Consecutive Application of the α‐Alkynone Cyclization: Total Synthesis of (±)‐Δ 9(12) ‐Capnellene

The synthesis of the (±)‐form of the marine sesquiterpene (–)‐Δ 9(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 (±)‐Δ 9(12) ‐capnellene ( 1 ) were then accomplished from 7 by known methods.