Vilsmeier reagent was prepared from dry N,N-dimethylformamide (0.3 g.) and phosphory chloride (0.5 g.) in dry ethyl acetate (1.2 ml.) in a usual manner. 2-(2-Formamidothiazol-4-yl)-2-(2-chloroethoxyimino)acetic acid (syn isomer, 0.9 g.) and dry ethyl acetate (10 ml.) were added to the mixture under ice-cooling and then stirred at the same temperature for 30 minutes. [Solution A]Trimethylsilylacetamide (2.9 g.) was added to a solution of 7-amino-3-(1-allyl-1H-tetrazol-5-yl)thiomethyl-3-cephem-4-carboxylic acid (1.0 g.) in dry ethyl acetate (20 ml.) and stirred at 40° C. for 30 minutes. To the solution was added the solution A at -10° C. and stirred at -5°to -10° C. for an hour. Water was added to the resultant solution and the organic layer was separated. Water was added to the organic layer and adjusted to pH 7.0 with sodium carbonate. The aqueous layer was separated, washed with ethyl acetate and adjusted to pH 4.5 with 10% hydrochloric acid. The solution was extracted with ethyl acetate. The extract was washed with a sodium chloride saturated aqueous solution, dried over magnesium sulfate and concentrated in vacuo. The residue was pulverized with diisopropyl ether. The precipitates were collected by filtration, washed with diisopropyl ether and dried to give 7-[2-(2-formamidothiazol-4-yl)-2-(2-chloroethoxyimino)acetamido]-3-(1-allyl-1H-tetrazol-5-yl)thiomethyl-3-cephem-4-carboxylic acid (syn isomer, 0.83 g.).
