Lactam acetals

The cis-trans isomerization in a number of enamines — 1-methyl-2-(2'-R-2'-R'-methylene)pyrrolidines,-piperidines, and -hexahydroazepines — depends substantially on the nature of the 2'-substituent and the ring size. The energy barrier to this process decreases as the ability of the substituents to delocalize the anionic center in the transition state increases. The free energy of activation of rotation about the C=C bond is appreciably lower for six-membered enamines than for the corresponding seven- and five-membered analogs.