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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.