Comparative effects of conjugated and deconjugated isomeric enones on the transannular Diels-Alder reaction

[GRAPHICS] Two isomeric cyclic trienones 2 and 3 (with four methyl esters at positions 3 and 10) underwent transannular Diels-Alder (TADA) reaction at very different temperatures. This drastic difference could be traced to the capacity of the enone dienophile to be conjugated or unconjugated at the transition state. Molecular modeling could confirm this explanation. The calculated enone proved to be very distorted in transition state ts2, and the TADA reaction temperature was accordingly much higher than the one corresponding to ts3 in which the enone was flat.