Nickel(II)-Catalyzed Asymmetric Inverse-Electron-Demand Diels–Alder Reaction of 2-Pyrones with Styrenes and Indenes

With electronically unbiased styrenes and indenes as dienophiles, a highly enantioselective all-carbon-based inverse-electron-demand Diels–Alder reaction of electron-deficient 2-pyrones is reported. Using C1-symmetric imidazolidine-pyrroloimidazolone pyridine as the tridentate ligand and Ni(OTf)2 as the Lewis acid, diverse bridged bicyclic lactones were obtained in high yields (88–97% yields), diastereoselectivities (>95:5 dr), and enantioselectivities (90–99% ee). Cyclic enamine and 2,3-dihydrofuran were also suitable dienophiles. DFT calculations supported a concerted 4 + 2 cycloaddition mechanism for the Ni(II) complex-catalyzed reaction with high enantioselectivity caused by steric factors.