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Into concentrated sulfuric acid (150 mL) was added 4-chloro-1H-pyrrolo[2,3-b]pyridine (50.0 g) in portions while maintaining the internal temperature of the mixture below 10° C. (Note: exothermic upon addition). A mixture of concentrated nitric acid (30.0 mL) and concentrated sulfuric acid (60.0 mL) was added slowly while maintaining the internal temperature of the mixture below 10° C. (Note: exothermic upon addition). The reaction mixture was stirred at 0° C. for 30 min. Cold water (750 mL) was added slowly while maintaining the internal temperature below 20° C. (Note: exothermic upon addition). The resulting solid was collected by filtration and washed with a pH 7 buffer solution, water, and then acetonitrile. The product was collected by filtration and dried in a vacuum oven at 50° C. overnight to give the 52 g (92% yield) of 4-chloro-3-nitro-1H-pyrrolo[2,3-b]pyridine as an light yellow solid. 1H NMR (300 MHz, DMSO-d6) δ 13.5 (s, 1H), 8.91 (s, 1H), 8.36 (d, J=5.2 Hz, 1H), 7.50 (d, J=5.2 Hz, 1H). 13C NMR (75 MHz, DMSO-d6) δ 148.0, 145.7, 134.7, 132.5, 127.2, 120.9, 110.0. HRMS calcd. For C7H3ClN3O2 [M-H]−: 199.9919, found 199.9921.

1. temperaturewhile maintaining
2. additionthe internal temperature of the mixture below 10° C. (Note: exothermic upon addition)
3. additionwas added slowly
4. temperaturewhile maintaining
5. additionthe internal temperature of the mixture below 10° C. (Note: exothermic upon addition)
6. temperaturewhile maintaining
7. additionthe internal temperature below 20° C. (Note: exothermic upon addition)
8. filtrationThe resulting solid was collected by filtration
9. washwashed with a pH 7 buffer solution, water
10. filtrationThe product was collected by filtration
11. customdried in a vacuum oven at 50° C. overnight