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Due to the excessive use of antifungal agents, drug resistance and ecological problems are increasing. Some antifungal agents are difficult to degrade and have high toxicity and several side effects. In this study, 15 novel tetrahydrogeranyl quaternary ammonium salts (8a–8o) were synthesized from the natural compound citral. The structures of the quaternary ammonium salts were characterized by Fourier transform infrared, proton nuclear magnetic resonance, carbon-13 nuclear magnetic resonance spectroscopy, and mass spectrometry, and the antifungal activities of these compounds at a concentration of 0.25 mg/mL against 10 plant pathogenic fungi were tested. The results showed that compound 8i had the best antifungal activity, and its inhibition rates against Rhizoctonia solani, Phytophthora parasitica var. nicotianae, Sphaeropsis sapinea, Fusarium oxysporum f. sp. niveum, and Poria vaporaria reached 100%. For Fusarium verticillioides, the inhibition rate of compound 8i was 93.28%, which was higher than that of chlorothalonil. In addition, it was found that the inhibition rates of compounds with N,N-di- n-propyl group (8l, 8m) against R solani, F oxysporum f. sp. niveum, S sapinea, P parasitica var. nicotianae, F verticillioides, Colletotrichum acutatum, and Coriolus versicolor were higher than compounds with N,N-diethyl and N,N-dimethyl groups (8a, 8b, 8j, 8k). The inhibition rates of compounds with morpholine groups (8n, 8o) were generally low.

The development of new antimicrobials has always been a research hotspot. In this study, β-pinene-based derivatives were synthesized, and their antimicrobial activity was evaluated. The purpose was to develop some novel, promising new fungicides. Three β-pinene derivatives containing bis-hydronopyl were prepared, and their antifungal and antibacterial activities were evaluated against 6 plant pathogenic fungi and 4 bacterial species; a preliminary structure-activity relationship is discussed. The results indicated that the derivatives containing the blend of alkyl group and bis-hydronopyl had potent inhibitory activities against plant fungal pathogens and bacteria. Among these molecules, bis-hydronopyl dimethyl ammonium bromide showed excellent effects on Colletotrichum acutatum with a half-maximal effective concentration of 0.538 µg/mL, which was lower than that of carbendazim. Scanning electron microscope showed that after administration of bis-hydronopyl dimethyl ammonium bromide (compound 3a), the C. acutatum mycelia were sunken and deformed in comparison with the control group. Furthermore, the inhibitory activities of the methyl derivatives against the plant pathogens were better than those of the ethyl derivatives. These results provide new insights into the inhibition of fungi and bacteria by β-pinene derivatives, which may lead them to be used as precursor molecules for novel pesticides and antimicrobials in further research.

β-Pinene can be used as a cheap source to synthesize a large number of high value-added derivatives. In this study, a series of β-pinene derivatives was prepared, and the antifungal activities of the compounds were assessed against phytopathogenic fungi. Eight N-alkyl hydronopyl diethyl ammonium halide salts were synthesized by the reaction of hydronopyl diethyl ammonium halide with 8 halogenated alkanes. The structures of the synthesized products were characterized by Fourier-transform infrared spectroscopy and nuclear magnetic resonance spectroscopy and mass spectrometry. The antifungal activities of these derivatives were tested against 11 plant pathogens, and the preliminary structure-activity relationship is discussed. Some derivatives exhibited moderate to significant antifungal activity due to the fusion of the hydronopyl, a long-chain alkyl, bromine, and iodine anionic groups. In contrast to the structure-activity relationship of compounds 2a, 2b, and 2c, iodine ions in 2f, 2g, and 2f had a significant effect on enhancing the antifungal activity against Colletotrichum gloeosporioides, S clerotinia sclerotiorum, Phytophthora capsici, Phomopsis, Sphaeropsis sapinea, Glomerella cingulata, and Fusicoccum aesculi. A higher molecular weight could increase the antifungal activity against Fusarium proliferatum, Alternaria kikuchiana, Sclerotinia sclerotiorum, P. capsici, Phomopsis, and S. sapinea. Compounds 2d and 2e exhibited broad-spectrum antifungal activity against the tested strains. These derivatives are expected to be used as precursor molecules for novel pesticide development in further research.