Several related macrocyclic trienes have been found to interconvert thermally via [1,5]sigmatropic hydrogen shifts at the diene sites. These macrocyclic trienes further undergo thermal transannular Diels-Alder cycloaddition, thus yielding complex mixtures of adducts. This difficult problem has been tackled and qualitatively solved by means of the semiempirical methods AM1 and PM3. Most of the starting material ground states have been characterized as well as all the [1,5]sigmatropic hydrogen shift and Diels-Alder transition structures. The results of these calculations indicate that some [1,5]sigmatropic hydrogen shift enthalpies of activation are within the same range of most Diels-Alder enthalpies of activation. As a result the Curtin-Hammett principle does not apply for the entire system but only on limited regions. The method proved successful in uncovering the previously missassigned identity of one of the Diels-alder adducts : an unexpected 5-6-7-membered tricycle.