Sequential Visible-Light and N-Heterocyclic Carbene Catalysis: Stereoselective Synthesis of Tetrahydropyrano[2,3-b]indoles.

An efficiently stereoselective [4 + 2] cycloaddition of 3-alkylenyloxindoles and α-diazoketones through sequential visible-light photoactivation and N-heterocyclic carbene catalysis was achieved. A series of tetrahydropyrano[2,3-b]indoles with an all-carbon quaternary stereocenter were obtained in good yields with excellent diastereo- and enantioselectivities.