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 alpha -cedrene and beta -cedrene delivered the compounds (3 beta ,3a beta ,7 beta)-(+/-)-6,6-ethylenedioxy-3,8,8-trimethyl-2,3,3a,4,5,6,7,8-octahydro-3a,7-methanoazulen-2-one, C16H22O3, and (3 beta ,3a beta ,7 beta ,8a alpha)-(+/-)-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.