Cooperative B(C6F5)3/Chiral Secondary Amine-Catalyzed Asymmetric [4 + 2] Cycloaddition Reaction.

A highly stereoselective [4 + 2] cycloaddition reaction of pyrazolone derivatives with enals via dienamine/Lewis acidic borane synergistic activation has been developed. The reaction proceeded under mild reaction conditions with a wide functional group tolerance. A series of spirooxindole-pyran-pyrazole derivatives with three contiguous stereocenters were obtained in moderate to good yields with excellent enantioselectivities and diastereoselectivities. This protocol provides the compatibility of chiral secondary amines and B(C6F5)3 catalysts and enriches the synergistic catalytic system of Lewis acidic boranes and chiral organocatalysts.