Determination of the orientation of the phenyl substituent in isomeric 1-ethyl-2,5-dimethyl-4-phenyl-4-piperidinols by two-dimensional nuclear overhauser effect (NOESY) spectroscopy. Conformation of the β isomer of 1,2,5-trimethyl-4-phenyl-4-piperidinol

In the case of three isomers (α, Β, and γ) of 1-ethyl-2,5-dimethyl-4-phenyl-4-piperidinol it is shown that NOESY spectroscopy can be successfully used in the determination of the orientation of the phenyl substituent. It was observed that the chemical shifts of the protons and the carbon atoms in the ortho position and of the quaternary C atom of the phenyl substituent and the {Mathematical expression}, direct spin-spin coupling constant (SSCC) satisfactory reflect the change in the orientation of the phenyl substituent on passing from the a isomer to the β and γ isomers. A chair 2a 5e) ⇄ twist-boat (2e, 5e) conformational equilibrium in which the twist-boat conformation is stabilized by an intramolecular hydrogen bond is proposed for the Β isomer of 1,2,5-trimethyl-4-phenyl-4-piperidinol on the basis of the NMR data. © 1991 Plenum Publishing Corporation.