Chromium(0)-Promoted Multicomponent Cycloaddition of Tethered Diynes with Cyclic Trienes: Application to the Total Synthesis of 9-<i>e</i><i>pi</i>-Pentalenic Acid

An efficient protecting group controlled regioselective chromium(0)-mediated three-component higher order cycloaddition of tethered diynes with cyclic trienes that generates five rings and six stereogenic centers in one step is described. Following a sequence of reactions featuring a chemoselective Baeyer−Villiger rearrangement and a regioselective cyclopropane hydrogenolysis, the total synthesis of 9-epi-pentalenic acid was achieved.