A solution of Vilsmeier reagent was prepared from dry dimethylformamide (0.39 g.), dry ethyl acetate (1.2 ml.) and phosphorus oxychloride (0.84 g.) in a usual manner. To the solution was added a solution of 2-(1,2,3-thiadiazol-4-yl)-2-methoxyiminoacetic acid (syn isomer, 0.93 g.) in ethyl acetate (10 ml.) at -15° C. to prepare the activated acid solution. On the other hand, a p-nitrobenzyl 7-amino-3-cephem-4-carboxylate (1.5 g.), trimethylsilylacetamide (4.6 g.) and bis(trimethylsilyl)acetamide (1 ml.) in dry ethyl acetate (50 ml.) was stirred at 45° C. for 5 hours to give a solution. To the solution was added all at once the activated acid solution obtained above at -10° C. with stirring, and the mixed solution was stirred at -5° C. for 1.5 hours. Water was added to the reaction mixture, and the insoluble product was separated by filtration, washed with ethyl acetate and water in turn, and then dried to give pale yellow powder of p-nitrobenzyl 7-{2-(1,2,3-thiadiazol-4-yl)-2-methoxyiminoacetamido}-3-cephem-4-carboxylate (syn isomer, 1.9 g.), m.p. 243° to 245° C. (dec.).
