General procedure for the synthesis of 5,6-dibromoindoline-2,3-dione from 6-bromodihydroindole-2,3-dione: Bromine (0.88 mmol) was slowly added to a solution of acetic acid (9 mL) containing 6-bromoindigo red (0.88 mmol). The reaction mixture was stirred under reflux conditions for 48 hours. Upon completion of the reaction, the mixture was cooled to 0 °C, the solid product was collected by filtration and washed with acetic acid. Subsequently, the crude product was dried in an oven. Purification of the crude product by recrystallization from acetic acid afforded an orange-colored pure product in 57% yield; melting point >270 °C (decomposition). The structure of the product was confirmed by nuclear magnetic resonance hydrogen (1H NMR, 300 MHz, DMSO-d6) and carbon (13C NMR, 75 MHz, DMSO-d6) spectra: 1H NMR (DMSO-d6) δ 7.23 (1H, s, H-7), 7.82 (1H, s, H-4), 11.22 (1H, brs, NH); 13C NMR (DMSO-d6) δ 117.40, 117.64, 119.61, 129.33, 133.81, 150.67, 159.83, 183.12.
