S(=O)(=O)(O)O.[N+:6]([C:9]1[CH:17]=[C:16]2[C:12]([CH:13]=[C:14]([C:18]([OH:20])=[O:19])[NH:15]2)=[CH:11][CH:10]=1)([O-:8])=[O:7].[C:21](=O)(O)[O-].[Na+]>CO>[N+:6]([C:9]1[CH:17]=[C:16]2[C:12]([CH:13]=[C:14]([C:18]([O:20][CH3:21])=[O:19])[NH:15]2)=[CH:11][CH:10]=1)([O-:8])=[O:7]Sulfuric acid (1 mL, 18.76 mmol) was added to a suspension of 6-nitro-1H-indole-2-carboxylic acid (0.5 g, 2.425 mmol) in methanol (5 mL). The mixture was heated to reflux for 2 h. The reaction mixture was cooled to RT, poured into ice/water, neutralized with sodium bicarbonate, and extracted with ethyl acetate. The organic extracts were combined, dried over sodium sulfate, and concentrated to give the title compound (0.433 g, 81%) as a yellow solid. 1H NMR (400 MHz, CDCl3): δ ppm 9.31 (s, 1H), 8.41 (s, 1H), 8.05 (dd, 1H), 7.79 (d, 1H), 7.29 (s, 1H), 3.70-4.24 (m, 3H). MS: m/z 221 (M+1).
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