ASYMMETRIC-SYNTHESIS OF YOHIMBAN ALKALOIDS - TOTAL SYNTHESIS OF (-)-PSEUDOYOHIMBAN AND (-)-ALLOYOHIMBAN

A route to (-)-pseudo and (-)-allo isomers of yohimban is described. This asymmetric synthetic approach to indole alkaloids is based on the stereocontrolled alkylation of alpha-amino carbanions mediated by chiral form-amidines. The stereochemically pure enantiomer of the alkylated beta-carboline 7 is utilized to bias the subsequent intramolecular Diels-Alder cycloaddition of the N-dienamide 8 or the homologous N-acrylamide 12 derived from the alkylated carboline 11. In this fashion, suitable choice of diene and dienophile partners led to the pseudo-and alloyohimban isomers 1b and 1c, respectively.