INVESTIGATION OF THE STEREOCHEMISTRY OF N-SUBSTITUTED 2,5-DIMETHYL-4-PIPERIDINONES BY NMR-SPECTROSCOPY

The conformations of the cis and trans isomers of N-substituted 2,5-dimethyl-4-piperidinones were studied by means of the1H and13C NMR parameters. It was established that in the case of bulky and electron-acceptor substituents attached to the N atom the cis isomers are virtually completely represented by the chair (2a,5e) conformation, while the trans isomers are characterized by the chair (2e,5e) ⇄ twistboat (2a,5e) ⇄ chair (2a,5a) conformational equilibrium. It is demonstrated that 1-tert-butyltrans-2,5-dimethyl-4-piperidinone hydrochloride has the twist (2a,5e) conformation.