[CH2:1]1[O:19][C:4]2=[CH:5][C:6]3[O:11][C:10](=[O:12])[C:9]([C:13]([O:15]CC)=[O:14])=[CH:8][C:7]=3[CH:18]=[C:3]2[O:2]1.O.Cl>CO.[OH-].[K+]>[CH2:1]1[O:19][C:4]2=[CH:5][C:6]3[O:11][C:10](=[O:12])[C:9]([C:13]([OH:15])=[O:14])=[CH:8][C:7]=3[CH:18]=[C:3]2[O:2]1To a suspension of ethyl 6,7-methylenedioxy-2-oxo-1-benzopyran-3-carboxylate (0.26 g) in methanol (20 ml), 2N KOH (1.5 ml) was added, followed by stirring at room temperature for 2 hours. The reaction mixture was poured into water and acidified with hydrochloric acid; the resulting crystal was collected by filtration to yield 6,7-methylenedioxy-2-oxo-1-benzopyran-3-carboxylic acid (0.1 g, 44%), which was then recrystallized from N,N-dimethylformamide-water to yield yellow prisms having a melting point of 279°-280° C.
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