A mixture of butyl(2E)-3-{3-[7-chloro-3-(2-{[4-fluoro-2-(trifluoromethyl)phenyl]amino}-2-oxoethyl)-6-methyl-2-oxo-2H-chromen-4-yl]phenyl}acrylate (3.0 g), ethanol (9 ml) and 2N NaOH (9 ml) was stirred at 50° C. for 1.5 hours. After completion of the reaction, acetone (27 ml) was added to the reaction mixture at 50° C., and 6N HCl (3.5 ml) was added dropwise thereto to adjust pH to 0.6. The mixture was stirred at 50° C. for 30 minutes and then at 25° C. for 1 hour, and crystals were collected by filtration, and washed with acetone/water (2/1, 6 ml). To a mixture of the resulting crystals (2.79 g), acetone (2 ml) and water (6 ml) was added dropwise 5% aqueous ammonia (2.1 ml) with stirring at room temperature to adjust pH to 9.2 and dissolve the mixture. Toluene (4.5 ml) was added to the resulting solution and the mixture was stirred for 5 minutes and allowed to stand for 5 minutes to separate an aqueous layer. Acetone (12 ml) and activated charcoal (0.15 g) were added to the obtained aqueous layer and the mixture was stirred at room temperature for 15 minutes. The activated charcoal was removed by filtration, and washed with acetone (6 ml). The filtrate and washings were combined and warmed to 50° C., and 6N HCl (1.25 ml) was added dropwise thereto with keeping the temperature almost the same and stirring to adjust pH to 0.5 and precipitate crystals. The mixture was stirred at 50° C. for 30 minutes and then at about 25° C. for 1 hour, and the crystals were collected by filtration and washed with acetone/water (2/1, 6 ml) to obtain the title compound (2.54 g, yield 84.8%). This product contained 0.9 mol of acetone.
