To dimethylformamide (6.42 g.) was added dropwise phosphorus oxychloride (12.5 g.) over 20 minutes with stirring under cooling to 5° to 10° C., and the mixture was stirred for 30 minutes at 40° C., and then ethyl acetate (200 ml.) was added thereto with vigorous stirring. After the mixture was cooled to 3° C., 2-methoxyimino-2-(2-formylaminothiazol-4-yl)acetic acid (syn isomer), which can be represented as 2-methoxyimino-2-(2-formylimino-2,3-dihydrothiazol-4-yl)acetic acid (syn isomer), (18.34 g.) was added thereto, and then the mixture was stirred for 40 minutes at 3° to 5° C. Thus obtained solution was added to a solution, which was prepared by stirring a mixture of 2-methyl-7-amino-3-cephem-4-carboxylic acid (17.1 g.) and trimethylsilylacetamide (84 g.) in ethyl acetate (300 ml.) for 1 hour at room temperature, with vigorous stirring under cooling to -25° C. The mixture was stirred for 1 hour at -20° to -15° C. and for 30 minutes at -10° to -5° C. To the mixture was added water (200 ml.) at room temperature, and the mixture was further stirred for 20 minutes at the same temperature. After a saturated aqueous sodium bicarbonate solution was added to the mixture in order to dissolve the precipitates, the aqueous layer was separated. To the aqueous layer was added ethyl acetate, and the mixture was adjusted to pH 2 with 2 N hydrochloric acid, and then the ethyl acetate layer was separated. The remaining aqueous layer was further extracted with ethyl acetate. The ethyl acetate layers were combined together, washed with a saturated aqueous sodium chloride solution, dried over magnesium sulfate, treated with an activated charcoal and then concentrated. Thus obtained crystalline residue was pulverized in diethyl ether, collected by filtration and then dried to give white crystals of 2-methyl-7-[2-methoxyimino-2-(2-formylaminothiazol-4-yl)acetamido]-3-cephem-4-carboxylic acid (syn isomer), which can be represented as 2-methyl-7-[2-methoxyimino-2-(2-formylimino-2,3-dihydrothiazol-4-yl)acetamido]-3-cephem-4-carboxylic acid (syn isomer), (32.2 g.). This compound was recrystallized from methanol to give white crystals of the pure object compound, mp 174° to 204° C. (dec.).
