[reaction: see text] The direct Michael addition of aldehydes and ketones to nitrostyrene, catalyzed by N-i-Pr-2,2'-bipyrrolidine, is described. The desired 1,4-adducts are obtained in excellent yield with enantioselectivities up to 85% ee and dr up to 95:5 of the syn product.
Abstract:The direct Michael addition of aldehydes and ketones to nitroolefins, catalyzed by N-i-Pr-2,2'-bipyrrolidine, is described. The desired 1,4-adducts are obtained in excellent yields with enantioselectivities up to 95% ee and dr up to 95 : 5 of the syn aldehyde addition product. An unexpected inversion of diastereoselectivity was observed for the addition of a-hydroxy ketones to b-arylnitroolefins with enantioselectivities up to 98% ee. The formation of an internal hydrogen bond between the OH group of the ahydroxy ketone and the tertiary nitrogen of the catalyst leads to the formation of a rigid cis enamine intermediate that accounts for the inversion of the expected diastereoselectivity and the very high ees.
[reaction: see text] The regio-, stereo-, and enantioselective direct Michael addition of alpha-hydroxyketones to beta-arylnitroolefins catalyzed by N-iPr-2,2'-bipyrrolidine is described. The formation of an internal hydrogen bond between the OH group of alpha-hydoxyacetone and the tertiary nitrogen of the catalyst leads to the formation of a rigid cis enamine intermediate that explains the inversion of the expected diastereoselectivity and the very high ee's.
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