Application of palladium-catalyzed cycloalkenylation reaction to terpenoid synthesis - Novel approach to tricyclo[5.3.1.0 ^2,6 ]undecane derivative and its transformation into bicyclo[5.3.1]undecane ring system

A novel synthesis of the tricyclo[5.3.1.0 ^2,6 ]undecane compound 16, basic carbon skeleton of gymnomitrane class of sesquiterpenoids, and the transformation into the bicyclo[5.3.1]undecane derivative 18, the AB carbon framework of taxol (6), have been achieved. Beginning with the bicyclo[3.2.1]octane ring system 2, prepared from the cross-conjugated silyl enol ether 1 employing palladium-catalyzed cycloalkenylation reaction, the tricyclo[5.3.1.0 ^2,6 ]undecane ring system 16 was synthesized by using samarium-promoted reductive cyclization of 15. After conversion of 16 to the hydroxy tosylate 17, basic treatment of 17 afforded the desired bicyclo[5.3.1]undecane derivative 18. Interestingly, the highly strained oxetane 19 was also obtained together with 18.