ChemInform Abstract: Facile Access to the ABC Ring System of the Taxane Diterpenes via Anionic Oxy-Cope Rearrangements.

The anionic oxy-Cope rearrangement of bicyclo[2.2.2]octadienols serves as a key step for the construction of substituted bicyclo[5.3.1]undecenones and provides a novel entry to the AB ring system of the taxane diterpenes. The bicyclo[2.2.2]octadienols 11a-e underwent facile anionically-accelerated [3,3]-sigmatropic rearrangements to give bicyclo[5.3.1]undecenones 14a-e. Such reorganizations were also found to proceed in more highly substituted systems as evidenced by the transformations 27a-c-->28a-c, 30-->32, and 31-->33. These bicyclo[5.3.1]undecenones were further functionalized by introducing a double band conjugated to the carbonyl group (27a-->40); moreover, oxygen functionality at C(13) could be introduced by allylic oxidation (28a-->41 and 40-->42). The enolates produced in situ by the anionic oxy-Cope rearrangement could be trapped by alkylating agents. These alkylations were highly stereoselective when the ketone enolate was trisubstituted as exemplified by the reactions 44-->47 and 45-->48. This entry to the taxane diterpenes allows access to the tricyclic ABC framework of the taxanes by annelation of the C ring onto an AB ring subunit as illustrated by the sequence 30-->49-->50.