In a mixed solvent consisting of 30 ml of methylene chloride and 15 ml of methanol was dissolved 3.00 g of (3R,5R,6R)-3-(3-benzylideneamino-2-oxoimidazolidin-1-yl)-3-(p-nitrobenzyloxycarbonyl)-7-oxo-6-phenylacetamido-4-thia-1-azabicyclo[3.2.0]heptane. To the solution were added 1.89 g of 2,4-dinitrophenylhydrazine and 850 mg of p-toluenesulfonic acid monohydrate in this order. The mixture was stirred at room temperature for 1.5 hours. Then, the insolubles were removed by filtration. The filtrate was concentrated under reduced pressure. The residue was mixed with a mixed solvent consisting of 30 ml of methylene chloride and 7 ml of methanol. The insolubles were removed by filtration. The filtrate was concentrated under reduced pressure. The residue was purified by column chromatography (eluant: chloroform/methanol=100/1 to 25/1) to obtain 1.56 g (yield: 60.5%) of (3R,5R,6R)-3-(3-amino-2-oxoimidazolidin-1-yl)-3-(p-nitrobenzyloxycarbonyl)-7-oxo-6-phenylacetamido-4-thia-1-azabicyclo[3.2.0] heptane.
