Synthesis and antioxidant activities of hydroxylated coumarinyl chalcones

Abstract

Hydroxylated coumarins and chalcones are known to possess potent antioxidant activities. The present study highlights the synthesis of two hydroxycoumarins namely 3-acetyl-7-hydroxycoumarin and 3-acetyl-6- hydroxycoumarin. Both compounds were synthesized using the Knoevenagel condensation method with respective hydroxybenzaldehydes under basic conditions. The synthesized hydroxycoumarins served as an intermediate for the subsequent coupling reaction to produce three new coumarinyl chalcones known as 7-hydroxy-3- [3-(4'-hydroxyphenyl)prop-2-enoyl]-2H-1-benzopyran-2-one, 6-hydroxy-3-[3-(4'- hydroxyphenyl)prop-2-enoyl]-2H-1-benzopyran-2-one and 7-hydroxy-3-[3-(3',4'- dihydroxyphenyl)prop-2-enoyl]-2H-1-benzopyran-2-one respectively. The target products were synthesized via the Claisen-Schmidt condensation reaction utilizing the Lewis acid, boron trifluoride-etherate (BF3-Et2O) in 1,4-dioxane. The structures of the synthetic compounds were confirmed by spectroscopic techniques which includes the Infrared (IR) and Nuclear Magnetic Resonance (NMR) (1H, 13C and DEPT) Spectroscopies. The antioxidant activities of all synthesized compounds were evaluated using three antioxidant assays known as the 2,2-diphenyl-1-picrylhydrazyl (DPPH), 2,2'-azinobis-(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS) and Ferric Reducing Ability of Plasma (FRAP) assays using the Bio-Tek Epoch microplate reader. Among the compounds tested, the coumarinyl chalcones exhibited a better antioxidant activity compared to the hydroxycoumarins. 7-Hydroxy-3-[3-(3',4'- dihydroxyphenyl)prop-2-enoyl]-2H-1-benzopyran-2-one proved to be the best antioxidant in all assays with SC50 values of 0.57 mM and 0.036 mM in the ABTS and DPPH assays respectively. The FRAP value of this compound falls between the range of 0.09 mM to 1.67 mM. The values were comparable with the positive control used in these assays known as butylated hydroxyanisole (BHA).