Cycloaddition reactions in ionic liquids for the synthesis of biologically relevant heterocycles

The heterocyclic frameworks (pyrrole, furan, quinolone, indole, oxazole, imidazole, triazole, etc.) represent the key structures of medicinal, pharmaceutical, and biologically important molecules as well as many bioactive natural products, for example, alkaloids and macrolides. Development of greener, efficient, and rapid approaches for the synthesis of these scaffolds is very challenging and yet very important. Among many approaches known, the cycloaddition reaction-based strategies are of significant importance as they directly provide the cyclic products from their acyclic precursors via multiple C-C as well as C-heteroatom bonds in one step. In modern times, the focus on the development of greener reagents, solvents, and alternate reaction conditions for the synthesis of important molecules is of high interest to synthetic community. The use of ionic liquids as solvents (and also as catalysts) for organic synthesis has been growing faster and customized ionic liquids for a particular purpose or reactions can also be made available. The ionic liquids are considered as an alternate and green solvent media for organic reactions in place of traditional organic solvents, due to their unique physical and chemical properties such as noninflammable, nonvolatility (high boiling points), controlled miscibility, and thermal stability. They can be easily reused and have the ability to dissolve wide ranges of organic and inorganic reagents. There are several reports of great improvement in reaction yields as well as rates when used ionic liquids as solvents in place of organic solvents. In this chapter, we limited our discussion to the synthesis of various types of pharmaceutically and medicinally important heterocyclic frameworks, by employing the cycloaddition reactions in ionic liquids (as solvents). We hope that the present review or chapter is helpful for the synthetic community to educate themselves about the titled topic.