A solution of methanesulfonyl chloride (20.5 ml, 265 mmol) in methylene chloride (20 ml) was dropwise added to a solution of (2S,4S)-N-allyloxycarbonyl-2-hydroxymethyl-4-tritylthiopyrrolidine (119 g, 259 mmol, compound of Reference Example 3-1 of Japanese Patent Application No. 192,093/1990) and triethylamine (40.3 ml, 289 mmol) in methylene chloride (1 l) under cooling with ice, and the mixture was stirred at the same temperature for 30 minutes. The reaction solution was washed sequentially with water (500 ml), a saturated sodium hydrogen carbonate aqueous solution (250 ml) and a saturated sodium chloride aqueous solution (250 ml), then dried over anhydrous sodium sulfate and concentrated. To the residue, ethyl acetate (30 ml) and diisopropyl ether (270 ml) were added, and the precipitated crystals were collected by filtration to obtain (2S,4S)-N-allyloxycarbonyl-2-methanesulfonyloxymethyl-4-tritylthiopyrrolidine (130.8 g, yield: 94%).
