Asymmetric Synthesis of 3,3′-Spirooxindoles Fused with Cyclobutanes through Organocatalytic Formal [2+2] Cycloadditions under H-Bond-Directing Dienamine Activation

The first organocatalytic asymmetric synthesis of a spirooxindole skeleton incorporated with a cyclobutane moiety has been successfully developed on the basis of H-bond-directing dienamine activation. Structurally complex spirocyclobutyl oxindoles, which possess four contiguous stereocenters, including one spiro quaternary center, were obtained in good yields (up to 83%) with excellent beta,gamma-regioselectivity (>19:1) and stereocontrol (up to >19:1 dr and 97% ee).