To a solution of 4-nitrobenzyl (4R)-2-diazo-4-[(2R, 3S)-3-{(1R)-1-hydroxyethyl}-4-oxoazetidin-2-yl]-3-oxopentanoate (1.0 g) in dichloroethane (50 ml) was added rhodium (II) acetate (2 mg) under refluxing. After refluxing for 1 hour, the reaction mixture was evaporated in vacuo to give a residue. The residue was dissolved in anhydrous benzene (10 ml) and then evaporated. This operation was repeated once again to give 4-nitrobenzyl (4R, 5R, 6S)-6-[(1R)-1-hydroxyethyl]-4-methyl-3, 7-dioxo-1-azabicyclo[3.2.0]heptane-2-carboxylate. The compound obtained above was dissolved in anhydrous acetonitrile (20 ml) and cooled to 0° C. under an atmosphere of nitrogen. To this solution were added N,N-diisopropyl-N-ethylamine (0.59 ml) and diphenyl phosphorochloridate (0.61 ml) successively, and the solution was stirred at 0°-5° C. for 40 minutes. To the resulting solution were added a solution of (2S, 4S)-2-(2-aminothiazol-4-yl)-4-mercapto-1-(4-nitrobenzyloxycarbonyl) pyrrolidine (1.07 g) in acetonitrile and N,N-diisopropyl-N-ethylamine (0.59 ml) and the stirring was continued at the same temperature for 2 hours. The reaction mixture was poured into ethyl acetate (60 ml) and washed with 0.1N hydrochloric acid, saturated aqueous sodium bicarbonate and brine successively. The dried organic layer was evaporated and the oily residue was chromatographed on silica gel (60 g) eluting with a mixture of dichloromethane and acetone (5:2 V/V) to give 4-nitrobenzyl (4R, 5S, 6S )-3- [(2S, 4S)-2-(2-aminothiazol-4-yl)-1-(4-nitrobenzyloxycarbonyl) pyrrolidin-4-ylthio]-6-[(1R)-1-hydroxyethyl]-4-methyl-7-oxo-1-azabicyclo [3.2.0]hept-2-ene-2-carboxylate (900 mg).
