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The identification of natural and environmentally friendly pesticides is a key area of interest for the agrochemical industry, with many potentially active compounds being sourced from numerous plant species. In this study, we report the bioassay-guided isolation and identification of phytotoxic and antifungal compounds from the ethyl acetate extract of Helietta parvifolia stems. We identified eight compounds, consisting of two coumarins and six alkaloids. Among these, a new alkaloid, 2-hydroxy-3,6,7-trimethoxyquinoline-4-carbaldehyde ( 6 ), was elucidated, along with seven known compounds. The phytotoxicity of purified compounds was evaluated, and chalepin ( 4 ) was active against Agrostis stolonifera at 1 mM with 50% inhibition of seed germination and it reduced Lemna pausicotata (duckweed) growth by 50% (IC 50 ) at 168 μM. Additionally, we evaluated the antifungal activity against the fungal plant pathogen Colletotrichum fragariae using a thin-layer chromatography bioautography assay, which revealed that three isolated furoquinoline alkaloids (flindersiamine ( 3 ), kokusagenine ( 7 ), and maculine ( 8 )) among the isolated compounds had the strongest inhibitory effects on the growth of C. fragariae at all tested concentrations. Our results indicate that these active natural compounds, i.e., ( 3 ), ( 4 ), ( 7 ), and ( 8 ), could be scaffolds for the production of more active pesticides with better physicochemical properties.

Natural products are a main source of new chemical entities for use in drug and pesticide discovery. In order to discover lead compounds with high herbicidal activity, a series of new pyrido[2,3- d ] pyrimidine derivatives were designed and synthesized using 2-chloronicotinic acid as the starting material. Their structures were characterized with 1 H NMR, 13 C NMR and HRMS, and the herbicidal activities against dicotyledonous lettuce ( Lactuca sativa ), field mustard ( Brassica campestris ), monocotyledonous bentgrass ( Agrostis stolonifera ) and wheat ( Triticum aestivum ) were determined. The results indicated that most of the pyrido[2,3- d ] pyrimidine derivatives had no marked inhibitory effect on lettuce at 1 mM. However, most of the pyrido[2,3- d ] pyrimidine derivatives possessed good activity against bentgrass at 1 mM. Among them, the most active compound, 3-methyl-1-(2,3,4-trifluorophenyl)pyrido[2,3- d ]pyrimidine-2,4(1 H ,3 H )-dione ( 2o ), was as active as the positive controls, the commercial herbicides clomazone and flumioxazin. Molecular simulation was performed with molecular docking and DFT calculations. The docking studies provided strong evidence that 2o acts as an herbicide by inhibition of protoporphyrinogen oxidase. However, the physiological results indicate that it does not act on this target in vivo, implying that it could be metabolically converted to a compound with a different molecular target.

Natural products are a source for pesticide or drug discovery. In order to discover lead compounds with high fungicidal or herbicidal activity, new niacinamide derivatives derived from the natural product niacinamide, containing chiral flexible chains, were designed and synthesized. Their structures were confirmed by 1 H NMR, 13 C NMR and HRMS analysis. The fungicidal and herbicidal activities of these compounds were tested. The fungicidal activity results demonstrated that the compound ( S )-2-(2-chloronicotinamido)propyl-2-methylbenzoate ( 3i ) exhibited good fungicidal activity (92.3% inhibition) against the plant pathogen Botryosphaeria berengriana at 50 μg/mL and with an EC 50 of 6.68 ± 0.72 μg/mL, which is the same as the positive control (fluxapyroxad). Compound 3i was not phytotoxic and could therefore be used as a fungicide on crops. Structure-activity relationships (SAR) were studied by molecular docking simulations with the succinate dehydrogenase of the fungal mitochondrial respiratory chain.