The relative stabilities of 6-membered cyclic allylamine/enamine systems

Evidence on the relative stabilities of acyclic allylamine/enamine systems and methods for the isomerisation of the former into the latter are reviewed. Previous evidence on the related question of the thermodynamic stabilities of 3-piperideines/2-piperideines is presented. The view that 6-membered cyclic allylamines are thermodynamically preferred over their enamine isomers is refuted by demonstrating the base-catalysed equilibrative isomerisations of several 1-alkyl-4-acyl-1,2,5,6-tetrahydropyridines into their 1-alkyl-4-acyl-1,4,5,6-tetrahydropyridine isomers. It is shown that the conjugated CO group in these examples is unnecessary to effect isomerisation: for example even in the simplest possible situation, 1-methyl-1,2,5,6-tetrahydropyridine can be isomerised into 1-methyl-1,4,5,6-tetrahydropyridine, though a stronger base is necessary.