[N+:1]([C:4]1[CH:5]=[CH:6][CH:7]=[C:8]2[C:12]=1[NH:11]C(=O)[C:9]2=[O:14])([O-:3])=[O:2].C(O)(=O)CC(CC(O)=O)(C(O)=O)[OH:18]>[OH-].[Na+].OO>[NH2:11][C:12]1[C:4]([N+:1]([O-:3])=[O:2])=[CH:5][CH:6]=[CH:7][C:8]=1[C:9]([OH:14])=[O:18]To a ice cold solution of 7-nitro-1H-indole-2,3-dione (9 g, 47 mmol) in 2 M aqueous sodium hydroxide (50 mL), 30% hydrogen peroxide (9 mL) was added drop wise. The mixture was warmed to room temperature and stirred overnight. The mixture was carefully acidified by addition of a saturated citric acid solution. The solid precipitate was collected by filtration, washed with water and dried in a vacuum oven to get 6 g of the required product as a yellow coloured solid (70%).
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