To a solution of (2S,4S)-2-(methanesulfonyloxy)methyl-1-(4-nitrobenzyloxycarbonyl)-4-(triphenylmethylthio)pyrrolidine (7.1 g) in dimethylformamide (70 ml) was added sodium azide (1.09 g) and ammonium chloride (0.90 g) and the mixture was stirred at 80°-90° C. for 3 hours. The reaction mixture was poured into ice-water (200 ml) and extracted twice with ethyl acetate (200 ml). The extract was washed with saturated aqueous sodium chloride, dried over magnesium sulfate and evaporated in vacuo. The resulting residue was chromatographed on silica gel (200 g) eluting with n-hexane and ethyl acetate (2:1, V/V). The fractions containing the desired compound were collected and evaporated in vacuo to give (2S,4S)-2-azidomethyl-1-(4-nitrobenzyloxycarbonyl)-4-(triphenylmethylthio)pyrrolidine (5.20 g).
