Synthesis and Antioxidant Properties of 1,3,4-Oxadiazole Derivatives of Benzoic and Monohydroxybenzoic Acids

2-Aryl- and 2-aryl-5-alkyl-1,3,4-oxadiazoles were synthesized by the interaction of benzoic acid hydrazides with orthoesters of carboxylic acids, and their antiradical action was studied in the reaction with 2,2-diphenyl-1-picrylhydrazyl (DPPH^•). In the reaction with DPPH^•, phenoloxadiazoles showed higher antiradical activity compared to the corresponding monohydroxybenzoic acids due to intramolecular synergism, which is a result of the combined action of the phenol and oxadiazole fragments within one molecule, leading to resonance stabilization of the resulting phenoxyl radical. For 1,3,4-oxadiazoles, which do not contain a phenolic fragment in their composition and exhibit lower antiradical activity compared to phenoloxadiazoles, a mechanism of their antioxidant action has been proposed, which is associated with the formation of a radical adduct.