KCS

Ngouniensine, isolated and structurally determined in 1982, is one of the major indole alkaloids of Strychnos ngouniensis. Unlike other indole alkaloids, it has unusual structural features; ngouniensine is a tetracyclic scaffold and possesses 7-membered ring. In addition, ngouniensine has an exocyclic methylene group bridged to the C-2 position of the indole scaffold and the C-3 position. Furthermore, the two hydrogens at the C-3 and C-20 positions have a cis-relationship. The previous total synthesis of ngouniensine commenced with a starting material bearing the piperidine scaffold with the desired substituents and right stereochemistry. The indole was introduced by the SN2 reaction of the piperidine with tryptophyl bromide. Subsequent intramolecular Friedel-Crafts type reaction allowed the seven-membered ring formation. The final introduction of the methylene completed the synthesis of ngouniensine. However, this strategy generally required a lengthy sequence and the control of the stereochemistry was often challenging. In this poster presentation, a novel strategy proposal for the synthesis of ngouniensine will be presented. Pyridine containing substituents at the C-3 and C-20 positions was used as a surrogate for the key piperidine scaffold. Alkylation of ethenone compound with 2-aminocinnamate and ring formation afforded the corresponding indoline intermediate. Subsequent indolization will allow 2-(2-pyridylcarbonyl) indole-3-acetate. After that, through the condensation and reduction will be utilized as a precursor of the piperidine ring. Syn-selective reduction of the pyridinium ring will afford the piperidine ring with right stereochemistry, and the Wittig reaction of the carbonyl group to methylene group will complete the total synthesis of ngouniensine.