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-Δ¹-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.