Rearrangement approaches to sesquiterpenes containing multiple contiguous quaternary carbon atoms. Total synthesis of (+/-)-myltayl-8(12)-ene and (+/-)-6-epijunicedranol

Details of the first total syntheses of the sesquiterpenes myltayl-8(12)-en e and 6-epijunicedran-8-ol are described. The aldehyde 13, obtained by Clai sen rearrangement of cyclogeraniol, was transformed into the dienones 12 an d 18. Boron trifluoride-diethyl ether mediated cyclization and rearrangemen t transformed the dienones 12 and 18 into the tricyclic ketones 16 and 17, efficiently creating three and four contiguous quaternary carbon atoms, res pectively. Wittig methylenation of 16 furnished (+/-)-myltayl-8(12)-ene (11 ), whereas reduction of the ketone 17 furnished (+/-)-6-epijunicedranol (23 ).