Ray Structural Investigation of Quinolizidine Alkaloids

The X-ray data available suggest that some alkaloids have a different conformation in solution than in crystal form (41). The presence of a nitrogen atom, capable of inversion, ensures certain conformational mobility of quinolizidine molecules, and the reaction conditions should be considered in establishing their 

Among quinolizidine alkaloids, sparteine and its stereoisomers have been studied in detail by X-ray analysis (42-50). It was demonstrated that proper conformation was not reorganized in monohydrates (42), diperchlorates (43), or methyliodides of a-isosparteine (11) (53). Unlike in the case of a-isosparteine, in spareteine diperchlorate rings C/D appear to have a boat-chair conformation (44-46). On the basis of spectroscopy data a cis conformation for sparteine methyliodide (12) was proposed (57,52). However, radiographic examination (53) of this compound showed it to have the trans conformation (13). 

In p-isosparteine (14) all rings have a chairlike shape (54). Protonation of the N-16 atom makes the distance between N-1 and N-16 equal to 2.61 A, owing to the presence of an intramolecular hydrogen bond. Molecules of sparteine stereoisomers in crystals are sterically conjugated, and in all cases the angles between atoms C-6—C-7—C-17 and C-10—C-9—C-11 were increased to 116-120° (42-50). The B and C rings are flattened at their N termini as a result of noncovalent interaction of atoms with those situated next to them. The conformation of such strained molecules is stabilized by intramolecular hydrogen bonds (46). 

The crystal structures of lupanine (15) and its derivatives were investigated as free bases (54-56) as well as protonated forms (57-60). In all structures examined ring A was a half-chair. The conformation of ring C is a boat, and rings B and D have chair conformations. In other cases rings B, C, and D had chair conformations (54-57,59). Lupanine derivatives mamanine (16) and pohakuline (17), possible metabolites in the biosynthesis of sparteine, were studied by 

X-Ray analysis of the cytisine alkaloid dehydroalbine did not confirm the structural formula determined earlier, and the compound was renamed de- 

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Our appreciation is expressed to F. Kamayev for assistance in writing Section II, i C-NMR Spectroscopy of Quinolizidine Alkaloids, and to B. Ibragimov for assistance in writing Section III, X-Ray Structural Investigation of Quinolizidine Alkaloids. ... 

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