Synthetic study of 1,3-butadiene-based IMDA approach to construct a [5-7-6] tricyclic core and its application to the total synthesis of C8-epi-guanacastepene O.

An efficient intramolecular Diels-Alder (IMDA) strategy for the construction of the [5-7-6] tricyclic core (18) of guanacastepenes has been developed from cis- and trans-1,3-butadiene-tethered 4-oxopent-2-ynoic acid ethyl esters 10 and 11. This method facilitates the synthesis of C8-epi-guanacastepene O (36) in a very efficient manner.