Consecutive Application of the α-Alkynone Cyclization: Total Synthesis of (±)-Δ9(12)-Capnellene

The synthesis of the (±)-form of the marine sesquiterpene (–)-Δ⁹⁽¹²⁾-capnellene ( 1 ) by double application of the a-alkynone cyclization is described. Starting with 2, 2, 5-trim ethylcyclopentanone ( 2 ), the elaboration of the tricyclo [6.3.0.0^2,6]undecane C-skeleton of 1 proceeded through the a-alkynone 3 , which was cyclized thermally to the bicyclo [3.3.0]octenone 4. For the anellation of the third five-membered ring, 4 was transformed into the a-alkynone 5 and the latter cyclized thermally to a mixture of the angular triquinenone 6 and the linear triquinenone 7. The last steps in the synthesis of (±)-Δ⁹⁽¹²⁾-capnellene ( 1 ) were then accomplished from 7 by known methods.