Access to Both Ehantiomers of α-Chloro-β-keto Esters with a Single Chiral Ligand: Highly Efficient Enantioselective Chlorination of Cyclic P-Keto Esters Catalyzed by Chiral Copper(II) and Zinc(II) Complexes of a Spiro-2,2'-bischroinan-Based Bisoxazoline Ligand

摘要

The spiro-2,2'-bichroman-based chiral bisoxazoline ligands (SPANbox) were found to be highly efficient in copper(II)- and zinc(II)-catalyzed asymmetric chlorinations of cyclic (3-keto esters with N-chlorosuccinimide (NCS) as the chlorination reagent, to give the corresponding α-chloro-β-keto esters in excellent yields in 5-30 min with ee values up to 97%. The copper(II) triflate and zinc(II) tri-flate complexes of a single SPANbox ligand demonstrated complementary results to each other with respect to the enantioselection, affording both antipodes of the chlorinated product enantiomers with good to excellent optical purities.