(R)-4-Nitrobenzyl 2-diazo-4-((2R,3S)-3-((R)-1-hydroxyethyl)-4-oxoazetidin-2-yl)-3-oxopentanoate → 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 (0.35 g) in dichloroethane (10 ml) was added rhodium acetate (1 mg) under reflux in a nitrogen stream. The mixture was refluxed for 30 minutes and concentrated under reduced pressure 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 acetonitrile (10 ml). To the solution was added diphenyl phosphorochloridate (0.20 ml) at -10°~5° C. in nitrogen stream and dropwise added N,N-diisopropyl-N-ethylamine (0.20 ml) at the same condition. The mixture was stirred at the same condition for 1 hour. To the solution were added N,N-diisopropyl-N-ethylamine (0.2 ml) and then a solution of (2S,4S)-4-mercapto-1-(4-nitrobenzyloxycarbonyl)-2-[2-(4-nitrobenzyloxycarbonylamino)ethyloxymethyl]pyrrolidine (0.46 g) in acetonitrile (2 ml) at -20° C. The mixture was stirred at the same temperature for 30 minutes and then at 0°-10° C. for 3 hours. The mixture was poured into a mixture of water (60 ml) and ethyl acetate (90 ml). The organic layer was washed with water (90 ml×2) and brine (90 ml) successively, dried over magnesium sulfate, and concentrated under reduced pressure to give a syrup. The syrup was subjected to a column chromatography on silica gel (20 g) and eluted with a mixture of acetone and dichloromethane (5:95, 10:90, and 15:85, in turn) to give 4-nitrobenzyl (4R,5S,6S)-6-[(1R)-1-hydroxyethyl]-4-methyl-3-[(2S,4S)-1-(4-nitrobenzyloxycarbonyl)-2-{2-(4-nitrobenzyloxycarbonylamino)ethyloxymethyl}pyrrolidin-4-yl]thio-7-oxo-1-azabicyclo[3.2.0]hept-2-ene-2-carboxylate (0.45 g).

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