Semisynthesis of Bis-Indole Alkaloid (-)-Melodinine K Enabled by a Combination of Biotransformation and Chemical Synthesis.

Enzymatic biotransformation has become a widely used technique in synthetic chemistry to achieve difficult chemical transformations. Cytochrome P450 monooxygenase enzymes found in nature carry out a wide range of difficult chemical reactions, such as the oxidation of the monoterpene indole alkaloid (-)-tabersonine at the unreactive 16th position on the indoline benzene ring in the biosynthesis of biologically active natural products such as the bis-indole alkaloid (-)-melodinine K. Herein, we describe the first semisynthesis of (-)-melodinine K enabled by a biological gram scale route to the northern fragment, (-)-16-hydroxytabersonine, as well as a chemical route to the southern fragment, (-)-pachysiphine, both derived from (-)-tabersonine and subsequently coupled in only eight linear steps. (-)-16-Hydroxytabersonine is produced through an enzymatic biotransformation with a genetically modified Saccharomyces cerevisiae yeast strain expressing a tabersonine 16-hydroxylase enzyme to enable regioselective oxidation on multigram scale, and (-)-pachysiphine is produced through stereoselective and regioselective epoxidation of the disubstituted alkene.