Détermination de la Structure Cristalline et de la Configuration Absolue du Bromhydrate d'Usambarensine

Usambarensine is a bisindolinic alkaloid found in the bark of Strychnos usambarensis roots. The crystal and molecular structure of usambarensine.2HBr.2H2O has been solved by direct methods thanks to a Hilger-Watts four-circle diffractometer. The crystals are monoclinic; refinement of the parameters was carried out with 2459 reflexions. The absolute configuration of the molecule, determined by X-ray anomalous scattering, confirms the 3 alpha cis configuration ( 11alpha cis, IUPAC) assigned earlier by spectral, chromatographic and biogenetic data.This configuration seems dependent on the presence of an ethylidenic chain in the corynane part of the molecule.