A Highly Abbreviated Synthesis of Pentalenene by Means of the Squarate Ester Cascade
(Equation Presented) The sequential addition of 5-methylcyclopentyllithium and propynyllithium to diisopropyl squarate results in the efficient formation of a functionalized angular triquinane having two of its five-membered rings substituted precisely as in the target sesquiterpene. Only seven additional steps are then required to access pentalenene.