1.11 g of sodium hydride (as a 55% w/w dispersion in mineral oil) was added, whilst ice-cooling, to a solution of 3.51 ml of 4-methoxybenzyl mercaptan dissolved in 60 ml of dry dimethylformamide, and the mixture was stirred at room temperature for 30 minutes. At the end of this time, a solution of 8.13 g of (2S, 4R)-1-(t-butoxycarbonyl)-4-methanesulfonyloxy-2-methoxycarbonylpyrrolidine [prepared as described in step (7) above] in 20 ml of dry dimethylformamide was added dropwise to the mixture, which was then stirred at room temperature for 15 minutes and then at 40° C. for 4 hours. The reaction mixture was then poured into an aqueous solution of sodium chloride and extracted with ethyl acetate. The extract was washed with water, dried and freed from the solvent by distillation under reduced pressure. The residue was purified by column chromatography through silica gel (eluted with a 15:1 by volume mixture of benzene and ethyl acetate), to afford 6.89 g of the title compound as an oil.
