Bioinspired Divergent Oxidative Cyclization of Strictosidine and Vincoside Derivatives: Second Generation Enantioselective Total Synthesis of (–)-Cymoside
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We report our second generation synthesis of (–)-cymoside as well as the formation of a new hexacyclic-fused furo[3,2-b]indoline framework. After a Pictet-Spengler condensation between secologanin tetraacetate and tryptamine, the course of the cyclization of the 7-hydroxyindolenine intermediate generated by oxidation with an oxaziridine, depends on the stereochemistry of the 3-position. The 3-(S)-strictosidine stereochemistry delivered efficiently the scaffold of cymoside via intramolecular coupling with the C16-C17 enol ether, while the 3-(R)-vincoside stereochemistry directed towards the reaction with the C18-C19 terminal alkene and the formation of the unexpected caged compound.