Enantioselective aldol reaction with bromofluoroketene silyl acetals

The aldol reaction of aldehydes with bromofluoroketene ethyl trimethylsilyl acetal in the presence of a catalytic amount of a chiral Lewis acid at -78 degrees C provides a mixture of the corresponding syn- and anli-alpha-brom o-alpha-fluoro-beta-hydroxy esters with high enantioselectivities (up to 99 % ee). Reaction temperature has a great influence on the stereoselectivity. The aldol reaction at -20 degrees C proceeds with high enantio- and diaste teoselectivities to preferentially afford the anti-aldols having opposite s igns of optical rotation to those at -78 degrees C. (C) 1999 Elsevier Scien ce Ltd. All rights reserved.