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An interesting class of heterocyclic compounds, 1,2,4-oxadiazoles and their derivatives have attracted the attention of chemists because of their use in the development of new pharmaceuticals and functional materials. Herein, a catalyst-free reaction of N-hydroxyamidoxime with various ketones or aldehydes has been investigated. The reaction was carried out in methanol under mild conditions and gave 3,5,5-trisubstituted 4-hydroxy-4 H,5 H-1,2,4-oxadiazole derivatives in excellent yields. The structure of product 3f was confirmed by single-crystal X-ray diffraction studies. [ABSTRACT FROM AUTHOR]