Highly Diastereoselective Methylation of Five-Ring N,O-Acetals

Highly diastereoselective methylation of (2S,4S)- and (2R,4S)-3-tert-butyl 4-methyl 2-tert- butyloxazolidine-3,4-dicarboxylate (1a/b) is reported. The relative and absolute configuration of the methylated products was assigned by NOESY and confirmed by a crystal structure of 1a.