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.
Treatment of the g-N-hydroxylamino-a,b-acetylenic esters, obtained by the reaction of nitrones with alkyl 3-lithiopropiolates, with H 2 over Raney nickel, followed by in situ protection of the formed amines, gives N-Boc-g-amino esters. This method has been applied to a synthesis of (S) and (R)-Vigabatrin ® .
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