Clarification of the Vinpocetine structure by X-ray analysis

Abstract

The structure of a semi-synthetic alkaloid Vinpocetine was clarified with the use of the X-ray diffraction method with a lower value of the divergence factor (R-factor) compared to that previously described in the literature. The Vinpocetine crystal was obtained by slow evaporation of its solution in methanol. According to the X-ray diffraction data, the fragment of the linear tetrahydrocarboline system of the Vinpocetine molecule is characterized by nitrogen atom N(2) of the hydrogenated pyridine ring exiting from the common plane of the remaining atoms by 0.653 Å. In its turn, the angular tetrahydropyrido[1,2-a]indole system is distinguished by carbon atoms C(13) and C(14) exiting from the plane by 0.806 and 0.327 Å, respectively. Yet another six-membered cycle is located at the 83.9 ° angle in relation to the tetracyclic system of indolo[3,2,1-de][1,5]naphthyridine and has a chair conformation. In the present study the torsion angle of the C(12)-C(13)-C(14)-C(15) bonds (–85.5°) and the orientation of the ethyl substituent, characterized by the corresponding C(14)-C(13)-C(18)-C(19) torsion angle (67.65°) are redefined. The main structural motif of organization of molecules in the crystal is formation of double ribbons along the b axis by means of weak hydrogen bonds of the C-H…O type (the D-H…A distance is 2.66 Å, DHA 121°, the D…A distance is 2.27 Å).