1.2.3- Triazolo pyridinium salts

By analogy with the conversion of 1,2-dialkylpyridinium salts into 2-alkylaminopyridines known as the Kost-Sagitullin rearrangement, the pyridinium salt 252 was treated with two different hydrazides (Equation 33). The [l,2,4]triazolo[4,3-tf]-pyridines 253 with Y = 4-pyridyl and NH2 were obtained with 35% and 38% yields, respectively <2003CHE275>. 

The oxidation of A -phenylhydrazones in the presence of pyridine leads to the formation of 5-triazolo[4,3a]pyridinium salts hy attack of pyridine as a nucleophile on the nitrilimine intermediate (Scheme 54) [77]. 

In 1981 we published the first paper [22] on the synthesis of s-triazolo[4,3-a]pyridinium salts, 4, by the anodic oxidation of hydrazones 3 in the presence of pyridine (Scheme 5). In our working mechanistic scheme we proposed nitrilimine as the possible intermediate and pointed out that this reaction opens the door to a wide range of heterocyclic systems via anodic oxidation of aldehyde hydrazones through 1,3-dipolar cycloaddition reactions of the nitrilimine involved. 

Conversion of [l,3,4]oxadiazolo[3,2-a]pyridinium salts (51) to [1,2,4]-triazolo[l,5-a]pyridines can take two pathways using aniline 1-phenyltria-zolopyridinium salts (52) are obtained,67 whereas using cyanamide and triethylamine, the 2-aminotriazolopyridine 53 is formed.68... 

There is a single example of formation of the l,8a-bond. The pyridinium salts 66 can be cyclized by chloranil or by DDQ to give 3-substituted triazolo-pyridines.94... 

The quaternary [l,2,3]triazolo[4,5-c]pyridinium salt 147 is oxidized by ferricyanide to the triazolopyridin-5-one.169... 

A novel way of preparing s-triazolo[3,4-a]pyridinium salts (105) by anodic oxidation of aryl hydrazones (104) of 2-acetylpyridine, 2-benzoylpyri-dine, and 2-formylpyridine in CH3CN-Et4NX (X=C104) p-TsO, or BF4) has been reported165 [Eq. (80)]. 

Only one report of synthetic value of anodic transformations of pyridyl aldehydes and ketones is known.210 In this work, a series of hydrazones was converted to the s-triazolo[3,4-a]pyridinium salts (Scheme 50). The yields were generally high (80-90%). 

The intermediate l-aza-2-azoniallene cations can undergo 1,3-dipolar cycloaddition reactions with inverse electron demand [119]. Thus, oxidation of a A -phenyl hydrazone in the presence of pyridine leads to the formation of a. y-triazolo[4,3-a]-pyridinium salt by attack of pyridine as a nucleophile on the intermediate nitrilimine [Eq. (17)] [120]. Other examples are reported by Jugelt [121]. 

There are two routes to [l,2,4]triazolo[l,5-a]pyridines from N-aminopyridinium salts. In one, Ar-(2-pyridyl)hydrazides (34) are converted by MSH (mesitylsulfonylhydroxylamine) to AZ-aminopyridinium salts, and these cyclize on heating, giving 1-aminotriazolopyridinium salts C35).4 In the other, AZ-aminopyridinium salts are treated with AZ-ethoxycarbonylacet-imidate to give pyridinium ylides (36), which cyclize on heating, giving mixtures of triazolopyridines and imidazopyridines.47,48... 

The reaction of N-aminoazonium salts with aliphatic and aromatic aldehydes in the absence of base gives the Schiff base type compounds 28.151,153-156 Some aliphatic ketones also react with N-aminoazonium salts.40,155 Hexane-2,5-dione reacts with N-aminopyridinium perchlorate to give l-(l -pyrrolyl)pyridinium cation 29 together with some of the bisper-chlorate 30.157 Certain esters such as diethyl malonate and ethyl cyano-acetate react with N-aminopyridinium salts in the presence of base to give the corresponding N-acylimines 31.48 However, the reaction of ethyl aceto-acetate and acetylacetone with N-aminoazonium salts in the presence of base gives 1,3-dipolar cycloaddition products (Section IV,C,1).36,154,158 The reaction of ethyl acetoacetate with 1-alkyl-l,2,4-triazole 4-imine affords zwitterionic triazolo[4,3-/>]pyridazines 32.139,159... 

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