2-Pyridones, formation from pyridinium

In an Initio study of the tautomerism of 2- and -hydroxy-pyridines, 4 -hydroxypyridine was calculated to be 2.4 kcal/mol more stable than 4-pyridone. 2-Pyridone was calculated to be 0.3 kcal /mol more stable than 2-hydroxypyridine and this is in good agreement with experimental values obtained from tautomeric studies in the gas phase.A study of the bromination of the 2-pyridone/2-hydroxypyridine system has revealed that reaction occurs via the principal "one" tautomer at pH<6 and via the conjugate anion at pH>6. Attack on the "one" occurs preferentially at the 3-position, whereas on the anion it probably occurs mainly at the 5-position. The facile formation of 3f5-dibromo-2-pyridone results from the comparable reactivity of the monobromopyridones at pH<1 and pH>4- Practical procedures have been reported for the preparation of 3-bromo-2-pyridone and 3,5-dibromo-2-pyridone Cycloaddition of 2-substituted pyridinium betaines with unsymmetrical alkenes gives products of mixed orientation for example, treatment of (40) with methyl... 

Both N-N and N-C bond fission occurs on irradiation of the hydrazone derivatives (191). The photodegradation of the phenylhydrazone and the hydrazone of benzil have also been described. a-Ketoiminyl radicals are formed on irradiation of oximino ketones at low temperature. A study of the photochemical decomposition of sulfamic esters and their use as initiators of cross-linking of a melamine resin have been described. The bispyridinyl radical (192) is formed by one electron reduction of the corresponding pyridinium salts. The irradiation of this biradical at 77 K results in C-N bond fission with the formation of benzene-1,3-diyl. The predominant products from the irradiation (X,> 340 nm) of (193) in methanol were identified as A -hydroxy-2-pyridone and (194) from the fission of the C-O bond. Other products were 2-pyridone, (195) and (196) that arise from O-N bond fission. The reaction is to some extent substituent dependent and a detailed analysis of the reaction systems has identified an intramolecular exciplex as the key intermediate in the C-O bond heterolysis. 

When l-[14C]methyl-pyridinium aldoxime iodide or radioactive pralidoxime [14C]-labelled in the oxime group was parenterally administrated to rats, 90% of the radioactivity was recovered in urine and 6% in the faeces, irrespective of the position of the label. About 90% of the urinary radioactivity was associated with intact pralidoxime. In addition, some 5% of the dose was excreted as l-methyl-2-pyridone, indicating some cyanogenesis (Enandcret al., 1962). In humans, the l-methyl-2-cyanopyridinium ion was detected in urine of male volunteers without significantly increased urinary thiocyanate. Since 90% of pralidoxime chloride, 5 mg kg 1 IV, was recovered from urine, cyanide formation is probably of no toxicological concern (Garrigue etal., 1990). 

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... 

Chloro-l-methylpyridinium iodide (1) reacts with a mixture of a carboxylic acid and an alcohol, in the presence of two equivalents of base, to form an ester (eq 1). The pyridinium salt (2) is formed rapidly by displacement of chloride from (1) by the carboxylate subsequent reaction with the alcohol results in formation of the ester, along with 1 -methyl-2-pyridone (3). A variety of solvents may be employed, but yields are highest in dichloromethane or pyridine. Tri-n-butylamine or Triethylanune are often used as base. The co-product (3) is insoluble in dichloromethane and so precipitates from this solvent. Good results are obtained even for hindered carboxylic acids and alcohols. 

---