The First Modular Route to Core-Chiral Bispidine Ligands and Their Application in Enantioselective Copper(II)-Catalyzed Henry Reactions

The first modular and flexible synthesis of core-chiral bispidines was achieved by using an inside-out strategy. The key intermediate, a NBoc-activated bispidine lactam, was constructed in enantiomerically pure form from a chirally modified beta-amino acid and 2-(acetoxymethyl)acrylonitrile in just five steps and good 48% yield. A simple addition-reduction protocol permitted a highly endo-selective introduction of substituents and, thus, a fast and variable access to 2-endo-substituted and 2-endo, N-fused bi- and tri-cyclic bispidines. The new diamines were evaluated as the chiral ligands in asymmetric Henry reactions. Excellent enantioselectivities of up to 99% ee and good diastereomeric ratios of up to 86: 14 were reached with a copper(II) complex modified by a 2-endo, N-(3,3-dimethylpyrrolidine)-annelated bispidine. Its performance is superior to that of the well-known bispidines (-)-sparteine .and the (+)-sparteine surrogate.