To a solution of (2S,4S)-4-acetylthio-2-hydroxymethyl-1-(4-nitrobenzyloxycarbonyl)pyrrolidine (15 g) in a mixture of methanol (150 ml) and tetrahydrofuran (150 ml) was added a 28% solution of sodium methoxide in methanol (8.95 ml) under ice-cooling and the mixture was stirred at the same temperature for 10 minutes. To the mixture was added triphenylmethyl chloride (12.39 g) on ice-bath, followed by stirring at the same temperature for 1 hour. The precipitates were filtered off and the filtrate was evaporated in vacuo to give a residue. The residue was dissolved in ethyl acetate (200 ml), washed with water, dried over magnesium sulfate, and concentrated under reduced pressure to give a syrup. The syrup was chromatographed on silica gel (200 g) eluting with a mixture of dichloromethane and acetone (9:1 V/V). The fractions containing the desired compound were collected and evaporated in vacuo to give (2S,4S)-2-hydroxymethyl-1-(4-nitrobenzyloxycarbonyl)-4-(triphenylmethylthio)pyrrolidine (22.24 g).
