[CH2:1]([C:3]1[CH:4]=[C:5]2[C:9](=[CH:10][CH:11]=1)[NH:8][CH2:7][CH2:6]2)[CH3:2].[N+:12]([O-])([O-:14])=[O:13].[K+].[OH-].[Na+]>OS(O)(=O)=O>[CH2:1]([C:3]1[CH:4]=[C:5]2[C:9](=[CH:10][C:11]=1[N+:12]([O-:14])=[O:13])[NH:8][CH2:7][CH2:6]2)[CH3:2]To a solution of 5-ethylindoline (2.5 g, 17 mmol) in H2SO4 (98%, 20 mL) was slowly added KNO3 (1.7 g, 17 mmol) at 0° C. The mixture was stirred at 0-10° C. for 10 minutes. The mixture was then carefully poured into ice, basified with NaOH solution to pH 9, and extracted with ethyl acetate. The combined extracts were washed with brine, dried over Na2SO4 and concentrated to dryness. The residue was purified by silica column to give 5-ethyl-6-nitroindoline (1.9 g, 58%).
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