the mukaiyama aldol reaction and the sakurai reaction refer to the addition of silyl enol ethers and allylsilanes to carbonyl compounds, respectively. only under lewis acid catalysis do these reactions occur under synthetically useful conditions. acyclic transition states are believed to be operating in both reactions for either 1,2- or 1,4- addition, and steric factors control stereoselectivity. this is in contrast with the rigid zimmerman-traxler cyclic transition state that has been widely accepted for the aldol reaction with lithium, boron, and titanium enolates. as a consequence, the double bond geometry in the silyl enol ether or allylsilane does not translate well into product stereochemistry. a model for the sakurai 1,2-addition, proposed by kumada, is presented in the scheme below; the "syn" diastereomer is predominant when the "(e)" silane is used, and also slightly favored when the "(z)" silane is used. a similar analysis by heathcock explains the fact that, with simple substrates, there is essentially no diastereoselectivity for the intermolecular mukaiyama aldol reaction.