[F:1][C:2]([F:17])([F:16])[C:3]1[C:8]2[S:9][C:10]([C:12]([O:14]C)=[O:13])=[CH:11][C:7]=2[CH:6]=[CH:5][CH:4]=1.O.[OH-].[Li+].O>CO>[F:16][C:2]([F:1])([F:17])[C:3]1[C:8]2[S:9][C:10]([C:12]([OH:14])=[O:13])=[CH:11][C:7]=2[CH:6]=[CH:5][CH:4]=1A mixture of 800 mg of methyl 7-(trifluoromethyl)benzo[b]thiophene-2-carboxylate, 154 mg of lithium hydroxide monohydrate, 4 ml of water and 12 ml of methanol was stirred at 75° C. for 2 hours. The reaction mixture was cooled to room temperature, and then concentrated under reduced pressure. To the residue was added water, and then washed with tert-butyl methyl ether 3 times. To the aqueous layer was added concentrated hydrochloric acid, and then extracted with tert-butyl methyl ether 3 times. The combined organic layer was washed with saturated aqueous sodium chloride solution, dried over magnesium sulfate, and then concentrated under reduced pressure to obtain 632 mg of 7-(trifluoromethyl)benzo[b]thiophene-2-carboxylic acid (the present compound 4).
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