Synthesis of imidazole-fused pentacyclic pyrrolo[3,4-c]coumarins via base-promoted rearrangement of coumarin-substituted N-heterocyclic carbene precursors

Abstract

Pentacyclic pyrrolocoumarin scaffolds are unique structural motifs that form the backbone of many bioactive compounds. Herein, we report the first-time synthesis of imidazole-fused pentacyclic pyrrolo[3,4-c]coumarin derivatives (2) through the stoichiometric reaction between coumarin-substituted N-heterocyclic carbene (NHC) precursors (1) and Pd(OAc)₂, in the presence of an external base. Control experiments revealed that the base-promoted rearrangement of the NHC precursors yields N-(alkyl)aniline-substituted pyrrolo[3,4-c]coumarin derivatives (3), wherein the imidazole ring is opened. Additionally, the formation of 2via the reaction of 3 and Pd(OAc)₂ indicated that the formation of pentacyclic pyrrolo[3,4-c]coumarin occurs via the base-promoted rearrangement product of NHC precursors.