| Summary: | Carotane sesquiterpenes are commonly found in plants but are infrequently reported in the fungal kingdom. Chemical investigation of <i>Trichoderma virens</i> QA-8, an endophytic fungus associated with the inner root tissue of the grown medicinal herb<i> Artemisia argyi</i> H. Lév. and Vaniot, resulted in the isolation and characterization of five new carotane sesquiterpenes trichocarotins I–M (<b>1</b>–<b>5</b>), which have diverse substitution patterns, and seven known related analogues (<b>6</b>–<b>12</b>). 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 <b>1</b>–<b>3</b>, <b>5</b>, <b>9</b>, and <b>10</b> were confirmed by X-ray crystallographic analysis. In the antibacterial assays, all isolates exhibited potent activity against <i>Escherichia coli</i> EMBLC-1, with MIC values ranging from 0.5 to 32 µg/mL, while 7<i>β</i>-hydroxy CAF-603 (<b>7</b>) strongly inhibited <i>Micrococcus luteus</i> 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 <i>T. virens</i> QA-8 from the planted medicinal herb <i>A. argyi</i> 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.
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