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| - Catalytic, Highly Enantioselective Friedel−Crafts Reactions ofAromatic and Heteroaromatic Compounds to Trifluoropyruvate. ASimple Approach for the Formation of Optically Active Aromaticand Heteroaromatic Hydroxy Trifluoromethyl Esters
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| - A new catalytic enantioselective synthetic method for the formation of optically active aromaticand heteroaromatic hydroxy-trifluoromethyl ethyl esters is presented. This catalytic enantioselectiveFriedel−Crafts reaction of trifluoromethyl pyruvate with aromatic and heteroaromatic compoundsis catalyzed by a chiral bisoxazoline copper(II) complex and proceeds in good yield and with highenantiomeric excess. For a series of substituted indoles, the corresponding 3-substituted hydroxy-trifluoromethyl ethyl esters are formed in up to 93% yield and 94% ee. Pyrrole and 2-substitutedpyrroles also react with trifluoromethyl pyruvate in a highly enantioselective aromatic electrophilicreaction and up to 93% ee and good yields are obtained. Furanes and thiophenes give thecorresponding 2-hydroxy-trifluoromethyl ethyl esters in high enantiomeric excess; however, theyields of the products are only moderate. Various types of aromatic compounds react in this catalyticreaction with trifluoromethyl pyruvate to give the aromatic electrophilic addition product in goodyield. To obtain high enantiomeric excess (>80% ee) it is necessary that aromatic amines areprotected with sterically demanding protecting groups such as benzyl or allyl. This preventscoordination of the amine nitrogen atom to the catalyst, as aromatic amines having a N,N-dimethylgroup probably coordinate to the catalyst, leading to a significant reduction of the enantioselectiveproperties of the catalyst. On the basis of the experimental results and the absolute configurationof the formed chiral center, the mechanism for the catalytic enantioselective Friedel−Crafts reactionis discussed.
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