Enantioselective Synthesis of Indole-Fused Dihydropyranones via Catalytic Cycloaddition of Ketenes and 3-Alkylenyloxindoles

Chiral N-heterocyclic carbenes were found to be efficient catalysts for the formal [4+2] cycloaddition reaction of alkyl(aryl)ketenes and 3-alkylenyloxindoles to give the corresponding 3,4-dihydropyrano[2,3-b]indol-2-ones in excellent yields with good diastereo- and enantioselectivities.