Base-Promoted [4 + 1]/[3 + 1 + 1] Bicyclization for Accessing Functionalized Indeno[1,2‑c]furans

A novel three-component bicyclization strategy for the metal-free synthesis of densely functionalized indeno­[1,2-c]­furans with generally good yields has been established from readily accessible o-phthalaldehydes (OPA), isocyanides, and α-diazoketones. The reaction pathway involves aldol-type addition, 1,2-hydride shift, 5-exo-trig cyclization, and 1,4-addition as well as an oxo-5-exo-dig cyclization sequence, resulting in continuous multiple bond-forming events including C–C and C–O bonds to rapidly build up functional oxo-heterocycles.