The Vilsmeier reagent was prepared from N,N-dimethylformamide (0.4 g.) and phosphoryl chloride (0.86 g.) in a usual manner. After the reagent was suspended in ethyl acetate (10 ml.), 2-(2-formamidothiazol-4-yl)-2-n-propoxyiminoacetic acid (syn isomer, 1.3 g.) was added to the stirred suspension under ice cooling and stirred at the same temperature for 30 minute. The solution was added to a solution of 4-nitrobenzyl 7-amino-3-chloro-3-cephem-4-carboxylate hydrochloride (2.0 g.), trimethylsilylacetamide (5.2 g.) and ethyl acetate (40 ml.) at -25° C., and stirred at -20° to -10° C. for 1.5 hours. After adding water into the resultant solution, the solution was extracted with ethyl acetate (60 ml.). The aqueous layer was extracted with ethyl acetate (50 ml.). The extracts were combined together, washed with a saturated aqueous solution of sodium bicarbonate and a saturated aqueous solution of sodium chloride in turn, and dried over magnesium sulfate. The solution was concentrated in vacuo, and the residue was triturated with diethyl ether. The precipitates were collected by filtration, washed and dried to give 4-nitrobenzyl 7-[2-(2-formamidothiazol-4-yl)-2-n-propoxyiminoacetamido]-3-chloro-3-cephem-4-carboxylate (syn isomer, 2.55 g.).
