3.57 g of sodium hydride (as a 55% w/w suspension in mineral oil) was added, whilst ice-cooling, to a solution of 11.31 ml of 4-methoxybenzyl mercaptan dissolved in 150 ml of dry dimethylformamide and the mixture was stirred at 0° to 5° C. for 30 minutes. A solution of 26.00 g of (2S, 4S)-1-t-butoxycarbonyl-2-ethylcarbamoyl-4-methanesulfonyloxypyrrolidine in 100 ml of dry dimethylformamide was then added to the resulting mixture, and the whole was stirred at room temperature for 2 hours. At the end of this time, the reaction mixture was poured into an aqueous solution of sodium chloride and extracted with ethyl acetate. The extract was washed with an aqueous solution of sodium chloride and dried over anhydrous magnesium sulfate. At the end of this time, the solvent was removed by distillation under reduced pressure, and the residue was purified by column chromatography through silica gel using a 1:1 by volume mixture of hexane and ethyl acetate as an eluent, to afford 17.00 g of the title compound as colorless crystals, melting at 92°-94° C.
