Spiro-2,2′-bichroman-based bisoxazoline (SPANbox) ligands for ZnII-catalyzed enantioselective hydroxylation of β-keto esters and 1,3-diester

Abstract

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 β-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.