To 6 ml of methylene chloride was added 335 mg of (N,N-dimethylformamide-dimethyl sulfate) complex compound, and 65 mg of sodium methoxide was added thereto at 0° C., after which the resulting mixture was subjected to reaction at 0° to 10° C. for 1 hour. Subsequently, 300 mg of ethyl 2-[2-(2,4-difluorophenylamino)-5-fluoro-6-(2,4,6-trimethylbenzenesulfonyloxy)nicotinoyl]acetate and 115 mg of acetic anhydride were added thereto. The resulting mixture was subjected to reaction at room temperature for 2 hours, and 0.31 ml of 2N hydrochloric acid and 3 ml of ethanol were added to the reaction mixture, after which the resulting mixture was subjected to reaction at room temperature for 1.5 hours. The reaction mixture was added to a mixture of 6 ml of methylene chloride and 6 ml of water, and the organic layer was separated, washed with 6 ml of saturated aqueous sodium chloride solution, and then dried over anhydrous magnesium sulfate. The solvent was removed by distillation under reduced pressure, and to the crystalline material thus obtained was added 2 ml of diisopropyl ether, after which crystals were collected by filtration to obtain 245 mg (yield 80.2%) of ethyl 1-(2,4-difluorophenyl)-6-fluoro-1,4-dihydro-4-oxo-7-(2,4,6-trimethylbenzenesulfonyloxy)-1,8-naphthyridine-3-carboxylate. The physical properties of this compound were identical with those of the compound obtained in Example 41.
