An Update with Recent Green Synthetic Approaches to Coumarins

Coumarin and its derivatives are privileged heterocyclic motifs and important building blocks for developing the biologically active compound due to its significant role in the development of new drugs. As a result, many methodologies have been developed to synthesize this important class of compounds. However, some methods are associated with toxic and corrosive catalysts, longer reaction time, poor yield, less purity, and by-products along with the desired product. In order to minimize the utilization and generation of toxic organic substances, green synthetic methods are applied in this manner. Green chemistry methods cover a wide range of methods, including the application of ultrasound and microwaves, ionic liquids and deep eutectic solvents, solvent-free and catalyst-free synthesis, and mechanosynthesis. These green synthetic methods have successfully performed all typical condensation reactions for coumarin synthesis like Knoevenagel, Perkin, Kostanecki-Robinson, Pechmann, and Reformatsky reactions. Compared to conventional methods, these methods not only minimize the use and generation of harmful chemicals but also improve reaction efficiency in terms of product yields, purity, energy consumption, and post-synthetic procedures. Due to the implication of coumarin (2-oxo-2H-1-benzopyran) backbone as a biologically active ubiquitous fragment and the recent demands of reducing toxic solvents, catalysts, and energy consumption, this review summarized various green synthetic methods for coumarin synthesis. Moreover, researchers working on this coumarin scaffold synthesis can find handy information from this review on the green synthetic approaches to their synthesis.