Total Synthesis of Alstilobanine C, Undulifoline and Alstilobanine B

The total synthesis of pentacyclic monoterpene indole alkaloids is the focus of this thesis. Total syntheses of (±)-alstilobanine C, (±)-undulifoline and (±)-alstilobanine B, three natural products from the Ulean family of alkaloids, were accomplished. The characteristic features of our synthesis included: a) a domino sequence involving a highly diastereoselective Michael addition followed by chemoselective C-acylation of the resulting enolate by Mander¿s ester; b) an unprecedented Lewis acid mediated intramolecular addition of enolate to oxonium, generated in situ from MOM protective group; c) a sequence of chemoselective reduction/indolization/intramolecular aza-Michael addition. This domino process afforded directly the (±)-alstilobanine C, a pentacyclic monoterpene indole alkaloid in 87% yield. Reductive N-methylation of (±)-alstilobanine C provided (±)-undulifoline which, upon N-oxidation, was further converted to (±)-alstilobanine B.

Enantioselective total synthesis of the same natural products through a different synthetic route is presented. After exploring different unsuccessful strategies, an enzymatic desymmetrization of a meso compound was adopted for the synthesis of enantioenriched cyclopentenone intermediate. Applying the two domino sequence developed previously, namely, Michael addition/C-acylation and oxepane formation, an efficient synthesis of the oxobicyclo[4.2.1]nonane was realized. A palladium-catalyzed decarboxylative cross-coupling, a methodology developed in our group, afforded an advanced bicyclic intermediate with all the carbon and functional groups needed to accomplish the total synthesis. However, conversion of the vinyl group to alkylazide or its synthetic equivalent proved to be difficult in spite of the efforts dedicated to this seemingly trivial transformation.