Acetate → 1-Azabicyclo[3.2.0]heptane-2-carboxylic acid, 6-[(1R)-1-hydroxyethyl]-4-methyl-3,7-dioxo-, (4-nitrophenyl)methyl ester, (4R,5R,6S)-

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To a solution of 4-nitrobenzyl (4R)-2-diazo-4-[(2R, 3S)-3-{(1R)-1-hydroxyethyl}-4-oxoazetidin-2-yl]-3-oxopentanoate (0.5 g) in 1,2-dichloroethane (10 ml) was added rhodiim(II) acetate (2 mg) under refluxing. After refluxing for 30 minutes, the reaction mixture was cooled and 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 and the residue was dried in vacuo 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 (10 ml) and cooled to 0° C. under an atmosphere of nitrogen. To this solution were added N,N-diisopropyl-N-ethylanine (0.27 ml) and diphenyl phosphorochloridate (0.28 ml) successively, and the solution was stirred at 0° C. for 40 minutes. To the resulting solution were added dropwise a solution of (2S,4S)-2-(difluoromethyl)thiomethyl-4-mercapto-1-(4-nitrobenzyloxycarbonyl)pyrrolidine (0.54 g) in anhydrous acetonitrile (3 ml) and N,N-diisopropyl-N-ethylamine (0.29 ml) at 5° C. with stirring, and the stirring was continued at the same temperature for 2 hours. To the reaction mixture was added ethyl acetate (30 ml). The solution was washed twice with saturated aqueous sodium chloride (20 ml), dried over anhydrous magnesium sulfate and evaporated. The oily residue was chromatographed on silica gel (60 g) eluting with a mixture of dichloromethane and acetone (5:1,V/V) to give 4-nitrobenzyl (4R,5S,6S)-3-[(2S,4S)-2-(difluoromethyl)thiomethyl-1-(4-nitrobenzyloxycarbonyl)pyrrolidin-4-ylthio]-6-[(1R)-1-hydroxyethyl]-4-methyl-7-oxo-1-azabicyclo[3.2.0]hept-2-ene-2-carboxylate (0.58 g).

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