Bis(oxazoline) and bis(oxazolinyl)pyridine copper complexes as enantioselective Diels-Alder catalysts: Reaction scope and synthetic applications

The scope of the Diels-Alder reaction catalyzed by bis(oxazoline) copper complexes has been investigated. In particular, [Cu((S,S)-t-Bu-box)](SbF6)(2) (1b) has been shown to catalyze the Diels-Alder reaction between 3-propenoyl-2-oxazolidinone (2) and a range of substituted dienes with high enantioselectivity. This cationic complex has also been employed in the catalysis of analogous intramolecular processes with good success. The total syntheses of ent-Delta(1)-tetrahydrocannabinol, ent-shikimic acid, and isopulo'upone, featuring the use of this chiral catalyst in more complex Diels-Alder processes, are described. Similarly, the cationic copper complex 9a, [Cu((S,S)-t-Bu-pybox)](SbF6)(2), is effective in the Diels-Alder reactions of monodentate acrolein dienophiles while the closely related complex, 9d [Cu((S,S)-Bn-pybox)](SbF6)(2), is the preferred Lewis acid catalyst for acrylate dienophiles in reactions with cyclopentadiene.