Quassinoids. 1. Attempted Intramolecular Diels-Alder Approach for Assembling the ABCD Rings

A synthetic approach to the quassinoids involved the attempted intramolecular Diels-Alder reaction of the ester 31. The diene component, 4-(phenylthio)-3,5-hexadienoic acid (5) was prepared either by carboxylation of the anion of 3-(phenylthio)-1,3-pentadiene or preferably by regioselective addition of thiophenol to ethyl (2£,4E)-4,5-epoxy-2-hexenoate followed by dehydration and deconjugation. The dienophile component 28 was prepared from 2,6-dimethylcyclohexanone in nine steps. The failure of the intramolecular Diels-Alder reaction of the linked diene-dienophile 31 was attributed to a high kinetic barrier in the transition state where the resonance interaction of the linking group is minimized.