[Cl:1][C:2]1[CH:7]=[CH:6][N:5]=[C:4]2[NH:8][CH:9]=[C:10]([N+:11]([O-:13])=[O:12])[C:3]=12.[Br:14]N1C(=O)CCC1=O.O>C(O)(=O)C>[Br:14][C:7]1[C:2]([Cl:1])=[C:3]2[C:10]([N+:11]([O-:13])=[O:12])=[CH:9][NH:8][C:4]2=[N:5][CH:6]=1Into a suspension of 4-chloro-3-nitro-1H-pyrrolo[2,3-b]pyridine (50.0 g) in acetic acid (300 mL) were added N-bromosuccinimide (NBS, 70.9 g, 1.60 equiv) in small portions at 25° C. The reaction mixture was stirred at 25° C. for 18 h. Water (3.0 mL) was added slowly over 30 min to get a light yellow slurry. The resulting solid was collected by filtration and washed with a 2:2:1 (v/v/v) mixture of saturated sodium sulfite solution, water, and methanol, followed by water and a 1:1 (v/v) mixture of methanol and water. The cake was dried in a vacuum at 50° C. oven overnight to give the 44.9 g (65% yield) of 5-bromo-4-chloro-3-nitro-1H-pyrrolo[2,3-b]pyridine as an light yellow solid. 1H NMR (300 MHz, DMSO-d6) δ 13.7 (s, 1H), 8.93 (s, 1H), 8.66 (s, 1H). 13C NMR (75 MHz, DMSO-d6) δ 146.9, 146.4, 133.9, 133.2, 127.0, 116.7, 111.3. HRMS calcd. For C7H2BrC1N3O2 [M-H]−: 273.9024, found 273.9031.
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