[Cl:1][C:2]1[C:10]2[CH:9]=[C:8]([C:11]([O:13]C)=[O:12])[S:7][C:6]=2[CH:5]=[CH:4][CH:3]=1.O[Li].O>CO.O>[Cl:1][C:2]1[C:10]2[CH:9]=[C:8]([C:11]([OH:13])=[O:12])[S:7][C:6]=2[CH:5]=[CH:4][CH:3]=1Stir a mixture of methyl 4-chlorobenzo[b]thiophene-2-carboxylate (1.0 g, 4.44 mmol) and LiOH—H2O (0.56 g, 13.3 mmol) in MeOH (30 mL) and water (10 mL) at room temperature overnight. Concentrate the reaction mixture under vacuum and then dilute the residue with ice water (20 mL). Acidify the aqueous mixture with conc. HCl solution to pH=1. Extract the resultant mixture with EtOAc (15 mL×3). The combined organic layers are washed with brine, dried over anhydrous Na2SO4 and evaporate under vacuum to afford 4-chlorobenzo[b]thiophene-2-carboxylic acid as a white solid (0.94 g, 100%). 1H NMR (400 MHz, CDCl3) δ 8.10-8.02 (m, 2H), 7.61-7.51 (m, 2H)
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