A General Strategy for the Synthesis of Vincamajine-Related Indole Alkaloids: Stereocontrolled Total Synthesis of (+)-Dehydrovoachalotine, (−)-Vincamajinine, and (−)-11-Methoxy-17-epivincamajine as Well as the Related Quebrachidine Diol, Vincamajine Diol, and Vincarinol1

Abstract

[structures: see text] The highly convergent stereocontrolled total synthesis of (-)-vincamajinine (7), (-)-11-methoxy-17-epivincamajine (9), and the oxygen-bridged (+)-dehydrovoachalotine (22) are described. Key steps in the synthesis of 7 and 9 involved the stereospecific enolate-driven palladium-catalyzed cross-coupling reaction, a Tollens reaction, an acid-assisted intramolecular cyclization to form the C(7)-C(17) quaternary center, and two stereospecific reductions. The efficiency of this strategy is illustrated by the completion of the synthesis of 7 and 9 in 16 [from d-(+)-tryptophan methyl ester 17] and 17 (from the Schöllkopf chiral auxiliary 27) reaction vessels, respectively. This constitutes the first total synthesis of these indole alkaloids and provides the first regiospecific route to 11-methoxy-substituted ajmaline/vincamajine-related alkaloids. The synthesis of 22 required a novel DDQ-mediated cyclization to furnish the C(6)-O(17) bond, executed in stereospecific fashion. Completion of these syntheses illustrates a concise and versatile strategy for the synthesis of vincamajine-related alkaloids, which has also been employed to prepare the related compounds quebrachidine diol (53), vincamajine diol (56), and vincarinol (59).