L-Serine hydrochloride (10.0 g, 70.6 mmol) and 0.073 g (7.1 mmol) of triethylamine were dissolved in 100 ml of diethylene glycol dimethyl ether, and 16.8 g (141.2 mmol) of thionyl chloride was added dropwise at room temperature in a nitrogen gas atmosphere. After 2 hours of stirring at 60° C., 200 ml of water was added while maintaining the reaction system at 15° C. or below, and the resulting mixture was stirred at room temperature for 30 minutes. Further, 50 g of potassium carbonate was added to make the pH about 10 and, then, 17.9 g (141.2 mmol) of benzyl chloroformate was added dropwise. After overnight standing at room temperature, 10 g of potassium carbonate was again added to make the pH about 10 and, then, 10.7 g (97.1 mmol) of thiophenol was added dropwise at room temperature in a nitrogen gas atmosphere. After 2 hours of stirring at 60° C., the reaction mixture was cooled with ice and acidified with 50% sulfuric acid, and extracted with ethyl acetate. The solvent was distilled off and the residue was purified by column chromatography to give 8.7 g (26.2 mmol, 37%) of N-carbobenzyloxy-S-phenyl-L-cysteine. The product obtained had an optical purity of not less than 98% e.e. as determined by HPLC analysis in the same manner as in Example 13.
