Synthesis of Dibenzofurans by Rhodium-Catalyzed [2+2+2] Cycloaddition

Significance

Reported is a flexible and convenient synthesis of fused benzofuran derivatives under mild conditions from phenol-linked 1,6-diynes with acetylenes under Rh-catalyzed conditions. The reaction is based on methodology developed previously by Tanaka (Synlett, 2007, 1977) and is suggested to proceed via a cationic rhodium-catalyzed [2+2+2] cycloaddition process. Some reactions proceed with poor regioselectivity (benzofurans 3 and 4, from alkynes) or with meta-regioselectivity (enol ether 6). Using the same conditions in reactions of 1 with activated or unactivated nitriles 7 leads to the formation of azabenzofurans 8 with excellent regioselectivity but variable yields.