Highly diastereo and enantioselective [4+2] cycloadditions have been achieved between pyrrolidone-derived cyclic enones and α-haloaldehydes under mild conditions. Relying on extremely reactive in-situ generated chiral N-heterocyclic carbenes, this stereoselective annulation proceeds efficiently even on the gram scale with the catalyst loading as low as 0.025 mol% (250 ppm). A variety of cis-substituted bicyclic dihydropyranones can be produced in up to 96% yield with up to >99% ee. In addition, simple, inexpensive linear aldehydes such as n-propanal can be used directly in asymmetric cycloadditions via oxidative N-heterocyclic carbene organocatalysis with low catalyst loading. This method may provide an economical and practical approach for the asymmetric synthesis of medicinally relevant molecules.