Indazoles, acidity nitration

Indazoles are nitrated, mainly into the position 5 by a mixture of sulfuric and nitric acids or just by nitric acid (Scheme 2.1) with formation of 5-nitroindazole derivatives that are a part of so-called universal nucleosides [9],... 

In the section dealing with electrophilic attack at carbon some results on indazole homocyclic reactivity were presented nitration at position 5 (Section 4.04.2.1.4(ii)), sulfon-ation at position 7 (Section 4.04.2.1.4(iii)) and bromination at positions 5 and 7 (Section 4.04.2.1.4(v)). The orientation depends on the nature (cationic, neutral or anionic) of the indazole. Protonation, for instance, deactivates the heterocycle and directs the attack towards the fused benzene ring. A careful study of the nitration of indazoles at positions 2, 3, 5 or 7 has been published by Habraken (7UOC3084) who described the synthesis of several dinitroindazoles (5,7 5,6 3,5 3,6 3,4 3,7). The kinetics of the nitration of indazole to form the 5-nitro derivative have been determined (72JCS(P2)632). The rate profile at acidities below 90% sulfuric acid shows that the reaction involves the conjugate acid of indazole. 

The nitration of indazoles seems to have been little studied. The nitration of 1-phenylindazole (32) in 86% nitric acid gives 4-nitro-1-phenylindazole (33) but an uncharacterized dinitro derivative is obtained with potassium nitrate in sulfuric acid (36LA285). 

The synthesis of pazopanib (1) involves sequential animation of 2,4-dichloropyrimidine 25 with 6-amino-2,3-dimethylindazole 24 and 5-amino-2-methyl-benzenesulfonamide 28. The 6-amino-2,3-dimethylindazole 24, on the other hand, was prepared from 2-ethylphenylamine 20 via 5-nitration with fuming nitric acid and concentrated sulfuric acid, followed by treatment with isoamyl nitrite and acetic acid to produce 6-nitro-3-methylindazole 22. The 6-nitro group was reduced with stannous chloride and concentrated HC1 in glyme and subsequently methylated at the C2 position of the indazole ring with trimethyloxonium tetrafluoroborate in acetone to produce 6-amino-2,3-dimethylindazole 24. The resultant indazole 24 was condensed with 2,4-dichloropyrimidine 25 in the presence of sodium bicarbonate in ethanol/THF and subsequent iV-methylation with iodomethane and cesium carbonate to produce 27. The 2-chloro group of pyrimidine was then allowed to react with 5-amino-2-methyl-benzenesulfonamide 28 in catalytic HCl/isopropanol and heated to reflux to deliver pazopanib hydrochloride (1) in good yield. 

Amination at an azole ring nitrogen is known for N-unsubstituted azoles. Thus, 1,2,3-triazole and 4,5-diphenyl-l,2,3-triazole with hydroxylamine-O-sulfonic acid give 1-amino 145 and 2-amino 146 derivatives in a ratio of 4 1 and 1 3, respectively. 3-Amino-5-/isy/-butylpyrazole affords 147 and indazole and tetrazole give comparable amounts of the 1- and 2-amino derivatives, and ethyl benzothiazol-2-ylcarboxylate reacts at room temperature <2001JOC8528>. Pyrazole is easily N-nitrated with nitrogen dioxide in the presence of ozone <1996JCR(S)388>. 

The nitration of 2-phenylindazole at 0°C with sulfuric-nitric acid mixture leads to 5-nitro-2-phenylindazole and 7-nitro-2-phenylindazole. These compounds have been identified using NMR spectroscopy [19]. In spite of the fact that the indazole positions 5 and 7 are most reactive with respect to electrophilic substitution [20] it is difficult to know beforehand the competition between the aromatic positions of the indazole ring (C-4, C-5, C-6, C-7) and the /V-phcnyl ring. 

The nitration of benzazoles is usually effected using concentrated (65%) to fuming (100%) nitric acid generally at temperature between 0 and 5°C. Indazoles are usually nitrated into 5 position, benzimidazoles - as a rule - into 5- or 6-position of the phenylene fragment whereas benzotriazole into position 4 or 7. For the preparation of other nitrobenzazoles the reaction of heterocyclization is used. 

Halogenation of indazole occurs preferentially in the 5-position. Nitration with filming nitric acid gives 5-nitroindazole. Sulfonation with oleum, however, yields indazole-7-sulfonic acid. Indazole couples with diazonium salts in the 3-position. 

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