Sesquiterpene lactones : a total synthesis of (±) quadrone

Two routes towards (±) quadrone (1), via an intramolecular Diels-Alder reaction have been studied. The cycloaddition of 5 failed, probably because of prohibitive strain. In the alternative approach, the key step afforded a mixture of the endo- and exo-adducts 6 and 7. Both isomers were transformed into Danishefsky's intermediate 2, which has previously been converted to the title compound 1