Enantiomerically Pure α-Alkylidene β-Amino Esters from Asymmetric Addition of Metal Dienolates to N-Sulfinylimines

Abstract

Lithium dienolate of 3-butenoic methyl ester was reacted with enantiomerically pure N-arylsulfinyl phenylimines (1 and 2) under different conditions. The reactions were completely regioselective-the C-C coupling occurs at the 2-position of dienolate-and highly stereoselective at the iminic carbon. In the presence of different Lewis acids (ZnCl(2), ZnBr(2), and ScTf(3)) mixtures of two alpha-vinyl, beta-arylsulfinylamino esters (epimers at C-alpha) were obtained, being the stereoselectivity depending on the nature of the aryl sulfinyl moiety and the Lewis acid used. Desulfinylation of these mixtures followed by isomerization of the double bond with Na(2)CO(3) allowed the synthesis of the optically pure (E)-alpha-ethylidene-beta-amino ester 10 in quite high overall yield. The addition of the lithium dienolate to sulfinylimines in the absence of the Lewis catalysts yielded mixtures containing important amounts of the optically pure N-arylsulfinyl alpha-ethylidene-beta-amino esters, which became the exclusive product of the reaction when N-2-methoxynaphthylsulfinyl phenylimine 2 was used as starting product.