Recent advances in the microwave- and ultrasound-assisted green synthesis of coumarin-heterocycles

Given the importance of the highly reactive nature and attractiveness of coumarins as a precursor of natural products and pharmacological agents, tremendous efforts have been dedicated to their synthesis in the last decades. Starting from the Knoevenagel, Perkin, Kostanecki-Robinson, Pechmann, Reformansky, Baylis Hillman reactions to several new one-pot multicomponent, sequential tandem reactions have been devised. After the emergence of non-conventional energy sources like microwaves (MW) and ultrasound irradiation, the field of organic synthesis has reached an outstanding level in this century in terms of synthetic efficiency as well as green chemistry viewpoint. Enlightened by this, a great deal of attention has been paid to the synthesis of bioactive coumarin-heterocycles by employing the non-conventional approach and a vast array of synthetic procedures has been established. Therefore, a time to time investigation is required for the synthesis and biological aspects of these hybrid molecules. The present review aims to highlight the current progress achieved in the synthesis of coumarins either linked or fused with diverse bioactive five- and six-membered heterocycles by making the utilization of microwaves and ultrasound-assisted strategies from 2014 to date. Besides highlighting the development achieved in this field, we have attempted to point out the drawbacks and challenges associated with the reaction discovery, which would hopefully provide the impetus for future exploration.