Synthesis of a tricyclic enone (B/C/D ring system), a common key precursor for the aphidicolane- and stemodane-type diterpene, is described. The key reaction for the construction of the quaternary carbon center is allylation of epoxide at the more substituted carbon with an organotitanium reagent. Asymmetric reduction with DIP-Cl followed by stereoselective cyclization of spirocyclic ketone and the functional group modification gave the desired tricyclic enone in good yield.