DMAP-Catalyzed [3+3] Annulation of Cyclopropenones with α-Bromoketones for Synthesis of 2-Pyrones

DMAP-catalyzed [3+3] annulation of cyclopropenones with α-bromoketones is described, which provides a simple and convenient synthesis of 2-pyrones in good yields with a broad scope. The reaction features advantages of transition metal-free conditions, readily available starting materials, and excellent regioselectivity. Experimental investigation and DFT calculations suggest a mechanism encompassing pyridium ylide-initiated ring opening of cyclopropenones, elimination of DMAP catalyst, and final 6π-electrocyclization.