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]undec enones and provides a novel entry to the AB ring system of the taxane diterpenes. The bicyclo[2.2.2]octadienols 11a-e underwent facile anion ically-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 transformation s 27a-c-->28a-c, 30-->32, and 31-->33. These bicyclo[5.3.1]undecenones were further functionalized by introducing a double band conjugated t o the carbonyl group (27a-->40); moreover, oxygen functionality at C(1 3) could be introduced by allylic oxidation (28a-->41 and 40-->42). Th e enolates produced in situ by the anionic oxy-Cope rearrangement coul d be trapped by alkylating agents. These alkylations were highly stere oselective when the ketone enolate was trisubstituted as exemplified b y the reactions 44-->47 and 45-->48. This entry to the taxane diterpen es allows access to the tricyclic ABC framework of the taxanes by anne lation of the C ring onto an AB ring subunit as illustrated by the seq uence 30-->49-->50.