[OH:1][C:2]1[CH:13]=[CH:12][C:5]2[S:6][CH:7]=[C:8]([C:9]([OH:11])=[O:10])[C:4]=2[CH:3]=1.[C:14]1([S:20](Cl)(=[O:22])=[O:21])[CH:19]=[CH:18][CH:17]=[CH:16][CH:15]=1>O1CCCC1.[OH-].[Na+].O>[C:14]1([S:20]([O:1][C:2]2[CH:13]=[CH:12][C:5]3[S:6][CH:7]=[C:8]([C:9]([OH:11])=[O:10])[C:4]=3[CH:3]=2)(=[O:22])=[O:21])[CH:19]=[CH:18][CH:17]=[CH:16][CH:15]=15-Hydroxybenzo[b]thiophene-3-carboxylic acid (11) (M.Martin-Smith et al. J. Chem. Soc (C), 1899-1905 (1967) 8.63 g (44.4 mmol)) was dissolved in aqueous tetrahydrofuran (water content, 20%; 160 ml) and 1 N sodium hydroxide (44 ml). To the solution were added dropwise 0.56 N sodium hydroxide (87 ml) and benzenesulfonyl chloride (6.2 ml, 48.4 mmol) simultaneously with stirring under ice-cooling while maintaining the pH at 11-12. After the completion of the reaction, the mixture was diluted with water, basified, and washed with toluene. The aqueous layer was made slightly acidic by adding concentrated hydrochloric acid with stirring, and the deposited crystals were filtered, washed with water and dried to provide 14.33 g of 5-benzenesulfonyloxybenzo[b]thiophene-3-carboxylic acid (12). M.p. 202-203° C.
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