Vilsmeier reagent was prepared from phosphorus oxychloride (2.0 g) and dimethylformamide (0.95 g) in ethyl acetate (2.8 ml) in a usual manner. 2-Methylthiomethoxyimino-2-(2-formamidothiazol-4-yl) acetic acid (syn isomer) (3.0 g) was added to the stirred suspension of Vilsmeier reagent in tetrahydrofuran (30 ml) under ice cooling, and stirred for 30 minutes at the same temperature to produce an activated acid solution. On the other hand, 1-[(7-amino-4-carboxy-3-cephem-3-yl)methyl]pyridinium chloride hydrochloride dihydrate (4.0 g) was dissolved to the solution of sodium bicarbonate (2.5 g) in water (20 ml) and acetone (20 ml). To the solution was added the above activated acid solution at -3° to 3° C. and the solution was stirred for 30 minutes under keeping the pH 6.5 to 7.5 with 20% aqueous solution of sodium carbonate. Water and ethyl acetate were added to the reaction mixture and a separate aqueous layer was washed with ethyl acetate. The aqueous layer was adjusted to pH 4.5 with 10% aqueous hydrochloric acid. After removing the solvent under reduced pressuer, the aqueous solution was adjusted to pH 4.0 with 10% aqueous hydrochloric acid under ice cooling. The solution was subjected to column chromatography on macroporous non-ionic adsorption resin "Diaion HP-20" (Trademark: prepared by Mitsubishi Chemical Industries) and eluted with 10% aqueous solution of iso-propyl alcohol. The fractions containing the object compound were concentrated and lyophilized to give 7-[2-methylthiomethoxyimino-2-(2-formamidothiazol-4-yl)acetamido]-3-(1-pyridiniomethyl)-3-cephem-4-carboxylate (syn isomer)(1.3 g).
