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 (I and 2) exhibiting the 13Z-configuration afforded cheilanthane and rearranged cheilanthane terpenoids as main products, along with scalarane compounds. In particular, 14-epi-cheilanthanic ester (6) was obtained together with 18-epi-scalaranic ester (5) by cyclisation of 13Z,17Z-bicyclogeranylfarnesoic acid methyl ester (1), whereas the rearranged 22(8-->14)-abeo-cheilanthanic ester (8) was formed along with scalaranic ester (7) by cyclisation of 13Z, 17E-bicyclogeranylfarnesoic acid methyl ester (2). The structure and stereochemistry of the new compounds 6 and 8 were established on the basis of their spectral data. (C) 2002 Elsevier Science Ltd. All rights reserved.