the difference of S andMupon attacking the carbonyl group. It is important to note that the phenyl group must be regarded quite similar in its steric size to a methyl group, as seen by comparison of the selectivities obtained with phenyl- and methylmagnesium bromide. The fact that the phenyl group is regarded in 13 as the substituent L and the methyl group as M might be due to stereoelectronic reasons with phenyl being a weak electron acceptor. However, the balance of the factors seems to be quite delicate, and examples in which methyl rather than phenyl takes the role of L are also encountered.