In 132 ml of methylene chloride was dissolved 6.6 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. The solution was cooled to -60° C. Thereto were added 4.66 ml of N,N-dimethylaniline and 3.3 g of phosphorus pentachloride. The mixture was stirred at -40° to -20° C. for 1 hour and cooled to -60° C. To the reaction mixture was added 13.8 ml of anhydrous methanol. The temperature of the mixture was elevated to 0° C. in 30 minutes and mixed with 66 ml of water. The resulting mixture was stirred for 15 minutes with ice cooling. The resulting crystals were collected by filtration and suspended in a mixed solvent consisting of 50 ml of water and 100 ml of methylene chloride. The suspension was adjusted to pH 7.5 with a saturated aqueous sodium hydrogencarbonate solution. The organic layer was separated and dried over anhydrous magnesium sulfate. The solvent was removed by distillation under reduced pressure to obtain 4.5 g (yield: 84.0%) of (3R,5R,6R)-6-amino-3-(3-benzylideneamino-2-oxoimidazolidin-1-yl)-3-(p-nitrobenzyloxycarbonyl)-7-oxo-4-thia-1-azabicyclo[3.2.0]heptane.
