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

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 beta-keto esters with N-chlorosuccinimide (NCS) as the chlorination reagent, to give the corresponding a-chloro-beta-keto esters in excellent yields in 530 min with ee values up to 97%. The copper(II) triflate and zinc(II) triflate 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.