(+)-Absinthin, a structurally unique triterpene, has been efficiently constructed in nine reaction steps and in 18.6% overall yield from O-acetylisophotosantonic lactone. The synthesis features Mitsunobu arylselenylation, oxidative elimination of allylic arylselenides, biomimetic dimerization via regio-and stereospecific Diels-Alder reaction, and a four-step stereochemical inversion of a highly sterically congested tertiary alcohol. This approach ...