A mixture of dry ethyl acetate (1.82 ml) and dry DMF (0.36 ml) was cooled to --10° C. Phosphorus oxychloride (0.37 ml) was added thereto, and the mixture was stirred at said temperature for 20 minutes. A solution of the compound of Example 1 (1.35 g) in dry methylene chloride (8.0 ml) was added thereto, and the mixture was stirred at a temperature between -10° to -5° C. for 30 minutes. On the other hand, pivaloyloxymethyl 7-amino-3-cephem-4-carboxylate (0.95 g) was dissolved in dry methylene chloride (12 ml), and N-trimethylsilylacetamide (0.79 g) was added thereto, and the mixture was cooled to -15° C. Thereto was dropwise added the aforementioned solution, and the mixture was stirred at said temperature for 45 minutes. Ethyl acetate (200 ml) was added thereto, and the mixture was washed with 5% aqueous solution of sodium bicarbonate (100 ml) and saturated brine (100 ml). After drying over anhydrous sodium sulfate, the solvent was distilled off under reduced pressure. The residue was subjected to silica gel column chromatography eluted with benzene-ethyl acetate. The object fraction was concentrated to give 1.32 g of pivaloyloxymethyl 7-[2-(Boc-L-alanyl)aminothiazol-4-yl]-2-methoxyimino-3-cephem-4-carboxylate (synisomer).
