[F:1][C:2]1[CH:3]=[C:4]([S:9]([NH2:12])(=[O:11])=[O:10])[CH:5]=[CH:6][C:7]=1C.[C:13](=[O:16])([O-])[O-:14].[Na+].[Na+].[Mn]([O-])(=O)(=O)=O.[K+].C(O)=O>O>[NH2:12][S:9]([C:4]1[CH:5]=[CH:6][C:7]([C:13]([OH:14])=[O:16])=[C:2]([F:1])[CH:3]=1)(=[O:11])=[O:10]To a stirred solution of 3-fluoro-4-methyl-benzenesulfonamide (prepared above) in 50 mL of water was added sodium carbonate (0.53 g, 5 mmol) and potassium permanganate (3.16 g, 20 mmol) portionwise over three hours at 50˜60° C. The resulting mixture was stirred for further 8 hours at this temperature before 0.2 mL of formic acid was added to quench the excess of potassium permanganate. The mixture was then filtered through celite while it was still hot and further washed with the hot water. The filtrate was concentrated to ˜30 mL and adjusted to pH 9˜10. The filtration was applied again to remove non-oxidized starting material. The final filtrate was acidified with HCl (conc.) to ˜pH 1 and 4-(aminosulfonyl)-2-fluorobenzoic acid was precipitated and collected by filtration as white solid (1.10 g, 50%).
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