Further studies on the stereochemistry of sparteine, its isomers and derivatives : Part. I. Synthesis, structure and properties of 16,17-endo-methylene-lupaniumm perchlorate, 17-β-methyllupanine and 17-β-methylsparteine

The reduction of the product (V) obtained from the reaction of δ^16,17-dehydro- lupaninium perchlorate with diazomethane leads to 17-β-methyllupanine (III) (NaBH₄- reduction) or to 17-β-methylsparteine (X) (LiAlH₄-reduction). The structures of III and X were determined by spectroscopic methods (NMR and IR) and further proved by chemical methods. The β-methyl group at C-17 has a pronounced effect on the basicity of III and X. The basicity of in is higher than that of its parent lupanine by about 1.5 pK^*_MCSunits which can be explained by additional stabilization of the C/D boat-chair conformation in III and better solvation conditions than in lupanine. The same effect leads to a decrease of the δpK^*_MCS-value of the diamine X with reference to that of sparteine by ca. 2.6 units. The methyl group in X stabilizes the C/D boat-chair conformation and hinders the formation of an intramolecular hydrogen bond in monoprotonated X, consequently the addition of a second proton is favoured and it is assumed that the diprotonated cations of X and sparteine have different conformations. The reaction intermediate 16,17-endo-methylenelupaninium perchlorate (V) was isolated and its structure was determined.