The Vilsmeier reagent was prepared by conventional method from dimethylformamide (1.16 g.), phosphorus oxychloride (2.44 g.) and ethyl acetate (6 ml.). Ethyl acetate (10 ml.) was added thereto, and then a solution of 2-dichloroacetoxyimino-2-(3-hydroxyphenyl)acetic acid (syn isomer) (4.03 g.) in ethyl acetate (28 ml.) was added thereto with stirring at -10° C., after which the resulting mixture was stirred for 30 minutes at the same temperature to give a solution. On the other hand, a suspension of 7-amino-3-(1-carboxymethyl-1H-tetrazol-5-yl)thiomethyl-3-cephem-4-carboxylic acid (4.30 g.) in water (25 ml.) was dissolved by adding a saturated aqueous solution of sodium bicarbonate and acetone (50 ml.) was added thereto to adjust the pH of the solution at 7.5. To this solution was added dropwise with stirring at -10° C. the ethyl acetate solution obtained above, and then the resulting mixture was stirred for 1 hour at the same temperature, during which the pH of the mixture was kept at 6.5 to 7.0. The reaction mixture was adjusted to pH5.5 and washed with ethyl acetate (60 ml.). The aqueous layer was adjusted to pH2.0 and extracted with ethyl acetate (400 ml.). The extract was washed with a saturated aqueous solution of sodium chloride, dried over magnesium sulfate and concentrated. The residue was pulverized with diethyl ether and the powder was collected by filtration to give 7-[2-hydroxyimino-2-(3-hydroxyphenyl)acetamido]-3-(1-carboxymethyl-1H-tetrazol-5-yl)thiomethyl-3-cephem-4-carboxylic acid (syn isomer) (4.11 g.).
