MICROWAVE-ASSISTED SYNTHESIS OF 4-METHYL COUMARINS, THEIR ANTIOXIDANT AND ANTIBACTERIAL ACTIVITIES

Abstract

Coumarins are heterocyclic compounds naturally found in plants. Coumarin-derived compounds have been commonly synthesized and some have exhibited a wide range of bioactivities. In searching for coumarins-based antioxidants and antibacterial, in this study, various 4-methyl coumarins have been synthesized by reacting ethyl acetoacetate with several phenol compounds, e.g. resorcinol, catechol, hydroquinone, α-naphthol, and β-naphthol, catalyzed by multiple Lewis acids, e.g. SnCl2, SnCl2.2H2O, ZnCl2, and AlCl3, using microwave irradiation via the Pechmann condensation. The antioxidant activities of synthesized compounds were tested using the DPPH method, while the antibacterial activities were measured using the disc diffusion method. Of the five phenol and four Lewis acid variations, only resorcinol catalyzed by SnCl2.2H2O and AlCl3 produced the targeted coumarin, 7-hydroxy-4-methyl coumarin, in a pure compound. The synthesized compound was obtained with an optimum yield of 55.25% by reacting resorcinol/ethyl acetoacetate 1:1 (mol/mol) and 10 mol% SnCl2.2H2O for 260 s reaction time under solvent-free conditions. This compound was identified by TLC, melting point apparatus, and then further characterized using FTIR, UV-Vis, and NMR instruments. The antioxidant activity of 7-hydroxy-4-methyl coumarin showed moderately active towards DPPH with an IC50 value of 99.69 ppm. This compound also displayed moderate activity against E. coli with 9 mm of inhibition zone in 125 ppm. However, 7-hydroxy-4-methyl coumarin was inactive against S. aureus at all tested concentrations. This study reveals that various phenols and Lewis acids can affect coumarins-based antioxidant and antibacterial synthesis.