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Abstract
Sarpagine-Ajmaline-Koumine type monoterpenoid indole alkaloids represent a fascinating class of natural products with polycyclic and cage-like structures, interesting biological activities, and related biosynthetic origins. Herein we report a unified approach towards the asymmetric synthesis of these three types of alkaloids, leading to a collective synthesis of 14 natural alkaloids. Among them, akuammidine, 19-Z-akuammidine, vincamedine, vincarine, quebrachidine, vincamajine, alstiphylianine J, and dihydrokoumine are accomplished for the first time. Features of our synthesis are a new Mannich-type cyclization to construct the key indole-fused azabicyclo[3.3.1]nonane common intermediate, a SmI2 mediated coupling to fuse the aza-bridged E-ring, stereoselective olefinations to install either the 19-E or 19-Z terminal alkenes presented in the natural alkaloids, and an efficient iodo-induced cyclization to establish the two vicinal all-carbon quaternary centers in the Koumine-type alkaloids.
Highlights
Sarpagine-Ajmaline-Koumine type monoterpenoid indole alkaloids represent a fascinating class of natural products with polycyclic and cage-like structures, interesting biological activities, and related biosynthetic origins
Synthetic efforts have resulted in a number of elegant strategies and culminated with the synthesis of a series of sarpagineajmaline-koumine type alkaloids
Numerous synthetic methods have been developed for sarpagine-ajmaline alkaloids as well as structurally related analogues, flexible and unified synthetic routes that lead to collective synthesis of these types of alkaloids, especially towards both 19-E and 19-Z isomers, are rarely documented[17–25]
Summary
Sarpagine-Ajmaline-Koumine type monoterpenoid indole alkaloids represent a fascinating class of natural products with polycyclic and cage-like structures, interesting biological activities, and related biosynthetic origins. The sarpagine alkaloids feature a polycyclic ring system with an azabicyclo[3.3.1]-nonane core These alkaloids serve as the biogenetic precursors for the more complex ajmaline and koumine type indole alkaloids[1–3]. Because of their characteristic indole-fused azabicyclo[3.3.1]-nonane structures and prominent biological activities, sarpagine-ajmaline-koumine related alkaloids have attracted attention from the organic synthetic community for decades[17–25]. Numerous synthetic methods have been developed for sarpagine-ajmaline alkaloids as well as structurally related analogues, flexible and unified synthetic routes that lead to collective synthesis of these types of alkaloids, especially towards both 19-E and 19-Z isomers, are rarely documented[17–25]
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