To a suspension of 2-methyl-7-[2-(2-formylaminothiazol-4-yl)glyoxylamido]-3-cephem-4-carboxylic acid, which can be represented as 2-methyl-7-[2-(2-formylimino-2,3-dihydrothiazol-4-yl)glyoxylamido]-3-cephem-4-carboxylic acid, (792 mg.) in water (20 ml.) was added sodium bicarbonate (168 mg.) with stirring. To thus obtained solution were added sodium acetate trihydrate (272.2 mg.) and O-methylhydroxylamine hydrochloride (334 mg.), and the mixture was stirred for 2 hours at 48° to 50° C. After cooling the reaction mixture, a saturated aqueous sodium bicarbonate solution (10 ml.) and ethyl acetate (15 ml.) were added thereto in order to dissolve the precipitated insoluble material. The aqueous layer was separated, washed with ethyl acetate, adjusted to pH 1 with 2 N hydrochloric acid and then extracted with ethyl acetate. The ethyl acetate extract was washed with a saturated aqueous sodium chloride solution, dried over magnesium sulfate and then concentrated. The residue was pulverized in diethyl ether, collected by filtration and then dried to give 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-carboxylic acid (syn isomer), (505 mg.). This compound was recrystallized from methanol to give white crystals of the pure object compound.
