The phytotoxicities of a selection of eudesmanolides and guaianolides, including natural products and new derivatives obtained by semisynthesis from plant-isolated sesquiterpene lactones, were evaluated in bioassays against three weeds of concern in agriculture (Amaranthus viridis L., Echinochloa crus-galli L., and Lolium perenne L.). Both eudesmanolides and guaianolides were active against the root and shoot growth of all the species, with the eudesmanolides generally showing improved activities. The IC50 values obtained for the herbicide employed as positive control (on root and shoot growth, respectively, A. viridis: 27.8 and 85.7 μM; E. crus-galli: 167.5 and 288.2 μM; L. perenne: 99.1 and 571.4 μM) were improved in most of the cases. Structure-activity relationships were discussed, finding that hydroxylation of the A-ring and C-13 as well as the position, number, and orientation of the hydroxyl groups and the presence of an unsaturated carbonyl group can significantly influence the level of phytotoxicity. γ-Cyclocostunolide was the most active compound in the series, followed by others such as dehydrozaluzanin C and α-cyclocostunolide (outstanding their IC50 values on A. viridis)─natural products that can therefore be suggested as models for herbicide development if further research indicates effectiveness on a larger scale and environmental safety in ecotoxicological assessments.
Keywords: bioassays; herbicide development models; phytotoxicity studies; sesquiterpene lactones; structure−activity relationships; weed control.