2- Hydrazino-3 nitropyridines

As a result of this observation, reacting 2-hydrazino-3-nitropyridine with aliphatic acids (a typical route to [l,2,4]triazolo[4,3-a]pyridines, see later) led directly to various 8-nitro-... 

A variety of 3-substituted 8-nitro-[l,2,4]triazolo[4,3-a]pyridines (70JHC1019) have been prepared via a number of the aforementioned ring closure methods with 2-hydrazino-3-nitropyridine. These products readily undergo rearrangement to the isomeric [1,5-fl] series as described elsewhere in this chapter. Reduction of the 8-nitro substituent of several of these derivatives resulted in the corresponding 8-amino products that offer the potential starting point for other 3,8-disubstituted systems of this type. 

Aminotriazolo[4,5-6]pyridine (155) has been synthesized [69JCS(C) 1758] starting from 2-hydrazino-3-nitropyridine, [Eq. (39)], a route which is similar to the Italian method in Eq. (38). In the same way, amine 156 was obtained as well as the quinoline analogues of compounds 155 and 156. The Italian method [Eq. (38)] was used to obtain A-aminotriazole 157 from 5-amino-6-nitrobenzimidazoles (70CC1458). 

In the presence of an ortho- or a para-nitxo substituent, a nitramino function in the 2- or 4-position is easily replaced by a hydrazino group. Thus, a solution of 2-nitramino-3-nitropyridine in methanol gives 2-hydrazino-3-nitropyridine when treated with hydrazine hydrate. Various hydrazinonitropyridines are prepared similarlythese are listed in Table IX-51. 

The parent heterocycle of pyrido[2,3-e][l,2,4]triazine and its phenyl derivative 39 were prepared (89JHC475) by cyclization with polyphos-phoric acid of 3-acyIhydrazino-2-aminopyridines 36, obtained by reduction of the corresponding 3-acylhydrazino-2-nitropyridines 35. Compounds 35 were obtained from 3-fluoro-2-nitropyridine 34 either by reaction with benzoylhydrazine or by reaction with hydrazine and subsequent for-mylation of the initially formed 3-hydrazino-2-nitropyridine 38. Attempts to prepare 38 from 3-chloro-2-nitropyridine gave 2-hydrazino-3-chloropyridine 37. These results could be explained by semiempirical calculations (CNDO and MNDO calculations). 

Pyrazolo[3,2-c]pyrido[4,3- ][l,2,4]triazine oxides704 were prepared by cyclization of the hydrazone derivatives 703 with ethanolic sodium hydroxide. Subsequent reduction of 704 gave 705 (76JPR835). Hydrazone 703 was prepared by condensation of 4-hydrazino-3-nitropyridine with 702. The mass spectral fragmentation patterns for 704 and 705 and their benzo analogues were studied (77ZC142). 

The rearrangement also occurs when 2-hydrazino-5-nitropyridine is treated with cyanogen bromide and carbon disulfide to result in 2-amino-6-nitro-[l,2,4]triazolo[l,5-fljpyridine and 6-nitro-[l,2,4]triazolo[l,5-fl]pyridine-2-thiol, respectively (70JHC1019). The rearrangement has also been used (76JOC3124) for the synthesis of nucleosides of the [1,5-a] system. 

Acyclo C-nucleosides 229 were synthesized by cyclocondensation of the thioimidates 228 with 2-hydrazino-3- or 5-nitropyridines (83JHC1169) (Scheme 68). 

Thermolysis of iV-oxides of type (64) yields pyridyl ethers (65) on cleavage with hydrazine, these give a catechol (67) and 2-hydrazino-5-nitropyridine (66). In addition to the ort/io-substituted phenol (65), thermolysis (if R = H) also gives a small amount of para-substituted product. Since the starting materials can be prepared from the reaction of 2-chloro-5-nitropyridine and a phenol, followed by N-oxidation, this represents overall a four-stage process by which a phenol can be converted into a catechol (Scheme 27). ... 

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