Diels-Alder reactions cinchona alkaloid-catalysed

Scheme 6.3 Cinchona alkaloid-catalysed Diels-Alder reactions of 2-pyrones.

A readily available 9-NH2 cinchona alkaloid has proved to efficiently catalyse the asymmetric Diels-Alder reaction of simple a,p-unsaturated ketones with 2-pyrones. In the presence of TFA as an additive, the reaction afforded the exo-cycloadduct as the major product in good enantiomeric excess of up to 99% ee in almost all cases of substrates, as shown in Scheme 6.3. Moderate to excellent diastereoselectivities of 50-94% de were obtained, but it is important to note that both the diastereoselectivity and the enantioselectivity of the reaction did not fluctuate significantly when the aromatic substituent of the a,p-unsaturated ketone was changed to an aliphatic substituent. 

There are only few examples of asymmetric inverse hetero-Diels Alder reactions using a,P-unsaturated aldehydes. As a recent example, a cinchona alkaloid was successfully used to catalyse the oxa-Diels Alder reaction of heterodienes such as 3-formylchromones with acetylene dicarboxylates, providing the corresponding tricyclic benzopyrones in moderate to high yields (54-87%) and enantioselectivities (46-84% ee), as shown in Scheme 6.12. ... 

Benzoate salt of the vicinal diamine (249) has been found to catalyse a domino reaction of enones R CH=CHCOR with R CH=CHCOR that affords cyclic products containing three to four contiguous stereocentres (with 92-99% ee and >30 1 dr) " The self-assembly of proline and cinchona alkaloid-derived thioureas has been reported to dramatically improve the enantioselectivity of the inverse-electron-demand hetero-Diels-Alder reactions between aldehydes R R CHCH=0 and electron-deficient enones R CH=CHCOR , affording (250) after subsequent oxidation with PCC. °... 

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