To a solution of 4-nitrobenzyl(2R,5R,6S)-3,7-dioxo-6-[1-methyl-1-(4-nitrobenzyloxycarbonyloxy)ethyl]-1-azabicyclo[3.2.0]heptane-2-carboxylate (148.5 mg) and 4-N,N-dimethylaminopyridine (3.35 mg) in acetonitrile (7.4 ml) was added dropwise a solution of di-isopropylethylamine (57.3 μl) in acetonitrile (0.516 ml). To the mixture was further added a solution of diphenyl chlorophosphate (59.6 μl) in acetonitrile (0.536 ml) at 0° C. After stirring for an hour at 0° C., a solution of diisopropylethylamine (190.9 μl) in acetonitrile (1.72 ml) was added to the mixture. To the mixture was further added a solution of N-(4-nitrobenzyloxycarbonyl)cysteamine (73.8 mg) in acetonitrile (0.664 ml) at -15° C. The mixture was allowed to stand overnight at -15° C. The resulting solution was diluted with ethyl acetate (15 ml) and washed in turn with water and saturated aqueous sodium chloride. After drying over magnesium sulfate, the ethyl acetate extract was filtered and evaporated in vacuo. The residue was chromatographed on silica gel (3 g) eluting with a mixture of methylene chloride and ethyl acetate (50:1 to 5:1) to give 4-nitrobenzyl(5R,6S)-6-[1-methyl-1-(4-nitrobenzyloxycarbonyloxy)ethyl]-3-[2-(4-nitrobenzyloxycarbonylamino)-ethylthio]-7-oxo-1-azabicyclo[3.2.0]hept-2-ene-2-carboxylate (167.8 mg) as an amorphous solid.
