Functionalisation of terpenoids at C-4 via organopalladium dimers: cyclopropane formation during oxidation of homoallylic σ-organopalladium intermediates with lead tetraacetate

The synthesis of new potential adjuvant saponin aglycons was investigated by selective palladium mediated C–H functionalisation of appropriately functionalised derivatives of lanosterol, cholesterol, and friedelin. The desired equatorial aldehyde functionality was successfully introduced into the lanosterol skeleton as expected. Cyclopalladation of a cholesterol derivative proceeded as expected, but during oxidation of the organopalladium intermediate, participation of the adjacent alkene functionality led to stereoselective formation of a cyclopropane and introduction of an acetate group into the steroid backbone at C-6. Further investigation of this unusual cyclopropane formation on a model decalin system confirmed the result, but C–H activation on a related open chain system was prevented by complexation of the alkene functionality to the palladium.