Structural Modification of Turpentine with Natural Chiral Preservation and Low-Risk Application Prospects in Crop Protection

For the purpose of chiral pesticide development, the natural chiral structure of turpentine should be applied rationally. Two series of amide derivatives were prepared to study the effect of chiral center retention on the fungicidal activity. The investigation of fungicidal activity against three pathogenic microbes was carried out extensively. Some satisfactory conclusions were obtained from the activity evaluation. Above all, compounds (5'a-l) derived from cis-myrtlecanic acid (a chiral compound obtained from beta-pinene) exhibited better activity than compounds (5a-l) derived from dehydrocumic acid (achiral compound obtained from beta-pinene). The overall effect was good, and it was remarkable that compounds 5'd (substitute with N-(4-(trifluoromethyl)phenyl), 5'e (substitute with N-(4-fluorophenyl)), and 5'f (substitute with N-(4-chlorophenyl)) demonstrated extremely excellent activity, with EC50 values of 1.604, 1.822, and 2.296 mu g/mL against Valsa mali. The above three compounds also showed a good control effect against V. mali in vitro on an apple branch. Treatment with 5'd, 5'e, and 5'f against V. mali resulted in significantly influenced physiological and biochemical indices, as well as markedly reduced pectinase activity in comparison with untreated controls. The preliminary structure-activity relationship (SAR) was summarized, showing that compounds obtained with chiral centers, a halogen atom, and small steric hindrance showed better performance. At the same time, the quantitative structure-activity relationship (QSAR) model (R-2 = 0.9288, F = 62.01, S-2 = 0.0138) was obtained, and two most important structural features were maximum net atomic charge for a F atom and shadow indices. Guided by this study, the highly efficient, low-toxicity, and environmentally friendly fungicide can be prepared through reasonable modification of terpene.