Tricyclic and Spirobicyclic Norsesquiterpenes from the Endophytic Fungus Pseudolagarobasidium acaciicola

The new tricyclic and spirobicyclic norsesquiterpenes 1 and 2 and the new nor-chamigrane endoperoxide 3, together with the known sesquiterpenes steperoxide A (4), merulin B (5), and merulin C (6), were isolated from the endophytic fungus Pseudolagarobasidium acaciicola. The structures and absolute configurations of 1-3 were determined by analysis of spectroscopic data, as well as by single-crystal X-ray analysis. Compound 1 has a previously unobserved 6/5/5 fused tricyclic ring system, in which the five-membered (middle) ring is, as revealed by X-ray analysis, perpendicular to a six-membered ring. Compound 2 possesses a hitherto unknown 5/6 fused spirobicyclic ring system. The cytotoxic activities of the isolated compounds 1-6 were evaluated.