16.1 g (0.092 mole) of 23% sodium hydroxide, 10 ml of water and 4.7 g (0.04 mole) of L-valine were placed in a 300-ml reaction flask, and stirred at room temperature for 30 minutes. Thereto was dropwise added 5.9 g.(0.048 mole) of isopropyl chlorocarbonate at room temperature, followed by stirring for 1 hour. The resulting mixture was neutralized with concentrated hydrochloric acid. Thereto were added 50 ml of toluene and 0.06 g (0.0004 mole) of N,N-dimethylaminobenzylamine. Then, 4.7 g (0.038 mole) of isopropyl chlorocarbonate was added dropwise at room temperature, followed by stirring for 1 hour. Thereafter, there was dropwise added a solution of 7.5 g (0.038 mole) of (R)-1-(6-fluorobenzothiazol-2-yl)ethylamine (purity: 98.3%, optical purity: 99.0% ee) dissolved in 50 ml of toluene, produced according to the above Reference Example, followed by stirring at room temperature for 2 hours. 50 ml of water was added; the resulting mixture was heated to 70° C. and subjected to phase separation; the toluene layer was washed with 50 ml of hot water and subjected to solvent removal to obtain 13.4 g (yield: 92.4%) of isopropyl [(S)-1-[(R)-1(6-fluorobenzothiazol-2-yl)ethylcarbamoyl]-2-methylpropyl]carbamate (purity: 96.3%, the proportion of formed intended substance in four diastereomers: 98.5%).
