Access to Fluorazones by Intramolecular Dehydrative Cyclization of Aromatic Tertiary Amides: A Synthetic and Mechanistic Study.

Abstract

An efficient synthesis has been developed for the preparation of 9H-pyrrolo[1,2-a]indol-9-ones (fluorazones) from readily available anthranilic acid derivatives via a one-pot amide- and pyrrole-formation step, followed by an intramolecular cyclodehydration. The cyclodehydration process is mediated by the activation of aromatic tertiary amides by triflic anhydride (Tf2O). Comparison of various benzo-substituents is shown to demonstrate the high functional group tolerance of this transformation. In addition, study of the reaction mechanism is also presented to unfold the exact role of the applied base additive. Herein, as a first example, we report our findings that Tf2O-mediated amide activation is obstructed by the easy protonation of amides by the formed triflic acid during the activation step. Additionally, it has been also proven that the base additive is not involved in the transformation of O-triflyliminium triflates into reactive species (e.g., nitrilium triflates) and is only responsible to neutralize the superacid to avoid the protonation of both the secondary or tertiary amides.