Cationic Copper(II)-Oxazoline-Sulfoxide Catalysts: Application to Asymmetric Diels-Alder Reactions

New chiral sulfoxides bearing a chiral 1,3-oxazoline ring were prepared and used as chiral ligands in copper(Ⅱ)-catalyzed Diels-Alder reactions.The copper(Ⅱ)-catalyzed cycloaddition reactions using the new chiral ligands were carried out at -78℃ to afford adducts with rather high (up to 75 %) enantiomeric excess.Introduction of a counterion (triflate or hexafluoroantimonate) into the catalysts represented higher degree of asymmetric induction; namely, the more cationic copper( Ⅱ )-oxazoline sulfoxide catalysts improved the enantioselectivity in the cycloaddition reaction.An asymmetric Diels-Alder reaction is one of the most powerful and versatile methods for the creation of six-membered rings in organic synthesis. 1Catalytic enantioselective processes with chiral Lewis acid-derived catalysts significantly extended the scope and utility of this reaction. 2,3In the design of chiral Lewis acidic catalysts, the choice of a matched chiral ligand for the reactions is seriously crucial for the achievement of the superior endo/exo selectivity as well as the endo enantioselectivity. 4Hitherto we have developed a number of chiral sulfoxide ligands in transition metal-catalyzed asymmetric reactions, in which palladium-catalyzed asymmetric allylic alkylations with them were exemplified. 5 We wish to communicate herein the synthesis of O-N type ligands (2 and 3) containing both chiral sulfoxide bearing a bulky aryl group (2-methoxy-1-naphthyl) and 1,3-oxazoline functions, and their application as the chiral ligands in copper(Ⅱ)-catalyzed enantioselective Diels-Alder reactions. 6