Tandem Use of Cobalt-Mediated Reactions to Synthesize (+)-Epoxydictymene, a Diterpene Containing a <i>Trans</i>-Fused 5−5 Ring System

Abstract

The diterpene (+)-epoxydictymene has been synthesized in 20 steps using the asymmetry of (R)-pulegone and several substrate-controlled diastereoselective reactions to prepare the natural product in its natural configuration. Three of the four rings were assembled with two consecutive intramolecular reactions involving dicobalt hexacarbonyl complexes of alkynes: a Lewis acid-promoted Nicholas reaction and a Pauson−Khand reaction. The construction of the strained trans-3-oxabicyclo[3.3.0]octane ring system of the natural product presented a significant challenge. To this end, several radical and anionic cyclizations were studied, the latter leading to (+)-epoxydictymene.