Chlorodicyclohexylborane-mediated aldol additions of alpha,alpha '-dioxygenated ketones

[GRAPHICS] Boron aldol additions of variously O-protected alpha,alpha'-dioxygenated ke tones using dicyclohexylboron chloride and a tertiary amine have been inves tigated. The stereoselectivity of the process was dependent on the protecti ng group on the alpha -oxygen atoms. Notably, ketones with bulky silyloxy g roups gave syn aldols, most likely via Z enolates, This rules out the parti cipation of chelates during the enolization process, at least in the presen ce of such sterically crowded protecting groups, An alternative explanation is offered.