Additives and Chiral Catalysts

A different and more useful approach is to prepare a chiral catalyst that will provide facial selectivity when it coordinates to the substrate (usually the dienophile) to form a chiral transition state containing the transient 

Yamamoto reported an asymmetric hetero-Diels-Alder reaction that was catalyzed by a chiral organo-aluminum reagent (300).244 when diene 299 reacted with benzaldehyde in the presence of 10 mol% of 300, a mixture of ci -dihydropyrone (301) and rran -dihydropyrone (302) was obtained in 77 and 7% yield, respectively. The optical activity of(301 was 95% ee, and that of 302 was 53% ee. 44 jf, ( g complex that mediates this reaction, the diene approaches benzaldehyde with an endo alignment that minimizes steric repulsion between the incoming diene and the front triphenylsilyl moiety. 

Narasaka et al. developed a titanium catalyst generated by complexation with chiral diols.245 xhe dienophile must contain functionality that will coordinate with the metal catalyst to form a chiral complex, and these catalysts are less effective with dienes and dienophiles that do not contain heteroatoms. A related titanium-BINOL complex has been used to catalyze Diels-Alder reactions.246 Kelly prepared a transient 

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