Intramolecular Diels-Alder Additions of Benzynes to Furans. Application to the Total Synthesis of Biflorin, and the Mansonone-E, I and F

Benzynes, generated either by the debromination of an appropriately substituted (o-dibromobenzene, or by the thermolysis of a substituted diazotized anthranilic acid, have been trapped intramolecularly by an attached furan moiety. One such adduct (41) has been transformed into 6,9-dimethylnaphtho[1,8-bc]pyran-3(2H)-one (43), which served as the key intermediate for the total synthesis of the title compounds, all of which are derivatives of 6,9-dimethylnaphtho[1,8-bc]pyran-7,8-quinone.