To dimethylformamide (0.2 g) was added dropwise phosphorus oxychloride (0.4 g) under stirring and ice-cooling, and the mixture was stirred for 30 minutes at 40° C. and then suspended in ethyl acetate (10 ml). To the suspension was added 2-methoxyimino-2-(4-thiazolyl)acetic acid (syn isomer) (0.4 g) under stirring and ice-cooling and then stirred for 30 minutes under ice-cooling. Thus obtained solution was added to a solution, which was prepared by stirring a mixture of 7-amino-3-(1-allyl-1H-tetrazol-5-yl)thiomethyl-3-cephem-4-carboxylic acid (0.76 g) and trimethylsilylacetamide (2.3 g) in ethyl acetate (20 ml), under cooling to -25° C. The mixture was stirred for 1 hour at -20° C. to -10° C., and a saturated aqueous solution of sodium chloride (100 ml) was added thereto. Thus obtained mixture was adjusted to pH 7.5 with 20% aqueous solution of sodium carbonate, and the aqueous layer was separated. To the aqueous layer was added ethyl acetate, and adjusted to pH 2 with 10% hydrochloric acid, and then extracted with ethyl acetate. The ethyl acetate layer was washed with a saturated aqueous solution of sodium chloride, dried over magnesium sulfate and then evaporated. The residue was pulverized in diethyl ether, collected by filtration and then dried to give 7-[2-methoxyimino-2-(4-thiazolyl)-acetamido]-3-(1-allyl-1H-tetrazol-5-yl)thiomethyl-3-cephem-4-carboxylic acid (syn isomer) (1.01 g).
