Total synthesis of naturally occurring 1-oxygenated carbazole alkaloids - clausine E, clausenapin, indizoline and formal synthesis of clausenaline D

Abstract

Total synthesis of indizoline and clausenapin have been accomplished via a facile functional group transformation of the ester functionality at the C-3 carbon of the key ester intermediate derived from clausine E by employing o-Claisen rearrangement and Wittig homologation as the key steps. An improved synthesis of clausine E was achieved using Eaton's reagent for the annulation step. O-allylation of clausine E followed by o-Claisen rearrangement and acetylation enabled the formal synthesis of clausenaline D.