0.59 g of (2S, 4S)-4-(4-methoxybenzylthio)-2-[3-(4-nitrobenzyloxycarbonylamino)azetidin-1-ylcarbonyl]-1-(4-nitrobenzyloxycarbonyl)pyrrolidine [prepared as described in step (b) above] was suspended in 0.94 ml of anisole, and 3.32 ml of trifluoroacetic acid and 0.15 ml of trifluoromethanesulfonic acid were added dropwise to the suspension, whilst ice-cooling, after which the mixture was stirred at the same temperature for 1 hour. At the end of this time, 1,2-dichloroethane was added to the reaction mixture, and the solvent was removed by distillation under reduced pressure. The residue was then washed by repeated decantation, in turn, with hexane and with diethyl ether. The residue was diluted with ethyl acetate and made alkaline by the addition of an aqueous solution of sodium hydrogencarbonate. The ethyl acetate layer was separated and washed with water and with an aqueous solution of sodium chloride, in that order, after which it was dried over anhydrous magnesium sulfate and concentrated by evaporation under reduced pressure to give 0.44 g of the title compound, in the form of an amorphous powder.
