To a suspension of N-{1-butyl-4-(3-hydroxyphenyl)-1,2-dihydro-2-oxo-1,8-naphthyridin-3-yl }-N'-(2,6-diisopropylphenyl)urea (300 mg, 0.58 mmol), potassium carbonate (96 mg, 0.7 mmol) and potassium iodide (19 mg) in DMF (5 ml) was added 1-iodopropane (99 mg, 0.58 mmol) at room temperature, and the mixture was stirred at 40°-50° C. for five hours. After allowed to stand for cooling, the mixture was poured into water, and the mixture was extracted with ethyl acetate. The extracted was washed with water, washed with a saturated aqueous sodium chloride solution, and dried over anhydrous magnesium sulfate. The resultant was concentrated under reduced pressure, and the residue was purified by silica gel column chromatography (20% ethyl acetate in hexane), and crystallized from ether/hexane to give the title compound (253 mg, 0.48 mmol).
