[N+:1]([O-:4])([O-])=[O:2].[Na+].[Cl:6][C:7]1[CH:8]=[C:9]2[C:17](=[CH:18][CH:19]=1)[NH:16][C:15]1[CH:14]=[N:13][CH:12]=[C:11]([F:20])[C:10]2=1.CO.C(=O)(O)[O-].[Na+]>FC(F)(F)C(O)=O>[Cl:6][C:7]1[CH:8]=[C:9]2[C:17](=[C:18]([N+:1]([O-:4])=[O:2])[CH:19]=1)[NH:16][C:15]1[CH:14]=[N:13][CH:12]=[C:11]([F:20])[C:10]2=1Sodium nitrate (1.5 eq, 53 mmol, 4.5 g) was added portionwise to a solution of Intermediate 65 (1.0 eq, 35 mmol, 7.8 g) in trifluoroacetic acid (200 mL) and the resulting mixture heated at 70° C. for 3 h. After cooling to RT the trifluoroacetic acid was removed on a rotary evaporator to afford a crude solid which was suspended in a small volume of methanol and added dropwise to a vigorously stirred mixture of saturated sodium bicarbonate solution (500 mL). The resulting slurry was stirred for 15 min then the precipitated solids were collected by suction filtration, washed with water (300 mL) and then dried in vacuo to afford 6-chloro-4-fluoro-8-nitro-9H-β-carboline (about 9.5 g) which was used in the subsequent step without further purification.
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