Conjugate addition of lithium N-tert-butyldimethylsilyloxy-N-(α-methylbenzyl)amide: asymmetric synthesis of β2,2,3-trisubstituted amino acids

Conjugate addition of the homochiral ammonia equivalent lithium N-tert-butyldimethylsilyloxy-N-(alpha-methylbenzyl)amide to a range of alpha,beta-unsaturated esters gives the corresponding beta-amino esters in moderate to good levels of diastereoselectivity. O-Desilylation and cyclisation furnishes homochiral soxazolidin-5-ones in >99:1 dr after purification. Sequential alkylation of these templates proceeds to give the corresponding 3,4-anti-disubstituted and 3,4,4-trisubstituted derivatives as single diastereoisomers after purification. The first alkylation occurs with high levels of diastereoselectivity on the face of the enolate anti to the C(3)-substituent, whereas the facial selectivity of the second alkylation is governed by a chiral relay effect, which depends upon the relative steric bulk of both the C(3)- and C(4)-substituents. Subsequent hydrogenolysis promotes cleavage of both the N-alpha-methylbenzyl group and the N-O bond within the isoxazolidin-5-one ring in one pot to give the corresponding beta(2,2,3)-tri-substituted amino acids directly. (C) 2010 Elsevier Ltd. All rights reserved.