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.