375 mg of 3-chloroperbenzoic acid (purity of 65% or more) was added to a mixture of 306 mg of 2-(4-chloro-2-ethylsulfanylphenyl)-4-ethylamino-7-trifluoromethylquinazoline and 5 ml of chloroform, under ice cooling, and the mixture was stirred at room temperature for 4 hours. A 10% aqueous sodium thiosulfate solution was added to the reaction mixture, and the mixture was extracted with chloroform. The organic layer was washed with a saturated aqueous sodium bicarbonate solution and dried over anhydrous magnesium sulfate, and then concentrated under reduced pressure. The resulting residue was applied to a silica gel column chromatography to obtain 270 mg of 2-(4-chloro-2-ethylsulfonylphenyl)-4-ethylamino-7-trifluoromethylquinazoline (Compound of Present Invention 16).
