| Summary: | Ten-membered lactones (nonenolides) demonstrate phytotoxic, antimicrobial, and fungicidal activity promising for the development of natural product-derived pesticides. The fungus <i>Stagonospora cirsii</i> is able to produce phytotoxic stagonolides A (<b>1</b>), J (<b>2</b>), K (<b>3</b>) and herbarumin I (<b>4</b>) with high yield. The aim of this study was to create a set of structurally related nonenolides and to reveal the structural features that affect their biological activity. Stagonolide A (<b>1</b>) and C-7 oxidized stagonolide K (<b>11</b>) showed the highest phytotoxicity in leaf puncture assay and agar seedlings assay. The oxidation of C-7 hydroxyl group (as in <b>1</b>, acetylstagonolide A (<b>10</b>) and (<b>11</b>) led to the manifestation of toxicity to microalgae, <i>Bacillus subtilis</i> and Sf9 cells regardless of the configuration of C-9 propyl chains (<i>R</i> in <b>1</b> and <b>10</b>, <i>S</i> in <b>11</b>). C-7 non-oxidized nonenolides displayed none or little non-target activity. Notably, 7<i>S</i> compounds were more phytotoxic than their 7<i>R</i> analogues. Due to the high inhibitory activity against seedling growth and the lack of side toxicity, mono- and bis(acetyl)- derivatives of herbarumin I were shown to be potent for the development of pre-emergent herbicides. The identified structural features can be used for the rational design of new herbicides.
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