Tricyclic and Spirobicyclic Norsesquiterpenes from the Endophytic Fungus Pseudolagarobasidium acaciicola

Abstract 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.