Key tricyclic synthetic intermediates for the preparation of the sesquiterpenes α- and β-cedrene

A completely novel and direct route towards the synthesis of the natural sesquiterpenes α-cedrene and β-cedrene delivered the compounds (3β,3aβ,7β)-(±)-6,6-ethyl­ene­dioxy-3,8,8-tri­methyl-2,3,3a,4,5,6,7,8-octa­hydro-3a,7-methano­azulen-2-one, C₁₆H₂₂O₃, and (3β,3aβ,7β,8aα)-(±)-6,6-ethyl­ene­dioxy-3,8,8-tri­methyl-1,2,3,3a,4,5,6,7,8,8a-deca­hydro-3a,7-methano­azulen-2-one, C₁₆H₂₄O₃, at key stages of the preparative programme. Structural elucidation showed the latter compound to have added an H atom to the same face of the cyclo­pentenone ring as that occupied by the methyl substituent, and also allowed correct isomer identification for further reaction.