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Isoxazole is an azole with an oxygen atom adjacent to the nitrogen atom in the ring. The isoxazole ring is considerably less aromatic than other five membered heterocycles including oxazole and furan. Isoxazole is best represented as a resonance hybrid of several resonance structures. Both the heteroatoms influence the rate of electrophilic substitution in the isoxazole ring. Isoxazole is very labile towards the action of nucleophilic agents or strong bases, which results in the cleavage of the isoxazole ring yielding beta-keto nitriles as end products.
Isoxazoles and their derivatives have been widely exploited for the synthesis of other heterocycles and complex molecules. This is due to their versatility as synthetic building blocks, their latent functionalities as enaminones, 1,3-dicarbonyl compounds, gamma-amino alcohols, and beta-hydroxy nitriles. (Kumar, K. A., et al., Isoxazoles: Molecules with Potential Medicinal Properties, Int. J. Pharma. Chem. Bio. Sci. 2013, 3(2), 294-304). For a review on the synthesis of natural products via Isoxazoles, see: Baraldi, P. G. et al., Synthesis, 1987, (10), 857-869. Isoxazole derivatives form a part of many natural products and drugs including COX-2 inhibitors, and beta-lactam antibiotics. Furthermore, isoxazole fictionalization technologies enable the construction of tetracycline derivatives, which are antibiotics effective against a broad spectrum of microorganisms including Gram-positive bacteria.