To a solution of (3S,4R)-3-[(1R)-1-(t-butyldimethylsilyloxy)ethyl]- 4-{(1R)-1-methyl-2-oxo-2-(2-thioxothiazolidin-3-yl)ethyl]-2-azetidinone (3.17 g) in tetrahydrofuran (30 ml) were added 1-allyloxycarbonyl-2-(2-fluoroethyloxymethyl)-4-mercaptopyrrolidine (2.21 g) and triethylamine (1.46 ml) at 5° C., and the mixture was stirred at room temperature for 8.5 hours. Acetic acid (0.63 ml) was added to the mixture and the mixture was evaporated. The residue was dissolved in water and ethyl acetate. The organic layer was separated, washed with water, dried and evaporated. The residue was purified by chromatography on silica gel eluting with a mixture of dichloromethane and ethyl acetate (2:1 V/V) to give (3S,4S)-4-[(1R)-2-{(2S,4S)-1-allyloxycarbonyl-2-(2-fluoroethyloxymethyl)pyrrolidin-4-yl-thio}-1-methyl-2-oxoethyl]-3-{(1R)-1-(t-butyldimethylsilyl-oxy)ethyl}-2-azetidinone (1.82 g).
