An Effective Synthetic Entry to Fused Benzimidazoles via Iodocyclization

A protocol for the synthesis of the fused heterocyclic polycyclic compounds pyrrole[1,2-a]benzimidazoles, piperidine[1,2-a]benzimidazoles and oxa-fused benzimidazoles using iodine and silver nitrate by an exo-dig or endo-dig cyclization pathway at room temperature has been developed. Silver nitrate is a key additive for improving the yield, and the improvement is a result of this additive eliminating the influence of iodine ions (I(-)), which would otherwise lead to the formation of bis-iodine by-products. Cyclizations involving terminal and substituted alkynes were performed. Further functionalizations demonstrated that the iodo derivatives obtained are potential synthetic intermediates that can increase the molecular complexity.