To a mixture of 2-(2-aminothiazol-4-yl)-2-(2,2,2-trichloroethoxycarbonylmethoxyimino)acetic acid (syn isomer) (1.13 g) and phosphorus oxychloride (0.56 g) in ethyl acetate (20 ml) was added trimethylsilylacetamide (0.30 g) at 4° C., and the mixture was stirred at 4° to 6° C. for 15 minutes. To the mixture was added additional phosphorus oxychloride (0.56 g), followed by stirring at the same temperature for 15 minutes. Thereto was added N,N-dimethylformamide (0.26 g), and the mixture was stirred at 4° to 6° C. for 50 minutes to prepare the activated acid solution. This solution was added to a solution of benzhydryl 7-amino-3-vinyl-3-cephem-4-carboxylate hydrochloride (1.1 g) and trimethylsilylacetamide (2.0 g) in ethyl acetate (20 ml) at -20° C., and the mixture was stirred at -20° to 0° C. for an hour. After addition of ethyl acetate and water, the organic layer was separated and washed with a saturated aqueous sodium bicarbonate and an aqueous sodium chloride, followed by drying over magnesium sulfate. The solution was evaporated, and the residue was pulverized with diethyl ether to give benzhydryl 7-[2-(2-aminothiazol-4-yl)-2-(2,2,2-trichloroethoxycarbonylmethoxyimino)acetamido]-3-vinyl-3-cephem-4-carboxylate (syn isomer) (1.6 g).
