Pyridinium compounds, reviews

Provide a retrosynthetic analysis and synthesis for each of the following compounds. Review Problem 12.9 Permitted starting materials are phenylmagnesium bromide, oxirane, formaldehyde, and alcohols or esters of four carbon atoms or fewer. You may use any inorganic reagents and oxidizing agents such as pyridinium chlorochromate (PCC). 

We have discussed primarily our own work, especially that which is still in progress. No discussion of pyridinyl radical Tr-mers would be complete without citing the discovery of the dimerization of pyridinyl radicals by Itoh and Nagakura (14) and the work of Ikegami (15) on different ir-mer triplets. The original impetus for the study of pyridinyl radical reaction with halocarbons was given by the discovery by Westhelmer, Kurz and Hutton (16) that dihy-dropyridines were converted into pyridinium compounds via free radical reactions in the presence of tetrachloromethane. An extensive review of pyridinyl radicals has been published elsewhere (IJ. New ways of measuring the low temperature spectra of pyridinyl radicals are under development (18). 

The synthesis of the four monocarboxylic acids of dibenzothiophene has been recorded in the previous review. However, several modified preparations have since been described. Ethyl 1-dibenzothiophene-carboxylate has been synthesized from 2-allylbenzo[6]thiophene (Section IV,B, 1) hydrolysis afforded the 1-acid (57% overall). In a similar manner, 3-methyl-1-dibenzothiophenecarboxylic acid was obtained from the appropriately substituted allyl compound. This method is now the preferred way of introducing a carbon-containing substituent into the 1-position of dibenzothiophene. 2-Dibenzothiophenecarboxylic acid has been prepared by oxidation of the corresponding aldehyde or by sodium hypoiodite oxidation of the corresponding acetyl compound. Reaction of 2-acetyldibenzothiophene with anhydrous pyridine and iodine yields the acetyl pyridinium salt (132) (92%), hydrolysis of which yields the 2-acid (85%). The same sequence has been carried out on 2-acetyldibenzothiophene 5,5-dioxide. The most efficient method of preparing the 2-acid is via carbonation of 2-lithio-... 

Oxidation of 2,5-dialkylfurans with pyridinium chlorochromate results in high yields of a,(3-unsaturated y-dicarbonyl compounds (Scheme 25) (82S245, 80T661). Similar results are obtained by peracid oxidation of furans (see review (90MI206-01)). The acid most frequently used is MCPBA. It is assumed that the first step involves epoxidation as shown in Scheme 25 (83CL1771). 

In addition to the preparation of a- and /3-hydrastine described above from the betaine (64), another conversion of a tetrahydroberberine into hydrastine has been reported. Acetylophiocarpine, on treatment with ethyl chloroformate, gives the acetoxy-derivative of (88), which can be hydrolysed to the hydroxymethyl compound and then oxidized to the aldehyde by pyridinium perchlorate. Hydrolysis of the acetoxyl group afforded the hemi-acetal (93 R = H), conversion of which into the mixed acetal (93 R = Et) protected the aldehyde system during reduction of N—C02Et to NMe by lithium aluminium hydride. Hydrolysis of the acetal, followed by oxidation, then gave a-hydrastine, and a similar sequence of reactions starting from O-acetyl-13-epi-ophiocarpine afforded / -hydrastine.119 Methods of synthesis of alkaloids of this group have been reviewed.120... 

The uses of pyridine derivatives, such as aminopyridines, quinaldine, and acridine, together with quaternary salts and photochromic compounds are reviewed in CHEG-II(1996) <1996CHEC-II(5)245>. Pyridinium-based stilba-zolium cationic dyes 43 and 44 were induced to give intense fluorescence by anionic peptide amphiphiles <2006CC1073>. 

More than 130 different organic chemicals are currently employed as herbicides in the U.S. All of the main families of organic compounds are represented aromatic, aliphatic, and heterocyclic. Herbicidal activity is found in a variety of classes of compounds haloaliphatic, phenoxy, and benzoic acids carbamates dinitroanilines acetanilides amino triazines quaternary pyridinium salts uracils and ureas. A few selected key examples are reviewed below. 

The UV and fluorescence characteristics of simple substituted oxazoles have been discussed in the early review, which also made mention of the utility of 2,5-diaryl derivatives as scintillators (3). Among the natural products, the 2,5-diaryl compounds halfordinol (16), halfordine (17), O-isopentenylhalfordinol (19), balsoxin (25), O-methylhalfordinoI (22), compound 24, texamine (26), and texa-line (27) reportedly display a high intensity (log e 3.61-4.63) band in the range 323-354 nm (Table III). In the 2-pyridyl-5-phenyl derivatives this band undergoes a bathochromic shift of 17-23 nm on acidification (Table III), which may be rationalized by the formation of the pyridinium salt (e.g., 204) for O-isopentenylhalfordinol (19). In 204 the 2-pyridinium substituent is obviously... 

Although it has been claimed that rate constants are slightly diminished in the inverted region (Creutz and Sutin, 1977), most studies so far have yielded the same conclusion as that of Rehm and Weller, namely that no such rate decrease is discernible. Examples of such investigations are quenching of excited ruthenium complexes by substituted pyridinium ions and back electron transfer between the species formed (Nagle et al., 1979), quenching of excited chromium and iridium complexes (Ballardini et al., 1978), and electron transfer between triplet dyes and aromatic compounds (Tamura et al., 1978). A brief review is available (Balzani et al., 1979 see also Sutin, 1979). 

Cationic polymerizations induced by thermally and photochemically latent N-benzyl and IV-alkoxy pyridinium salts, respectively, are reviewed. IV-Benzyl pyridinium salts with a wide range of substituents of phenyl, benzylic carbon and pyridine moiety act as thermally latent catalysts to initiate the cationic polymerization of various monomers. Their initiation activities were evaluated with the emphasis on the structure-activity relationship. The mechanisms of photoinitiation by direct and indirect sensitization of IV-alkoxy pyridinium salts are presented. The indirect action can be based on electron transfer reactions between pyridinium salt and (a) photochemically generated free radicals, (b) photoexcited sensitizer, and (c) electron rich compounds in the photoexcited charge transfer complexes. IV-Alkoxy pyridinium salts also participate in ascorbate assisted redox reactions to generate reactive species capable of initiating cationic polymerization. The application of pyridinium salts to the synthesis of block copolymers of monomers polymerizable with different mechanisms are described. 

We present an overview of the structures for 3-alkylpyridine and 3-alkylpyridinium compounds that have been isolated, taking into account the structural characterization of alkylpyridinium compounds and discussing the central role played by mass spectrometry. Concerning their synthesis, both the natural pathways and organic synthesis of pyridinium alkaloids are reviewed. Synthetic aspects of 3-alkylpyridines have been taken into account only where relevant for structural modifications or for the assignment of absolute configurations. Finally, biological activities of both 3-alkylpyridine and 3-alkylpyridinium compounds and the potential use of the latter are discussed. 

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