A Concise Synthesis of 1-Oxygenated Carbazole Alkaloids, Clausine E and Clausine F

Abstract

A rapid entry into naturally occurring and biologically important 1-oxygenated carbazole clausine E was established through Wittig reaction and annulation. Further O-prenylation of clausine E by Mitsunobu reaction followed by p-Claisen rearrangement delivered clausine F.

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

“…2 We have earlier reported the synthesis of clausine E using Wittig reaction and annulation as the key steps. 14 Argade and coworkers reported the first total synthesis of clausenaline D starting from Bocprotected 3-formylindole and dimethyl maleate featuring Wittig reaction, selective monoalkylation, oxidative cleavage of terminal alkene and dehydrative intramolecular cyclization as the crucial steps. 16 Prior to its isolation clausenapin was synthesized as Huang Minlon reduction product of indizoline.…”

“…11 Unsuccessful attempts were made in the past to synthesize indizoline as well as clausenapin. 17 We realized that a library of 1-oxygenated carbazole alkaloids could be synthesized via a common intermediate 10 which could be obtained from clausine E. Continuing our interest in the application of Wittig reaction in the synthesis of naturally occurring carbazole alkaloids, 14 we now wish to report the synthesis of clausenapin (3) and indizoline (2). An improved synthesis of clausine E (1) and the formal synthesis of clausenaline D were also accomplished.…”