Cooperative B(C ₆ F ₅ ) 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(C 6 F 5 ) 3 catalysts and enriches the synergistic catalytic system of Lewis acidic boranes and chiral organocatalysts.