Asymmetric Diels–Alder Reactions of Some Chiral Dienophiles Derived from Cinchona Alkaloids

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Abstract
Acrylates, crotonate, and fumarates derived from cinchona alkaloids were easily prepared. The highly re-facial selective additions of the dienophiles derived from cinchonidine or quinine to some dienes were achieved in the presence of SnCl4. Similarly, si-facial selective additions were also achieved by the use of cinchonine instead of cinchonidine or quinine as a chiral auxiliary alcohol with equal ease. The inverse diastereofacial selectivity was observed by the use of TiCl4 instead of SnCl4 as a Lewis acid in the case of the reaction of the acrylates cited above with cyclopentadiene. It was suggested by the infrared spectra of the dienophile–SnCl4 complexes and by some circumstantial evidence that in the complexes, SnCl4 coordinates with the oxygen atom of the C–O bond (not C=O bond) and probably also with the nitrogen atom of the quinuclidine framework to form a rigid five-membered ring structure.