Vilsmeier reagent, which was prepared from N,N-dimethylformamide (0.37 ml) and phosphorus oxychloride (0.44 ml) in a conventional manner, was suspended in dried tetrahydrofuran (20 ml). 2-(tert-Butoxycarbonylmethoxyimino)-2-(6-formamidopyridin-2-yl)acetic acid (syn isomer) (3.0 g) was added thereto under ice-cooling with stirring, and the stirring was continued at the same temperature for an hour to prepare the activated acid solution. This solution was added at a time to a solution of benzhydryl 7-amino-3-vinyl-3-cephem-4-carboxylate (2.72 g) and trimethylsilylacetamide (5.5 g) in methylene chloride at -20° C. with stirring, and the stirring was continued at -20° to -10° C. for an hour. To the reaction mixture were added water (50 l ml) and ethyl acetate (200 ml), and the separated organic layer was washed with 5% aqueous sodium bicarbonate and then a saturated aqueous sodium chloride, followed by drying over magnesium sulfate. Removal of the solvent gave benzhydryl 7-[2-(tert-butoxycarbonylmethoxyimino)-2-(6-formamidopyridin-2-yl)acetamido] -3-vinyl-3-cephem-4-carboxylate (syn isomer) (4.8 g), mp 154°-157° C.
