Metal-Free Radical Carbamoylation/Cyclization for Divergent Synthesis of Six- to Eight-Membered Benzimidazole-Fused Heterocycles.

A metal-free radical tandem carbamoylation/cyclization strategy for the divergent synthesis of benzimidazole-fused heterocycles has been described. Using commercially available oxamic acids and imidazoles tethered with unactivated alkenes under mild conditions, this method provides the first efficient access to stable [6,5,7]- and [6,5,8]-fused frameworks along with [6,5,6] systems. It exhibits excellent functional group tolerance and broad substrate scope, including 2-heteroarylbenzimidazoles. Preliminary studies also revealed that monoalkyl oxalates can serve as effective radical precursors in this transformation, offering potential for further derivatization of the products. The method is also applicable to the late-stage diversification of bioactive molecules, such as theophylline.