Synthesis and Conformational Analysis of All Eight Diastereoisomers of 5-Methylyohimbane

Abstract

Some time ago, Siddiqui et al. proposed structure 1 for the naturally occurring indole alkaloid yohambinine, which they had isolated from Rauwolfia serpentina Benth. In the present paper, enantioselective syntheses of all eight diastereoisomers endowed with the proposed 5-methylyohimbane structure are disclosed. However, none of the synthetically prepared compounds showed spectroscopic properties identical with those reported for the natural product yohambinine, which, therefore, must possess an altogether different constitutional formula. The ground-state conformations of the diastereoisomers 1, 14, 18, 19, 21, 22, 25, and 26 were deduced by spectroscopic methods, and the outcome was compared with the results of extensive force-field, semi-empirical, and ab-initio calculations.