(R)-2-BOC-amino-3-(4-methoxy-benzylsulfanyl)-propionic acid (43 g, 132 mmol) prepared in Step B, 1-methylmorpholine (14.5 mL, 132 mmol) and ethylchloroformate (14.1 mL, 132 mmol) were dissolved in tetrahydrofuran (500 mL), and the mixture was stirred for 1 h at −25° C. At the same time, potassium hydroxide (75 g, 1336 mmol) was dissolved in water (75 mL) and diethylether (750 mL), N-methyl-nitrosourea (26 g, 252 mmol) was added in drops thereto over 2 h at 0° C., and the mixture was stirred for 30 min. The two solutions thus obtained were mixed together, and stirred for 3 h at −25° C. to room temperature. After completion of the reaction, water was added to the reaction mixture, which was then washed with saturated aqueous sodium hydrogen carbonate solution and saturated aqueous ammonium chloride solution in the order. The organic layer was concentrated to give the title compound (46.0 g, Yield 95%).
