A concise total synthesis of sespenine, a structurally unusual indole terpenoid from Streptomyces

Abstract

Sespenine is a structurally unusual indole sesquiterpenoid isolated from endophytic Streptomyces sp. HKI0595. Herein, we report a ten-step (the longest linear sequence) synthesis of this molecule from commercially available materials, on the basis of our first generation synthesis. Sharpless asymmetric epoxidation and Stille-Miyata coupling were used to construct a functionalized epoxy ester, which underwent Ti(iii) mediated reductive radical cyclization to give a trans-decalin intermediate with a 2-methoxycarbonylindole side chain. Oxidation of this compound afforded a pair of epimeric 3-hydroxyindolenines, and the major isomer entered a bioinspired cascade of Prins cyclization/Friedel-Crafts/retro Friedel-Crafts under acidic conditions, to furnish the polycyclic core of sespenine. Sespenine analogues bearing different C2 substituents were prepared with similar chemistry. Xiamycin A, a carbazole congener of sespenine, was synthesized from the minor hydroxyindolenine epimer as well.