Synthesis of dibenzofuran-1,4-diones using the Dötz benzannulation

The chromium Fischer carbene complexes of benzofuran and benzothiophene have been prepared and can be used in Dötz benzannulations with alkynes for the regioselective and converg ent synthesis of dibenzofuran-1,4-dione heterocycles. The use of alkynylboronates led to model systems of the tricyclic ring system of popolohuanone E after oxidation. It would appear that the combination of alkynyl boronates with furan type Fischer carbene complexes leads to substantial amounts (∼50%) of protodeboronated products.