[Br:1][C:2]1[CH:3]=[C:4]2[C:8](=[CH:9][CH:10]=1)[N:7]([C:11]([C:13]([O:15][CH2:16][CH3:17])=[O:14])=[O:12])[CH2:6][CH2:5]2.[N+:18]([O-])([O:20]C(C)C)=[O:19].O>S(=O)(=O)(O)O>[Br:1][C:2]1[CH:3]=[C:4]2[C:8](=[C:9]([N+:18]([O-:20])=[O:19])[CH:10]=1)[N:7]([C:11]([C:13]([O:15][CH2:16][CH3:17])=[O:14])=[O:12])[CH2:6][CH2:5]2To a solution of 5-bromo-N-ethoxalylindoline (5.23 g, 17.5 mmol) in concentrated sulfuric acid was added slowly isopropyl nitrate (1.87 mL, 18.5 mmol) at 0° C. The mixture was stirred for 1 h at 0° C. and poured into a mixture of water and crashed ice. The mixture was extracted with ethyl acetate and the organic layer was washed with brine, dried over magnesium sulfate, and concentrated. The crude residue was purified by silica gel column chromatography to give 5.02 g of 5-bromo-7-nitro-N-ethoxalylindoline (83%): 1H NMR (270 MHz, CDCl3) δ7.91 (d, 1H, J=1 Hz), 7.62 (d, 1H, J=1 Hz), 4.39 (t, 2H, J=8 Hz), 4.38 (q, 2H, J=7 Hz), 3.22 (t, 2H, J=8 Hz), 1.39 (t, 3H, J=7 Hz).
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