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Oxadiazoles are five-membered organic heteroaromatic rings containing two nitrogens, and one oxygen atom, and they exist in different regioisomeric forms. Oxadiazoles are often used in drug synthesis with the intention of being bioisosteric replacements for ester and amide functionalities. Oxadiazole is a very weak base owing to its heteroatoms. Electrophilic substitutions on the carbon atom in the oxadiazole ring are extremely difficult to synthesize because of the electron withdrawing effect of the pyridine-type nitrogen atom. However, the attack of electrophiles occurs at nitrogen, if there are electron-releasing groups on the ring. While they are generally resistant, halogen-substituted oxadiazoles undergo nucleophilic substitutions with replacement of the halogen atom.
Out of its four possible isomers, 1, 3, 4-oxadiazole is the most widely exploited for various applications. Derivatives of 1,3,4-oxadiazole exhibit a wide range of biological activities including anticancer, anti-inflammatory, analgesic, anticonvulsant, anti-HIV, antibacterial, and plant growth regulator activities. Oxadiazole rings have been introduced into drug discovery programs for several different purposes. They have been employed either as an essential part of the pharmacophore, or as a flat aromatic linker to place substituents in the appropriate orientation as well as for modulating molecular properties. For a review on Oxadiazoles in drug discovery, see Boström, J., Oxadiazoles in Medicinal Chemistry, J. Med. Chem., 2012, 55 (5), 18171830.