Asymmetric Synthesis of Multi-substituted Prolines via a Catalytic 1,3-Dipolar Cycloaddition Using a Monocationic ZnIIOAc Complex of a Chiral Bisamidine Ligand, Naph-diPIM-dioxo-R

A monocationic zinc acetate complex of chiral bisamidine-type bidentate ligand (R)- or (S)-Naph-diPIM-dioxo-iPr (L(R)orL(S)) catalyzes a 1,3-dipolar cycloaddition between tridentate-type imino esters and acrylates in the absence of an external base to give the corresponding multi-substituted prolines with high reactivity, diastero-/enantio-/regio-selectivity, productivity, and broad generality. An anionic acetate ligand of the Lewis acidic Zn complex acts as a Bronsted base to facilitate a smooth intramolecular deprotonation to generate an iminoN,O-cis-Zn enolate. The oxygen atoms of two dioxolanes of theL(R)/L(S)ligand form aC(2)chiral scaffold that coordinates the Zn enolate via a non-bonding n-pi* interaction. This view is supported by the lack of enantioselectivity obtained with an sp(2)N-based Ph-BOX ligand that has no oxygen atom in the reaction site.