CONFORMATIONAL EQUILIBRIUM IN 2,13-DIOXO-11-ALPHA-SPARTEINE

2,13-dioxo-11 alpha-sparteine (13-oxolupanine, 1), a compound obtained by oxidation of lupin alkaloid 13 alpha-hydroxylupanine, was found to occur in solution as a conformational mixture of two forms differing in the conformation of ring C. Analysis of H-1 NMR spectrum shows that the contribution of ''C chair'' conformer (in) is ca. 44%, which seem s to be corroborated qualitatively by H-1, H-1 COSY and NOESY spectra. This unusually high contribution of ''C chair'' conformer in 1, when compared with ca. 10% in lupanine (5), may result from the distortion of ring D by a carbonyl group which decreases the destabilization of t his conformer.