Electrocyclic Reactions Ground State

There are no reports of thermal electrocyclic reactions involving simple pyridines. 2-Pyridones, however, participate as 4n components in Diels-Alder additions, especially under high pressure.  

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

A-Tosyl-2-pyridones with a 3-alkoxy or 3-arylthio substituent, undergo cycloaddition with electron-deficient alkenes under milder conditions, as illustrated below.  

Sodium in ethanol treatment generally gives mixtures in which the 1,2,5,6-tetrahydropyridine is predominant, whereas sodium in liquid ammonia, in the presence of ethanol, affords the l,4-dihydropyridine7 Metal/acid combinations, which in other contexts bring about reduction of iminium groups, are without effect on pyridines. 

The combination lithium/chlorotrimethylsilane produces a 1,4-dihydro doubly silylated product the enamine character in which can be utilised for the introduction of 3-alkyl groups via reaction with aldehydes7 Trimethylsilane in the presence of palladium gives 1,4-dihydro-l-trimethylsilylpyridine, together with silylated dimer.  

4-Pyridones are reduced to 2,3-dihydro-derivatives by lithium in liquid 

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