(E)-6-Hydroxy-2-oxo-2H-chromen-7-yl 3-(4-hydroxy-3-methoxyphenyl)acrylate

Abstract

A hybrid compound 5: (E)-6-hydroxy-2-oxo-2H-chromen-7-yl 3-(4-hydroxy-3-methoxyphenyl)acrylate composed of (E)-3-(4-hydroxy-3-methoxyphenyl)acrylic acid (ferulic acid) 1 and 6,7-hydroxycoumarin (esculetin) 3 was prepared in a 61% yield by the esterification reaction of protected ferulic acid 2a with esculetin 3 in the presence of triethylamine in dichloromethane for 3 h, followed by deprotection using 3M HCl. The structure of compound 5 was confirmed by ¹H, ¹³C NMR spectroscopy, mass-spectrometry and elemental analysis.

1. Introduction

Many coumarin-based derivatives are important structural scaffolds for the synthesis of potential biologically active compounds with different pharmacological applications [1]. They continue to be designed and synthesized [2] because of their remarkable biological properties, including anticancer [3], anticonvulsant [4], antimicrobial [5], and antiviral [6] activities. Coumarins with an intramolecular charge transfer character have also been investigated for fluorescence sensors [7,8]. Among them, 6,7-dihydroxycoumarin (esculetin) 3 (Figure 1) displayed various biological activities such as anticancer [9,10], free radical scavenging [11], anti-inflammatory [12], anti-arthritic [13], and hepatoprotective [14]. On the other hand, (E)-3-(4-hydroxy-3-methoxyphenyl)acrylic acid (ferulic acid) 1 (Figure 1), a phenolic acid widely present in seeds, vegetables, and fruits, has many pharmacological effects, including antioxidant [15], anticancer [16], neuroprotective [17], and anti-metabolic syndrome [18]. It has been widely used in the food, pharmaceutical, and cosmetic industries. However, there have been no reports on the synthesis of the hybrid compounds composed of 1 and 3. We report herein the synthesis of a new hybrid compound 5 that is of potential biological interest, (E)-6-hydroxy-2-oxo-2H-chromen-7-yl 3-(4-hydroxy-3-methoxyphenyl)acrylate.

2. Results

The new compound 5 was prepared as shown in Scheme 1. The hydroxy group of the starting material 1 was first protected with acetic anhydride and pyridine according to the previously reported procedure [19] to give 2, (E)-3-(4-acetoxy-3-methoxyphenyl)- acrylic acid. After 2 was activated with oxalyl chloride, including DMF, the resultant 2a was allowed to react with 3 in dichloromethane at room temperature for 3 h in the presence of triethylamine to afford an esterified product 4, (E)-6-hydroxy-2-oxo-2H-chromen-7-yl 3-(4-acetoxy-3-methoxyphenyl)acrylate at a 72% yield. The deprotection of the acetyl group of 4 was achieved through the use of a 3M HCl solution in acetone at room temperature for 24 h to give a conjugate compound 5 in an 85% yield.

The ¹H NMR spectrum of 4 showed an expected pattern with two sharp singlets at δ 3.82 and 2.24 ppm, which were attributed to methoxy and acetyl protons, respectively, and two doublets at δ 7.81 and 6.93 ppm (J = 16.0 Hz) due to trans vinyl protons in the ferulic acid moiety. It also showed two doublets at δ 7.89 and 6.24 ppm (J = 9.5 Hz) due to the cis vinyl protons of esculetin moiety, and the aromatic protons were shown as two singlets at δ 7.48, 6.86 and three doublets at δ 7.59 (d, J = 1.7 Hz), 7.36 (dd, J = 8.2, 1.7 Hz) and 7.13 ppm (d, J = 8.2 Hz). A sharp singlet at δ 10.93 of the low field was shown for a hydroxy proton of esculetin moiety (Supplementary Materials). In the ¹³C NMR spectrum, compound 4 displayed three peaks δ 168.8, 165.0, 160.7 ppm for the two carbonyls and newly formed ester carbon, including sixteen peaks for aromatic and vinyl carbons at δ 153.6, 153.2, 151.7, 146.4, 144.5, 141.9, 136.1, 133.3, 123.8, 122.5 (2C), 117.7, 112.9, 112.8, 111.4, 104.0 ppm, and two peaks for two methyl carbons at δ 56.6, 20.9 ppm. The mass spectrum showed m/z = 395 (M⁺ − 1) corresponding to the molecular formula, C₂₁H₁₆O₈, and elemental analysis also provided satisfactory results.

Compound 5 was confirmed by signals δ 10.88 (s, OH, 1H), 9.64 (s, OH, 1H), 7.88 (d, cis vinyl proton, J = 9.5 Hz, 1H), 7.71 (d, trans vinyl proton, J = 15.9 Hz, 1H), 7.45 (s, 1H), 7.39 (d, Ar, J = 1.6 Hz, 1H), 7.18 (dd, Ar, J = 8.2, 1.7 Hz, 1H), 6.85 (s, Ar, 1H), 6.79 (d, Ar, J = 8.2 Hz, 1H), 6.69 (d, trans vinyl proton, J = 15.9 Hz, 1H), 6.24 (d, cis vinyl proton, J = 9.5 Hz, 1H), 3.80 (s, OMe, 3H) in the ¹H NMR, and signals δ 165.3, 160.7, 153.7, 153.1, 150.3, 148.5, 147.6, 144.5, 136.3, 125.9, 124.1, 122.5, 116.1, 113.6, 112.8, 112.1, 111.3, 104.0, 56.6 in the ¹³C NMR spectrum. It showed the absence of signals such as acetyl protons at δ 2.24 ppm in the ¹H NMR and carbonyl carbon of acetyl at δ 168.8 ppm in the ¹³C NMR spectrum, compared to the spectra of compound 4. Two singlets due to two hydroxy groups, including deprotection, were shown at δ 10.88 and 9.64 ppm in the ¹H NMR spectrum. The mass spectrum provided m/z = 353 (M⁺-1) corresponding to the molecular formula, C₁₉H₁₄O₇, and elemental analysis gave satisfactory results. The preliminary biological test of DPPH’s free radical scavenging activity [20,21] for 4 and 5 as an antioxidant exhibited SC₅₀ values of 40.4 and 2.36 μg/mL, respectively, compared to 1 (2.58 μg/mL) and 3 (0.82 μg/mL) with ascorbic acid (1.65 μg/mL) as the positive control.

In conclusion, a new hybrid compound 5 was effectively prepared at a 61% yield by the esterification reaction of a protected ferulic acid 2a with esculetin 3 in the presence of triethylamine in dichloromethane for 3 h, followed by the deprotection of the acetyl group using 3M of HCl in acetone. This compound could be useful as a potential material with various biological activities.

3. Materials and Methods

3.1. General Information

Ferulic acid, esculetin, oxalic chloride, acetic anhydride, triethylamine, 1,1-diphenyl-2-picryhydrazyl (DPPH), ascorbic acid, and the dry organic solvents were purchased from Sigma-Aldrich (St. Louis, MO, USA) and TCI (Tokyo, Japan). The melting point was determined on the Kofler apparatus. Thin-layer chromatography (TLC) was used to monitor reactions and was performed using aluminum sheets precoated with silica gel 60 (HF₂₅₄, Merck, Waltham, MA, USA) and visualized with UV radiation (Fisher Scientific, Waltham, MA, USA). The ¹H and ¹³C NMR spectrum was recorded in deuterated DMSO with TMS as the standard on a JEOL JNM-ECZ600R 500 FT-NMR (Tokyo, Japan). The mass spectrum was obtained with AGILENT1100 LCMS (Santa Clara, CA, USA) under electrospray ionization (ESI) conditions. The absorbance for the compounds was measured using a SpectraMax Paradigm multi-mode microplate reader (San Jose, CA, USA).

3.2. Synthesis of (E)-6-hydroxy-2-oxo-2H-chromen-7-yl 3-(4-acetoxy-3-methoxyphenyl)acrylate (4)

To a stirred solution of 2 (1.0 g, 4.23 mmol) in dry dichloromethane (20 mL), a few drops of DMF was added in the form of oxalyl chloride (1.07 g, 8.45 mmol) and stirred at room temperature for 2 h. After the evaporation of the solution, the mixture was diluted with dichloromethane (20 mL) and was added with 3 (0.75 g, 4.23 mmol) and triethylamine (1.19 mL, 8.50 mmol). The resulting solution was stirred at room temperature for 3 h with monitoring. When the reaction was complete, the mixture was washed with a 0.1M HCl solution (10 mL) and water (10 mL) and extracted with dichloromethane (2 × 15 mL). The organic extracts were dried over MgSO₄, filtered, and concentrated to dryness. The crude product was purified by column chromatography (eluent: ethyl acetate/n-hexane = 1/1, v/v) and recrystallized from ethanol to give a white solid of 4 at a 72% yield (1.20 g). Mp 212–213 °C; TLC Rf = 0.48 (dichloromethane/MeOH = 90/10). ¹H NMR (500 MHz, DMSO-d₆) (ppm) δ 10.93 (s, 1H), 7.89 (d, J = 9.5 Hz, 1H), 7.81 (d, J = 16.0 Hz, 1H), 7.59 (d, J = 1.7 Hz, 1H), 7.48 (s, 1H), 7.36 (dd, J = 8.2, 1.7 Hz, 1H), 7.13 (d, J = 8.2 Hz, 1H), 6.93 (d, J = 16.0 Hz, 1H), 6.86 (s, 1H), 6.24 (d, J = 9.5 Hz, 1H), 3.81 (s, 3H), 2.24 (s, 3H). ¹³C NMR (126 MHz, DMSO-d₆) (ppm) δ 168.8, 165.0, 160.7, 153.6, 153.2, 151.7, 146.4, 144.5, 141.9, 136.1, 133.3, 123.8, 122.5 (2C), 116.1, 113.6, 112.8, 112.1, 111.3, 104.0, 56.3. MS (ESI) m/z = 395 (M⁺ − 1). Anal. calcd. for C21H16O₈, %: C, 63.64; H, 4.07. Found, %: C, 63.88; H, 4.20.

3.3. Synthesis of (E)-6-hydroxy-2-oxo-2H-chromen-7-yl 3-(4-hydroxy-3-methoxyphenyl)acrylate (5)

A solution of 4 (1.0 g, 2.82 mmol) in acetone (15 mL) containing 3M HCl (1 mL) was heated at 60 °C while stirring for 24 h. After the reaction was complete, the mixture was added to saturated aqueous sodium bicarbonate (10 mL) and was extracted with ethyl acetate (2 × 15 mL). The organic extracts were dried over MgSO₄, filtered, and concentrated under reduced pressure. The crude product was purified by column chromatography (eluent: dichloromethane/MeOH = 95/5, v/v) and recrystallized from ethanol to give a white solid of 5 at an 85% yield (0.84 g). Mp 232–233 °C; TLC Rf = 0.38 (dichloromethane/MeOH = 90/10). ¹H NMR (500 MHz, DMSO-d₆) (ppm) δ 10.88 (s, 1H), 9.64 (s, 1H), 7.88 (d, J = 9.5 Hz, 1H), 7.71 (d, J = 15.9 Hz, 1H), 7.45 (s, 1H), 7.39 (d, J = 1.6 Hz, 1H), 7.18 (dd, J = 8.2, 1.7 Hz, 1H), 6.85 (s, 1H), 6.79 (d, J = 8.2 Hz, 1H), 6.69 (d, J = 15.9 Hz, 1H), 6.24 (d, J = 9.5 Hz, 1H), 3.80 (s, 3H). ¹³C NMR (126 MHz, DMSO-d₆) (ppm) δ 165.3, 160.7, 153.7, 153.1, 150.3, 148.5, 147.6, 144.5, 136.3, 125.9, 124.1, 122.5, 116.1, 113.6, 112.8, 112.1, 111.3, 104.0, 56.6. MS (ESI) m/z = 353 (M⁺ − 1). Anal. calcd. for C19H14O₇, %: C, 64.41; H, 3.98. Found, %: C, 64.30; H, 4.09.

3.4. DPPH Radical Scavenging Assay for the Compounds

Each sample was dissolved in methanol at various concentrations ranging from 0 to 100 μg/mL. Then, 50 μL of the sample solution was mixed with 450 μL of a DPPH solution (400 μM) and incubated for 30 min at 4 °C. The absorbance was measured at 517 nm using a microplate reader (SpectraMax Paradigm). The SC₅₀, which is the minimum concentration (μg/mL) required scavenging at 50% of the DPPH radicals, was calculated based on the measured absorbance. Ascorbic acid was used as a positive control.