On the structure and spectroscopic properties of two 13-hydroxylupanine epimers

The crystal structures of 13a- and 13b-hydroxylupanines present a rare case of unexpected configuration and conformation of bisquinolizidine alkaloid free bases. The configuration of A/B and C/D ring junctions is trans–cis and quasi trans–cis in a and b epimers, respectively. Ring C adopts a distorted chair form in both of them which is unusual for lupin alkaloids free bases. Such a drastic rearrangement of electronic configuration at N16 is caused by intermolecular hydrogen bonds formation between the OH and CaO groups. In spite of different orientations of the OH groups - an axial or an equatorial in a and b epimers, respectively, the hydrogen bond geometry is the same in both crystal structures. This causes different distortions in all four rings of bis-quinolizidine skeletons. The Duax-Norton approach has been used to analyze them. In solution, both epimers occur in conformational equilibrium with considerable domination of the C-boat conformer (85–90%). This conformation is stabilized by the other type of intermolecular hydrogen bonds in comparison with that operating in the solid. Intermolecular deuterium bonds between 13a- and/or 13b-hydroxylupanine molecules and the solvent CDCl3 molecules as well as the N16/H–O association have been found to play a crucial role. q 2005 Elsevier B.V. All rights reserved