(3S, 4S)-3-(tert-Butoxycarbonyl) amino-4-methylpyrrolidine (12a) was synthesized from L- aspartic acid (4) on a large scale. Methylation of dimethyl (2S)-N-benzyl-N-(9-phenylfluoren- 9-yl) aspartate (5), easily derived from 4, with methyl iodide gave (3R)-3-methylaspartate (6a) in 91% yield with high diastereoselectivity. Reduction of 6a with lithium aluminum hydride, followed by hydrogenolysis, protection with di-tert-butyl dicarbonate, and ...