(-)-Wistarin from the marine sponge Ircinia sp.: the first case of enantiomeric sesterterpenes

The sestertepene (-)-wistarin has been isolated from a Red Sea sponge of the genus Ircinia. This metabolite is the enantiomer of (+)-wistarin, a tetracyclic metabolite previously isolated from the sponge Ircinia wistarii. The enantiomeric relationship was proved by comparison of the CD spectrum of (-)-wistarin with that of a synthetic sample of the (+)-isomer. A hypothetical biosynthesis of (-)-wistarin is discussed starting from a putative linear precursor.