Synthetic applications of N–N linked heterocycles. Part 12. The preparation of 4-alkylthio- and 4-arylthio-pyridines by regiospecific attack of thioalkoxide lons on N-(4-oxopyridin-1-yl)pyridinium salts

Abstract

Thiolate ions add regiospecifically to N-(4-oxopyridin-1-yl)pyridinium salts (2)–(7) to give in good to excellent yields only the 1,4-dihydropyridine adducts (8)–(13), regardless of whether or not the pyridone moiety carries substituents for sterically shielding the 2- and 6-positions of the pyridinium ring. The addition is believed to be thermodynamically controlled. Decomposition of the dihydro-adducts under free-radical conditions, or by pyrolysis, gives good yields of pyridin-4-yl thioethers (14) and (16) though the reaction failed with the 2-methyl adducts (9). An improved synthesis of 6-methyl-4-oxopyran-2-carboxylic acid is also described.