High Value-Added Application of Natural Plant Products in Crop Protection: Honokiol Monoester/Diester Derivatives Containing the Novel Core Scaffold of Benzodihydrofuran and Their Agricultural Bioactivities and Control Effects

To discover new potential botanical insecticides from plant secondary metabolites, a series of new honokiol-type monoester/diester derivatives containing the core scaffold of benzodihydrofuran were synthesized by structural modification of honokiol. Against Mythimna separata Walker, 2-hydroxymethy-5-(2'-(para-chlorobenzoyloxy)-5'-(1″,2″-epoxypropanyl))phenyl-2,3-dihydrobenzofuran (5) and 2-(2″-chloropyridin-5″-ylcarbonyloxy)methylene-5-(2'-(2″-chloropyridin-5″-ylcarbonyloxy)-5'-(1″,2″-epoxypropanyl))phenyl-2,3-dihydrobenzofuran (37) displayed >2.1-fold promising insecticidal activity of the precursor honokiol. Against Aphis citricola Van der Goot, 2-hydroxymethy-5-(2'-(tridecylcarbonyloxy)-5'-(1″,2″-epoxypropanyl))phenyl-2,3-dihydrobenzofuran (21) (LD₅₀: 0.049 μg/nymph) and 2-(para-fluorobenzylcarbonyloxy)methylene-5-(2'-(para-fluorobenzylcarbonyloxy)-5'-(1″,2″-epoxypropanyl))phenyl-2,3-dihydrobenzofuran (31) (LD₅₀: 0.040 μg/nymph) showed 3.5- and 4.3-folds potent aphicidal activity of honokiol (LD₅₀: 0.171 μg/nymph), respectively. Interestingly, 2-(tridecylcarbonyloxy)methylene-5-(2'-(tridecylcarbonyloxy)-5'-(1″,2″-epoxypropanyl))phenyl-2,3-dihydrobenzofuran (46) (LC₅₀: 0.186 mg/mL) and 2-(dodecylcarbonyloxy)methylene-5-(2'-(dodecylcarbonyloxy)-5'-(1″,2″-epoxypropanyl))phenyl-2,3-dihydrobenzofuran (53) (LC₅₀: 0.159 mg/mL: >6.4-fold of honokiol (LC₅₀: 1.024 mg/mL)) exhibited promising acaricidal activity and control efficiency against Tetranychus cinnabarinus Boisduval. Structure-activity relationships indicated that a specific length of the aliphatic chain is necessary for the agricultural activities of honokiol monoester/diester derivatives, especially for the acaricidal activity of diester derivatives.