To a solution of (2S,4S)-4-acetylthio-2-(3-methyl-2-oxoimidazolidin-1-yl)methyl-1-(4-nitrobenzyloxycarbonyl)-pyrrolidine (1.72 g) in methanol (40 ml) was added 28% sodium methoxide in methanol solution (0.83 ml) and the mixture was stirred under ice-cooling for 30 minutes. Acetic acid (0.25 ml) was added to the reaction mixture at the same temperature. The mixture was evaporated in vacuo to give a residue. The residue was dissolved in a mixture of ethyl acetate (80 ml) and tetrahydrofuran (30 ml) and the solution was washed with saturated aqueous sodium chloride, dried over anhydrous magnesium sulfate and evaporated in vacuo. The resulting residue was chromatographed on silica gel (100 g) eluting with a mixture of chloroform and methanol (19:1, V/V). The fractions containing the desired compound were collected and evaporated in vacuo to give (2S,4S)-4-mercapto-2-(3-methyl-2-oxoimidazolidin-1-yl)methyl-1-(4-nitrobenzyloxycarbonyl)pyrrolidine (1.23 g).
