Vilsmeier reagent was prepared from phosphorous oxychloride (2.5 g) and N,N-dimethylformamide (1.2 g) in ethyl acetate (4.8 ml) in a usual manner. 2-(t-Butoxycarbonylmethoxyimino)-2-(2-formamido-5-chlorothiazol-4-yl)acetic acid (syn isomer) (5.0 g) was added to the stirred suspension of Vilsmeier reagent in tetrahydrofuran (70 ml) under ice-cooling, and stirred for 30 minutes at the same temperature to produce an activated acid solution. Trimethylsilylacetamide (16.4 g) was added to the stirred suspension of 1-[(7-amino-4-carboxy-3-cephem-3-yl)methyl]pyridinium chloride hydrochloride dihydrate (5.0 g) in tetrahydrofuran (100 ml). To the solution was added the above activated acid solution at -10° C. and stirred at -10° C.-5° C. for 40 minutes. A saturated aqueous sodium chloride was added to the reaction mixture and adjusted to pH 3.0 with 10% aqueous sodium hydroxide. The separated aqueous layer was extracted three times with tetrahydrofuran. The organic layer was washed with a little saturated aqueous sodium chloride, dried over magnesium sulfate and evaporated to give 7-[2-t-butoxycarbonylmethoxyimino-2 -(2-formamido-5-chlorothiazol-4-yl)acetamido]-3-(1-pyridiniomethyl)-3-cephem-4-carboxylate (syn isomer) (2.13 g).
