Opposite π-face selectivity for the DMD and m-CPBA epoxidations of chiral 2,2-dimethyloxazolidine derivatives of tiglic amides:: Control by steric interactions versus hydrogen bonding

A high extent but opposite sense in the diastereoselectivity has been observed for the DMD and m-CPBA epoxidations of the optically active tiglic amides (S)-1 with 2,2-dimethyloxazolidines as chiral auxiliaries, This unprecedented reversed pi-facial differentiation for these two peroxidic oxidants is rationalized in terms of like (lk) and unlike (ul) transition structures: For DMD, steric interactions dominate, such that the unlike transition structure is favored, while for m-CPBA, hydrogen bonding effects overcome these steric repulsions and the like one is preferred.