Spiro-2,2 '-bichroman-based bisoxazoline (SPANbox) ligands for Zn-II-catalyzed enantioselective hydroxylation of beta-keto esters and 1,3-diester

A new type of spiro bisoxazoline ligand (SPANbox) has been developed on the basis of the spiro-2,2'-bichroman motif and bisoxazoline chelating units. The Zn(II) complexes of SPANbox showed excellent activity (98-99% yield) and enantioselectivity (90-98% ee) in the catalysis of hydroxylation of various beta-keto esters and 1,3-diester with catalyst loading of 0.1-5 mol%. The performance of SPANbox in the present catalytic system was found to be significantly superior to the commonly used classic bisoxazoline ligands, such as (R,R)-Ph-Box or (S,S)-tBu-PyBox, indicating the advantage of the present spiro ligand skeleton.