Sonochemical Protocols for Heterocyclic Synthesis: A Representative Review.

In the present era of the industrial revolution, we all are familiar with ever-increasing environmental pollution released from various chemical processes. Chemical production has had a severe impact on the environment and human health. For the betterment of our environment, the chemical community has turned their interest to developing green, harmless and sustainable synthetic processes. To accomplish these goals of green chemistry, the extraordinary properties of sonication play an important role. It is well known that sonochemistry can make decisive contributions to creating high pressures of almost 1000 atm and very high temperatures in the range of 4500-5000 °C. The implementation of ultrasound in chemical transformations somehow fulfils the measures of green chemistry, as it reduces energy consumption, enhances product selectivity, and uses lesser amounts of hazardous chemicals and solvents. Furthermore, heterocyclic synthesis under ultrasonication offers several environmental and process-related advantages compared with conventional methods. The remarkable contribution of ultrasonics to the development of green and sustainable synthetic routes inspired us to write this article. Herein, we have discussed only some of the various synthetic methodologies developed for the construction of heterocyclic cores under ultrasonic irradiation, accompanied by mechanistic insights. In some cases, a comparison between sonochemical conditions and conventional conditions has also been investigated. We emphasized principally 'up to date' developments on various sono-accelerated chemical transformations comprising aza-Michael, aldol reactions, C-C couplings, oxidation, cycloadditions, multi-component reactions, etc. for the synthesis of heterocycles.