1.80 g of sodium hydride (as a 55% w/w dispersion in mineral oil) was added, whilst ice-cooling, to a solution of 5.70 ml of 4-methoxybenzyl mercaptan dissolved in 100 ml of dry tetrahydrofuran, and the mixture was then stirred at 0° to 5° C. for 30 minutes. At the end of this time, a solution of 11.00 g of (2S, 4R)-1-(t-butoxycarbonyl)-4-methanesulfonyloxy-2-methylcarbamoylpyrrolidine [prepared as described in step (2) above] in 80 ml of dry dimethylformamide was added to the mixture, and the mixture was stirred at 34° C. for 4.5 hours. The reaction mixture was then 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. The solvent was then removed by distillation under reduced pressure. The residue was purified by column chromatography through silica gel (eluted with a 5:1 by volume mixture of ethyl acetate and hexane) to afford 4.35 g of the title compound as an oil.
