Studies towards the synthesis of cheilanthane sesterterpenoids: superacidic cyclisation of methyl 13Z,17Z- and 13Z,17E-bicyclogeranylfarnesoates

Superacidic low temperature cyclisation of bicyclogeranylfarnesoic acid methyl esters ( and ) exhibiting the 13Z-configuration afforded cheilanthane and rearranged cheilanthane terpenoids as main products, along with scalarane compounds. In particular, 14-epi-cheilanthanic ester () was obtained together with 18-epi-scalaranic ester () by cyclisation of 13Z,17Z-bicyclogeranylfarnesoic acid methyl ester (), whereas the rearranged 22(8→14)-abeo-cheilanthanic ester () was formed along with scalaranic ester () by cyclisation of 13Z,17E-bicyclogeranylfarnesoic acid methyl ester (). The structure and stereochemistry of the new compounds and were established on the basis of their spectral data.