Asymmetric Total Synthesis of (−)-Lemnalemnane A

The asymmetric total synthesis of lemnalemnane A, a rare rearranged sesquiterpenoid, has been accomplished in 13 steps from (S)-carvone. The key features of the synthesis are the removal of the isopropenyl group derived from (S)-carvone via a radical intermediate, the formation of the bicyclo[3.3.1]nonane skeleton using the Dieckmann condensation, the stereocontrolled construction of five continuous chiral centers by chemo- and stereoselective reduction and stereoselective introduction of the alkyne group, and the formation of the spirolactone moiety via a hemiacetal intermediate.