1- Substituted pyridinium-2-carboxylates

The catalytic constants for acids in aqueous solution correlate well with pK, and there is no apparent difference between uncharged and cation acids (carboxylic acids and pyridinium cations), though some steric inhibition is evident for 2,6-substituted pyridinium cations. The catalytic constants for bases behave somewhat more erratically, and it has been suggested by Gruen and MoTigue (1963b) that errors are intro-... 

Ester hydrolysis.1 Pyridinium chloride selectively demethylates methyl esters of o-substituted aromatic carboxylic acids. 

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-... 

Pyridine has another useful attribnte, in that it behaves as a nncleophilic catalyst, forming an intermediate acylpyridinium ion, which then reacts with the nucleophile. Pyridine is more nucleophilic than the carboxylate anion, and the acylpyridinium ion has an excellent leaving group (pATa pyridinium 5.2). The reaction thus becomes a double nucleophilic substitution. 

Orotic acid (uraciI-6-carboxylic acid), an intermediate in the biosynthesis of uracil, also reacts smoothly with CF3SCI in pyridine to give the 5-substituted compound. The pyridinium salt initially formed can be easily cleaved with dilute hydrochloric acid ... 

A number of syntheses of substituted 2,3 -bipyridines are worthy of note. Tetracyclone heated at 215°C with nicotinonitrile affords 3,4,5,6-tetraphenyl-2,3 -bipyridine, whereas 3,4-di(2-pyridyl)pyridine is obtained by an oxidative degradation of the corresponding 6,7-disubstituted thiazolo[3,2-a]-pyridinium salt. Nicotinic acid on UV irradiation in aqueous solution at pH 4-6 gives 2,3 -bipyridine-5-carboxylic acid, whereas irradiation of picolinic acid in the same pH range in the absence of metal ions gives some 2,3 -bipyridine. 6,6 -Diphenyl-2,3 -bipyridine is thought to be formed from... 

The aqueous methanolysis of 3,4-dihydro-27f-pyrano[3,2-(r]pyridinium a-D-A -acetylneuraminoate has been studied and is believed to proceed via dissociative transition states with no intramolecular nucleophilic participation by the anomeric carboxylate group <2004JP0478>. The synthesis of the starting substituted sugar is also reported. 

Fig. 13.56. Mechanism of the Knoevenagel condensations in Figure 13.55. The C,H( )-acidic reaction partneris malonicacidin the form of the malonic acid enolate D (malonic acid "monoanion"). The decarboxylation proceeds as a fragmentation of the pyridinium-substituted malonic acid carboxylate F to furnish the ,/Tunsaturated ester (G) and pyridine. This fragmentation resembles the decomposition of the sodium salts H of ,/Tdibrominated carboxylic acids to yield the a,/Tunsaturated bromides I and sodium bromide.

Although not officially a transformation involving carboxylate, the anion formed by two electron reduction of l-methyl-4-(methoxycarbonyl)pyridinium iodide has been extensively used as an electron transfer agent in the study of aliphatic nucleophilic substitution reactions [80]. 

If carboxylates are subjected to Kolbe electrolysis in the presence of olefins, the generated radicals add to the double bonds to afford mainly additive dimers (Table 8, entries 12-20). In vicinal disubstituted styrenes, upon addition of the Kolbe radical Me02CCH2, the yields of adducts decrease with increasing size of the /f-substituent H = 42%, Me = 27%, Et = 11%, /Pr = 5%, tBu = 2% [125]. The ratio of additive dimer 87 (Eq. 11) to monomer 89 can be changed to some extent by the current density i. Upon electrolysis of trifluoroacetate in MeCN-H20-(Pt) in an undivided cell in the presence of electron-deficient olefins, additive dimers and additive monomers are obtained. The selectivity can be controlled by current density, temperature and the substitution pattern of the olefin [126]. Trifluoromethylation of various aromatic compounds with -M substituents has been achieved in satisfactory yield via electrolysis of pyridinium trifluoroacetate in acetonitrile [127]. 

Many synthetic porphyrins have been prepared which are substituted at the meso positions (5,10,15,20). Porphyrins that have only alkyl or aryl substituents are insoluble in water. Water soluble porphyrins are made by introducing charged groups, such as carboxylate, sulfonate, trimethylammonium, or pyridinium. Some common synthetic porphyrins and their abbreviated notations are shown below. 

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