Isolation and Characterization of Antibacterial Carotane Sesquiterpenes from Associated Endophytic QA-8.

Carotane sesquiterpenes are commonly found in plants but are infrequently reported in the fungal kingdom. Chemical investigation of QA-8, an endophytic fungus associated with the inner root tissue of the grown medicinal herb H. Lév. and Vaniot, resulted in the isolation and characterization of five new carotane sesquiterpenes trichocarotins I-M (-), which have diverse substitution patterns, and seven known related analogues (-). The structures of these compounds were established on the basis of a detailed interpretation of their NMR and mass spectroscopic data, and the structures including the relative and absolute configurations of compounds -, , , and were confirmed by X-ray crystallographic analysis. In the antibacterial assays, all isolates exhibited potent activity against EMBLC-1, with MIC values ranging from 0.5 to 32 µg/mL, while 7-hydroxy CAF-603 () strongly inhibited QDIO-3 (MIC = 0.5 µg/mL). Structure-activity relationships of these compounds were discussed. The results from this study demonstrate that the endophytic fungus QA-8 from the planted medicinal herb is a rich source of antibacterial carotane sesquiterpenes, and some of them might be interesting for further study to be developed as novel antibacterial agents.