Synthetic approaches to Forskolin.

In a series of studies directed towards the total synthesis of the natural product forskolin, an intramolecular Diels-Alder approach to the construction of the AB ring system is described, with the stereocontrolled synthesis of the tricyclic adducts (128), (129) and (140). An unusual stereochemical outcome in the intramolecular Diels-Alder reaction of ester (122) to give (129) is also described, and evidence defining the origin of this result considered. Chemical elaboration of the aforementioned tricyclic adducts towards forskolin is presented, together with the intramolecular Diels-Alder cyclisation of acetylene dienophiles to two further tricyclic adducts (90) and (176), possessing the same relative stereochemistry as centres 10a and 10b in forskolin. Finally, the synthesis is extended to include a complementary intermolecular Diels-Alder route to the AB ring system of forskolin, providing an insight into the chemistry of these important decalin intermediates in an approach to the synthesis of forskolin.