Synthetic approaches to phorbols via the intramolecular Diels–Alder reaction of furans: influence of the dienophile upon the stereochemical control

A carbotricycle (13) having the same relative stereochemistry as the phorbols at 6 centres has been constructed via stereoselective Intramolecular Diels–Alder reaction of furans (IMDAF) of a precursor (7) possessing a Z-vinylogous ketoester followed by selective epimerisation at C-10 of the initial cycloadduct (11); whereas the corresponding E-isomer (8) or alkyne (3) show no stereoselection in the IMDAF.