Total Syntheses of Catharanthine and Keramaphidin B by an Iridium-Catalyzed Reductive Cyclization Cascade

ended

During the course of this Fellowship a reductive cyclization cascade for dihydropyridinones, which consists of an iridium-catalyzed reduction to the corresponding electron-rich dienes and subsequent [4+2] cycloaddition (Diels-Alder reaction) will be developed. A first promising proof of concept study establishing the feasibility of this powerful transformation has already been carried out in the Dixon group; however further optimization by varying the solvent, the reaction time, the temperature or by using suitable Lewis acids is required to perfect the methodology. The additional use of chiral Lewis acid or chiral amine catalysts is also intended to develop an enantioselective variant of this method. The methodology development should then be completed by determining the substrate scope. The resulting azabicyclo[2.2.2]octene motif is an important structural component of various alkaloids. Therefore, this reaction cascade should then be used as a late stage key step in the total syntheses of the natural products catharanthine and keramaphidin B. Catharanthine is an important member of the large class of iboga alkaloids, which usually carry as common structural feature an indole or an indoline moiety. Catharanthine can be considered as a direct precursor of the dimeric Catharanthus alkaloids vinblastine and vincristine, both of which are clinically valuable agents in the treatment of cancer due to their ability to inhibit mitosis the formation of microtubules. The marine alkaloid keramaphidin B belongs to the manzamine family and shows cytotoxicity against P388 leukaemia cells in mice (IC50 0.28 μg/mL) and against epidermal KB cancer cells in humans (IC50 0.30 μg/mL). Structurally, keramaphidin B is a pentacyclic compound comprising two piperidine rings (one bridged and one unbridged) and two macrocycles. Furthermore, this alkaloid has two (Z)-configured and one triple-substituted double bond, as well as four stereogenic centers, one of which is quaternary.