An Iminium Ion Cascade Annulation Strategy for the Synthesis of Akuammiline Alkaloid Pentacyclic Core Structures

The akuammiline alkaloids
are a family of indole monoterpene natural
products known for their polycyclic cage-like structures. An iminium
ion cascade annulation approach was developed, simultaneously synthesizing
both the C and D rings of these natural products by annulation onto
a protected indole ring. This reaction allowed the synthesis of a
key tetracyclic intermediate toward these natural products. This tetracycle
was used for the synthesis of the pentacyclic methano­quinolizidine
core present in such alkaloids as akuammiline and strictamine as well
as the pentacyclic furoindoline core found in pseudo­akuammigine.