Asymmetric synthesis of anti‐α‐alkyl‐β‐amino carboxamides

The alkylation reactions of enolates derived from the highly diastereoselective conjugate additions of lithium (R)‐N‐benzyl‐N‐α‐methylbenzylamide (R)‐1 to N, N‐dimethyl crotonamide 2 and N, N‐dimethyl cinnamide 3 have been investigated. The alkylations of enolates derived from 3 are shown to afford anti‐α‐alkyl‐β‐amino carboxamides with excellent stereocontrol: the stereochemistry is assigned with the aid of a single crystal X‐ray structure of (2R,3S,αR)‐N, N‐dimethyl 2‐benzyl‐3‐phenyl‐3‐(N‐benzyl‐N‐α‐methylbenzylamino)propanamide (2R,3S,αR)‐10. As a result of the difficulty encountered in hydrolysing these hindered β‐amino amides such as 10, an alternative procedure is developed involving conjugate addition‐alkylations with α,β‐unsaturated N‐acyloxazolidin‐2‐ones as substrates, giving selectively alkylated products which are susceptible to reductive cleavage. Copyright © 1995 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim