N-Heterocyclic carbene-catalyzed [4 + 2] cyclization of α-chloroaldehydes and aurones: Highly enantioselective synthesis of benzofuran-fused dihydropyran-2-ones

The chiral N-heterocyclic carbene-catalyzed [4 + 2] annulation of α-chloroaldehydes and aurones was developed, giving the corresponding benzofuran-fused dihydropyranones in good to high yields with good cis-selectivities and excellent enantioselectivities. The catalytic cycle features with the generation of enolate from chloroaldehdye and its following [4 + 2] cycloaddtion with aurones.