3-[5-Fluoro-3-(4-methoxyphenyl)-4-oxo-8-propoxy-4H-quinolin-1-yl]propionaldehyde (1.0 g, 2.61 mmol) was dissolved in water (10 ml), tert-butyl alcohol (20 ml) and dichloromethane (20 ml). Sodium chlorite (3.2 g, 35.4 mmol), 2-methyl-2-butene (19.86 gm, 283 mmol) and sodium-dihydrogenphosphate dihydrate (2 g, 2.61 mmol) were added to the resulting solution, and the solution was stirred at room temperature for 1 hour. Water was added to the reaction mixture, the mixture was extracted with dichloromethane, and then washed with water and dried over anhydrous sodium sulfate. The dried product was concentrated under reduced pressure. The residue was purified using silica gel column chromatography (dichloromethane:ethyl acetate=50:50→0:100). The purified product was concentrated to dryness under reduced pressure, giving a pale yellow powder of 3-[5-fluoro-3-(4-methoxyphenyl)-4-oxo-8-propoxy-4H-quinolin-1-yl]propionic acid (710 mg, yield: 68%).
