A Short Access to the Skeleton of Elisabethin A and Formal Syntheses of Elisapterosin B and Colombiasin A

A short
stereoselective synthesis of the Elisabethin A skeleton 4 is described, which opens a formal access to the diterpenes Elisapterosin
B and Colombiasin A as well. Key reactions were an intermolecular endo-selective Diels–Alder reaction to generate the
decalin part of the molecule, a chemo- and diastereoselective allylation
of an aldehyde with allylzinc, a palladium ene annulation of the cyclopentane
ring, and a novel sulfonium ylide induced fragmentation of a polycyclic
ketone. Additional insights have been gained for the crucial epimerization
at C-2.