Protonation effect on chemical shifts of some piperidones ⎯ unusual influence by anions

H and C NMR spectra are recorded for twelve picrate derivatives 4-15 derived from some 3-alkyl-2,6diarylpiperidin-4-ones and 3,5-dimethyl-2,6-diarylpiperidin-4-ones. The difference in the chemical shift of equatorial methylene proton and axial methylene proton at C(5) [Δ = δeq − δax] is highly negative in 4-13 which is in contrast to the value observed in the corresponding parent piperidin-4-ones and this is attributed to the syn 1,3-diaxial interaction between the axial N-H bond and axial hydrogen at C-5. The effect of protonation on the chemical shifts was studied in detail. The chemical shifts of the heterocyclic ring protons are influenced by the picrate anion.