Bifunctional N-heterocyclic carbene catalyzed [3+4] annulation of enals and aurones.

Bifunctional N-heterocyclic carbenes with a free hydroxy group are demonstrated as efficient catalysts for the [3+4] annulation of enals with aurones to give the corresponding benzofuran-fused epsilon-lactones in good yields with good diastereoselectivities and excellent enantioselectivities. Control experiments reveal that the [3+4] cycloadducts are kinetically favored and could be transformed to the thermodynamically favored [3+2] cycloadducts with a non-bifunctional NHC catalyst.