Abstract
Water is the most abundant and environmentally benign solvent with some unique physico-chemical properties. The present chapter describes the formation of three- to seven-membered aza-heterocycles in water as a solvent. Aziridine synthesis is described by transfer of nitrogen atom from appropriate nitrene-precursor to olefinic systems in water. Asymmetric Kinugasa reaction in aqueous medium leads to the formation of 2-azetidinones. The Paal–Knorr method for the synthesis of pyrroles has been revisited in aqueous medium. Among other five-membered aza-, oxaza-, and thiaza-heterocycles, syntheses of benzimidazoles, pyrazoles, thiazoles, isoxazolidines, 1,2,4-oxadiazoles, 1,4,2-dioxazoles, and 1,2,3-triazoles are described. Synthesis of diverse types of six-membered nitrogen-containing heterocycles such as di- and tetrahydropyridines, quinolines, quinazolines, pyrimidines, 1,2,4-oxadiazines, 1,4,2-dioxazinanes, etc. is described. Among seven-membered rings, formation of diazepines, benzodiazepines, benzoxazepines, and benzthiazepines rings is described. The biological relevance of the products synthesized is highlighted. The mechanistic rationale for the formation of products is discussed where appropriate.
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