[OH-].[K+].[O:3]1[C:7]2=[N:8][C:9]([C:17]([O:19]CC)=[O:18])=[C:10]([C:12]([O:14]CC)=[O:13])[CH:11]=[C:6]2[CH:5]=[CH:4]1.CC(C)=O>O.C(O)C>[O:3]1[C:7]2=[N:8][C:9]([C:17]([OH:19])=[O:18])=[C:10]([C:12]([OH:14])=[O:13])[CH:11]=[C:6]2[CH:5]=[CH:4]1Potassium hydroxide (5.60 g, 85%, 0.087 mol) in water (5 mL) is added to a stirred suspension of diethyl furo[2,3-b]pyridine-5,6-dicarboxylate (9.3 g, 0.035 mol) in absolute ethanol (100 mL). The reaction mixture is heated at 60° C. for one hour, then cooled and anhydrous acetone added. The precipitate is filtered off, dried, suspended in dry acetone and treated with hydrogen chloride to adjust to a pH of 2. Crystallization of the isolated solids from an ethyl acetate-acetone mixture affords furo[2,3-b]pyridine-5,6-dicarboxylic acid mp 189°-192° C.
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