Investigation of the role of stereoelectronic effects in the conformation of piperidones by NMR spectroscopy and X-ray diffraction.

This paper reports the synthesis of a series of piperidones 1-8 by the Mannich reaction and analysis of their structures and conformations in solution by NMR and mass spectrometry. The six-membered rings in 2,4,6,8-tetraphenyl-3,7-diazabicyclo[3.3.

NMR structural study of the prototropic equilibrium in solution of Schiff bases as model compounds.

An NMR titration method has been used to simultaneously measure the acid dissociation constant (pKa) and the intramolecular NHO prototropic constant ΔKNHO on a set of Schiff bases. The model compounds were synthesized from benzylamine and substituted ortho-hydroxyaldehydes, appropriately substituted with electron-donating and electron-withdrawing groups to modulate the acidity of the intramolecular NHO hydrogen bond. The structure in solution was established by 1H-, 13C- and 15N-NMR spectroscopy.

(E)-4-Hy-droxy-2-{[(2-phenyl-eth-yl)iminium-yl]meth-yl}phenolate.

The title Schiff base compound, C(15)H(15)NO(2), crystallized as the iminium-phenolate zwitterion. The H atom is localized on the imine N atom, forming a strong intra-molecular hydrogen bond with the phenolate O atom, and giving rise to an S(6) ring motif. The mol-ecule has an E conformation about the C=N bond.

3-(Piperidin-1-ium-1-yl)-6-azoniaspiro-[5.5]undecane dibromide monohydrate.

The title compound, C(15)H(30)N(2) (2+)·2Br(-)·H(2)O, was synthesized by reaction of 4-piperidino-piperidine with dibromo-pentane. The dication is built up from three linked piperidine rings, two of which have one quaternary N atom in common (azoniaspiro), whereas the third is N-C bonded to the azoniaspiro system and protonated on the N atom (piperidinium). All three piperidine rings adopt chair conformations.