ChemInform Abstract: Limitations in the Application of Anionic Oxy-Cope Sigmatropy to Elaboration of the Forskolin Nucleus.

ChemInform Abstract

The bicyclooctenone (II) undergoes 1,2-addition with lithiated dihydropyran (III) to give the alcohols (IV) and (V). The potassium salt of (V) undergoes oxy-Cope rearrangement in refluxing THF and subsequent PhSeCl trapping to open a route to the tricyclic ketone (VII) and its methylated derivative (IX), both of which partially reflect the structure of forskolin (X), the major diterpene from the roots of Coleus forskohlii. Several more highly functionalized dihydropyran adducts similar to (V) are prepared, some of which originate in enantiomerically pure forms from carbohydrate precursors, but no oxyanionic Cope rearrangement reactions are observed for these adducts.