The Vilsmeier reagent was prepared from dry dimethylformamide (0.74 g), phosphorus oxychloride (1.56 g) and dry ethyl acetate (2.0 ml) by the conventional method. Dry tetrahydrofuran (15 ml) was added thereto and then 2-(3-isoxazolyl)methoxyimino-2-(2-formamidothiazol-4-yl)acetic acid (syn isomer) (1.50 g) was added thereto at 0° C. The mixture was stirred for 30 minutes at the same temperature. The resulting mixture was added dropwise at -5° to 0° C. to a stirred solution of 7-amino-3-(1-allyl-1H-tetrazol-5-yl)thiomethyl-3-cephem-4-carboxylic acid (2.34 g) in a mixture of water (11.5 ml) and acetone (11.5 ml) keeping the pH at 7.5 by triethylamine, and the mixture was stirred for 30 minutes at the same temperature at pH 7.5. To the reaction mixture was added ethyl acetate (60 ml) and the mixture was adjusted to pH 2.5 with 10% hydrochloric acid. Insoluble material was filtered off and the filtrate was extracted twice with ethyl acetate. The combined extracts were washed twice with a saturated sodium chloride aqueous solution and dried over magnesium sulfate. The solvent was distilled off and the residue was pulverized with diethyl ether to give yellowish powder of 7-[2-(3-isoxazolyl)methoxyimino-2-(2-formamidothiazol-4-yl)acetamido]-3-(1-allyl-1H-tetrazol-5-yl)thiomethyl-3-cephem-4-carboxylic acid (syn isomer) (2.15 g).
