Electronic and steric effects of bis(oxazolinyl)pyridine ligands on asymmetric Diels-Alder reactions

A series of bis(oxazolinyl)pyridine (Pybox) ligands, with different electronic and steric properties were synthesized and evaluated in the Sc(III)catalyzed asymmetric Diels-Alder reaction of alkenoyl-1,3-oxazolidin-2-ones with cyclopentadiene. The results show that electron-withdrawing groups increase the enantioselectivity, which is more significantly influenced by steric effects arising near the metal center. Up to 96% ee was obtained under mild reaction conditions when using a ligand containing the sterically bulky tBu substituent and electron-withdrawing chloride. (c) 2008 Elsevier B.V. All rights reserved.