Access to Chiral 2,5-Pyrrolidinyl Dispirooxindoles via Dinuclear Zinc-Catalyzed Asymmetric Cascade Reactions

A series
of new nonsymmetric semi-azacrown ether ligands were developed
and applied to the asymmetric Michael/cyclic keto-imine formation/Friedel–Crafts
alkylation reactions of 3-amino oxindole hydrochlorides and β,γ-unsaturated
α-keto amides. A diversity of 2,5-pyrrolidinyl dispirooxindoles
containing two nonadjacent spiro-quaternary stereocenters were obtained
in excellent diastereoselectivities and moderate to excellent enantioselectivities
(up to 95% ee). A possible catalytic cycle was proposed to explain
the origin of the asymmetric induction.