Divergent Access to Polycyclic Fluorenofuran Systems via Palladium-Catalyzed Cascade Furan-Participated Diels-Alder Cycloaddition Reactions

The Pd-catalyzed reaction of 1-(2-iodophenyl)-3-arylprop-2-yn-1-ones with benzene-tethered furan-ynes produces novel fused hexacyclic indenone-furan scaffolds, while the reaction with propargyl furfuryl ethers affords the pentacyclic indenone-spirodifuran system. The thiophene-containing alkynes also proceed by similar transformations. A mechanism involving the Pd-catalyzed coupling and propargylic Alder-ene reaction to generate the indenone-allene intermediate, followed by an intramolecular furan/thiophene-participated inverse electron-demand Diels-Alder reaction, is proposed.