Synthetic studies directed toward guianolides: an organoiron route to the 5,7,5 tricyclic ring system

A diastereoselective route to the 5,7,5-tricyclic core of the guianolides is presented. This route relies on Cope rearrangement of a divinylcyclopropane prepared by alkenyl Grignard addition to a (pentadienyl)iron(+1) cation, followed by oxidative decomplexation. An additional key reaction involves oxidative rearrangement of a 3,4-epoxy-1,7-diol to generate a γ-lactone. The relative stereochemistry of this product was established by X-ray crystallography.