Stereocontrolled Synthesis of 5α- and 5β-Substituted Kainic Acids

A general and efficient method for the stereoselective synthesis of racemic 5 beta- and 5 alpha-substituted kainic acids 3 and 4 has been developed starting from the bicyclic pyroglutamate derivative 6, readily available on a large scale. Compound 6 proved to be a versatile synthon from which straightforward functionalizations at both C-5 beta and C-5 alpha were accomplished in a stereoselective manner without compromising the stereogenic integrity of the potentially labile C-2 center. The key steps involved the stereoselective nucleophilic addition of organocopper reagents to the N-acyliminium ion I, and the stereoselective hydrogenation of the cyclic imine 9 derived from 6. Transformations of the bicyclic intermediates 7 and 8 into the final substituted kainic acids 3 and 4 were accomplished via stepwise sequences that avoid the facile and undesirable intramolecular Claisen and epimerization reactions. Compounds 3 and 4 have shown no significant binding affinity for the kainate receptors, which reflects the sensitivity of the recognition site to steric and conformational factors.