Synthesis of Furo[2,3-f]dibenzotropones via Intramolecular Wittig Reaction of Alkylidene Dibenzo-β-tropolones

A strategy was developed for synthesizing furan-fused dibenzotropones via intramolecular Wittig reactions of alkylidene dibenzo-β-tropolones and acylating reactants in catalytic or stoichiometric conditions. With these protocols, a series of furo[2,3-f]dibenzotropones were obtained in 49–99% yields within 1–10 hours. Mechanistic studies of the base-free phenomena revealed that the organophosphane acts both as a mediator and a regenerable Brønsted base under heating.