[NH2:1][C:2]1[C:7](=[O:8])[C:6]2[CH:9]=[C:10]([O:18][C:19]3[CH:24]=[CH:23][CH:22]=[CH:21][CH:20]=3)[C:11]([NH:13][S:14]([CH3:17])(=[O:16])=[O:15])=[CH:12][C:5]=2[O:4][CH:3]=1.[O-:25][C:26]#[N:27].[Na+]>C(O)(=O)C.O>[CH3:17][S:14]([NH:13][C:11]1[C:10]([O:18][C:19]2[CH:20]=[CH:21][CH:22]=[CH:23][CH:24]=2)=[CH:9][C:6]2[C:7](=[O:8])[C:2]([NH:1][C:26]([NH2:27])=[O:25])=[CH:3][O:4][C:5]=2[CH:12]=1)(=[O:15])=[O:16]500 mg of 3-amino-7-methylsulfonylamino-6-phenoxy-4H-1-benzopyran-4-one was dissolved in 20 ml of acetic acid and 10 ml of water. The solution was heated to 35° C. Thereto was dropwise added a solution of 190 mg of sodium cyanate dissolved in 5 ml of water, in 5 minutes. The mixture was stirred for 30 minutes at the same temperature. 20 ml of water was added thereto. The resulting crystal was collected by filtration and recrystallized from acetic acid to obtain 350 mg (yield: 62.3%) of 7-methylsulfonylamino-6-phenoxy-3-ureido-4H-1-benzopyran-4-one having a melting point of >250° C.
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