To a solution of (3R,4R)-4-acetoxy-3-[(1R)-1-t-butyldimethylsilyloxyethyl]-2-oxoazetidine (2.2 g) and triethylamine (1.1 ml) in dichloromethane (20 ml) was added trimethylsilyl trifluoromethanesulfonate (1.5 ml) at -60° C. under nitrogen, and the mixture was stirred at 0° C. for 30 minutes (Solution A). To a solution of 4-acetyl-1-allyloxycarbonylpyrrolidine (1.0 g) and triethylamine (0.71 ml) in dichloromethane (10 ml) was added trimethylsilyl trifluoromethanesulfonate (0.98 ml) at -60° C. under nitrogen and the resulting mixture was stirred at 0° C. for 30 minutes. To this mixture was added dropwise the Solution A at 0° C. under nitrogen, and the mixture was stirred for additional 4 hours at 0° C. The reaction mixture was taken up into a mixture of ethyl acetate and water and the mixture was stirred at ambient temperature for 2 hours. After adjusting pH to around 6.5 with aqueous sodium hydrogen carbonate, the organic layer was separated, washed with brine, dried over magnesium sulfate and evaporated in vacuo. The residue was chromatographed on silica gel eluting with a mixture of n-hexane and ethyl acetate (1:2, V/V) to give (3S,4R)-4-[2-(1-allyloxycarbonylpyrrolidin-3-yl)-2-oxoethyl]-3-[(1R)-1-t-butyldimethylsilyloxyethyl]-2-oxoazetidine (1.2 g).
