A solution of diazomethane in diethyl ether was added little by little to a solution of 4-nitrobenzyl 7-[2-(2-amino-4-thiazolyl)-2-hydroxyiminoacetamido]-3-cephem-4-carboxylate (syn-isomer: 0.3 g.) in methanol (30 ml.) until the reaction terminated. The resultant solution was concentrated under reduced pressure, and the residue was pulverized with diethyl ether, collected by filtration and dried to give 4-nitrobenzyl 7-[2-(2-amino-4-thiazolyl)-2-methoxyiminoacetamido]-3-cephem-4-carboxylate (syn-isomer: 0.26 g.) This product was identified with the authentic sample.
