Hydroxypyridines quaternary salts

The structures of other charged pyridines such as quaternary salts (7), zwitterions (8) arising from /3-hydroxypyridines (9), another aspect of tautomerism, and ylides (10) will also be considered. 

The quaternary salts of 3-hydroxypyridines are converted by mild base into zwitterionic, organic-solvent-soluble species for which no neutral canonical form can be drawn. These 3-oxidopyridiniums undergo a number of dipolar cycloaddition reactions, especially across the 2,6-positions. 

Hydroxypyridine is also quaternized by methyl iodide 393-6 chlorace-tic acid i and jS-chloropropionic acid 52. The quaternary salts are readily converted into betaines, and alkylation in alkali gives these rather than... 

The nucleophilic substitution might be effected by chloride ion liberated in the first step ( S n2) or, internally, by a chlorine atom still attached to phosphorus ( ). A similar description could apply to the case of l-alkyl-2-pyridones, except that the intermediates would be quaternary salts. Clearly, the iSnI mechanism is not open to 4-hydroxypyridines. 

The nitro group in quaternary salts of 4-nitropyridine is easily replaced. Recrystallization of the methiodide from undried acetone gives l-methyl-4-pyridone . Reaction of 4-nitropyridine with benzyl chloride yields 1-benzyl-4-pyridone, and with benzyl bromide, l-benzyl-3,5-dibromo-4-pyridone (nuclear bromination is thought to result from the oxidation of hydrobromic acid by nitrous acid) the experimental description suggests that in these reactions nucleophilic replacement of nitro by halide may occur initially . The consequences of the autoquaternization of 4-nitropyridine have already been mentioned. The formation of 4-hydroxypyridine from 4-nitropyridine and acetic anhydride a presumably involves the acetyl-pyridinium salt. 4-Nitropyridine 1-oxides give with acetic anhydride mainly 4-hydroxy-or 4-acetoxy-3-nitropyridine l-oxides sic but the presence... 

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