Convergent Total Synthesis of Erchinines A and B and Their C20 Epimers.

The total synthesis of two monoterpenoid indole alkaloids (MIAs), erchinines A and B, is described. Isolated from the plant Ervatamia chinensis, both compounds display antimicrobial activity against Trichophyton rubrum and Bacillus subtilis. Their structures are composed of a unique caged system, bearing a 1,4-diazepine fused oxazolidine moiety and containing three successive N,O-acetals. Key features of the synthesis include the construction of the 2-aza-3-oxobicyclo[2.2.2]octene core via an intramolecular pyridone Diels-Alder reaction and a Zr-mediated bisamide reduction that induces a tandem N,O-acetal forming cascade reaction.