Efficient Asymmetric Synthesis of (S)- and (R)-N-Fmoc-S-Trityl-α-methylcysteine Using Camphorsultam as a Chiral Auxiliary

(1R)-(+)-2,10- and (1S)-(−)-2,10-camphorsultam were acylated with ethyl 2-phenylthiazoline 4-carboxylate to afford (+)- and (−)-2-phenylthiazolinylcamphorsultam, which were stereoselectively alkylated with MeI in the presence of n-BuLi. Alkylation of these phenylthiazolinylcamphorsultams occurred from the β-face rather than α-face, resulting in the formation of (S)-α-methylcysteine from (1R)-(+)-2,10-camphorsultam and (R)-α-methylcysteine from (1S)-(−)-2,10-camphorsultam after acidic hydrolysis. Subsequent protection of the side chain thiol group with trityl alcohol and α-amine function with Fmoc-OSu furnished fully protected (S)- and (R)-N-Fmoc-S-trityl-α-methylcysteine in overall 20% yield.