Opposite pi-face selectivity for the DMD and m-CPBA epoxidations of chiral2,2-dimethyloxazolidine derivatives of tiglic amides: Control by steric interactions versus hydrogen bonding

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