Pentafluorophenol-Catalyzed Metal-Free Fischer Indole Synthesis: A Novel Approach to Carbazole Derivatives and Desbromoarborescidine A*

Abstract

Carbazoles represent significant heterocyclic compounds with a wide range of applications in organic synthesis. Conventional techniques, such as the Fischer indole synthesis, have been extensively utilized for the construction of these structures. In this study, we report the first use of pentafluorophenol (PFP) as a catalyst in a metal-free approach for the synthesis of carbazoles. The strong electron-withdrawing properties and low pKa of PFP facilitate the effective activation of electrophiles, leading to improved yields and selectivity. A comprehensive exploration of substrate diversity has been conducted, resulting in the synthesis and thorough characterization of various carbazole derivatives using ¹H NMR, ¹³C NMR, DEPT-135, IR, and mass spectrometry. A scale-up synthesis was also executed to validate the method‘s scalability. Additionally, a natural carbazole alkaloid was synthesized, followed by post-synthetic modifications to enhance the diversity of the compound library. This approach provides a valuable alternative to traditional acid-catalysed methods, thereby broadening the synthetic avenues for carbazoles and their derivatives.