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 ) (LD 50: 0.049 μg/nymph) and 2-( para -fluorobenzylcarbonyloxy)methylene-5-(2′-( para -fluorobenzylcarbonyloxy)-5′-(1″,2″-epoxypropanyl))phenyl-2,3-dihydrobenzofuran ( 31 ) (LD 50: 0.040 μg/nymph) showed 3.5- and 4.3-folds potent aphicidal activity of honokiol (LD 50: 0.171 μg/nymph), respectively. Interestingly, 2-(tridecylcarbonyloxy)methylene-5-(2′-(tridecylcarbonyloxy)-5′-(1″,2″-epoxypropanyl))phenyl-2,3-dihydrobenzofuran ( 46 ) (LC 50: 0.186 mg/mL) and 2-(dodecylcarbonyloxy)methylene-5-(2′-(dodecylcarbonyloxy)-5′-(1″,2″-epoxypropanyl))phenyl-2,3-dihydrobenzofuran ( 53 ) (LC 50: 0.159 mg/mL: >6.4-fold of honokiol (LC 50: 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.