Total synthesis of the alkaloids (±)-alpinigenine and (±)-cis-alpinigenine

Abstract

A stereoselective synthesis of the rhoeadine alkaloid (±)-cis-alpinigenine is described. Hofmann elimination of the known tetracyclic base (4) gave the trans-azecine (7) which afforded the diol (13) on oxidation with N-bromo-succinimide. Periodic acid cleavage of (13) yielded the dibenzaldehyde (14), which on photolysis resulted in the formation of (±)-cis-alpinigenine (20–30%) and (±)-alpinigenine (1%) involving endo(16) and exo(15)(4s + 2s) intramolecular additions respectively of the intermediate photodienol-E (Scheme 1).