Greener and expeditious synthesis of bioactive heterocycles using microwave irradiation

The utilization of green chemistry techniques is dramatically reducing chemical waste and reaction times as has recently been proven in several organic syntheses and chemical transformations. To illustrate these advantages in the synthesis of bioactive heterocycles, we have studied various environmentally benign protocols that involve greener alternatives. Microwave (MW) irradiation of neat reactants catalyzed by the surfaces of recyclable mineral supports, such as alumina, silica, clay, or their doped versions, enables the rapid one-pot assembly of heterocyclic compounds, such as flavonoids, related benzopyrans, and quinolone derivatives. The strategy to assemble oxygen and nitrogen heterocycles from in situ generated reactive intermediates via enamines or using hypervalent iodine reagents is described. Examples of multicomponent reactions that can be adapted for rapid parallel synthesis include solventless synthesis of dihydropyrimidine-2(1H)-ones (Biginelli reaction), imidazo[1,2-a]annulated pyridines, pyrazines, and pyrimidines (Ugi reaction). The relative advantages of greener pathways, which use MW irradiation and eco-friendly aqueous reaction medium, for the synthesis of various heterocycles, such as N-aryl azacycloalkanes, isoindoles, 1,3-dioxane, 1,3,4-oxadiazole, 1,3,4-thiadiazole, pyrazole, and diazepines, are also summarized.