Polarographic reduction of benzazolylsubstituted pyridinium salts

N-Methyl- and N-benzyl-substituted 2-, 3-, and 4-benzazolypyridinium (BAP) salts are reduced on a dropping mercury electrode (DME) to give a free radical in the first step and the corresponding dihydropyridine derivative in the second step. The free radicals of 2- and 4-BAP salts are distinguished by high stability as compared with other radicals with a pyridine structure owing to delocalization of the unpaired electron over the conjugated system and can be recorded by ESR spectroscopy. A peculiarity of the BAP salts is their facile reduction on the electrode; this is particularly true of the 2- and 4-benzoxazolyl- and benzothiazolyl-substituted salts.     

The ammonia-free partial reduction of substituted pyridinium salts

This paper reports a study into the partial reduction of N-alkylpyridinium salts together with subsequent elaboration of the intermediates thus produced. Activation of a pyridinium salt by placing an ester group at C-2, allows the addition of two electrons to give a synthetically versatile enolate intermediate which can be trapped with a variety of electrophiles. Furthermore, the presence of a 4-methoxy substituent on the pyridine nucleus enhances the stability of the enolate reaction products, and hydrolysis in situ gives stable dihydropyridone derivatives in good yields. These versatile compounds are prepared in just three steps from picolinic acid and can be derivatised at any position on the ring, including nitrogen when a p-methoxybenzyl group is used as the N-activating group on the pyridinium salt. This publication describes our exploration of the optimum reducing conditions, the most appropriate N-alkyl protecting group, as well as the best position on the ring for the methoxy group. Electrochemical techniques which mimic the synthetic reducing conditions are utilised and they give clear support for our proposed mechanism of reduction in which there is a stepwise addition of two electrons to the heterocycle, mediated by di-tert-butylbiphenyl (DBB). Moreover, there is a correlation between the viability of reduction of a given heterocycle under synthetic conditions and its electrochemical response; this offers the potential for use of electrochemistry in predicting the outcome of such reactions.     

Partial reduction of pyridinium salts as a versatile route to dihydropyridones

[reaction: see text] The addition of two electrons to a pyridinium salt turns it into a nucleophile. The intermediate generated by the reduction of such salts can be reacted successfully with a range of different electrophiles (acids, alkyl halides, and carbonyl compounds) and the intermediate hydrolyzed in situ to provide a wide range of dihydropyridones. Each position on the dihydropyridone ring is then accessible using standard synthetic manipulations.     

Synthesis of indoles from pyridinium salts.

The reaction of 1,2,5,6-tetramethyl-3-nitropyridinium perchlorate with acetone and piperidine on a solid carrier has given the novel, relatively stable 3-nitropyridine anhydro-base, 1,2,5-trimethyl-3-nitro-6-methylene-1,6-dihydropyridine, which bears alkyl groups only. This anhydro-base has been shown not to be an intermediate in the synthesis of indoles from 3-nitropyridinium salts.See [1] for Communication 7.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 9, pp. 1214–1216, September, 1990.     

Catalytic hydrogenation of pyridinium salts

Catalytic hydrogenation of polysubstituted pyridinium salts leads to piperidines and their condensed analogs The spatial properties and conformational properties of the saturated azaheterocycles have been determined by¹³C NMR spectorscopy. It was shown that hydrogenation of the pyridinium salts occurs stereoselectively to form cis isomers in most cases.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 1, pp. 68–72, January, 1994.     

Azulene‐substituted pyridines and pyridinium salts. Synthesis and structure. 2. Azulene‐substituted pyridinium salts

4‐Azulen‐1‐yl substituted 2,6‐dimethyl‐and 2,6‐diphenyl‐pyridinium salts are obtained in yields between 50 % and 100 % in the reaction of corresponding 4‐azulen‐1‐yl‐pyranylim salts and various amines. The effects of amine structure and of substitution in the heterocycle or at azulene moieties on the synthesis have been investigated. The uv‐vis and NMR spectra of reaction products are examined and discussed in correlation with their structure.     

New electrochromic polymeric pyridinium salts

A series of new anode-cathode conjugated electrochromic polymeric pyridinium salts containing mainchain triphenylamine units and demonstrating multicolor electrochromism has been synthesized. All of the polymers exhibit intense UV absorption with maxima of absorption and fluorescence at 340–343 and 336–338 nm in DMF solutions and 460–461 and 410–438 nm in films. The electrochromic properties of the films are investigated by electrochemical and spectroelectrochemical methods. Reversible redox signals with stable electrochromic characteristics are found in cyclic voltamperograms. The electrochromic properties of the polymers remain highly stable after 50 cycles in the range 0–1.2 V.     

Electrochemical reduction of N-(2-nitro-4-R-phenyl)pyridinium salts using redox-mediators

Electrochemical reduction of N-(2-nitroaryl)pyridinium chlorides was accomplished in acidic aqueous alcoholic medium in the galvanostatic mode using tin, titanium, vanadium, and iron chlorides as redox-mediators. The use of SnCl₂ as a mediator catalyst made it possible to shorten the electrosynthesis time as compared to the direct electroreduction on an electrode and prepare the intramolecular cyclization products, viz., substituted pyrido[1,2-a]benzimidazoles, in high yield. The influence of the mediator nature and its amount, current density, temperature, cathode material, medium acidity, and the substrate structure on the efficiency of reductive heterocyclization of N-(2-nitroaryl)pyridinium salts was studied.     

Synthesis of indoles from pyridinium salts

The detailed analysis of the products of the interaction of nitropyridinium salts with ketones and alkylamines resulted in new data testifying in favor of the previously proposed scheme for the formation of indoles from pyridinium salts.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 4, pp. 488–496, April, 1987.     

Imination of pyridinium and quinolinium salts

A new method for imination of N-alkyl pyridinium- and quinolinium salts is described. It involves a low temperature oxidation of a solution of appropriate substrates in liquid ammonia with potassium permanganate.     

Microwave-promoted synthesis of chiral pyridinium salts

The synthesis of several chiral pyridinium salts via Zincke’s reaction can be easily accomplished by domestic microwave oven irradiation. Yield enhancements, reduction of reaction time, and less racemization were observed under microwave heating when compared to conventional heating in similar conditions.     

N-Arylated pyridinium salts having reactive groups

The pyridinium salts having reactive amine and/or pyridyl groups were obtained by the reaction using Zincke salts. Optical properties and anion exchange behavior of the obtained pyridinium salts were investigated.     

The reaction of N-nitrosamines with pyridinium salts

N-Nitrosamines react readily with pyridinium salts containing an activated methylene group to give hydrazones. This reaction may serve as the basis for the routine determination of N-nitrosamines.     

Hexahydrobenzothiazolo[3,2-a]pyridinium salts

Salts of hexahydrobenzothiazolo[3,2-a]pyridine are obtained by exchange reactions. Anions are placed in the order [(NC)₂CCHC(CN)₂]- > ClO₄ ^–>I₃ ^–>Br^– according to the ability to displace each other. X-ray structural and spectroscopic studies establish the cis-attachment of the thiazolidine and cyclohexane rings in the hexahydrobenzothiazolo [3,2-a]pyridinium salts studied.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 12, pp. 1682–1688, December, 1988.     

Thiazolo [3,2-a] pyridinium salts

The sulfides formed by the reaction of α-halo ketones or α-halo acetals with 2-mercaptopyridine may be cyclized in good yield to form thiazolo[3, 2-a]pyridinium salts. The presence of chloro or nitro substituents on the pyridine ring does not interfere with the synthesis. Nitration of 3-methylthiazolo[3, 2-a]pyridinium perchlorate has been found to occur at position 8.     

Oxazolo[3,2-a]pyridinium salts

The first synthesis of the aromatic oxazolo[3,2-a]pyridinium ring system has been accomplished by the cyclization of 1-acetonyl- or 1-phenacyl-2-pyridones. Through use of the analogous quinolones and 2-(p-bromophenacyl)-1-isoquinolone, benzologs of the new system have been prepared.     

Oxidative diphosphonylation of 1,4-dihydropyridines and pyridinium salts

An oxidative double phosphonylation of dihydropyridines 1 and pyridinium salts 2 is achieved through the use of dialkyl phosphites, DDQ, and triethylamine. Acceptable to good yields of 2, 6-diphosphonylated-1,2-dihydropyridines 3 are obtained in a one-pot reaction involving tandem nucleophilic addition/oxidation processes. Isomerization of 3 to the more stable 2,4-diphosphonylated-1, 4-dihydropyridine 4 was observed in some cases.