In accordance with a process described in JP-61-246183A, 4-hydroxy-1-phenyl-1,8-naphthyridin-2(1H)-one was synthesized. To a suspension of the synthesized compound (953 mg, 4.0 mol) in DMF (32 mL) was added sodium hydride (purity of about 60%, 192 mg, 4.8 mmol, 1.2 eq.), and the mixture was stirred until no more hydrogen was generated, to obtain a solution. Then, 2-phenylpropionyl chloride (1.2 eq.) was added thereto, and the mixture was stirred at a room temperature for 1 hour. To the mixture was added a saturated sodium hydrogencarbonate aqueous solution, and the resulting precipitate was separated by filtration, washed with water, and dried to give 1-phenyl-4-(2-phenylpropionyloxy)-1,8-naphthyridin-2(1H)-one as a form of crystal (1.45 g, yield 98%).
