[CH3:1][O:2][C:3]1[C:15]([O:16][CH3:17])=[CH:14][C:6]2[S:7][C:8]([C:10]([O:12]C)=[O:11])=[CH:9][C:5]=2[CH:4]=1.[OH-].[Na+].Cl>CO.C1COCC1.O>[CH3:1][O:2][C:3]1[C:15]([O:16][CH3:17])=[CH:14][C:6]2[S:7][C:8]([C:10]([OH:12])=[O:11])=[CH:9][C:5]=2[CH:4]=1To a stirred solution of methyl 5,6-dimethoxybenzo[b]thiophene-2-carboxylate (1.0 g, 4 mmol) in MeOH (20 mL) and THF (10 mL) was added 2N NaOH (10 mL) dropwise at room temperature. The resulting mixture was stirred at room temperature for 2 hours before adjusting the pH value to 3 with 1 N HCl and diluting with water. The resulting suspension was filtered and the solid was dried under vacuum to afford 5,6-dimethoxybenzo[b]thiophene-2-carboxylic acid as a white solid (890 mg, 94%). LC-MS (ES-API); rt 8.17 min; m/z calculated for C11H10O4S [M+Na]+ 261.0, found 260.9.
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