Enantioselective aldol reaction with bromofluoroketene silyl acetals

Abstract

The aldol reaction of aldehydes with bromofluoroketene ethyl trimethylsilyl acetal in the presence of a catalytic amount of a chiral Lewis acid at −78°C provides a mixture of the corresponding syn- and anti-α-bromo-α-fluoro-β-hydroxy esters with high enantioselectivities (up to 99% ee). Reaction temperature has a great influence on the stereoselectivity. The aldol reaction at −20°C proceeds with high enantio- and diastereoselectivities to preferentially afford the anti-aldols having opposite signs of optical rotation to those at −78°C.