To a solution of 0.127 g of DMF in 5 ml of methylene chloride is added 0.104 ml of diphosgene at -10° C. The mixture is stirred for 30 minutes at room temperature, followed by addition of a solution of 15 ml of methylene chloride containing 0.483 g of 2-(2-chloroacetamidothiazol-4-yl)-2-methoxyiminoacetic acid and 0.176 g of triethylamine at -70° C. The mixture is stirred for two hours at -25° C., then cooled to -70° C., followed by addition of 0.352 g of triethylamine, 0.40 g of tosyl salt of (3S,4S)-3-amino-4-azido-2-oxoazetidine and 2 ml of propylene oxide. The temperature of the mixture is raised up to room temperature in the course of one hour while stirring, followed by ice-cooling for one hour. The crystals separated out are collected by filtration to yield 0.383 g of (3S,4S)-4-azido-3-[2-(2-chloroacetamidothiazol-4-yl)-2-methoxyiminoacetamido]-2-oxoazetidine.
