2.55 ml of acetic anhydride was added to 850 mg of ethyl 2-(2-hydroxy-4-methylsulfonylamino-5-phenoxybenzoyl)acetate and 540 mg of sodium acetate. The mixture was stirred for 10 minutes at 110°-120° C. The reaction mixture was introduced into a mixture of 20 ml of ethyl acetate and 20 ml of water. The resulting mixture was adjusted to pH 2 with 2N hydrochloric acid. The organic layer was separated, washed with water and a saturated aqueous sodium chloride solution in this order, and dried with anhydrous magnesium sulfate. The solvent was removed by distillation under reduced pressure. The residue was purified by a column chromatography (eluant: a 5 : 1 mixture of toluene and ethyl acetate) to obtain 550 mg (yield: 55.6%) of 7-(N-acetyl-N-methylsulfonylamino)-3-ethoxycarbonyl-2-methyl-6-phenoxy-4H-1-benzopyran-4-one.
