Chapter 7 - Synthetic Approaches to Small- and Medium-Size Aza-Heterocycles in Aqueous Media

Abstract

This chapter describes the formation of three- to seven-membered aza-heterocycles in aqueous media by using conventional thermal energy or other energy sources such as microwave and ultrasonic irradiation. Aziridine synthesis is described in water from 2-amino alcohols or aza-Michael-initiated ring-closure reaction between vinyl selenone and primary amines. Treatment of sulfilimines with aqueous base leads to the formation of azetidines. Azetidin-2-ones have also been synthesized in aqueous media by Kinugasa reaction, and by photochemical or catalytic cyclization of diazoamides. The Paal–Knorr method and Hantzsch reaction for the synthesis of pyrroles and pyridines are also described in aqueous media. Some of the reactions proceed without any catalyst, whereas in some cases catalysts such as indium(III) chloride, nanomaterials, platinum, and palladium complexes have been used. The synthesis of spiro-fused pyrrolidine ring to 2-oxindole is described employing isatins. Among other five-membered aza-heterocycles, syntheses of indoles, benzimidazoles, and pyrazoles are also described. Among seven-membered rings, formation of azulenone and oxoazepan rings is described by appropriate multicomponent reactions.