New chiral sulfoxide ligands in catalytic asymmetric Diels–Alder reactions: double acceleration by the chiralities of the sulfoxides and oxazolines

New chiral sulfoxide ligands which are useful for catalytic asymmetric Diels–Alder reactions have been developed. The new ligands involve a chiral sulfinyl function and a 1,3-oxazoline ring with an asymmetric carbon center, in which the chiral sulfinyl group has been revealed to play a crucial role in achieving high enantioselectivity in asymmetric Diels–Alder reactions. Among the Lewis acid catalysts employed, magnesium iodide provided the highest chemical and stereochemical efficiency in the cycloaddition reactions. A mechanistic pathway for the asymmetric synthesis is proposed on the basis of the stereochemical outcomes obtained.