Water → 1-(1,3-dioxolan-2-ylmethyl)-7-hydroxy-1,5-naphthyridin-2(1H)-one

To a suspension of 0.50 g of 1-(1,3-dioxolan-2-ylmethyl)-7-fluoro-1,5-naphthyridin-2(1H)-one in 3 mL of tetrahydrofuran, 3 mL of a 20% aqueous sodium hydroxide solution was added, and the mixture was heated under reflux while stirring for 1 hour. Thereto was added 3 mL of water, and the mixture was heated under reflux while stirring for 3 hours 30 minutes. Thereto was added 3 mL of a 20% aqueous sodium hydroxide solution, and the mixture was heated under reflux while stirring for 4 hours. Tetrahydrofuran was distilled off under reduced pressure, and the mixture was then heated under reflux while stirring for 1 hour 30 minutes. After cooling to room temperature, the reaction mixture was added with 2 mol/L hydrochloric acid and adjusted to pH 5. Thereto was added ethyl acetate, the organic layer was separated, and the aqueous layer was extracted twice with ethyl acetate. The organic layer and the extract were combined, the resultant solution was washed with a saturated aqueous sodium chloride solution and dried over anhydrous magnesium sulfate, and the solvent was distilled off under reduced pressure. Diethyl ether was added to the resultant residue, and the solid was filtered off to obtain 0.32 g of 1-(1,3-dioxolan-2-ylmethyl)-7-hydroxy-1,5-naphthyridin-2(1H)-one as a yellow solid.