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.     

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.     

A new synthetic procedure for the preparation of pyridinium tetrafluoroborate and hexafluorosilicate

Pyridinium tetrafluoroborate (C₅H₅NHBF₄) and pyridinium hexafluorosilicate [(C₅H₅NH)₂SiF₆] have been prepared in good yields and high purity by the reaction of pyridinium poly(hydrogen fluoride) with oxides and acids of boron and silicon respectively. The salts have been characterised by melting points, IR, ¹H and ¹⁹F NMR spectroscopy and chemical analysis.     

Synthesis of dimeric pyridinium bromide under silica supported approach

Abstract

Synthesis of dimeric substituted ester derivative of pyridinium salt with flexible linker units under conventional/silica supported approach. Solid-phase approach is much superior to the conventional method due to nontoxic, solvent free, easy work up procedure, and lesser reaction period. Synthesized dimeric ester derivative of pyridinium salts is showed excellent catalytic response for the preparation of oxizinone derivatives under conventional/approach.     

An efficient protocol for the preparation of pyridinium and imidazolium salts based on the Mitsunobu reaction

We report herein that, in the absence of any nucleophilic counterions, tertiary nitrogen nucleophiles such as pyridines and imidazoles can be alkylated with alcohols, by simply using their ammonium form as the acidic component of the Mitsunobu reaction. This led to efficient preparation of ionic liquids under mild conditions, avoiding the usual anion exchange step.     

Novel synthetic method for ketones from substituted benzimidazolium salts and Grignard reagents

A new synthetic method of ketones from substituted benzimidazolium salts and Grignard reagents is reported. The influences of the various Grignard reagents on the yield of ketones and the mechanism are discussed.     

Recyclization of pyridinium salts with exchange as a method for the preparation of N-substituted aromatic amines

The conditions for the formation of N-substituted aromatic amines in the sulfite variant of recyclization of pyridinium salts, as well as the structural factors that affect the yields of the final substances, were investigated.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 8, pp. 1095–1098, August, 1985.     

The solid-phase Zincke reaction: preparation of omega-hydroxy pyridinium salts in the search for CFTR activation

A study of structural modifications of MPB-07 was undertaken as part of a synthetic program aimed at discovering small molecules with CFTR activation potential. Solid-phase synthesis techniques were used to prepare derivatives of MPB-07 employing the Zincke reaction for the construction of aromatic, quaternary ammonium salts such as those found in 2 or 3. In this transformation, primary amines react with highly electrophilic N-2,4-dinitrophenylpyridinium (DNP) salt 4 to afford pyridinium salt 8 with release of 2,4-dinitroaniline 6. Thus, the reaction of 1-(2,4-dinitrophenyl)pyridinium salts with various polymer-bound amino ethers, followed by cleavage from the resin, delivers the desired salts in good yield and high purity.     

Azulene‐substituted pyridines and pyridinium salts. Synthesis and structure. 1. Azulene‐substituted pyridines

4‐Azulen‐1‐yl substituted 2,6‐dimethyl‐and 2,6‐diphenyl‐pyridines are obtained in good yields from the reaction of corresponding 4‐azulen‐1‐yl‐pyranylium salts and ammonium acetate in ethanol or starting from 4‐chloro‐2,6‐diphenyl‐pyranylium salts in two steps: reaction with azulenes followed by in situ treatment with ammonium acetate. The effect of substitution at the 3‐position of the heterocycle was taken into account. The structure assignment was accomplished with NMR and uv‐vis spectra.     

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.     

A flexible approach to the preparation of polymer scaffolds for tissue engineering

Porous polyester thermoset xerogels have been produced via sol-gel chemistry as a first step in the development of sol-gel derived tissue engineering scaffolds templated by replica molding and/or salt leaching. The pore structure of these untemplated thermosets is tunable and can be altered independent of or in tandem with alterations in composition. Cytocompatibility studies on these xerogels imply the effects of both pore size and materials chemistry, with fully aliphatic polyesters with large pore structures allowing the growth of mammalian cells. To the best of our knowledge, this represents the first report examining the preparation and potential of sol-gel derived porous polymer xerogels as tissue engineering scaffolds.     

Convenient, scalable and flexible method for the preparation of imidazolium salts with previously inaccessible substitution patterns

A high yielding and modular approach to N,N'-disubstituted imidazolium salts is described, providing access to substitution patterns that are beyond the reach of established methodology.     

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.     

Synthesis of fluorescent and coloured pyrylium and pyridinium salts

The synthesis is described of fluorescent and coloured pyrylium and pyridinium salts, including water-soluble derivatives, designed to serve as marker reagents for primary amines.     

The reaction of thioesters with nitriles. A new synthetic approach to the preparation of substituted 4-alkylthio- and 4-arylthiopyrimidine derivatives

S-Alkyl and S-aryl thioesters react with nitriles in the presence of triflic anhydride to form substituted 4-alkylthio- and 4-arylthiopyrimidines. However, when methyl thiocyanate is used as nitrile, dithioimidocarbonates are formed. A mechanism to explain these differences is postulated.     

Greener approach toward the generation of dimedone derivatives

The use of dimedone in green chemistry has been described for the synthesis of selective heterocyclic motifs which are both pharmacologically and industrially important. The objective of this review is to summarize some of the selected recent advances of dimedone in the synthesis of organic compounds utilizing green chemistry procedures.     

Photocatalyst-controlled and visible light-enabled selective oxidation of pyridinium salts

This study proposes two different methods of photocatalytic-controlled and visible light-induced selective oxidation of pyridiniums with air as the terminal oxidant. The key to these transformations is to choose the appropriate light source and photocatalyst. Pyridiniums are successfully converted into pyrroles through oxygen-mediated cycloaddition, proton-coupled electron transfer(PCET), pyridine ring opening, and recyclization. The other route is that pyridiniums selectively form 4-carbonyl pyridines through free radical rearrangement/aerobic oxidation under the catalysis of cobalt(Ⅱ).