[CH3:1][O:2][C:3]1[N:8]=[CH:7][C:6]([N:9]2[C:18]3[C:13](=[CH:14][CH:15]=[CH:16][N:17]=3)[CH:12]=[C:11]([C:19]([O:21]CC)=[O:20])[C:10]2=[O:24])=[CH:5][CH:4]=1.C(=O)([O-])[O-].[K+].[K+].O>O1CCOCC1>[CH3:1][O:2][C:3]1[N:8]=[CH:7][C:6]([N:9]2[C:18]3[C:13](=[CH:14][CH:15]=[CH:16][N:17]=3)[CH:12]=[C:11]([C:19]([OH:21])=[O:20])[C:10]2=[O:24])=[CH:5][CH:4]=14.1 g (13 mmol) of ethyl (6-methoxy-3-pyridinyl)-2-oxo-1,2-dihydro-1,8-naphthyridine-3-carboxylate was dissolved in 1,4-dioxane (60 mL), and 2.6 g (19 mmol) of potassium carbonate and 100 mL of water were added thereto at room temperature. The resulting mixture was stirred for 3 hours at 60° C. The solvent was distilled off under reduced pressure from the reaction mixture, and the residue was dissolved in water and washed with chloroform. Concentrated hydrochloric acid was added to the aqueous phase to make the aqueous phase acidic, and the mixture was extracted with chloroform. The organic phase was dried and concentrated. Thus, 1.45 g of the title compound was obtained as colorless crystals (yield: 39%).
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