Catalytic asymmetric epoxidation of α,β-unsaturated esters with chiral yttrium-biaryldiol complexes

The full details of the asymmetric epoxidation of α,β- unsaturated esters catalyzed by yttrium complexes with biaryldiol ligands are described. An yttrium-biphenyldiol catalyst, generated from Y(OiPr) ₃-biphenyldiol ligand-triphenylarsine oxide (1:1:1), is suitable for the epoxidation of various α,β-unsaturated esters. With this catalyst, β-aryl α,β-unsaturated esters gave high enantioselectivities and good yields (≤99% ee). The reactivity of this catalyst is good, and the catalyst loading could be decreased to as little as 0.5-2 mol % (the turnover number was up to 116), while high enantiomeric excesses were maintained. For β-alkyl α,β-unsaturated esters, an yttrium-binol catalyst, generated from Y-(OiPr)₃-binol ligand-triphenylphosphine oxide (1:1:2), gave the best enantioselectivities (≤97% ee). The utility of the epoxidation reaction was demonstrated in an efficient synthesis of (-)-ragaglitazar, a potential antidiabetes agent.