Antibacterial Activities of 3-Substituted Coumarin-Scaffolds Synthesized under Microwave Irradiation

Global infections, such as diarrhea and acute respiratory disease, have urged the researchers in this area to discover coumarin-scaffold antibacterial agents, most widely used therapeutic drugs worldwide. This research aims to synthesize 3-substituted coumarins using microwave-assisted and evaluate their antibacterial activities against Escherichia coli and Staphylococcus aureus. In this study, salicylaldehyde with numerous activated methylenes has been used to synthesize 3-substituted coumarins following Knoevenagel condensation assisted by microwave irradiation. The highest yield of 3-acetylcoumarin was achieved using ethyl acetoacetate at the molar ratio of 1.2:1 (mol/mol) and 30 mol% of diethylamine using 100W microwave irradiation for 60 s under neat condition (43.65+0.50%). Moreover, ethyl coumarin-3-carboxylate (16.33%) was successfully synthesized using the optimum condition of 3-acetylcoumarin from diethyl malonate as activated methylene with a longer reaction time (900 s). However, the trial to afford 3-cyanocoumarin was failed from ethyl cyanoacetate and malononitrile as activated methylenes under the optimum condition of 3-acetylcoumarin. The inhibition zones of 3-acetylcoumarin and ethyl coumarin-3-carboxylate on disc diffusion assay towards E. coli at concentrations higher than 250 ppm showed weak activity, ranging from 1.9-3.6 mm. In contrast, both compounds showed no inhibition zone against S. aureus at all tested concentrations. This finding reveals that 3-substituted coumarins can be synthesized by low wattage microwave irradiation and solvent-free with moderate yield with ethyl acetoacetate is the most reactive methylene. Furthermore, this work proposes some corrections to assign proton and carbon chemical shifts in 3-substituted coumarins by analyzing HSQC and HMBC experiments in NMR measurement.