To a solution of p-nitrobenzyl 7-{2-(2-formamido-4-thiazolyl)-2-methoxyiminoacetamido}-3-cephem-4-carboxylate (syn isomer, 1.25 g.) in methanol (40 ml.) and tetrahydrofuran (50 ml.) was added 10% palladium carbon (0.65 g.), and the mixture was subjected to catalytic reduction at room temperature under atmospheric pressure for 3.5 hours. After removing the catalyst from the reaction mixture, the filtrate was concentrated under reduced pressure. Water (80 ml.) was added to the residue, and the mixture was adjusted to pH 7.5 with an aqueous solution of sodium bicarbonate, and then the insoluble substance was filtered off. The filtrate was washed with ethyl acetate (50 ml.), and then ethyl acetate (100 ml.) was added to the solution. After adjusting to pH 1.5 with 10% hydrochloric acid, the ethyl acetate layer was separated. The remaining aqueous layer was extracted with ethyl acetate (80 ml.) twice, and the extracts were combined with the ethyl acetate layer obtained above, washed with an aqueous solution of sodium chloride, dried over magnesium sulfate, and then concentrated under reduced pressure to give 7-{2-(2-formamido-4-thiazolyl)-2-methoxyiminoacetamido}-3-cephem-4-carboxylic acid (syn isomer, 0.60 g.), m.p. 176° to 183° C. (dec.).
