Gold-Catalyzed Cyclization of Furan-Ynes bearing a Propargyl Carbonate Group: Intramolecular Diels-Alder Reaction with In Situ Generated Allenes

Gold-catalyzed cyclization of various furan-ynes with a propargyl carbonate or ester moiety results in the formation of a series of polycyclic aromatic ring systems. The reactions can be rationalized through a tandem goldcatalyzed 3,3-rearrangement of the propargyl carboxylate moiety in furan-yne substrates to form an allenic intermediate, which is followed by an intramolecular DielsAlder reaction of furan and subsequent ring-opening of the oxa-bridged cycloadduct. It was found that the steric and electronic properties of phosphine ligands on the gold catalyst had a significant impact on the reaction outcome. In the case of 1,5-furan-yne, the cleavage of the oxa-bridge in the cycloadduct with concomitant 1,2-migration of the R-1 group occurs to furnish anthracen-1(2H)-ones bearing a quaternary carbon center. For 1,4-furanyne, a facile aromatization of the cycloadduct takes place to give 9-oxygenated anthracene derivatives.