On the structure and spectroscopic properties of two 13-hydroxylupanine (cas 15358-48-2) epimers

Add time:08/07/2019    Source:sciencedirect.com

The crystal structures of 13α- and 13β-hydroxylupanines present a rare case of unexpected configuration and conformation of bis-quinolizidine alkaloid free bases. The configuration of A/B and C/D ring junctions is trans–cis and quasi trans–cis in α and β 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 CO groups. In spite of different orientations of the OH groups - an axial or an equatorial in α and β 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 13α- and/or 13β-hydroxylupanine molecules and the solvent CDCl3 molecules as well as the N16⋯H–O association have been found to play a crucial role.

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