NMR study of 4-hydroxy-1-methylpiperidine betaine derivatives

Conformations of two 1-carbethoxymethyl-4-hydroxy-1-methylpiperidinium chlorides (1α and 1β), 4-hydroxy-1-methylpiperidine betaine hydrochlorides (2α and 2β), and 4-hydroxy-1-methylpiperidine betaine inner salts (3α and 3β), having a hydroxyl group at axial (α) or equatorial (β) positions, have been studied by the 1H and 13C NMR spectroscopy. The signals attributed to the equatorial and axial protons at C-2,6 have been found to differentiate the appropriate conformers. The NOE effect has been used to assign the equatorial and axial protons at C-3,5. The C-2,6 carbon atoms in the α-conformers have been shown to be shielded by the axial 4-hydroxyl group due to the γ-gauche substituent effect. The magnetic isotropic shielding tensors have been calculated by GIAO/B3LYP/6-31G(d,p) method and correlated with the experimental chemical shifts.