To a suspension of 7-aminocephalosporanic acid (0.99 g) and 1-(2-carboxy-2-methoxyiminoethyl)-1H-tetrazole-5-thiol (0.95 g) in dry acetonitrile (10 ml) was added boron trifluoride etherate (1.55 g) and the resulting solution was stirred at 50° C. for 2 hours. Water (10 ml) was added to the reaction mixture and the solution was adjusted to pH 3.5 with conc. aqueous ammonia under ice-cooling to give precipitates which were filtered, washed with water and dried to give 7-amino-3-[1-(2-carboxy-2-methoxyiminoethyl)-1H-tetrazol-5-yl]thiomethyl-3-cephem-4-carboxylic acid (1.0 g), mp 170° to 180° C. (dec.).
