient and Practical Method for the Synthesis of a Diverse Series of Cyclopropane α-Amino Acids and Amines

J. Org. Chem 2004, 69, 1262-1269.

A practical synthesis for the preparation of a diverse series of cyclopropane α-amino acids is described. Nitrocyclopropane carboxylates can be readily prepared through treatment of α-nitro-esters and iodobenzene diacetate or α-nitro-α-diazoesters with a Rh(II) catalyst and an olefin. Reduction of the nitro groupe using Zinc/HCl in i-PrOH affords substituted cyclopropane α-amino esters in modest to high yields (54-99%). A "one-pot" procedure involving sequential cyclopropanation and reduction is described. The method can also be applied to the preparation of arylcyclopropyl amines (three examples).

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2003


(82) CHARETTE, A. B.; WONG, K. C. W.; FOURNIER, J.-F.; PATRICK, B.; SCHEFFER, J. R.
Asymmetric Synthesis in a Norrish Type II Cleavage Reaction Induced by Crystal Chirality

Photochem. Photobiol. Sci. 2003, 2, 1052-1055.

Moderate to high enantiomeric excesses for the cis and trans olefinc products of a Norrish type II cleavage reaction have been obtained for the first time through the use of the solid-state ionic chiral auziliary approach.

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(81) WURZ, R. P.; LIN, W.; CHARETTE, A. B.
Trifluoromethanesulfonyl Azide: An Efficient Reagent for the Preparation of α-Cyano-α-Diazo Carbonyls and an α-Sulfonyl-α-Diazo Carbonyl

Tetrahedron Lett. 2003, 44, 8845-8848.

The reaction scope of trifluoromethanesulfonyl azide in diazo transfer reactions was extended to include the preparation of α-cyano-α-diazo-carbonyls, phenyl sulfonyl diazoacetophenone and diethyl diazomalonate in high yields. The effect of the bases used in the diazo transfer reactions were found to have a dramatic influence on the success of the reaction with pyridine being the base of choice.

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(80) BOEZIO, A. A.; PYTKOWICZ, J.; CÔTÉ, A.; CHARETTE, A. B.
Asymmetric, Catalytic Synthesis of α-Chiral Amines Using a Novel Bis(phosphine) Monoxide Chiral Ligand

J. Am. Chem. Soc. 2003, 125, 14260-14261.

We have shown that Me-DuPHOS monoxide (BozPHOS) is a very effective ligand in the copper-catalyzed addition of dialkylzinc to N-phosphinoylimines providing access to α-chiral amines. The new ligand is particularly effective for the addition of the lesser reactive dimethylzinc. The major advantages of this process are high yields, broad substrate scope, and high enantioselectivities with low catalyst loading (3 mol %).

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(79) DEROY, P. L.; CHARETTE, A. B.
Total Synthesis of (+)-Cystothiazole A

Org. Lett. 2003, 5, 4163-4165.

The total synthesis of cystothiazole A is described. Key steps of the synthesis include an Evans asymmetric catalytic aldol reaction, which established the required C4-C5 stereochemistry. the [2,4']-bis(thiazole) was obtained applying our methodology of electrophilic activation of amide. Semistabilized Wittig reaction between the phosphonium salt 3 and the aldehyde 2 afforded 1 in nine linera steps and 38% overall yield.

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(78) LEGAULT, C.; CHARETTE, A B.
Highly Efficient Synthesis of O-(2,4-Dinitropheny)hydroxylamine. Application to the Synthesis of Substituted N-Benzoyliminopyridinium Ylides

J. Org. Chem. 2003, 68, 7119-7122.

An efficient two-step synthesis of O-(2,4-dinitrophenyl)hydroxylamine is described along with a comparison of its aminating efficiency with O-mesitylenesulfonylhydroxylamine (MSF). It was used in an expedient N-amination/benzoylation procedure involving various substituted pyridines, leading to polysubstituted N-benzoyliminopyridinium ylides, and the scope of its amination power was studied.

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(77) WURZ, R. P.; CHARETTE, A. B.
Hypervalent Iodine (III) Reagents as Safe Alternatives to α-Nitro-α-Diazocarbonyls

Org. Lett. 2003, 5, 2327-2329.

A cyclopropanation reaction involving iodonium ylides generated in situ allows for efficient preparation of substituted 1-nitro-1-carbonyl cyclopropanes. This robust cyclopropanation reaction can be performed in organic solvents, biphasic aqueous media, or under solvent-free conditions with alkene substrates. The iodonium ylides genereated in situ display some surprising differences in reactivity when compared to α-nitro-α-diazocarbonyl compounds.

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(76) CHARETTE, A. B.; GRENON, M.
Mild Method for the Conversion of Amides to Thioamides

J. Org. Chem. 2003, 68, 5792-5794.

Aqueous ammonium sulfide was found to be an ideal substitute for hydrogen sulfide for the thiolysis of activated amides. High yields of the corresponding thioamides were obtained for a broad ranger of substrates, usin two different procedures that are both operationally simple and inexpensivve, as well as amenable to large-scale preparation. Preliminary results indicate that aqueous ammonium sulfide may also replace hydrogen sulfide in the synthesis of thionoesters from amides.

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(75) LEGAULT, C.; CHARETTE, A. B.
Complexation Promoted Additions to N-Benzoyliminopyridinium Ylides. A Novel and Highly Regioselective Approach to Polysubstituted Piperidines

J. Am. Chem. Soc. 2003, 125, 6360-6361.

Unprecedented regioselectivities were observed for the addition of different organometallic nucleophiles to N-benzoyliminopyridinium ylides. Even benzylic and branched aliphatic Grignard reagents, which usually give substantial amounts of 1,4-adducts, showed excellent 1,2-regioselectivities. The electronic nature of the pyridinium ylide is taken into account to explain these results.

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(74) BOEZIO, A. A.; SOLBERGHE, G.; LAUZON, C.; CHARETTE, A. B.
Orthoacylimines: A New Class of Chiral Auxiliaries for Nucleophilic Addition of Organolithium Reagents to Imines

J. Org. Chem. 2003, 68, 3241-3245.

A new class of orthoacylimine-derived chiral auxiliaries has been synthesized and tested in the nucleophilic addition of organolithium reagents to imines. The precursors can be prepared by an aza-Wittig reaction between the corresponding orthoacyl azide and a variety of aldehydes in the presence of trialkylphosphines. The nucleophilic addition of organolithium reagents led to the addition products in good yields and with good to excellent diastereoselectivities (from 85:15 to 99:1). The ciral, nonracemic secondary amines could be readily obtained under mild hydrolytic condition. Furthermore, the chiral auxiliary can be recovered in quantitative yield and reconverted to the starting orthoacyl azide precursor. This method was applied to the synthesis of (S)-t-leucine.

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(73) LEBEL, H.; MARCOUX, J.-F.; MOLINARO, C.; CHARETTE, A. B.
Stereoselective Cyclopropanation Reactions

Chem. Rev. 2003, 103, 977-1050.

Organic chemists have always been fascinated by the cyclopropane subunit. The smallest cycloalkane is found as a basic structural element in a wide range of naturally occurring compounds. Moreover, many cyclopropane-containing unnatural products have been prepared to test the bonding features of this class of highly strained cycloalkanes and to study enzyme mechanism or inhibition. Cyclopropanes have also been used as versatile synthetic intermediates in the synthesis of more functionalized cycloalkanes and acyclic compounds. In recent years, most of the synthetic efforts have focused on the enantioselective synthesis of cyclopropanes. This has remained a challenge ever since it was found that the members of the pyrethroid class of compounds were effective insecticides. New and more efficient methods for the preparation of these entities in enantiomerically pure form are still evolving, and this review will focus mainly on the new methods that have appeared in the literature since 1989. It will elaborate on only three types of stereoselective cyclopropanation reactions from olefins: the halomethylmetal-mediated cyclopropanation reactions, the transition metal-catalyzed decomposition of diazo compounds, and the nucleophilic addition-ring closure sequence. These three processes will be examined in the context of diastereo- and enantiocontrol. In the last section of the review, other methods commonly used to make chiral, nonracemic cyclopropanes will be briefly outlined.

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(72) CHARETTE, A. B.; JANES, M. K.; LEBEL, H.
Bis(oxazoline)·Copper(I)-Catalyzed Enantioselective Cyclopropanation of Cinnamate Esters with Diazomethane

Tetrahedron: Asymmetry 2003, 14, 867-872.

Chiral bis(oxazoline)-copper(I) complexes were found to be effective catalysts for the enantioselective cyclopropanation reaction of trans-cinnamate esters exploiting an argon flow mediated diazomethane addition method. After optimization of the catalyst structure, good yields and enantiomeric excesses were obtained with electron-poor methyl cinnamate derivatives. Sterically demanding esters gave lower yields and enantiomeric excesses were obtained with electron-poor methyl cinnamate derivatives. Sterically demanding esters gave lower yields and enantioselectivities. The correlation between the product enantiopurities and the δ+ values of the aromatic para-substituents was shown to be linear in a Hammet-type plot.

>> sur invitation dans le numéro spécial portant sur le thème de Catalytic Asymmetric Carbene Transfer Reaction

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(71) BOEZIO, A. A.; CHARETTE, A. B.
Catalytic Enantioselective Addition of Diethylzinc to N-Diphenylphosphinoylimines. A Practical Synthesis of α-Chiral Amines

J. Am. Chem. Soc. 2003, 125, 1692-1693.

The enantioselective addition of dialkylzinc reagents to N-diphenylphosphinoylimines derived from aryl-, furyl-, and cyclopropylaldehydes is efficiently catalyzed by a copper(II) triflate/(R,R)-MeDUPHOS complex. The yields are high (51-98%), and the enantiomeric excesses vary from 85 to 96%. This route provides a practical route to α-chiral amines.

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(70) CHARETTE, A. B.; WURZ, R. P.
Progress Towards Asymmetric Intermolecular and Intramolecular Cyclopropanations Using α-Nitro-α-Diazo Carbonyl Substrates

J. Mol. Catal. A: Chem. 2003, 196, 83-91.

A variety of Rh(II) catalysts were screened for enantioselectivity in the cyclopropanation of styrene with α-nitro-α-diazo carbonyl compounds and found to give modest to high yields in a wide range of solvents but modest enantioselectivities (up to 41% e.e. for substrate 9c). Copper catalysts with bis(oxazoline) ligands gave higher enantioselection, with e.e. up to 72% for the major diastereoisomer and yields up to 55%. The use of ethyl diazoacetate as an additive was necessary to obtain improved yields. The first example of intramolecular cyclopropanation involving α-nitro-α-diazo carbonyls is also reported. Rh(II) carboxylate 4b catalyzed the formation of 9-membered nitro cyclopropyl lactones giving up to 66% yield and 61% e.e.

>> dédié au Professeur Howard Alper

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2002


(69) WURZ, R. P.; CHARETTE, A. B.
Transition Metal-Catalyzed Cyclopropanation of Alkenes in Water: Catalyst Efficiency and In Situ Generation of the Diazo Reagent

Org. Lett. 2002, 4, 4531-4533.

A cyclopropanation reaction involving ethyl diazoacetate and olefins proceeds with surprisingly high efficiency in aqueous media using Rh(II) carboxylates. Nishiyama's Ru(II) Py-box and Katsuki's Co(II) Salen complexes that allow for highly enantioselective cyclopropanations in organic solvents can also be applied to aqueous cyclopropanations with similar results. In situ generation of ethyl diazoacetate and cyclopropanation also proceeds efficiently. A chemoselective O-H insertion is also possible in water when hydrophobic catalysts and alcohols are used.

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(68) CHARETTE, A. B.; WURZ, R. P.; OLLEVIER, T.
Synthesis of α-Nitro-α-Diazocarbonyl Derivatives and their Applications in the Cyclopropanation of Alkenes and O-H Insertion Reactions

Helv. Chim. Acta 2002, 85, 4468-4484.

A facile and highly efficient method for the preparation of α-nitro-α-diazocarbonyl derivatives by a diazo-transfer reaction involving (trifluoromethyl)sulfonyl azide has been developed. These substrates undergo a rhodium-catalyzed cyclopropanation reaction with a variety of alkenes. A systematic study of the reaction indicated that the diastereoselectivity of the cyclopropanation could be effectively controlled through the modification of the steric bulk of the diazo reagent. A novel O-H insertion reaction of the metal-carbene complex derived from the α-nitro-α-diazocarbonyl reagent afforded the corresponding novel α-nitro-α-alkoxycarbonyl derivatives.

>> dédié au Professeur Dieter Seebach à l'occasion de sa retraite

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(67) CHARETTE, A. B.; LACASSE, M.-C.
Cyclopropanation of Protected Chiral, Acyclic Allylic Alcohols: an Expedient Access to the anti-Cyclopropylcarbinol Derivatives

Org. Lett. 2002, 4, 3351-3353.

The diastereoselective cyclopropanation of silyl-protected chiral allylic alcohols using Shi's carbenoid (TFA-Et2Zn-CH2I2) gave access to the anti-cyclopropylcarbinyl silyl ethers with excellent diastereocontrol. The level of stereocontrol was shown to depend on the sizes of the protective group and the allylic substituents.

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(66) CHARETTE, A. B.; BEAUCHEMIN, A.; FRANCOEUR, S.; BÉLANGER-GARIÉPY, F.; ENRIGHT, G. D.
Photoinduced Alkyl Group Exchange of Ethylzinc Alkoxides: X-Ray Crystal Structure of an Iodomethylzinc Methoxide

Chem. Commun. 2002, 466-467.

Irradiation of a solution of ethyl zinc alkoxides and CH2I2 leads to clean formation of iodomethylzinc alkoxides; these intermediates are important species generated in stereoselective cyclopropanation processes; no alkyl group exchange is observed in the absence of irradiation; the solid-state structure of (MeO)8Zn7(CH2I)6 is also reported.

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(65) CHARETTE, A. B.; WILB, N.
Cyclopropanation of Allylic Alcohols Using Substituted Haloalkylzinc Reagents: Synthesis of gem-Dimethylcyclopropanes

Synlett 2002, 176-178.

gem-Dimethylcyclopropanes were obtained by a cyclopropanation of allylic alcohols and ethers using a mixture of diethylzinc and 2,2-diiodopropane in 53-96% yields. Enantioenriched gem-dimethylcyclopropanes were also synthesized using a glucose-derived auxiliary.

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(64) CHARETTE, A. B.; GAGNON, A.; FOURNIER, J.-F.
First Evidence for the Formation of a Geminal Dizinc Carbenoid: A Highly Stereoselective Synthesis of 1,2,3-substituted Cyclopropanes

J. Am. Chem. Soc. 2002, 124, 386-387.

Significant amounts of novel gem-dizinc carbenoids (RZnCHIZnR) are formed when diethylzinc is mixed with iodoform in CH2Cl2 at 0 °C. This reagent was shown to be effective in the cyclopropanation of butenediol derivatives to generate a cyclopropylzinc intermediate that could be trapped with a variety of electrophiles. 1,2,3-Substituted cyclopropane derivatives are formed with excellent diastereoselectivities by using this simple procedure.

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2001


(63) CHARETTE, A. B.; MOLINARO, C.; BROCHU, C.
Stability, Reactivity, Solution and Solid-State Structure of Halomethylzinc Alkoxides

J. Am. Chem. Soc. 2001, 123, 12160-12167.

In this paper, we report our findings regarding the development of a Lewis acid-catalyzed cyclopropanation of allylic alcohols with bis(iodomethyl)zinc. Iodomethylzinc alkoxides can be formed by treatment of an alcohol with bis(iodomethyl)zinc. These species are not prone to undergo cyclopropanation at low temperature but the addition of a Lewis acid in catalytic amounts induces the cyclopropanation reaction. Using this procedure, we demonstrated that the Lewis acid-catalyzed pathway significantly overwhelms the uncatalyzed one. The paper describes fundamental issues regarding the preparation and stability of halomethyl zinc alkoxides in solution as well as their aggregation state in solution and solid-state structures. Furthermore, the competition reaction between the inter- vs intramolecular cyclopropanation will be studied. Finally, we will discuss the possible activation pathways to explain the Lewis acid activation of halmethylzinc alkoxides. These findings provided new insights on the reactivity of ROZnCH2I and established the groundwork for the elaboration of an enantioselective version of the reaction.

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(62) CHARETTE, A. B.; MOLINARO, C.; BROCHU, C.
Catalytic Asymmetric Cyclopropanation of Allylic Alcohols with Titanium-TADDOLate: Scope of the Cyclopropanation Reaction

J. Am. Chem. Soc. 2001, 123, 12168-12175.

A substoichiometric amount of titanium-TADDOLate complex was effective at catalyzing the cyclopropanation reaction of allylic alcohols in the presence of 1 equiv of bis(iodomethyl)zinc. After initial optimization of the catalyst structure, excellent yields and enantiomeric ratios were obtained for 3-aryl- or 3-heteroaryl-substituted allylic alcohols (up to 97:3). Alkyl-substituted allylic alcohols gave modest yields and enantiomeric ratios (up to 87:13) but these compare favorably with those observed with other substoichiometric chiral ligands. The full synthetic scope of the reaction is presented in this paper.

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(61) CHARETTE, A. B.; GRENON, M.; LEMIRE, A.; POURASHRAF, M.; MARTEL, J.
Practical and Highly Regio- and Stereoselective Synthesis of 2-Substituted Dihydropyridines and Piperidines: Application to the Synthesis of (-)-Coniine

J. Am. Chem. Soc. 2001, 123, 11829-11830.

In this publication, we have reported an expedient regio- and stereoselective approach to 2-substituted dihydropyridines that relies on the powerful directing ability of the imidate group.

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(60) CHARETTE, A. B.; JOLICOEUR, É.; BYDLINSKI, G. A. S.
Enantioselective Synthesis of Spiropentanes from Hydroxymethylallenes

Org. Lett. 2001, 3, 3293-3295.

Hydroxymethylallenes are efficiently converted to the corresponding spiropentanes in high yields and enantiomeric ratios upon treatment with Zn(CH2I)2 and dioxaborolanes ligand 1. The reaction also proceeds with complete diastereocontrol. The application of this methodology to the synthesis of spiropentaneacetic acid is also presented.

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(59) CHARETTE, A. B.; GRENON, M.
Spectroscopic Studies of the Electrophilic Activation of Amides with Triflic Anhydride and Pyridine

Can. J. Chem. 2001, 79, 1694-1703.

The reaction of amides with trifluoromethanesulfonic (triflic) anhydride in the presence of pyridine was thoroughly investigated by NMR spectroscopic techniques. Different pyridinium intermediates were generated from secondary amides, tertiary amides with enolizable protons and tertiary amides lacking enolizable protons. It was found that the actual triflating reagent is N-(trifluoromethylsulfonyl)pyridinium triflate 11 which is formed by the initial reaction of triflic anhydride with pyridine. The alcoholysis of these intermediates yields O-alkyliminium ethers which can then be easily hydrolyzed under mild acidic conditions to the corresponding esters.

>> dédié au Professeur Victor Snieckus

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(58) CHARETTE, A. B.; BEAUCHEMIN, A.; FRANCOEUR, S.
Acyloxymethylzinc Reagents: Preparation, Reactivity and Solid-State Structure of this Novel Class of Cyclopropanating Reagents

J. Am. Chem. Soc. 2001, 123, 8139-8140.

In this publication, we have shown that acyloxymethylzinc derivatives are promising cyclopropanating reagents for unfunctionalized alkenes.

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(57) CHARETTE, A. B.; BERTHELETTE, C.; ST-MARTIN, D.
An Expedient Approach to E,Z-Dienes Using the Julia Olefination

Tetrahedron Lett. 2001, 42, 5149-5153.

New reaction conditions were developed for the synthesis of E,Z-dienes from α, β-unsaturated aldehydes and heteroarylsulfones using the Julia reaction. In most cases under optimal conditions, the selectivity of the olefination reaction is better that 88:12 when a pyridylsulfone was used as the precursor. In addtion, novel reaction conditions for the chemoselective oxidation of heteroarylthioethers that are compatible with alkenes and dienes are also reported.

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(56) CHARETTE, A. B.; JANES, M. K.; BOEZIO, A. A.
Mitsunobu Reaction Using Triphenylphosphine Linked to Non-Cross-Linked Polystyrene

J. Org. Chem. 2001, 66, 2178-2180.

The Mitsunobu Reaction using triphenylphosphine linked to non-cross-linked polystyrene was found to be particularly effective to invert the configuration of chiral secondary alcohols since a clean SN2 process is generally observed.

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(55) CHARETTE, A. B.; BEAUCHEMIN, A.
Reinvestigation of the Chemoselective Cyclopropanation of Allylic Alcohols, Allylic Ethers and Alkenes: Comparison Between Different Reagents and Protocols

J. Organomet. Chem. 2001, 617-618, 702-708.

the relative reactivities of various Zn, Al and Sm carbenoids in the chemoselective Simmons-smith cyclopropanation of geraniol and its benzyl ether derivative were studied. Directed cyclopropanation was obtained in all cases, even with a iodomethylaluminum reagent. By variation of the reaction protocols using aluminum carbenoids, cyclopropanation can favor the proximal or distal double bond.

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2000



(54) CHARETTE, A. B.; WURZ, R. P.; OLLEVIER, T.
Trifluoromethanesulfonyl Azide: A Powerful Reagent for the Preparation of α-Nitro-α-Diazocarbonyl Derivatives

J. Org. Chem. 2000, 65, 9252-9254.

The importance of α-diazocarbonyl reagents has been recognized for a number of years. These reagents are ideal precursors for several transition metal-catalyzed processes including cyclopropanations and X-H insertion reactions (X = C, O, N, S, P, etc.) and the phosphorus or sulfur ylide formation. As part of our research program aiming at developing new methods for the stereoselective synthesis of unnatural amino acids, we became interested in using α-nitro-α-diazocarbonyl derivatives as potential synthetic precursors. Herein, we report a much improved procedure to prepare these reagents which is amenable to the synthesis of a large number of structurally diverse derivatives.

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(53) CHARETTE, A. B.; BOEZIO, A. A.; JANES, M.
Synthesis of a Triphenylphosphine Reagent on Non-Cross-Linked Polystyrene Support: Application to the Staudinger/Aza-Wittig Reaction

Org. Lett. 2000, 2, 3777-3779.

A new triphenylphosphine reagent linked to a linear polystyrene was synthesized. The reactivity of this phosphine-bound polymer is superior to that of the phosphine-bound to cross-linked polystyrene. The polymer reacted very rapidly with azides to generate iminophosphoranes which could then react with aldehydes to generate imines in good yields and high purities.

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(52) CHARETTE, A. B.; FRANCOEUR, S.; MARTEL, J.; WILB, N.
New Family of Cyclopropanating Reagents: Synthesis, Reactivity and Stability Studies of Iodomethylzinc Phenoxides

Angew. Chem. Int. Ed. 2000, 39, 4539-4542.

The iodomethylzinc phenozides offer a valuable alternative to the traditional Simmons-Smith reagents due to their ease of preparation, high reactivities towards unfunctionalized olefins, and good stabilities.

>> souligné dans Chem. Eng. News par la rédaction

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(51) CHARETTE, A. B.; GAGNON, A.; BÉLANGER-GARIÉPY, F.
(Z)-N-(Bromomagnesio)-1-(2-Naphthylmethoxy)propan-2-Imine

Acta Cryst. 2000, C55, 538-540.

The title compound, [Mg2Br2(C14H14NO)2(C4H8O)4]·2C7H8, has been cyrstallized as a C2-symmetric dimer and the Mg atom has a distorted octahedral geometry. The metal is chelated by the N atom of the ketiminate and the O atom of the ether moiety, giving a rigid structure.

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(50) CHARETTE, A. B.; MARCOUX, J.-F.; MOLINARO, C.; BEAUCHEMIN, A.; BROCHU, C.; ISABEL, É.
Preparation, Solid-State Structure and Synthetic Applications of Isolable and Storable Haloalkylzinc Reagents

J. Am. Chem. Soc. 2000, 122, 4508-4509.

We have shown that the bipyridine complexes of haloalkylzinc reagents are fully characterized and quite stable reagents that can be stored and used in haloalkylzinc mediated processes.

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(49) CHARETTE, A. B.; GRENON, M.
Mild Method for the Synthesis of Amidines by the Electrophilic Activation of Amides

Tetrahedron Lett. 2000, 41, 1677-1680.

The synthesis of amidines was achieved by the addition of amines to amides that were previously activated with trifluoromethanesulfonic anhydride (triflic anhydride) and pyridine. Various disubstituted and trisubstituted amidines were prepared in yields up to 84%.

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1999


(48) EVANS, D. A.; CARTER, P. H.; CARREIRA, E. M.; CHARETTE, A. B.; PRUNET, J. A.; LAUTENS, M.
Total Synthesis of Bryostatin 2

J. Am. Chem. Soc. 1999, 121, 7540-7552.

The total synthesis of the marine macrolide bryostatin 2 is described. The synthesis plan relies on aldol and directed reduction steps in order to construct the anti-1,3-diol array present in each of the principal subunits (A, B, and C). These fragments were coupled using a Julia olefination and subsequent sulfone alkylation. A series of functionalization reactions provided a bryopyran seco acid, which was macrolactonized under Yamaguchi conditions. Installation of the two enolate moieties took advantage of asymmetric phosphonate and aldol condensation strategies. Reduction of the C20 ketone and simple protecting group operations then completed the synthesis of bryostatin 2. This flexible approach should provide access to a series of new analogues of this clinically important marine natural product.

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(47) CHARETTE, A. B.; GAGNON, A.
Stereoselective Synthesis of α,α-Disubstituted Amines by a Sequential Nucleophilic Addition to O-Protected Cyanohydrins

Tetrahedron: Asymmetry 1999, 10, 1961-1968.

The first enantioselective double nucleophilic addition reaction to O-protected cyanohydrin was achieved in 71% yield and 82:18 enantiomeric ratio by using a bis(BINOL)titanium Lewis acid complex. New (4S,5S)-2,2-dialkyl-4,5-bis(hydroxymethyl)-1,3-dioxolane titanium complexes were synthesized and they displayed good levels of enantioselection in the nucleophilic addition reaction.

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(46) CHARETTE, A. B.; LEBEL, H.
Preparation of (2S,3S)-(+)-(3-Phenylcyclopropyl)methanol

Org. Synth. 1999, 76, 86-100; Organic Syntheses; Wiley & Sons: New York, 2004; Collect. Vol. X, pp 613-626.

Abstract not available.

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(45) CHARETTE, A. B.; LEBEL, H.; GAGNON, A.
The Stereoselective Cyclopropanation of Chiral Allylic Alcohols Using a Chiral Dioxaborolane Ligand: A New Route to anti-Cyclopropylmethanol Derivatives

Tetrahedron 1999, 55, 8845-8856.

The diastereoselective cyclopropanation reaction of allylic alcohols in the presence of the chiral dioxaborolane ligand 1 was studied. Unprecedently high anti-selectivities were obtained with E-olefins when 2.2 equivalents of bis(iodomethyl)zinc and 1.1 equivalent of 1 were used.

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(44) CHARETTE, A. B.; BEAUCHEMIN, A.; MARCOUX, J.-F.
Preparation and Reactivity of Some Functionalized Halomethylzinc Carbenoids

Tetrahedron Lett. 1999, 40, 33-36.

Various mixed diorganozinc reagents were prepared by the irradiation of diethylzinc and functionalized alkyl iodides. These species were shown to react with dihalomethane to form zinc carbenoids in a highly regioselective manner. In some cases, the latter reagents proved to be effective cyclopropanating reagents.

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1998


(43) CHARETTE, A. B.; JUTEAU, H.; LEBEL, H.; MOLINARO, C.
Enantioselective Cyclopropanation of Allylic Alcohols with Dioxaborolane Ligands: Scope and Synthetic Applications

J. Am. Chem. Soc. 1998, 120, 11943-11952.

A very effective chiral controller has been found for the conversion of allylic alcohols into the corresponding enantiomerically enriched cyclopropanes using bis(iodomethyl)zinc. A variety of chiral non-racemic cyclopropylmethanols could be obtained according to this method. This methodology was extended with success to the cyclopropanation of unconjugated and conjugated polyenes and homoallylic alcohols. The cyclopropanation of allylic carbamates has also been investigated with this system, but it was found that enantiorenriched cyclopropylmethylamines are best prepared from enantioenriched cyclopropylmethanols.

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(42) CHARETTE, A. B.; NAUD, J.
Regioselective Opening and Electrophilic Trapping of Substituted (Cyclopropylmethyl)lithium

Tetrahedron Lett. 1998, 39, 7259-7262.

Substituted cyclopropylmethanol derivatives were successfully opened with excellent regiocontrol by a three-step protocol. The resulting homoallylic carbanion could be trapped with a variety of electrophiles.

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(41) EVANS, D. A.; CARTER, P. H.; CARREIRA, E. M.; PRUNET, J. A.; CHARETTE A. B.; LAUTENS, M.
Asymmetric Synthesis of Bryostatin 2

Angew. Chem. Int. Ed. Engl. 1998, 37, 2354-2359.

Herein we describe the first total synthesis of bryostatin 2.

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(40) CHARETTE, A. B.; BEAUCHEMIN, A.; MARCOUX, J.-F.
Photoinduced Synthesis of Diorganozinc and Organozinc Iodide Reagents

J. Am. Chem. Soc. 1998, 120, 5114-5115.

Our interest in the synthesis of chiral, functionalized zinc carbenoid reagents (D) for the cyclopropanation reaction of unfunctionalized olefins prompted us to devise a unified route to diorganozinc (A, B) and organozinc iodides (C).

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(39) CHARETTE, A. B.; GAGNON, A.; JANES, M.; MELLON, C.
Synthesis of α,α-Disubstituted-α-Amino Acids by Double Nucleophilic Addition to Cyanohydrins

Tetrahedron Lett. 1998, 39, 5147-5150.

The synthesis of tertiary carbinamines was achieved by the double nucleophilic addition of Grignard reagents to cyanohydrins. Titanium isopropoxide was found to promote the process. In a typical example, the rapid conversion of a carbinamine to the corresponding α,α-disubstituted---amino acid was also demonstrated.

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(38) CHARETTE, A. B.; MELLON, C.
Enantioselective Synthesis of α,α-Disubstituted-α-Amino Acids by a Sequential Nucleophilic Addition to Nitriles

Tetrahedron 1998, 54, 10525-10535.

The sequential addition of two different nucleophiles to a tartaric acid-derived nitrile produced carbinamines. The adducts from chelation-controlled addition, which are obtained in high diastereoselectivities and yields, were easily converted to α,α-disubstituted-α-amino acids.

>> sur invitation dans le numéro de Tetrahedron Symposium-In-Print portant sur le thème de Heterocycles in Asymmetric Synthesis

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(37) CHARETTE, A. B.; CHUA, P.
Mild Method for the Synthesis of Thiazolines from Secondary and Tertiary Amides

J. Org. Chem. 1998, 63, 908-909.

Herein we report that, at low temperatures in a medium buffered with excess pyridine, iminium and imino triflates can be generated and reacted with amino thiols to allow efficient access to thiazolines without any racemization.

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(36) CHARETTE, A. B.; CHUA, P.
Enantioselective Synthesis of Biarylphenylglycines and their Use as Chiral Ligands in the Lewis Acid Catalyzed Diels-Alder Reaction

Molecules Online 1998, 2, 63-75.

The enantioselective syntheses of naphthyl-substituted-N-tosylphenylglycines are described. These unnatural amino acids were tested as ligands for the chiral oxazaborolidine catalyzed Diels-Alder reaction between cyclopentadiene and 2-bromacrolein.

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(35) CHARETTE, A. B.; CHUA, P.
A New Mild Method for the Cleavage of the Amide Bond: Conversion of Secondary and Tertiary Amides to Esters

Synlett 1998, 2, 163-165.

We have found an approach towards the longstanding problem of cleaving the amide bond. Since this transformation can be accomplished in the presence of various functionality, a greater use of amides as amine and carboxylic acid protecting groups may develop. Secondary and tertiary amides can be converted to highly electrophilic anhydride and pyridine. These species can be used to generate alkyl imino esters and alkyl iminium esters which are readily converted to esters.

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(34) CHARETTE, A. B.; CHUA, P.
Thiolysis and Hydrolysis of Imino and Iminium Triflates: Synthesis of Secondary and Tertiary Thioamides and 18O-Labeled Amides

Tetrahedron Lett. 1998, 39, 245-248.

Secondary and tertiary amides were treated with trifluoromethanesulfonic (triflic) anhydride in the presence of pyridine at low temperatures to generate imino and iminium triflates. Subsequent treatment with hydrogen sulfide immediately gave rise to the corresponding thioamide in good to excellent yields at low temperature. Alternatively, treatment with stoichiometric amounts of 18O-labeled water produced the 18O-labeled amide.

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1997


(33) CHARETTE, A. B.; CHUA, P.
A New Mild Method for the Conversion of secondary and Tertiary Amides to Bridged Ortho Esters

Tetrahedron Lett. 1997, 38, 8499-8502.

Secondary and tertiary amides were treated with trifluoromethanesulfonic (triflic) anhydride in the presence of pyridine at low temperatures to generate imino and iminium triflates. Successive treatment with 2,2-bishydroxymethyl-1-propanol in the pyridine buffered solution gave the corresponding 2,6,7-trioxabicyclo[2.2.2]octane orthoesters in good to excellent yields at below room temperature.

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(32) CHARETTE, A. B.; PEREIRA DE FREITAS-GIL, R.
Synthesis of Contiguous Cyclopropanes by Palladium-Catalyzed Suzuki-Type Cross-Coupling Reactions

Tetrahedron Lett. 1997, 38, 2809-2812.

Suzuki cross-coupling reactions between a variety of iodocyclopropanes and cyclopropylboronate esters to produce symmetrical or unsymmetrical contiguous cyclopropanes was achieved in good yields.

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(31) CHARETTE, A. B.; JUTEAU, H.
Application of the Chemo- and Enantioselective Cyclopropanation of Polyenes to the Total Synthesis of (+)-Bicyclohumulenone

Tetrahedron 1997, 53, 16277-16286.

The chemo- and enantioselective cyclopropanation of a monoprotected bis(allylic alcohol) using a chiral dioxaborolane-derived ligand and Zn(CH2I)2 was used as a key step in the total synthesis of (+)-bicyclohumulenone. The synthesis of this important perfume component was efficiently accomplished in 16 steps from diethyl 3,3-dimethylglutarate in 9% overall yield.

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(30) CHARETTE, A. B.; LEMAY, J.
Diastereo- and Enantioselective Synthesis of 1,2,3-Substituted Cyclopropanes Using Zinc Carbenoids

Angew. Chem. Int. Ed. Engl. 1997, 36, 1090-1092.

We have developed an expedient and efficient synthesis of enantiomerically enriched 1,2,3-substituted cyclopropanes from allylic alcohols. In addition, we have presented the first cyclopropanation reaction of a functionalized iodoalkylzinc reagent.

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1996


(29) CHARETTE, A. B.; JUTEAU, H.; LEBEL, H.; DESCHÊNES, D.
The Chemo- and Enantioselective Cyclopropanation of Polyenes: Chiral Auxiliary vs. Chiral Reagent-Based Approach

Tetrahedron Lett. 1996, 37, 7925-7928.

The chemo- and enantioselective cyclopropanation of allylic alcohols containing additional olefins was investigated using the dioxaborolane-derived reagent. High chemo- and enantioselectivities were usually observed if a mixture of the chiral dioxaborolane ligand/allylic alcohol was treated with a solution of Zn(CH2I)2·DME complex.

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(28) CHARETTE, A. B.; GIROUX, A.
Palladium-Catalyzed Suzuki-Type Cross-Couplings of Iodocyclopropanes with Boronic Acids: Synthesis of trans-1,2-Dicyclopropyl Alkenes

J. Org. Chem. 1996, 61, 8718-8719.

We have reported the first cross-coupling reaction of substituted cyclopropyl iodides with boronic acids. This methodology represents the first insertion of palladium(0) in a cyclopropyl halide bond, and it opens the door to other palladium-catalyzed coupling reactions involving cyclopropyl iodides.

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(27) CHARETTE, A. B.; GIROUX, A.
Asymmetric Hydrogenation of N-Tosylimines Catalyzed by BINAP-Ruthenium(II) Complexes

Tetrahedron Lett. 1996, 37, 6669-6672.

The asymmetric hydrogenation of N-tosylimines was accomplished with a catalytic amount of Ru[(R)-2,2'-bis(diphenylphosphino)-1,1'-binaphtyl](O2CCH3)2 to afford the corresponding amines in high enantiomeric excesses.

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(26) CHARETTE, A. B.; LEBEL, H.
Enantioselective Total Synthesis of (+)-U-106305

J. Am. Chem. Soc. 1996, 118, 10327-10328.

The synthesis of (+)-U-106305 was accomplished in 14 steps (ca. 5% overall yield) from alcohol 7 and is suitable for obtaining large quantities of this material. It also clearly illustrates the power of the reagent-based asymmetric cyclopropanation reaction developed in our laboratories. Furthermore, the final olefination reaction, occurring without opening or scrambling of the cyclopropane stereochemistry, significantly simplifies the synthesis of this class of compounds.

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(25) CHARETTE, A. B.; MARCOUX, J.-F.; BÉLANGER-GARIÉPY, F.
X-Ray Crystal Structure of a Zinc Carbenoid Cyclopropanating Reagent: The IZnCH2I•18-Crown-6 and Benzo-18-Crown-6 Complexes

J. Am. Chem. Soc. 1996, 118, 6792-6793.

The solid-state structure of IZnCH2I is reported for the first time. This reagent may be the actual Simmons-Smith reagent, but this remains to be established. The structural features of this reagent will serve as groundwork to gain further insight on the mechanism of stereoselective cyclopropanation reactions involving the various iodomethyl-zinc-derived reagents.

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(24) CHARETTE, A. B.; MARCOUX, J.-F.
Spectroscopic Characterization of (Iodomethyl)zinc Reagents Involved in Stereoselective Reactions: Spectroscopic Evidence that IZnCH2I Is Not Zn(CH2I)2 + ZnI2 in the Presence of an Ether

J. Am. Chem. Soc. 1996, 118, 4539-4549.

We have shown that low temperature 13C NMR spectroscopy is an extremely powerful technique for characterizing the (iodomethyl)zinc-derived reagents involved in the cyclopropanation reactions. This technique has allowed us to spectroscopically characterize and unambigously differentiate the Furukawa reagent (EtZnCH2I), the Simmons-Smith reagent (IZnCH2I), and the Wittig reagent (Zn(CH2I)2). Unique spectra are obtained for each of these reagents when they are complexed to a chiral diether. We have also demonstrated the IZnCH2I is not converted into Zn(CH2I)2 + ZnI2 in CD2Cl2 in the presence of a chiral complexing agent. Furukawa's reagent "EtZnCH2I", however, is in equilibrium with Et2Zn and Zn(CH2I)2 and it eventually decomposes into PrZnI and EtZnI at room temperature. The decomposition of Zn(CH2I)2 into IZnCH2I and of IZnCH2I into ZnI2 was monitored by NMR. We have also demonstrated that the general trends observed for the various equilibria involving (iodomethyl)zinc-derived reagents follow those observed with ethylzinc-derived organometallic compounds.

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1995


(23) CHARETTE, A. B.; BROCHU, C.
Lewis Acid-Catalyzed Cyclopropanation of Allylic Alcohols

J. Am. Chem. Soc. 1995, 117, 11367-11368.

In this communication, we unveil a new strategy for the Lewis acid-catalyzed cyclopropantion of allylic alcohols in which the rate of the uncatalyzed process is significantly slower than that of the catalyzed reaction.

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(22) CHARETTE, A. B.; MELLON, C.; MOTAMEDI, M.
The 2-Benzyloxytetrahydropyranyl Group as a Chiral Auxiliary for the Nucleophilic Addition to α-Alkoxy Aldehydes

Tetrahedron Lett. 1995, 36, 8561-8564.

The addition of Grignard reagents, hydride reagents and crothylstannanes to an α-alkoxy carbonyl bearing the 2-benzyloxytetrahydropyranyl group showed good levels of diastereofacial control. The sense of induction was the same as that reported for the addition of allylSnBu3. An important solvent effect consistent with the proposed transition state model was observed in these reactions.

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(21) CHARETTE, A. B.; MARCOUX, J.-F.
The Asymmetric Cyclopropanation of Acyclic Allylic Alcohols: Efficient Stereocontrol with Iodomethylzinc Reagents

Synlett 1995, 1197-1207.

The asymmetric cyclopropanation of acyclic allylic alcohols can now be achieved with unprecendently high level of stereocontrol with halomethylzinc reagents. The ability of proximal basic groups to "deliver" the methylene unit to a nearby olefin has been used to develop efficient reaction conditions for the cyclopropanation of allylic alcohols. This property was employed to improve the diastereoselection observed in the cyclopropanation of chiral, acyclic allylic alcohols; in the development of an efficient chiral auxiliary; and finally in the design of an efficient stoichiometric chiral reagent for the cyclopropanation of allylic alcohols.

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(20) CHARETTE, A. B.; CÔTÉ, B.
Stereoselective Synthesis of all Four Isomers of Coronamic Acid: A General Approach to 3-Methanoamino Acids

J. Am. Chem. Soc. 1995, 117, 12721-12732.

A general approach to the synthesis of enantiomerically enriched 3-methyanoamino acids is presented where the cyclopropyl unit was introduced by a stereoselective cyclopropanation using a D-glucose-derived chiral auxiliary. This methodology was applied to the synthesis of the four stereoisomers of coronamic acid 14, 17, 20, and 22. Starting with the readily available allylic alchohol 5,β-D-glucopyranoses (E)-6 and (Z)-7 have been selectively prepared using trichloroacetimidate 2. These olefins were cyclopropanated with very high diastereoselectivities using Et2Zn/CH2I2 (>98% de) and Et2Zn/CH2ICl (>97% de) in yields greater than 90%. After cleavage of the chiral auxiliary by ring contraction of the 2-O-trifyl derivative of cyclopropanes 8 and 9, cyclopropylmethanols (-)-10 and (+)-11 have been tranformed to all four isomeric protected amino acids by selective protecting group manipulations, oxidation of primary alcohols, and Curtius rearrangement.

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(19) CHARETTE, A. B.; CÔTÉ, B; MONROC, S; PRESCOTT, S.
Synthesis of Monoprotected 2-Alkylidene-1,3-Propanediols by an Unusual SN2' Mitsunobu Reaction

J. Org. Chem. 1995, 60, 6888-6894.

A simple and efficient route to monoprotected (E)- and (Z)-2-alkylidene-1,3-propanediols is described. The key step involves an unusual regio- and stereoselective SN2' Mitsunobu reaction of substituted 3-hydroxy-2-methylenealkanoates which are readily available from a Baylis-Hillman reaction between methyl acrylate and an aldhyde. These allylic alcohols, when treated with PPh3, a carboxylic acid, and DEAD in THF at temperatures ranging from -40 °C to 22 °C, produced the corresponding 2-alkylidene-3-hydroxypropanoate derivatives (or (E)-2-(hydroxymethyl)3-substituted-2-alkenoate derivative (in >70% with SN2':SN2 ratio of 22:1 to >50:1. It was found that weak and bulky carboxylic acids and low temperatures favor SN2' addition. The reaction conditions were effective for alkyl substituted derivatives, but the addition of Et3N to the Mitsunobu conditions was necessary to improve the SN2':SN2 ratios for the vinyl 19 and phenyl 20 derivatives. The monoprotected (Z)- and (E)-2-alkylidene-1,3-propanediols can be efficiently synthesized by a three-step sequence involving either a transesterification, protection, and DIBAL-H reduction (>80% overall yield) or by the chemoselective reduction, protection, and ester cleavage (67% overall yield).

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(18) CHARETTE, A. B.; BENSLIMANE, A. F.; MELLON, C.
The Tetrahydropyranyl Group as a Chiral Auxiliary for the Nucleophilic Addition to α-Alkoxy Ketones

Tetrahedron Lett. 1995, 36, 8557-8560.

The protected 2-hydroxytetrahydropyranyl group is an efficient chiral auxiliary for the stereoselective nucleophilic addition of Grignard reagents to α-alkoxy ketones. We also report that allylSnBu3/MgBr2·OEt2 is a very efficient system for the chelation controlled stereoselective addition of the allyl group to α-alkoxy ketones.

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(17) CHARETTE, A. B.; LEBEL, H.
Diastereoselective Cyclopropanation of Chiral Allylic Alcohols: A More Efficient Reagent for the Relative Stereocontrol

J. Org. Chem. 1995, 60, 2966-2967.

We have demonstrated that a number of structurally related iodomethylzinc reagents can produce significantly different results in a given reaction. The selection of the iodomethylzinc reagent is, therefore, essential for the controlling the relative stereochemistry in the cyclopropanation of chiral allylic alcohols. Excellent diastereoselectivities are observed with a variety of olefins.

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(16) CHARETTE, A. B.; PRESCOTT, S.; BROCHU, C.
Improved Procedure for the Synthesis of Enantiomerically Enriched Cyclopropylmethanol Derivatives

J. Org. Chem. 1995, 60, 1081-1083.

We have described a safe and simple procedure for the generation of enantiomerically enriched cyclopropylmethanol derivatives on >1 mmol scale and for the efficient recovery of the reaction components. This procedure is general and, to the best of our knowledge, is the first report on the use of a Zn(CH2I)2·DME complex solution in CH2Cl2 as the cyclopropanation reagent.

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< 1995


(15) CHARETTE, A. B.; JUTEAU, H.
Design of Amphoteric Bifunctional Ligands: Application to the Enantioselective Simmons-Smith Cyclopropanation of Allylic Alcohols

J. Am. Chem. Soc. 1994, 116, 2651-2652.

Dioxaborolane 1 is currently the most efficient chiral ligand for the enantioselective Simmons-Smith cyclopropanation reaction of substituted allylic alcohols.

>> souligné dans Org. Proc. Res. Dev. par la rédaction

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(14) CHARETTE, A. B.; TURCOTTE; N., CÔTÉ, B.
One-Pot Synthesis of Substituted Allyl-α-D-Glucopyranosides by an in situ Anomerization Protocol

J. Carbohydr. Chem. 1994, 13, 421-432.

A new 3-step, 1-pot procedure for the stereoselective α-glycosylation of substituted allylic alcohols has been developed. The key step involves an in situ anomerization of the β-glycoside, obtained by Schmidt's glycosylation method, upon treatment with TiCl4.

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(13) CHARETTE, A. B.; TURCOTTE, N.; MARCOUX, J.-F.
The Use of α-D-Glucopyranosides as Surrogates for the L-Glucose in the Stereoselective Cyclopropanation Reaction

Tetrahedron Lett. 1994, 35, 513-516.

Treatment of substituted allyl α-D-glucopyranosides with Et2Zn/CH2I2 in t-butyl methyl ether produced the corresponding cyclopropane derivatives in >90% yields with diastereoselectivities ranging from 11:1 to 17:1.

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(12) CHARETTE, A. B.; CÔTÉ, B.
Synthesis of Monoprotected 2-Alkylidene-1,3-Propanediols by an Unusual SN2' Mitsunobu Reaction

Tetrahedron Lett. 1993, 34,6833-6836.

A simple and efficient route to monoprotected E- and Z-2-alkylidene-1,3-propanediols was developed. The key step involves an unusual regio- and stereoselective SN2' Mitsunobu reaction of substituted 3-hydroxy-2-methylenealkenoate.

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(11) CHARETTE, A. B.; CÔTÉ, B.
Carbohydrates as Chiral Auxiliaries: The Asymmetric Epoxidation Reaction of Olefins

Tetrahedron: Asymmetry 1993, 4, 2283-2286.

Under optimized conditions, the epoxidation of 1-O-trans-2-butenyl-3,4,6-tri-O-benzyl-β-D-glucopyranose gave a 9:1 mixture of diastereomers. The diastereoselectivities observed were shown to be highly dependent on the nature of the reagent used and of the protecting group at C-2 of the auxiliary.

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(10) CHARETTE, A. B.; MARCOUX, J.-F.
The Use of 1,2-trans-Cyclohexanediol as an Efficient Chiral Auxiliary for the Asymmetric Cyclopropanation of allylic Ethers

Tetrahedron Lett. 1993, 34, 7157-7160.

A new and simple chiral auxiliary derived from optically pure 1,2-trans-cyclohexanediol was developed for the asymmetric Simmons-Smith cyclopropanation of substitued allylic alcohols. Excellent yields and diastereoselectivities were obtained with a variety of olefins.

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(09) CHARETTE, A. B.; CÔTÉ, B.
Asymmetric Cyclopropanation of Allylic Ethers: Cleavage and Regeneration of the hiral Auxiliary

J. Org. Chem. 1993, 58, 933-936.

The ring contraction reaction of 2-O-[[(trifluoromethyl)sulfonyl]oxy]β-D-glucopyranosides and their corresponding α-anomers is reported. The rearrangement was shown to occur under extremely mild basic conditions to allow isolation of acid-sensitive optically active substituted cyclopropylmethanols. The chiral auxiliary can be regenerated by converting the C-glucofuranoside, byproduct of the rearrangement, into tri-O-benzyl-D-glucal (two steps).

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(08) CHARETTE, A. B.; MELLON, C.; ROUILLARD, L.; MALENFANT, É.
Design of New Chiral Auxiliaries for the Stereoselective Nucleophilic Addition to α-Alkoxy Aldehydes: A Remarkable Remote Protecting Group Effect

Synlett 1993, 1, 81-82.

The MgBr2·OEt2 mediated addition of allyltributyltin to an α-alkoxyaldehyde bearing a protected 2-hydroxytetrahydropyranyl group was shown to be highly diastereoselective. The sense and the level of induction was shown to depend on the nature of the protecting group at C-3.

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(07) CHARETTE, A. B.; MELLON, C.; ROUILLARD, L.; MALENFANT, É.
Design, Synthesis and Applications of New Oxygenated Chiral Auxiliaries

Pure & Appl. Chem. 1992, 64, 1925-1931.

The magnesium bromide etherate-mediated addition of allyltributyltin to an α-alkoxyaldehyde bearing a protected 3-hydroxytetrahydropyranyl- moiety was shown to be highly diastereoselective. The sense and the level of induction depend on the nature of the protecting group at C-3. Esters and benzyl ether gave the opposite relative stereochemistry in comparison to silyl ethers. It was also found that the other enantiomer of an optically active 1,2-diol could be obtainted using the same chiral auxiliary simply by changing the stereochemistry at the anomeric position. The exceptionally high level of 1,4-induction in these systems was attributed to the formation of a tridentate chelate involving one oxygen atom of the auxiliary and both oxygen atoms of the aglycone.

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(06) CHARETTE, A. B.; MARCOUX, J.-F.; CÔTÉ, B.
Carbohydrates as Chiral Auxiliaries: Synthesis of 2-Hydroxy-β-D-Glucopyranosides

Tetrahedron Lett. 1991, 32, 7215-7218.

A new 2-step, 1-pot procedure for the stereoselective glycosylation of allylic alcohols with 1,2-di-O-benzoyl-β-D-glucopyranosides to produce 2-hydroxy-β-D-glucopyranosides has been developed. The required precursor for the glycosylation was readily obtained from tri-O-benzyl-D-glucal in 2 steps (91% overall).

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(05) CHARETTE, A. B.; CÔTÉ, B.; MARCOUX, J.-F.
Carbohydrates as Chiral Auxiliaries: Asymmetric Cyclopropanation Reaction of Acyclic Olefins

J. Am. Chem. Soc. 1991, 113, 8166-8167.

A highly diastereoselective cyclopropanation reaction of substituted allylic alcohols linked to a carbohydrate readily available from D-glucose is reported herein.

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(04) BOECKMAN, R. K. Jr.; ASBERON, T.; JOHNSTON, B. H.; CHARETTE, A. B.
The Chemistry of Cyclic Vinyl Ethers 6. Total Synthesis of Polyether Ionophore Antibiotics of the Valcimycin (A-23187) Vlass

J. Am. Chem. Soc. 1991, 113, 5337-5353.

An extremely convergent (longest linear sequence, 16 steps), fully stereoselective, and potentially general synthesis of the antibiotic ionophores of the Calcimycin (A-23187) class was devised. The key steps involve a coupling reaction between the chiral nonracemic subunits dihydropyran 41 (as the α-lithio anion) and bromide 49. Subsequent acid-promoted cyclization directly produces the spirocyclic ring system found in the ionophore X-14885A (3). Alternatively, cyclopropanation of substituted vinyl ether 55 followed by acid treatment afforded the spiroketal 58 that was subsequently converted into the polyether ionophore Calcimycin (1) and also Cezomycin (2).

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(03) EVANS, D. A.; GAUCHET-PRUNET, J. A.; CARREIRA, E. M.; CHARETTE, A. B.
Synthesis of 1,3-Diol Synthons From Epoxy Aromatic Precursors: An Approach to the Construction of Polyacetate-Derived Natural Products

J. Org. Chem. 1991, 56, 741-750.

The identification of stereoregular polyol subunits within polyacetate-derived natural products such as bryostatin 1 and amphotericin B has led to the convergent synthesis of six- and seven-carbon 1,3-diol fragments in high yields and stereoselectivities. This plan relies upon the enantioselective asymmetric epoxidation/kinetic resolution of cinnamyl alcohols and the use of meta-substituted anisyl rings as masked β-keto ester synthons. Birch reduction of 9 (R = SiiPr3) followed by ozonolysis afforded the hydroxyl keto ester 11 in 66% overall yield. Anti-selective β-selective β-hydroxy ketone reduction employing Me4NBH(OAc)3 afforded 12 in 93% yield and 13:1 diastereoselectivity while syn-selective reduction with Et2BOMe/NaBH4 gave 13 in 82% yield and 15:1 diastereoselectivity. An analogous sequence provides access to all stereoisomers of 3,5,6-trihydroxyhexanoic acid heptanoic acids which should be useful chiral subunits of polyacetate-derived natural products such as amphotericin and bryostatin.

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(02) BOECKMAN, R. K. Jr.; CHARETTE, A. B.; ASBEROM, T.; JOHNSTON, B. H.
A Convergent General Synthetic Protocol for Construction of Spirocyclic Ketal Ionophores: An Application to the Yotal Synthesis of (-)-A-23187 (Calcimycin)

J. Am. Chem. Soc. 1987, 109, 7553-7555.

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(01) BOECKMAN, R. K. Jr.; ENHOLM, E. J.; DEMKO, D. M.; CHARETTE, A. B.
An Efficient Enantioselective Total Synthesis of (-)-X-14547A (Indanomycin)

J. Org. Chem. 1986, 51, 4743-4745.

A highly efficient and stereocontrolled enantioselective total synthesis of the antibiotic ionophore X-14547A (indanomycin) (1) is described. A particularly concise approach to the key pyranaldehyde intermediate 3 features the use of reductive lithiation to append the axial C(7) pyran side chain. A Wittig reaction was employed to couple the two major subunits 3 and 4 followed by intramolecular [4 + 2] cycloaddition to complete the framework.

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