TOTAL SYNTHESIS OF THE YOHIMBINES

A recently introduced scheme of indole alkaloid synthesis has been utilized for the six-step construction of pseudoyohimbine from methyl β-(β-pyridyl)acrylate. Thus the interaction of the A-tryptophyl salt of the latter with dimethyl sodiomalonate, followed by acid-catalyzed ring closure, affords an indoloquinolizidine, whose monodecarbomethoxylation and hydrogenation produce a diester, which on base-induced cyclization and hydrogenation leads to the pentacyclic alkaloid. The interconversion of the intermediate tri-and diesters results in formal total syntheses of also the alkaloids yohimbine, β-yohimbine, alloyohimbine, and «-yohimbine. A 13C NMR analysis of all indoloquinolizidines permits their ready conformational analysis. © 1979, American Chemical Society. All rights reserved.