Asymmetric aldol addition of aldehydes to a difluoroketene silyl acetal catalyzed by chiral Lewis acids

Aldol reaction of aldehydes with difluoroketene ethyl trimethylsilyl acetal (1) in the presence of a substoichiometric amount of chiral Lewis acid 2 or 3 provides the corresponding alpha,alpha-difluoro beta-hydroxy esters 4-12 with high enantioselectivities (up to 98% ee). Reaction temperature has a great influence on the enantiofacial selection of aldehydes; the reactions of benzyloxyacetaldehyde catalysed by Lewis acid 2 at -78 and -30'C gave the (+)- and (-)-alpha,alpha-difluoro beta-hydroxy esters 7 in optical yields of 98% and 85%, respectively. (C) 1997 Elsevier Science Ltd.