Synthesis and Evaluation of Novel Fluorinated 2-Styrylchromones as Antibacterial Agents

Momin, Mehbub; Ramjugernath, Deresh; Chenia, Hafizah; Koorbanally, Neil A.

doi:https://doi.org/10.1155/2013/436758

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Abstract Introduction Methods Results and Discussion Conclusion Acknowledgments References Copyright Related Articles

Research Article | Open Access

Volume 2013 | Article ID 436758 | https://doi.org/10.1155/2013/436758

Synthesis and Evaluation of Novel Fluorinated 2-Styrylchromones as Antibacterial Agents

Mehbub Momin,¹Deresh Ramjugernath,²Hafizah Chenia,³and Neil A. Koorbanally¹

Academic Editor: Hugo Verli

Received27 May 2013

Revised08 Aug 2013

Accepted16 Aug 2013

Published24 Sept 2013

Abstract

A range of fluorinated 2-styrylchromones (5a–g) of which six were new (5a–f) were prepared in three steps using the Baker-Venkataraman rearrangement along with two methoxylated derivatives (5h-i) and a methylenedioxy derivative (5j) and screened for their antibacterial activity using Gram-positive bacteria (Staphylococcus aureus, sciuri, and xylosus as well as Bacillus subtilis) and Gram-negative bacteria (Escherichia coli, Pseudomonas aeruginosa, and Klebsiella pneumonia). The compounds were most effective against B. subtilis followed by S. aureus and a single strain of E. coli (ATCC 25922). Difluorination on the phenyl ring was shown to enhance antibacterial activity, and fluorine substitution at the 6 position was shown to be far superior to substitution at the 7 position. In comparison to tetracycline, the activity indices of the fluorinated styrylchromones ranged from 0.50 to 0.75 against B. subtilis. The crystal structure of 2′-fluoro-2-styrylchromone is also presented, and the molecule was shown to be planar.

1. Introduction

Fluorinated compounds have a wide range of medical applications such as anti-inflammatory, antiviral, anti-HIV, antibacterial, anticancer, antimalarial, antidepressants, antipsychotics, anaesthetics, and steroids [1, 2]. Introducing fluorine atoms into drugs can also alter the rate and route of drug metabolism [1], and stereoelectronic factors associated with the fluorine atom can lead to changes in the biological action of molecules in comparison to its hydroxyl or hydrogen analogues [3]. The substitution of fluorine for hydrogen or hydroxy groups can lead to changes in the mechanism of the drug as well as enzyme inhibition [3]. The small size of the fluorine atom, the enhanced lipophilicity it imparts to the molecules, and the electronegativity of the atom often result in improved therapeutic drugs [2]. As part of an ongoing study on fluorinated pharmaceutical compounds, we have chosen to explore the antibacterial effects of fluorinated 2-styrylchromones.

The biological activities of 2-styrylchromones have recently been reviewed by Gomes et al. [4]. The 2-styrylchromones were shown to be A₃ adenosine receptor antagonists [5], have hepatoprotective activity [6], be potent antioxidants [7], have antiallergic properties [8], antiviral activity [9], and anticancer activity [10–12], and display xanthine oxidase inhibition to treat, for example, gout, hypertension, and hepatitis linked to xanthine oxidase activity [13].

The synthesis of these compounds has been reviewed by Silva et al. [14]. In one synthetic approach, it involves the aldol condensation between cinnamaldehydes and 2′-hydroxyacetophenones followed by an oxidative cyclisation [15]. It was also synthesised by the Baker-Venkataraman rearrangement, involving the O-acylation of 2′-hydroxyacetophenones with cinnamic acids, followed by rearrangement of the ester and then cyclisation into the styrylchromone [16]. They can also be made directly from 2′-hydroxyacetophenones with cinnamoyl chlorides [17].

The 2-styrylchromones have a structure analogous to the flavonoids, with an extra two-carbon olefinic bond between the chromone and the phenyl rings. Thus, instead of a phenyl group attached to the chromone portion as in the flavonoids, a styryl group is inserted instead (see 5 in Figure 1). Due to the double bond in the backbone of the structure, 2-styrylchromones are reactive molecules, being able to take part in pericyclic reactions acting as dienes forming xanthones [18] and as dienophiles forming flavones with ortho-benzoquinodimethane [19]. They can also be transformed into pyrazolines [20, 21], 1,2,3-triazoles [22], pyrazoles [23–25], and pyrimidines [26].

To our knowledge, there have only been two studies on fluorinated styrylchromones, where the 6-fluoro styrylchromones have shown antirhinovirus activity [27] and the 4′-fluoro-, the 4′-trifluoromethyl-, and 4′-trifluoromethoxy-styrylchromones were shown to have antitumor activity [28]. We report here on the synthesis and antibacterial activity of a series of fluorinated 2-styrylchromones as well as the crystal structure for the 2′-fluoro-2-styrylchromone.

2. Material and Methods

2.1. General Experimental Procedures

Reagents and chemicals used in this study were purchased from Sigma Aldrich via Capital Laboratories, South Africa, and were reagent grade. All organic solvents were redistilled and dried according to standard procedures. NMR spectra were recorded using a Bruker 400 MHz spectrometer at room temperature with chemical shifts (δ) recorded against the internal standard, tetramethylsilane (TMS). 2D NMR spectroscopy (COSY, NOESY, HSQC, and HMBC) were used for the structural elucidation of the synthesised compounds. IR spectra were recorded on a Perkin Elmer Spectrum 100 FT-IR spectrometer with universal ATR sampling accessory. For GC-MS analyses, the samples were analysed on an Agilent GC-MSD apparatus equipped with DB-5SIL MS (30 m 0.25 mm i.d., 0.25 μm film thickness) fused-silica capillary column. Helium (at 2 mL min⁻¹) was used as a carrier gas. The MS was operated in the EI mode at 70 eV. Optical rotation was recorded using a Perkin Elmer, Model 341 Polarimeter. Melting points were recorded on an Ernst Leitz Wetzlar microhot stage melting point apparatus.

2.2. Typical Procedure for the Preparation of Cinnamic Acids

For the preparation of the cinnamic acids 2a–c and 2h-i, the procedure in Qian et al. [29] was adopted with slight modifications. The required aromatic aldehydes (3.2 mmol), malonic acid (3.87 mmol), and piperidine (0.387 mmol) were dissolved in pyridine (10 mL) and stirred at 80–90°C for 4-5 hours. The pyridine was removed under vacuum, and the reaction mixture was poured into water (25 mL) and washed with HCl (3 10 mL). A precipitate formed which was filtered and washed thrice with hexane (3 × 10 mL), after which it was dried under vacuum to afford the cinnamic acids 2a–c and 2h-i (Figure 1), whose ¹H NMR was consistent with that in the literature [30–34]. The cinnamic acids 2d–g and 2j were purchased from Sigma Aldrich via Capital Laboratories, South Africa.

2′-Fluorocinnamic acid (2a) yield 91%; mp 175-176°C (lit. 176.5–177°C [30]); 3′-fluorocinnamic acid (2b) yield 84%, mp 166-167°C (lit. 167°C [35]); 4′-fluorocinnamic acid (2c) yield 88%, mp 206–208°C (lit. 208°C [32]); 4′-methoxycinnamic acid (2h) yield 92%, mp 172-173°C (lit. 173–175°C [33]); 3′,4′-dimethoxycinnamic acid (2i) yield 83%, mp 181–183°C (lit. 182–184°C [36]).

2.3. Typical Procedure for the Synthesis of Substituted 2-(Cinnamoyloxy) Acetophenones (3a–j)

POCl₃ (15.6 mmol, 2.39 g) was added to a solution of the appropriate 2-hydroxyacetophenone (12.0 mmol) and the appropriate cinnamic acid (15.6 mmol) in dry pyridine (10 mL). The solution was stirred at 60–70°C for 3 h and then poured into ice and water (20 mL), and the reaction mixture acidified with HCl (pH 3-4). The obtained solid was removed by filtration, dissolved in EtOAc (100 mL), and purified by silica gel column chromatography using a 7 : 3 mixture of EtOAc : n-hexane as the eluent. The solvent was evaporated to dryness, and the residue recrystallized from EtOH, resulting in compounds 3a–j.

2′-(2-Fluorocinnamoyloxy) Acetophenone (3a). Brown solid residue (90% yield); mp 68–70°C; IR (KBr) : 1682 (br C=O), 1627 (C=C), 1612 (aromatic C–C), 1483, 1456, 1284 (C–F), 1227 cm⁻¹; ¹H NMR (CDCl₃, 400 MHz) δ 8.00 (1H, d, J = 16.2 Hz, H-β), 7.85 (1H, dd, J = 7.9, 1.6 Hz, H-6′), 7.59 (1H, td, J = 7.9, 1.7 Hz, H-6′′), 7.54 (1H, td, J = 7.6, 1.6 Hz, H-4′), 7.39-7.40 (1H, m, H-4′′), 7.33 (1H, td, J = 7.6, 0.8 Hz, H-5′), 7.19 (1H, dd, J = 8.0, 0.8 Hz, H-3′), 7.18 (1H, t, J = 7.5 Hz, H-5′′), 7.11 (1H, dd, J = 10.3, 8.8 Hz, H-3′′), 6.76 (1H, d, J = 16.2 Hz, H-α), 2.55 (3H, s, CH₃-2); ¹³C NMR (CDCl₃, 100 MHz) δ 197.7 (C-1), 165.1 (C=O), 161.8 (d, = 252.6 Hz, C-2′′), 149.1 (C-2′), 140.0 (d, J = 2.7 Hz, C-β), 133.4 (C-4′), 132.2 (d, J = 14.2 Hz, C-4′′), 131.3 (C-1′′), 130.2 (C-6′), 129.4 (d, J = 2.7 Hz, C-6′′), 126.1 (C-5′), 124.6 (d, J = 3.6 Hz, C-5′′), 123.8 (C-3′), 122.2 (d, J = 11.6 Hz, C-1′′), 119.4 (d, J = 6.9 Hz, C-α), 116.3 (d, J = 21.7 Hz, C-3′′), 29.8 (C-2); ¹⁹F NMR (CDCl₃, 376.5 MHz) δ −113.57; EIMS (probe) 70 eV, m/z (rel. int.): 284 M⁺ (3), 149 (100), 121 (63), 101 (65), 75 (15); calculated molecular mass: 284.28.

2′-(3-Fluorocinnamoyloxy) Acetophenone (3b). Brown solid residue (68% yield): mp 55-56°C; IR (KBr) : 1733 and 1673 (C=O), 1637 (C=C), 1444, 1136 (C–F), 1073 cm⁻¹; ¹H NMR (CDCl₃, 400 MHz) δ 7.83 (1H, dd, J = 7.6, 1.6 Hz, H-6′), 7.82 (1H, d, J = 16.0 Hz, H-β), 7.55 (1H, td, J = 7.8, 1.6 Hz, H-4′), 7.35–7.37 (2H, m, H-5′, H-6′′), 7.33 (1H, td, J = 7.7, 0.8, H-5′′), 7.27 (1H, d, J = 9.6 Hz, H-2′′), 7.17 (1H, dd, J = 8.0, 0.7 Hz, H-3′), 7.10 (1H, tt, J = 8.2, 2.0 Hz, H-4′′), 6.55 (1H, d, J = 16.0 Hz, H-α), 2.55 (3H, s, CH₃-2); ¹³C NMR (CDCl₃, 100 MHz) δ 197.7 (C-1), 164.9 (C=O), 163.0 (d, = 245.6 Hz, C-3′′), 149.0 (C-2′), 145.8 (d, J = 2.7 Hz, C-β), 136.3 (d, J = 7.9 Hz, C-1′′), 133.4 (C-4′), 131.2 (C-1′), 130.6 (d, J = 8.0 Hz, C-5′′), 130.2 (C-6′), 126.2 (C-5′), 124.4 (d, J = 2.9 Hz, C-6′′), 123.8 (C-3′), 118.3 (C-α), 117.7 (d, J = 21.3 Hz, C-4′′), 114.6 (d, J = 21.9 Hz, C-2′′), 30.0 (C-2); ¹⁹F NMR (CDCl₃, 376.5 MHz) δ −112.27; EIMS (probe) 70 eV, m/z (rel. int.): 284 M⁺ (3), 149 (100), 121 (60), 101 (55), 75 (11); calculated molecular mass: 284.28.

2′-(4-Fluorocinnamoyloxy) Acetophenone (3c). Cream solid residue (72% yield); mp 80–82°C; IR (KBr) : 1729 (C=O), 1670 (C=O), 1624 (C=C), 1590, 1446, 1221 (C–F), 1202, 1159, 1050 cm⁻¹; ¹H NMR (CDCl₃, 400 MHz) δ 7.84 (1H, d, J = 16.0 Hz, H-β), 7.81 (1H, dd, J = 8.0, 1.6 Hz, H-6′), 7.58 (2H, dd, J = 8.6, 5.4 Hz, H-2′′/6′′), 7.53 (1H, dd, J = 8.0, 1.5 Hz, H-4′), 7.33 (1H, td, J = 8.1, 0.7 Hz, H-5′), 7.17 (1H, dd, J = 8.1, 0.7 Hz, H-3′), 7.09 (2H, t, J = 8.6 Hz, H-3′′/5′′), 6.58 (1H, d, J = 16.0 Hz, H-α), 2.54 (s, 3H, CH₃-2); ¹³C NMR (CDCl₃, 100 MHz) δ 197.8 (C-1), 165.1 (C=O), 164.3 (d, = 250.7 Hz, C-4′′), 149.1 (C-2′), 146.0 (C-β), 133.4 (C-4′), 131.3 (C-1′), 130.4 (d, J = 8.4 Hz, C-2′′/6′′), 130.32 (d, J = 3.6 Hz, C-1′′), 130.2 (C-6′), 126.1 (C-5′), 123.8 (C-3′), 116.6 (d, J = 2.4 Hz, C-α), 116.2 (d, J = 21.9 Hz, C-3′′/5′′), 29.7 (C-2); ¹⁹F NMR (CDCl₃, 376.5 MHz) δ −108.54; EIMS (probe) 70 eV, (m/z, rel. int.) 284 M⁺ (21), 149 (100), 121 (25), 101 (20); calculated molecular mass: 284.28.

2′-(3,5-Difluorocinnamoyloxy) Acetophenone (3d). Brown solid residue (70% yield); mp 58-59°C; IR (KBr) : 1729 (C=O), 1682 (C=O), 1249 (C–F), 1201, 1122 cm⁻¹; ¹H NMR (CDCl₃, 400 MHz) δ 7.82 (1H, dd, J = 7.9, 1.0 Hz, H-6′), 7.75 (1H, d, J = 16.0 Hz, H-β), 7.55 (1H, td, J = 7.6, 1.1 Hz, H-4′), 7.34 (1H, t, J = 7.6 Hz, H-5′), 7.16 (1H, dd, J = 7.9, 0.8 Hz, H-3′), 7.08–7.10 (2H, m, H-2′′/6′′), 6.85 (1H, tt, J = 8.7, 2.3 Hz, H-4′′), 6.64 (1H, d, J = 16.0 Hz, H-α), 2.54 (3H, s, CH₃-2); ¹³C NMR (CDCl₃, 100 MHz) δ 197.6 (C-1), 164.6 (C=O), 163.2 (dd, = 248.3, 12.8 Hz, C-3′′/5′′), 148.9 (C-2′), 144.5 (t, J = 2.8 Hz, C-β), 137.3 (d, J = 9.5 Hz, C-1′′), 133.5 (C-4′), 131.0 (C-1′), 130.3 (C-6′), 126.3 (C-5′), 123.7 (C-3′), 119.7 (C-α), 111.0 (dd, J = 18.8, 7.2 Hz, C-2′′/6′′), 105.9 (t, J = 25.4 Hz, C-4′′), 29.5 (C-2); ¹⁹F NMR (CDCl₃, 376.5 MHz) δ −108.75; EIMS (probe) 70 eV, (m/z, rel. int.): 302 M⁺ (3), 167 (100), 139 (79), 119 (60); calculated molecular mass: 302.27.

4′-Fluoro-2′-(4-fluorocinnamoyloxy) Acetophenone (3e). Off-white solid residue (68% yield); mp 60–62°C; IR (KBr) : 1724 (C=O), 1679 (C=O), 1361 (C–O), 1225 (C–F), 1143 cm⁻¹; ¹H NMR (CDCl₃, 400 MHz) δ 7.87 (1H, dd, J = 8.8, 6.3 Hz, H-6′), 7.84 (1H, d, J = 16.0 Hz, H-β), 7.58 (2H, dd, J = 5.4, 2.0 Hz, H-2′′/6′′), 7.10 (2H, dd, J = 8.7, 2.5 Hz, H-3′′/5′′), 7.03 (1H, td, J = 8.8, 2.5 Hz, H-5′), 6.92 (dd, J = 8.9, 2.5 Hz, H-3′), 6.56 (1H, d, J = 16.0 Hz, H-α), 2.53 (3H, s, CH₃-2); ¹³C NMR (CDCl₃, 100 MHz) δ 196.1 (C-1), 165.1 (C=O), 165.0 (d, = 254.1 Hz, C-4′), 164.4 (d, = 251.0 Hz, C-4′′), 151.0 (d, J = 11.2 Hz, C-2′), 146.6 (C-β), 132.2 (d, J = 10.1 Hz, C-6′), 130.5 (d, J = 8.5 Hz, C-2′′/6′′), 130.2 (d, J = 3.0 Hz, C-1′′), 127.6 (d, J = 3.5 Hz, C-1′), 116.3 (d, J = 21.9 Hz, C-3′′/5′′), 116.1 (d, J = 2.2 Hz, C-α), 113.3 (d, J = 21.2 Hz, C-5′), 111.7 (d, J = 24.0 Hz, C-3′), 29.7 (C-2); ¹⁹F NMR (CDCl₃, 376.5 MHz) δ −103.81, −103.17; EIMS (probe) 70 eV (m/z, rel. int.) 302 M⁺ (3), 149 (100), 121 (92), 101 (75); calculated molecular mass: 302.27.

4′-Fluoro-2′-cinnamoyloxy Acetophenone (3f). Brown solid residue (86% yield); mp 98–100°C; IR (KBr) : 1730 (C=O), 1678 (C=O), 1634, 1598, 1247 (C–F), 1100, 886 cm⁻¹; ¹H NMR (CDCl₃, 400 MHz) δ 7.88 (1H, d, J = 15.9 Hz, H-β), 7.86 (1H, dd, J = 8.6, 5.4 Hz, H-6′), 7.58 (2H, dd, J = 7.5, 1.9 Hz, H-2′′/6′′), 7.43–7.45 (3H, m, H-3′′/4′′/5′′), 7.03 (1H, ddd, J = 8.6, 7.9, 2.5 Hz, H-5′), 6.94 (1H, dd, J = 8.9, 2.5 Hz, H-3′), 6.63 (1H, d, J = 15.9 Hz, H-α), 2.53 (3H, s, CH₃-2); ¹³C NMR (CDCl₃, 100 MHz) δ 196.1 (C-1), 166.4 (d, = 255.8 Hz, C-4′), 164.8 (C=O), 151.0 (C-2′), 145.4 (C-β), 133.9 (C-1′′), 132.3 (d, J = 10.2 Hz, C-6′), 131.1 (C-4′′), 129.0 (C-2′′/6′′), 128.5 (C-3′′/5′′), 127.0 (C-1′), 116.3 (C-α), 113.4 (d, J = 21.1 Hz, C-3′), 111.7 (d, J = 24.1 Hz, C-5′), 29.8 (C-2); ¹⁹F NMR (CDCl₃, 376.5 MHz) δ −103.91; EIMS (probe) 70 eV (m/z, rel. int.) 284 M⁺ (3), 131 (100), 103 (71), 77 (39), 51 (11); calculated molecular mass: 284.28.

5′-Fluoro-2′-cinnamoyloxy Acetophenone (3g). Brown solid residue (90% yield); mp 81–83°C; IR (KBr) : 1731 (C=O), 1681 (C=O), 1632, 1581, 1131 (C–F), 983 cm⁻¹; ¹H NMR (CDCl₃, 400 MHz) δ 7.88 (1H, d, J = 15.9 Hz, H-β), 7.58-7.59 (2H, m, H-2′′/6′′), 7.49 (1H, dd, J = 8.7, 3.0 Hz, H-3′), 7.39–7.41 (3H, m, H-3′′/4′′/5′′), 7.23 (1H, dd, J = 7.8, 3.0 Hz, H-4′), 7.15 (1H, dd, J = 8.7, 4.7 Hz, H-6′), 6.64 (1H, d, J = 15.9 Hz, H-α), 2.53 (3H, s, CH₃-2); ¹³C NMR (CDCl₃, 100 MHz) δ 196.4 (C-1), 165.2 (C=O), 159.9 (d, = 245.1 Hz, C-5′), 147.8 (C-2′), 145.0 (C-β), 133.9 (C-1′′), 132.6 (d, J = 6.1 Hz, C-1′), 131.0 (C-4′′), 129.0 (C-3′′/5′′), 128.5 (C-2′′/6′′), 125.4 (d, J = 8.0 Hz, C-3′), 120.1 (d, J = 23.3 Hz, C-6′), 116.5 (d, J = 20.5 Hz, C-4′), 116.6 (C-α), 29.8 (C-2); ¹⁹F NMR (CDCl₃, 376.5 MHz) δ −115.35; EIMS (probe) 70 eV (m/z, rel. int.) 284 M⁺ (30), 266 (8), 145 (25), 131 (100), 103 (44), 77 (21); calculated molecular mass: 284.28.

2′-(4-Methoxycinnamoyloxy) Acetophenone (3h). Off-white solid residue (91% yield); mp 97–99°C (lit. 103–105°C [20]); IR (KBr) : 1711 (C=O), 1680 (C=O), 1600 (C=C), 1509, 1581, 1246, 1189 cm⁻¹; ¹H NMR (CDCl₃, 400 MHz) δ 7.83 (1H, d, J = 15.9 Hz, H-β), 7.80 (1H, dd, J = 8.0, 1.6 Hz, H-6′), 7.53 (2H, d, J = 8.7 Hz, H-2′′/6′′), 7.51 (1H, td, J = 7.6, 1.6 Hz, H-4′), 7.31 (1H, td, J = 8.0, 0.8 Hz, H-5′), 7.17 (1H, d, J = 8.0 Hz, H-3′), 6.91 (dd, J = 8.7, 2.6 Hz, H-3′′/5′′), 6.52 (1H, d, J = 15.9 Hz, H-α), 3.84 (3H, s, OCH₃), 2.54 (3H, s, CH₃-2); ¹³C NMR (CDCl₃, 100 MHz) δ 197.9 (C-1), 165.5 (C=O), 161.9 (C-4′′), 149.3 (C-2′), 147.2 (C-β), 133.3 (C-4′), 131.5 (C-1′), 130.2 (C-2′′/6′′), 130.0 (C-6′), 126.8 (C-1′′), 126.0 (C-5′), 123.8 (C-3′), 114.5 (C-3′′/5′′), 114.1 (C-α), 55.4 (OCH₃), 29.9 (C-2); EIMS (probe) 70 eV (m/z, rel. int.) 296 M⁺ (7), 161 (100), 133 (49), 118 (16), 90 (15), 77 (16); calculated molecular mass: 296.10.

2′-(3,4-Dimethoxycinnamoyloxy) Acetophenone (3i). Off-white solid residue (56% yield); mp 99–101°C (lit. 97–99°C [37]); IR (KBr) : 1728 (C=O), 1683 (C=O), 1633 (C=C), 1515, 1254 cm⁻¹; ¹H NMR (CDCl₃, 400 MHz) δ 7.82 (1H, d, J = 15.9 Hz, H-β), 7.81 (1H, dd, J =7.8, 1.7 Hz, H-6′), 7.54 (td, J = 7.9, 1.6 Hz, H-4′), 7.31 (1H, td, J = 7.6, 0.9 Hz, H-5′), 7.17 (1H, d, J = 8.0 Hz, H-3′), 7.16 (1H, dd, J = 8.2, 1.9 Hz, H-6′′), 7.10 (1H, d, J = 1.9 Hz, H-2′′), 6.87 (1H, d, J = 8.2 Hz, H-5′′), 6.52 (1H, d, J = 15.9 Hz, H-α), 3.91 (6H, s, 2 x OCH₃), 2.55 (3H, s, CH₃-2); ¹³C NMR (CDCl₃, 100 MHz) δ 197.9 (C-1), 165.5 (C=O), 151.7 (C-2′), 149.3 (C-4′′), 149.2 (C-3′′), 147.4 (C-β), 133.3 (C-4′), 131.5 (C-1′), 130.1 (C-6′), 127.0 (C-1′′), 126.0 (C-5′), 123.8 (C-3′), 123.3 (C-6′′), 114.4 (C-α), 111.1 (C-5′′), 109.8 (C-2′′), 55.9 (OCH₃), 56.0 (OCH₃), 29.9 (C-2); EIMS (probe) 70 eV (m/z, rel. int.) 326 M⁺ (20), 191 (100), 163 (36), 148 (19), 77 (22); calculated molecular mass: 326.10.

2′-(3,4-Methylenedioxycinnamoyloxy) Acetophenone (3j). Off-white solid residue (59% yield), mp 99-100°C, IR (KBr) : 1715 (C=O), 1679 (C=O), 1600 (C=C), 1449, 1202 (C–F), 925 cm⁻¹; ¹H NMR (CDCl₃, 400 MHz) δ 7.80 (1H, dd, J = 7.9, 1.6 Hz, H-6′), 7.78 (1H, d, J = 15.9 Hz, H-β), 7.53 (1H, td, J = 7.9, 1.6 Hz, H-4′), 7.31 (1H, td, J = 7.9, 1.6 Hz, H-5′), 7.16 (1H, d, J = 7.9 Hz, H-3′), 7.08 (1H, dd, J = 7.9, 1.6 Hz, H-6′′), 7.05 (1H, dd, J = 7.9, 1.6 Hz, H-2′′), 6.82 (1H, d, J = 7.9 Hz, H-5′′), 6.47 (1H, d, J = 15.9 Hz, H-α), 6.01 (2H, s, OCH₂O), 2.54 (3H, s, CH₃-2); ¹³C NMR (CDCl₃, 100 MHz) δ 197.8 (C-1), 165.4 (C=O), 150.2 (C-2′), 149.2 (C-4′′), 148.5 (C-3′′), 147.1 (C-β), 133.3 (C-4′), 131.5 (C-1′), 130.1 (C-6′), 128.5 (C-1′′), 126.0 (C-5′), 125.2 (C-3′), 123.8 (C-6′′), 114.6 (C-α), 108.6 (C-5′′), 106.7 (C-2′′), 101.7 (OCH₂O), 29.9 (C-2); EIMS (probe) 70 eV (m/z, rel. int.) 310 M⁺ (12), 175 (100), 145 (64), 117 (24), 89 (40), 63 (16); calculated molecular mass: 310.30.

2.4. Typical Procedure for the Synthesis of Substituted 3-Hydroxy-1-(2-hydroxyphenyl)-5-(phenyl)-2,4-pentadien-1-ones (4a-j)

KOH powder (0.05 mmol, 2.8 g) was added to a solution of 2-cinnamoyloxy acetophenones 3a–j (10.0 mmol) in Me₂SO (15 mL). The solution was stirred at room temperature until complete disappearance of the starting material, which was monitored by TLC. A typical reaction time was 2 h. The solution was then poured into ice water and HCl (20 mL) and the pH adjusted to 5. The obtained solid was removed by filtration, dissolved in EtOAc (150 mL), and purified by silica gel chromatography using EtOAc : n-hexane (7 : 3) as the eluent. The solvent was evaporated to dryness, and the residue recrystallized from EtOH, resulting in 4a-j.

3-Hydroxy-1-(2-hydroxyphenyl)-5-(2-fluorophenyl)-2,4-pentadien-1-one (4a). Pale yellow solid residue (93% yield); mp 158–160°C, IR (KBr) : 1680 (C=O), 1626, 1581, 1483, 1283 (C–F), 1227 (C–O) cm⁻¹; ¹H NMR (CDCl₃, 400 MHz) δ 14.55 (s, 3-OH), 12.17 (s, 2′-OH), 7.73 (1H, d, J = 16.0 Hz, H-5), 7.69 (1H, dd, J = 8.0, 1.4 Hz, H-6′), 7.54 (1H, td, J = 7.7, 1.5 Hz, H-6′′), 7.43 (1H, ddd, J = 8.5, 7.1, 1.4 Hz, H-4′), 7.31-7.32 (1H, m, H-4′′), 7.16 (1H, t, J = 7.6 Hz, H-5′′), 7.09 (1H, t, J = 8.2 Hz, H-3′′), 6.97 (1H, dd, J = 8.5, 0.7 Hz, H-3′), 6.88 (1H, t, J = 8.1 Hz, H-5′), 6.70 (1H, d, J = 16.0 Hz, H-4), 6.32 (1H, s, H-2); ¹³C NMR (CDCl₃, 100 MHz) δ 196.5 (C-1), 174.0 (C-3), 162.6 (C-2′), 161.4 (d, = 253.8 Hz, C-2′′), 136.1 (C-4′), 132.6 (d, J = 2.2 Hz, C-5), 131.4 (d, J = 8.8 Hz, C-4′′), 129.2 (d, J = 3.0 Hz, C-6′′), 128.6 (C-6′), 124.8 (d, J = 7.8 Hz, C-4), 124.5 (d, J = 3.5 Hz, C-5′′), 123.1 (d, J = 11.5 Hz, C-1′′), 119.06 (C-5′), 119.04 (C-1′), 118.7 (C-3′), 116.3 (d, J = 21.9 Hz, C-3′′), 97.4 (C-2); ¹⁹F NMR (CDCl₃, 376.5 MHz) δ −114.18; EIMS (probe) 70 eV (m/z, rel. int.) 284 M⁺ (26), 264 (7), 149 (100), 121 (59), 101 (20); calculated molecular mass: 284.28.

3-Hydroxy-1-(2-hydroxyphenyl)-5-(3-fluorophenyl)-2,4-pentadien-1-one (4b). Yellow solid residue (72% yield), mp 115–117°C, IR (KBr) : 1641 (C=O), 1626 (C=C), 1581, 1488, 1429, 1294 (C–F), 1236 cm⁻¹; ¹H NMR (CDCl₃, 400 MHz) δ 14.55 (s, 3-OH), 12.15 (s, 2′-OH), 7.68 (1H, dd, J = 8.0, 2.0 Hz, H-6′), 7.58 (1H, d, J = 15.8 Hz, 1H, H-5), 7.44 (1H, ddd, J = 8.5, 7.1, 1.5 Hz, H-4′), 7.34 (1H, dd, J = 7.9, 5.7 Hz, H-5′′), 7.30 (1H, d, J = 7.8 Hz, H-6′′), 7.24-7.25 (1H, m, H-2′′), 7.06-7.07 (1H, m, H-4′′), 6.89 (1H, ddd, J = 8.0, 7.1, 0.9 Hz, H-5′), 6.97 (1H, dd, J = 7.9, 0.9 Hz, H-3′), 6.56 (1H, d, J = 15.8 Hz, H-4), 6.32 (1H, s, H-2); ¹³C NMR (CDCl₃, 100 MHz) δ 196.3 (C-1), 173.6 (C-3), 164.9 (d, = 247.2 Hz, C-3′′), 162.7 (C-2′), 138.3 (d, J = 2.5 Hz, C-5), 137.3 (d, J = 7.8 Hz, C-1′′), 136.0 (C-4′), 130.5 (d, J = 8.23 Hz, C-5′′), 128.5 (C-6′), 124.1 (d, J = 2.8 Hz, C-6′′), 123.5 (C-4), 119.1 (C-5′), 119.0 (C-1′), 118.8 (C-3′), 116.9 (d, J = 21.6 Hz, C-4′′), 114.1 (d, J = 20.0 Hz, C-2′′), 97.4 (C-2); ¹⁹F NMR (CDCl₃, 376.5 MHz) δ −112.32; EIMS (probe) 70 eV (m/z, rel. int.) 284 M⁺ (25), 149 (100), 265 (8), 121 (88), 101 (17); calculated molecular mass: 284.28.

3-Hydroxy-1-(2-hydroxyphenyl)-5-(4-fluorophenyl)-2,4-pentadien-1-one (4c). Pale yellow solid residue (92% yield); mp 130–132°C, IR (KBr) : 1683 (C=O), 1627 (C=C), 1598, 1572, 1489, 1156 (C–F) cm⁻¹; ¹H NMR (CDCl₃, 400 MHz) δ 14.62 (s, 3-OH), 12.17 (s, 2′-OH), 7.68 (1H, dd, J = 8.0, 1.4 Hz, H-6′), 7.60 (1H, d, J = 16.0 Hz, H-5), 7.52 (2H, dd, J = 8.9, 5.4 Hz, H-2′′/6′′), 7.44 (1H, ddd, J = 8.5, 7.1, 1.4 Hz, H-4′), 7.08 (2H, t, J = 8.9 Hz, H-3′′/5′′), 6.97 (1H, dd, J = 8.5, 0.9 Hz, H-3′), 6.88 (1H, ddd, J = 8.1, 7.1, 0.9 Hz, H-5′), 6.49 (1H, d, J = 16.0 Hz, H-4), 6.29 (1H, s, H-2); ¹³C NMR (CDCl₃, 100 MHz) δ 196.0 (C-1), 174.3 (C-3), 163.8 (d, = 250.3 Hz, C-4′′), 162.6 (C-2′), 138.5 (C-5), 135.9 (C-4′), 130.2 (d, J = 3.5 Hz, C-1′′), 129.8 (d, J = 8.2 Hz, C-2′′/6′′), 128.5 (C-6′), 121.9 (C-4), 119.04 (C-1′/5′), 118.8 (C-3′), 116.2 (d, J = 21.9 Hz, C-3′′/5′′), 97.0 (C-2); ¹⁹F NMR (CDCl₃, 376.5 MHz) δ −109.55; EIMS (m/z, rel. int.) 284 M⁺ (21), 149 (100), 121 (71), 265 (4), 163 (16), 101 (18); calculated molecular mass: 284.28.

3-Hydroxy-1-(2-hydroxyphenyl)-5-(3,5-difluorophenyl)-2,4-pentadien-1-one (4d). Light brown solid residue (91% yield), mp 130–132°C, IR (KBr) : 1698 (C=O), 1658 (C=C), 1119 (C–F), 962, 843 cm⁻¹; ¹H NMR (CDCl₃, 400 MHz) δ 14.46 (s, 3-OH), 12.10 (s, 2′-OH), 7.67 (1H, dd, J = 8.0, 1.4 Hz, H-6′), 7.51 (1H, d, J = 15.7 Hz, H-5), 7.45 (1H, ddd, J = 8.5, 7.2, 1.6 Hz, H-4′), 7.04 (1H, dd, J = 8.2, 2.2 Hz, H-2′′/6′′), 6.98 (1H, dd, J = 8.5, 1.1 Hz, H-3′), 6.89 (1H, ddd, J = 8.1, 7.2, 1.1 Hz, H-5′), 6.80 (1H, tt, J = 8.8, 2.2 Hz, H-4′′), 6.55 (1H, d, J = 15.7 Hz, H-4), 6.32 (1H, s, H-2); ¹³C NMR (CDCl₃, 100 MHz) δ 196.4 (C-1), 172.8 (C-3), 163.3 (d, = 247.8 Hz, C-3′′/5′′), 162.7 (C-2′), 138.3 (t, J = 9.5 Hz, C-1′′), 137.0 (C-5), 136.2 (C-4′), 128.6 (C-6′), 124.8 (C-4), 119.1 (C-5′), 118.94 (C-1′), 118.85 (C-3′), 110.5 (dd, J = 18.5, 6.8 Hz, C-2′′/6′′), 105.1 (d, J = 25.6 Hz, C-4′′), 97.9 (C-2); ¹⁹F NMR (CDCl₃, 376.5 MHz) δ −109.10; EIMS (m/z, rel. int.) 302 M⁺ (28), 167 (100), 121 (76), 285 (10), 139 (29), 121 (76); calculated molecular mass: 302.27.

3-Hydroxy-1-(4-fluoro-2-hydroxyphenyl)-5-(4-fluorophenyl)-2,4-pentadien-1-one (4e). Yellow solid residue (82% yield); mp 143–145°C; IR (KBr) : 1726 (C=O), 1629 (C=C), 1234 (C–F), 1157, 975, 824, 803, 789 cm⁻¹; ¹H NMR (CDCl₃, 400 MHz) δ 14.42 (s, 3-OH), 12.47 (s, 2′-OH), 7.60 (1H, d, J = 15.9 Hz, H-5), 7.68 (1H, dd, J = 9.0, 6.4 Hz, H-6′), 7.52 (1H, dd, J = 8.7, 5.4 Hz, H-2′′/6′′), 7.08 (2H, t, J = 8.6 Hz, H-3′′/5′′), 6.65 (1H, dd, J = 10.4, 2.5 Hz, H-3′), 6.60 (1H, ddd, J = 8.8, 8.2, 2.2 Hz, H-5′), 6.51 (1H, d, J = 15.9 Hz, H-4), 6.20 (1H, s, H-2); ¹³C NMR (CDCl₃, 100 MHz) δ 194.9 (C-1), 174.2 (C-3), 166.4 (d, = 212.1 Hz, C-4′), 165.2 (d, J = 14.1 Hz, C-2′), 163.0 (d, = 252.6 Hz, C-4′′), 138.7 (C-5), 130.7 (d, J = 11.9 Hz, C-1′′), 130.4 (d, J = 10.8 Hz, C-6′), 129.9 (d, J = 8.6 Hz, C-2′′/6′′), 121.7 (C-4), 116.2 (d, J = 21.9 Hz, C-3′′/5′′), 116.0 (C-1′), 107.3 (d, J = 22.6 Hz, C-5′), 105.3 (d, J = 23.6 Hz, C-3′), 96.7 (C-2); ¹⁹F NMR (CDCl₃, 376.5 MHz) δ −100.64, −109.57; EIMS (m/z, rel. int.) 302 M⁺ (41), 149 (100), 283 (18), 207 (11), 163 (35), 139 (95), 121 (37), 101 (35); calculated molecular mass: 302.27.

3-Hydroxy-1-(4-fluoro-2-hydroxyphenyl)-5-phenyl-2,4-pentadien-1-one (4f). Yellow solid residue (64% yield); mp 143–145°C; IR (KBr) : 1632 (C=O), 1579 (C=C), 1178 (C–F) cm⁻¹; ¹H NMR (CDCl₃, 400 MHz) δ 14.48 (s, 3-OH), 12.55 (s, 2′-OH), 7.68 (1H, dd, J = 8.9, 6.4 Hz, H-6′), 7.64 (1H, d, J = 15.8 Hz, H-5), 7.53 (dd, J = 8.1, 2.1 Hz, H-2′′/6′′), 7.37–7.39 (3H, m, H-3′′/4′′/5′′), 6.65 (dd, J = 10.3, 2.5 Hz, H-3′), 6.60 (1H, td, J = 8.0, 2.50 Hz, H-5′), 6.57 (1H, d, J = 15.8 Hz, H-4), 6.21 (1H, s, H-2); ¹³C NMR (CDCl₃, 100 MHz) δ 194.9 (C-1), 174.4 (C-3), 165.2 (d, = 209.2 Hz, C-4′), 165.1 (d, J = 14.1 Hz, C-2′), 140.1 (C-5), 134.9 (C-1′′), 130.5 (d, J = 11.7 Hz, C-6′), 130.2 (C-4′′), 129.0 (C-3′′/5′′), 128.0 (C-2′′/6′′), 122.0 (C-4), 116.0 (C-1′), 107.3 (d, J = 22.7 Hz, C-5′), 105.3 (d, J = 23.4 Hz, C-3′), 96.8 (C-2); ¹⁹F (CDCl₃, 376.5 MHz) δ −100.72; EIMS (m/z, rel. int.) 284 M⁺ (33), 131 (100), 265 (14), 139 (64), 103 (42), 77 (39), 51 (11); calculated molecular mass: 284.28.

3-Hydroxy-1-(5-fluoro-2-hydroxyphenyl)-5-phenyl-2,4-pentadien-1-one (4g). Yellow solid residue (90% yield); mp 118–120°C; IR (KBr): 1632 (C=O), 1550, 1487, 1248, 1180, 960, 781, 754 cm⁻¹; ¹H NMR (CDCl₃, 400 MHz) δ 14.59 (s, 3-OH), 11.94 (s, 2′-OH), 7.66 (1H, d, J = 15.8 Hz, H-5), 7.54 (2H, dd, J = 7.9, 2.2 Hz, H-2′′/6′′), 7.39–7.41 (3H, m, H-3′′/4′′/5′′), 7.34 (1H, dd, J = 9.0, 3.0 Hz, H-6′), 7.17 (1H, ddd, J = 9.2, 3.0, 1.3 Hz, H-4′), 6.93 (1H, dd, J = 9.1, 4.7 Hz, H-3′), 6.58 (1H, d, J = 15.8 Hz, H-4), 6.20 (1H, s, H-2); ¹³C NMR (CDCl₃, 100 MHz) δ 194.8 (d, J = 2.7 Hz, C-1), 175.2 (C-3), 158.7 (C-2′), 155.1 (d, = 236.8 Hz, C-5′), 140.6 (C-5), 134.8 (C-1′′), 130.4 (C-4′′), 129.0 (C-3′′/5′′), 128.1 (C-2′′/6′′), 123.2 (d, J = 23.4 Hz, C-4′), 121.9 (C-4), 120.0 (d, J = 7.41 Hz, C-3′), 118.7 (d, J = 6.5 Hz, C-1′), 113.46 (d, J = 23.5 Hz, C-6′), 96.8 (C-2); ¹⁹F (CDCl₃, 376.5 MHz) δ −124.33; EIMS (probe) 70 eV (m/z, rel. int.) 284 M⁺ (5), 131 (100), 103 (80), 77 (35); calculated molecular mass: 284.28.

3-Hydroxy-1-(2-hydroxyphenyl)-5-(4-methoxyphenyl)-2,4-pentadien-1-one (4h). Yellow solid residue (90% yield); mp 167–169°C (lit. 162–164°C [38]); IR (KBr) : 1645 (C=O), 1599, 1514, 1462, 1258, 963, 828, 749 cm⁻¹; ¹H NMR (CDCl₃, 400 MHz) δ 14.72 (s, 3-OH), 12.24 (s, 2′-OH), 7.67 (1H, dd, J = 8.0, 1.6 Hz, H-6′), 7.61 (1H, d, J = 15.8 Hz, H-5), 7.49 (2H, d, J = 8.8 Hz, H-2′′/6′′), 7.42 (1H, ddd, J = 8.5, 7.5, 1.6 Hz, H-4′), 6.96 (1H, dd, J = 8.5, 2.1 Hz, H-3′), 6.91 (2H, d, J = 8.8 Hz, H-3′′/5′′), 6.88-6.89 (1H, m, H-5′), 6.45 (d, J = 15.8 Hz, H-4), 6.26 (1H, s, H-2), 3.83 (s, 3H, OCH₃); ¹³C NMR (CDCl₃, 100 MHz) δ 195.3 (C-1), 174.9 (C-3), 162.3 (C-2′), 161.1 (C-4′′), 139.5 (C-5), 135.3 (C-4′), 129.8 (C-1′′), 129.4 (C-2′′/6′′), 128.8 (C-6′), 119.4 (C-4), 118.9 (C-1′), 118.7 (C-5′), 118.44 (C-3′), 114.2 (C-3′′/5′′), 96.1 (C-2), 55.2 (OCH₃); EIMS (probe) 70 eV (m/z, rel. int.) 296 M⁺ (14), 161 (100), 207 (18), 133 (77), 118 (29); calculated molecular mass: 296.10.

3-Hydroxy-1-(2-hydroxyphenyl)-5-(3,4-dimethoxyphenyl)-2,4-pentadien-1-one (4i). Yellow solid residue (84% yield); mp 130–132°C (lit. 136–138°C [37]); IR (KBr): 1685 (C=O), 1621, 1564, 1488, 1252, 1161; ¹H NMR (CDCl₃, 400 MHz) δ 14.71 (s, 3-OH), 12.23 (s, 2′-OH), 7.67 (1H, dd, J = 8.1, 1.5 Hz, H-6′), 7.59 (1H, d, J = 15.8 Hz, H-5), 7.42 (1H, ddd, J = 8.5, 8.3, 1.5 Hz, H-4′), 7.11 (dd, J = 8.3, 1.9 Hz, H-6′′), 7.1 (d, J = 1.8 Hz, H-2′′), 6.96 (1H, dd, J = 8.4, 0.7 Hz, H-3′), 6.87 (1H, d, J = 8.3 Hz, H-5′′), 6.85 (1H, td, J = 8.3, 0.7 Hz, H-5′), 6.45 (1H, d, J = 15.8 Hz, H-4), 6.28 (1H, s, H-2), 3.92 (3H, s, 4′-OCH₃), 3.91 (3H, s, 3′-OCH₃); ¹³C NMR (CDCl₃, 100 MHz) δ 195.6 (C-1), 175.0 (C-3), 162.5 (C-2′), 151.1 (C-3′′), 149.3 (C-4′′), 140.0 (C-5), 135.6 (C-4′), 128.4 (C-6′), 128.0 (C-1′′), 122.6 (C-6′′), 119.9 (C-4), 119.1 (C-1′), 119.0 (C-5′), 118.7 (C-3′), 111.2 (C-5′′), 109.7 (C-2′′), 96.5 (C-2), 56.0 (3′′-OCH₃), 55.93 (4′′-OCH₃); EIMS (probe) 70 eV (m/z, rel. int.) 326 M⁺ (15), 191 (100), 207 (16), 163 (49), 148 (19), 133 (18), 77 (23); calculated molecular mass: 326.12.

3-Hydroxy-1-(2-hydroxyphenyl)-5-(3,4-methylenedioxyphenyl)-2,4-pentadien-1-one (4j). Light yellow solid residue (94% yield); mp 165–167°C; IR (KBr) : 1693 (C=O), 1621, 1602, 1566, 1484, 1446, 1239 (C–O), 1171, 1035, 925 cm⁻¹; ¹H NMR (CDCl₃, 400 MHz) δ 14.68 (s, 3-OH), 12.21 (s, 2′-OH), 7.66 (1H, dd, J = 8.0, 1.6 Hz, H-6′), 7.55 (1H, d, J = 15.8 Hz, H-5), 7.42 (1H, ddd, J = 8.5, 8.0, 1.6 Hz, H-4′), 7.04 (1H, bd, J = 0.35 Hz, H-2′′), 7.02 (1H, dd, J = 8.0, 1.2 Hz, H-6′′), 6.96 (1H, dd, J = 8.5, 0.5 Hz, H-3′), 6.87 (1H, td, J = 8.0, 0.5 Hz, H-5′), 6.81 (1H, d, J = 8.0 Hz, H-5′′), 6.39 (1H, d, J = 15.8 Hz, H-4), 6.26 (1H, s, H-2), 6.00 (2H, s, OCH₂O); ¹³C NMR (CDCl₃, 100 MHz) δ 195.7 (C-1), 174.8 (C-3), 162.6 (C-2′), 149.6 (C-3′′), 148.5 (C-4′′), 139.7 (C-5), 135.7 (C-4′), 129.5 (C-1′′), 128.4 (C-6′), 124.6 (C-6′′), 120.1 (C-4), 119.1 (C-1′), 119.0 (C-5′), 118.73 (C-3′), 108.7 (C-5′′), 106.3 (C-2′′), 101.6 (OCH₂O), 96.6 (C-2); EIMS (probe) 70 eV (m/z, rel. int.) 310 M⁺ (18), 175 (100), 207 (28), 145 (87), 157 (42), 117 (44), 89 (52), 43 (62); calculated molecular mass: 310.30.

2.5. Typical Procedure for the Synthesis of Substituted 2-Styrylchromones (5a–j)

p-Toluenesulfonic acid (3.42 mmol, 0.59 g) was added to a solution of the appropriate 3-hydroxy-1-(2-hydroxyphenyl)-5-(phenyl)-2,4-pentadien-1-ones 4a–j (6.5 mmol) in Me₂SO (20 mL). The reaction mixture was heated at 90°C for 2 h and then poured into ice and water (20 mL) and stirred for 10 min. The obtained solid was removed by filtration, dissolved in CHCl₃ (100 mL), and washed with a 20% aqueous solution of sodium thiosulphate (3 10 mL). The solvent was evaporated to dryness, and the residue was purified by silica gel chromatography, using CHCl₃ : n-hexane (7 : 3) as the eluent, to produce 5a–j.

2′-Fluro-2-styrylchromone (5a). Light yellow solid residue (68% yield); mp 150–152°C; UV (CH₃OH) nm (log ε): 325 (3.37); IR (KBr) : 1682 (C=O), 1625, 1589 (C–C), 1562, 1464, 1391 (C–F), 1125, 968 cm⁻¹; ¹H NMR (CDCl₃, 400 MHz) δ 8.17 (1H, dd, J = 7.9, 1.6 Hz, H-5), 7.72 (1H, d, J = 16.2 Hz, H-β), 7.66 (1H, ddd, J = 8.6, 7.2, 1.6 Hz, H-7), 7.59 (1H-td, J = 7.6, 1.5 Hz, H-6′), 7.53 (1H, d, J = 8.3 Hz, H-8), 7.37 (1H, td, J = 7.9, 0.8 Hz, H-6), 7.31-7.32 (1H, m, H-4′), 7.17 (1H, t, J = 7.9 Hz, H-5′), 7.11 (1H, ddd, J = 9.2, 8.2, 2.4 Hz, H-3′), 6.87 (1H, d, J = 16.2 Hz, H-α), 6.32 (1H, s, H-3); ¹³C NMR (CDCl₃, 100 MHz) δ 178.5 (C-4), 161.5 (C-2), 161.2 (d, = 253.3 Hz, C-2′), 156.0 (C-9), 133.9 (C-7), 131.3 (d, J = 8.7 Hz, C-4′), 129.5 (d, J = 3.1 Hz, C-β), 128.4 (d, J = 2.7 Hz, C-6′), 125.7 (C-5), 125.1 (C-6), 124.6 (d, J = 3.6 Hz, C-5′), 124.1 (C-10), 123.1 (d, J = 11.7 Hz, C-1′), 122.7 (d, J = 6.5 Hz, C-α), 117.9 (C-8), 116.2 (d, J = 21.8 Hz, C-3′), 111.2 (C-3); ¹⁹F NMR (CDCl₃, 376.5 MHz) δ −115.39; EIMS (m/z, rel. int.) 265 (M⁺-1) (100), 237 (12), 207 (20), 146 (36), 92 (25); HRMS (m/z) M⁺ 266.0733 (calculated for C₁₇H₁₁FO₂: 266.0743).

3′-Fluro-2-styrylchromone (5b). Brown solid residue (62% yield), mp 105–108°C; UV (CH₃OH) nm (log ε): 325 (3.34); IR (KBr) : 1694 (C=O), 1622, 1579 (C–C), 1465, 1389 (C–F), 1247, 1122, 967, 775 cm⁻¹; ¹H NMR (CDCl₃, 400 MHz) δ 8.18 (dd, J = 7.9, 1.6 Hz, H-5), 7.68 (dt, J = 8.6, 1.6 Hz, H-7), 7.52 (d, J = 8.3 Hz, H-8), 7.55 (1H, d, J = 16.0 Hz, H-β), 7.35–7.37 (3H, m, H-5′/6′/6), 7.26-7.27 (1H, m, H-2′), 7.06 (1H, d, J = 6.8 Hz, H-4′), 6.77 (1H, d, J = 16.0 Hz, H-α), 6.34 (1H, s, H-3); ¹³C NMR (CDCl₃, 100 MHz) δ 178.5 (C-4), 163.2 (d, = 245.5 Hz, C-3′), 161.2 (C-2), 156.0 (C-9), 137.3 (d, J = 7.8 Hz, C-1′), 135.6 (d, J = 2.8 Hz, C-β), 133.9 (C-7), 130.5 (d, J = 8.3 Hz, C-5′), 125.8 (C-5), 125.1 (C-6), 124.1 (C-10), 123.61 (d, J = 2.7 Hz, C-6′), 121.7 (C-α), 117.9 (C-8), 116.7 (d, J = 21.6 Hz, C-4′), 114.0 (d, J = 22.0 Hz, C-2′), 111.2 (C-3); ¹⁹F NMR (CDCl₃, 376.5 MHz) δ −108.99; EIMS (m/z, rel. int.) 265 (M⁺-1) (100), 237 (6), 209 (8), 173 (16), 146 (40), 121 (20), 92 (27); HRMS (m/z): 266.0726 M⁺ (calculated for C₁₇H₁₁FO₂: 266.0743).

4′-Fluoro-2-styrylchromone (5c). Off-white solid residue (70% yield), mp 158–160°C; UV (CH₃OH) nm (log ε): 328 (3.39); IR (KBr): 1691 (C=O), 1623, 1594, 1506, 1466, 1391 (C–F), 1224, 969, 817 cm⁻¹; ¹H NMR (CDCl₃, 400 MHz) δ 8.01 (1H, dd, J = 7.9, 1.4 Hz, H-5), 7.81-7.82 (1H, m, H-7), 7.79 (2H, m, H-2′/6′), 7.70 (1H, d, J = 16.2 Hz, H-β), 7.69 (1H, d, J = 8.5 Hz, H-8), 7.47 (1H, t, J = 7.4 Hz, H-6), 7.28 (2H, t, J = 8.8 Hz, H-3′/5′), 7.16 (1H, d, J = 16.2 Hz, H-α), 6.46 (1H, s, H-3); ¹³C NMR (CDCl₃, 100 MHz) δ 177.1 (C-4), 162.88 (d, = 240.6 Hz, C4’), 161.7 (C-2), 155.4 (C-9), 135.4 (C-β), 134.4 (C-7), 131.6 (d, J = 3.2 Hz, C-1′), 130.0 (d, J = 8.1 Hz, C-2′/6′), 125.3 (C-6), 124.8 (C-5), 123.4 (C-10), 120.4 (C-α), 118.2 (C-8), 116.0 (d, J = 24.3 Hz, C-3′/5′), 110.1 (C-3); ¹⁹F NMR (CDCl₃, 376.5 MHz) δ −110.72; EIMS (m/z, rel. int.) 265 (M⁺-1) (100), 237 (8), 207 (13), 173 (10), 146 (39), 120 (18), 92 (20); HRMS (m/z): 266.0721 M⁺ (calculated for C₁₇H₁₁FO₂: 266.0743).

3′,5′-Difluoro-2-styrylchromone (5d). Light brown solid residue (92% yield); mp 114–116°C; UV (CH₃OH) nm (log ε) 322 (3.49); IR (KBr) : 1701 (C=O), 1615, 1586, 1465, 1390 (C–F), 1309, 1272, 1117 (C–F), 966, 847, 751 cm⁻¹; ¹H NMR (CDCl₃, 400 MHz) δ 8.18 (1H, dd, J = 7.9, 1.6 Hz, H-5), 7.72 (1H, ddd, J = 8.5, 7.2, 1.6 Hz, H-7), 7.51 (1H, d, J = 8.3 Hz, H-8), 7.49 (1H, d, J = 16.0 Hz, H-β), 7.39 (1H, td, J = 7.9, 0.7 Hz, H-6), 7.08 (2H, dd, J = 8.1, 1.9 Hz, H-2′/6′), 6.76 (1H, d, J = 16.0 Hz, H-α), 6.81 (1H, tt, J = 8.7, 2.4 Hz, H-4′), 6.34 (1H, s, H-3); ¹³C NMR (CDCl₃, 100 MHz) δ 178.3 (C-4), 163.4 (dd, = 247.8, 12.9 Hz, C3′/5′), 160.6 (C-2), 156.0 (C-9), 138.3 (t, J = 11.2 Hz, C-1′), 134.0 (C-7), 134.2 (t, J = 3.0 Hz, C-β), 125.8 (C-5), 125.3 (C-6), 124.1 (C-10), 123.0 (C-α), 117.9 (C-8), 111.7 (C-3), 110.3 (dd, J = 18.5, 7.2 Hz, C-2′/6′), 105.0 (t, J = 25.4 Hz, C-4′); ¹⁹F NMR (CDCl₃, 376.5 MHz) δ −109.31; EIMS (m/z, rel. int.) 284 M⁺ (100), 267 (82), 191 (40), 164 (63), 121 (58), 92 (65), 64 (21); HRMS (m/z): 284.0633 M⁺ (calculated for C₁₇H₁₀F₂O₂: 284.0649).

7,4′-Difluoro-2-styrylchromone (5e). Pale yellow solid residue (45% yield); mp 182–184°C; UV (CH₃OH) nm (log ε) 322 (3.54); IR (KBr): 1659 (C=O), 1621 (C=C), 1598, 1511, 1438, 1377 (C–F), 1233, 1140, 1112, 967 cm⁻¹; ¹H NMR (CDCl₃, 400 MHz) δ 8.18 (1H, dd, J = 8.8, 6.4 Hz, H-5), 7.56 (2H, dd, J = 8.6, 5.6 Hz, H-2′/6′), 7.53 (1H, d, J = 16.0 Hz, H-β), 7.20 (1H, dd, J = 9.0, 2.4 Hz, H-8), 7.11-7.12 (1H, m, H-6), 7.10 (2H, t, J = 8.6 Hz, H-3′/5′), 6.67 (1H, d, J = 16.0 Hz, H-α), 6.28 (1H, s, H-3); ¹³C NMR (CDCl₃, 100 MHz) δ 177.4 (C-4), 167.1 (d, = 210.1 Hz, C-7), 165.0 (d, = 251.6 Hz, C-4′), 161.8 (C-2), 156.9 (C-9), 135.8 (C-β), 131.2 (d, J = 3.6 Hz, C-1′), 129.5 (d, J = 8.2 Hz, C-2′/6′), 128.2 (d, J = 10.5 Hz, C-5), 121.0 (C-10), 119.7 (C-α), 116.2 (d, J = 21.9 Hz, C-3′/5′), 113.7 (d, J = 22.5 Hz, C-6), 110.6 (C-3), 104.6 (d, J = 25.5 Hz, C-8); ¹⁹F NMR (CDCl₃, 376.5 MHz) δ −102.96, −109.89; EIMS (m/z, rel. int.) 283 (M⁺-1) (100), 267 (56), 255 (8), 227 (13), 173 (13), 146 (50), 120 (10); HRMS (m/z): 284.0642 (calculated for C₁₇H₁₀F₂O₂: 284.0649).

7-Fluoro-2-strychromone (5f). Off-white solid residue (94% yield); mp 116–118°C; UV (CH₃OH) nm (log ε) 312 (3.35); IR (KBr) : 1667 (C=O), 1599, 1538, 1438, 1382 (C–F stretch), 1143, 1012, 960 cm⁻¹; ¹H NMR (CDCl₃, 400 MHz) δ 8.19 (1H, dd, J = 8.9, 6.4 Hz, H-5), 7.59 (1H, d, J = 16.0 Hz, H-β), 7.58 (2H, dd, J = 8.1, 1.5 Hz, H-2′/6′), 7.39–7.41 (3H, m, H-3′/4′/5′), 7.21 (1H, dd, J = 9.1, 2.4 Hz, H-8), 7.10 (1H, td, J = 8.6, 2.4 Hz, H-6), 6.76 (1H, d, J = 16.0 Hz, H-α), 6.29 (1H, s, H-3); ¹³C NMR (CDCl₃, 100 MHz) δ 177.5 (C-4), 164.5 (d, = 240.0 Hz, C-7), 162.0 (C-2), 157.1 (d, J = 13.2 Hz, C-9), 137.2 (C-β), 134.9 (C-1′), 130.0 (C-4′), 129.05 (C-3′/5′), 128.2 (d, J = 10.6 Hz, C-5), 127.7 (C-2′/6′), 121.0 (C-10), 119.9 (C-α), 113.7 (d, J = 22.6 Hz, C-6), 110.6 (C-3), 104.6 (d, J = 25.4 Hz, C-8); ¹⁹F NMR (CDCl₃, 376.5 MHz) δ −103.04; EIMS (m/z, rel. int.) 265 (M⁺-1) (100), 250 (36), 237 (5), 209 (7), 128 (29), 102 (8); HRMS (m/z): 266.0730 (calculated for C₁₇H₁₁FO₂: 266.0743).

6-Fluoro-2-styrylchromone (5g). Light green solid residue (89% yield); mp 108–110°C; IR (KBr): 1710 (C=O), 1628, 1567, 1478, 1445, 1378, 1284, 1172, 967, 818, 751 cm⁻¹; ¹H NMR (CDCl₃, 400 MHz) δ 7.85 (1H, dd, J = 8.2, 3.2 Hz, H-5), 7.60 (1H, d, J = 16.1 Hz, H-β), 7.56 (2H, d, J = 8.0 Hz, H-2′/6′), 7.52 (1H, dd, J = 9.10, 4.15 Hz, H-8), 7.39–7.41 (4H, m, H-3′/4′/5′/7), 6.77 (1H, d, J = 16.1 Hz, H-α), 6.31 (1H, s, H-3); ¹³C NMR (CDCl₃, 100 MHz) δ 177.6 (d, J = 2.3 Hz, C-4), 162.0 (C-2), 159.5 (d, = 245.1 Hz, C-6), 152.20 (C-9), 137.4 (C-β), 134.9 (C-1′), 130.0 (C-4′), 129.1 (C-3′/5′), 127.7 (C-2′/6′), 125.5 (d, J = 7.1 Hz, C-10), 121.8 (d, J = 25.13 Hz, C-7), 120.03 (C-α), 119.9 (d, J = 7.9 Hz, C-8), 110.7 (d, J = 23.4 Hz, C-5), 109.9 (C-3); ¹⁹F NMR (CDCl₃, 376.5 MHz) δ −115.51; EIMS (m/z, rel. int.) 265 (M⁺-1) (100), 249 (43), 237 (9), 209 (12), 128 (56); calculated molecular mass: 266.67.

4′-Methoxy-2-styrylchromone (5h). Yellow solid residue (90% yield); mp 140-141°C (lit. 139-140 [11]; UV (CH₃OH) nm (log ε) 354 (3.33); IR (KBr) : 1645 (C=O), 1599, 1514, 1462, 462, 1258, 963, 828, 749 cm⁻¹; ¹H NMR (CDCl₃, 400 MHz) δ 8.18 (1H, dd, J = 8.0, 1.6 Hz, H-5), 7.65 (1H, ddd, J = 8.6, 7.1, 1.6 Hz, H-7), 7.55 (1H, d, J = 16.0 Hz, H-β), 7.52 (1H, d, J = 8.6 Hz, H-8), 7.48 (2H, d, J = 8.7 Hz, H-2′/6′), 7.36 (1H, t, J = 8.0 Hz, H-6), 6.92 (2H, d, J = 8.7 Hz, H-3′/5′), 6.64 (1H, d, J = 16.0 Hz, H-α), 6.28 (1H, s, H-3), 3.84 (3H, s, OCH₃); ¹³C NMR (CDCl₃, 100 MHz) δ 178.5 (C-4), 162.3 (C-2), 161.1 (C-4′), 156.0 (C-9), 136.7 (C-β), 133.6 (C-7), 131.0 (C-1′), 129.3 (C-2′/6′), 125.7 (C-5), 124.9 (C-6), 124.2 (C-10), 117.90 (C-α), 117.85 (C-8), 114.5 (C-3′/5′), 109.9 (C-3), 55.4 (OCH₃); EIMS (m/z, rel. int.) 277 (M⁺-1) (100), 247 (21), 207 (19), 158 (38), 115 (55); calculated molecular mass: 278.30.

3′,4′-Dimethoxy-2-styrylchromone (5i). Yellow solid residue (55% yield); mp 162-163°C (lit. 163-164°C [11]); UV (CH₃OH) nm (log ε) 367 (3.18); IR (KBr) : 1682 (C=O), 1617, 1558, 1509, 1464, 1381, 1261, 1138, 1025, 965, 780, 759 cm⁻¹; ¹H NMR (CDCl₃, 400 MHz) δ 8.01 (1H, dd, J = 7.9, 1.7 Hz, H-5), 7.81 (1H, ddd, J = 8.2, 7.2, 1.7 Hz, H-7), 7.70 (1H, d, J = 8.2 Hz, H-8), 7.65 (1H, d, J = 16.0 Hz, H-β), 7.47 (1H, ddd, J = 7.9, 7.2, 0.7 Hz, H-6), 7.36 (1H, d, J = 1.7 Hz, H-2′), 7.27 (1H, dd, J = 8.3, 1.7 Hz, H-6′), 7.11 (1H, d, J = 16.0 Hz, H-α), 7.02 (1H, d, J = 8.3 Hz, H-5′), 6.40 (1H, s, H-3), 3.80 (3H, s, 3′-OCH₃), 3.83 (s, 3H, 4′-OCH₃); ¹³C NMR (CDCl₃, 100 MHz) δ 177.0 (C-4), 162.3 (C-2), 155.4 (C-9), 150.5 (C-4′), 149.0 (C-3′), 136.9 (C-β), 134.2 (C-7), 127.8 (C-1′), 125.2 (C-6), 124.7 (C-5), 123.4 (C-10), 122.3 (C-6′), 118.1 (C-8), 118.0 (C-α), 111.7 (C-5′), 109.9 (C-2′), 109.2 (C-3), 55.5 (2 OCH₃); EIMS (m/z, rel. int.) 308 (M⁺) (100), 277 (22), 250 (10), 221 (14), 188 (70), 121 (19); calculated molecular mass: 308.33.

3′,4′-Methylenedioxy-2-styrylchromone (5j). Yellow solid residue (92% yield); mp 209-210°C (lit. 209-210°C [11]); UV (CH₃OH) nm (log ε) 329 (3.36); IR (KBr) : 1694 (C=O), 1625, 1461, 1499, 1447, 1383, 1251, 845 cm⁻¹; ¹H NMR (CDCl₃, 400 MHz) δ 8.17 (1H, d, J = 7.6 Hz, H-5), 7.65 (1H, ddd, J = 8.1, 7.1, 1.0 Hz, H-7), 7.51 (1H, d, J = 7.8 Hz, H-8), 7.50 (1H, d, J = 16.1 Hz, H-β), 7.37 (1H, t, J = 7.5, H-6), 7.08 (1H, s, H-2′), 7.05 (1H, d, J = 8.1 Hz, H-6′), 6.81 (1H, d, J = 8.1 Hz, H-5′), 6.59 (1H, d, J = 16.1, H-α), 6.28 (1H, s, H-3), 6.01 (2H, s, OCH₂O); ¹³C NMR (CDCl₃, 100 MHz) δ 178.5 (C-4), 162.0 (C-2), 156.0 (C-9), 149.3 (C-3′), 148.5 (C-4′), 136.7 (C-β), 133.7 (C-7), 129.5 (C-1′), 125.7 (C-5), 125.0 (C-6), 123.9 (C-6′), 123.3 (C-10), 118.4 (C-α), 117.8 (C-8), 110.2 (C-3), 108.7 (C-5′), 106.2 (C-2′), 101.6 (OCH₂O); EIMS (m/z, rel. int.) 291 (M⁺-1) (100), 275 (55), 233 (18), 205 (24), 172 (67), 114 (29); calculated molecular mass: 292.29.

2.6. X-Ray Crystallographic Study

Single-crystal X-ray diffraction data were collected on a Bruker KAPPA APEX II DUO diffractometer using graphite-monochromated Mo-Kα radiation (χ = 0.71073 Å). Data collection was carried out at 173(2) K. Temperature was controlled by an Oxford Cryostream cooling system (Oxford Cryostat). Cell refinement and data reduction were performed using the program SAINT [39]. The data were scaled, and absorption correction was performed using SADABS [40]. The structure was solved by direct methods using SHELXS-97 [40] and refined by full-matrix least-squares methods based on F² using SHELXL-97 [40] and using the graphics interface program X-Seed [41, 42]. The programs X-Seed and POV-Ray were both used to prepare molecular graphic images. All nonhydrogen atoms were refined anisotropically, and all hydrogen atoms could be found in the difference electron density maps but were placed in idealised positions and refined in riding models with U_iso set at 1.2 times those of their parent atoms and at a distance (C–H) of 0.95 Å. The structure was refined to a R factor of 0.0503.

CCDC 897969 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html (or from the CCDC, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033; e-mail: [email protected]).

2.7. Antibacterial Assay

In vitro evaluation of antibacterial activity was carried out on all synthesized, fluorinated, and oxygenated 2-styrylchromones by the disc diffusion method as described by Bauer et al. [43] against the Gram-positive bacteria, Bacillus subtilis, Enterococcus faecium, and three Staphylococcus species, aureus, sciuri, and xylosus, and the Gram-negative bacteria, Escherichia coli, Klebsiella pneumoniae, and Pseudomonas aeruginosa. The standard antibiotics, tetracycline (Te) and ampicillin (Amp), were used as controls and for comparison. Briefly, Mueller Hilton agar was prepared (38 g in 1 L of water) and poured into prelabeled sterile Petri dishes, which were then allowed to set and dry at room temperature. The bacterial organisms were standardized using a 0.5 McFarland standard turbidity and then swabbed onto the agar plates. Paper discs with dissolved sample and a control disc was placed onto the agar plates, which were inverted and incubated at 35–37°C for 24 hours. The diameter of the inhibition zones were then measured in mm. The tests were done in triplicate, and the results were reported as means of at least three determinations. The results are summarized in Tables 2 and 3.

The activity index of the product 2-styrylchromones was calculated as follows: Activity index (A.I.) = zone of inhibition of compound/Zone of inhibition obtained for the standard antibiotic drug.

3. Results and Discussion

3.1. Chemistry

Seven fluorinated 2-styrylchromones of which six (5a–f) were novel were prepared in good overall yields of between 60 and 90% with only one compound (5e) having a yield of 45%. The synthesis was carried out according to the three-step sequence shown in Figure 1 and based on the Baker-Venkataraman rearrangement [16]. This involved the formation of the desired 2′-cinnamoyloxyacetophenones from substituted ortho-hydroxyacetophenones and cinnamic acid derivatives in pyridine using POCl₃ as a condensing agent. A strong base such as potassium hydroxide abstracts a proton from the methyl ketone, and the resultant carbanion attacks the ester carbonyl resulting in the conversion of the cinnamoyloxyacetophenones to the ketoenols. A ring chain tautomerism then occurs in this intermediate which is subsequently protonated by the strong catalyst para-toluene sulfonic acid, and attack by the 2′-hydroxy group ultimately results in the more stable chromone product by elimination. The cinnamic acids, 2a–c and 2h-I, were prepared by an aldol condensation and elimination reaction from the corresponding benzaldehydes and malonic acid before being reacted with the corresponding acetophenones.

The series of 2-styrylchromones synthesized contained a single fluorine atom on the ortho-, meta-, and para-positions (5a–c) of the phenyl ring, two fluorine atoms at the 3′ and 5′ positions on the phenyl ring (5d), fluorine atoms on both of the aromatic rings (at the 7 and 4′ positions) (5e), and a single fluorine atom on the 6 (5f) and 7 (5g) positions on the chromone ring. These substitution patterns were chosen to observe the effect of fluorine at different positions on the phenyl ring as well as the effect of fluorine on the chromone ring. The difluorinated compounds would provide information on multiple sites of the molecule as well as substitution on both the phenyl and chromone rings simultaneously. Two methoxylated 2-styrylchromones, the 4′-methoxy- and the 3′,4′-dimethoxy-2-styrylchromones as well as the 3′,4′-methylenedioxy-2-styrylchromone (5h–j) were also synthesized to test alongside the fluorinated styrylchromones for comparison.

The structures of the prepared compounds were elucidated using 1D and 2D NMR spectroscopies along with mass spectrometry and IR spectroscopy. Compounds 5g–j and their intermediates were all prepared previously [11, 27]; however only the NMR data for 4g and 5g [27], 3h and 4h [20] and 3i and 4i [37] is available in the literature. Furthermore, only the ¹H NMR data is given for 5g [27], and only the ethylene resonance is reported for 5h–j [11]. The NMR data for 3g, 3j, 4j, and 5g–j is therefore also reported here along with the new compounds 5a–f and their intermediates 3a–f and 4a–f, to provide a complete set of NMR data for all the synthesized 2-styrylchromones and their intermediates.

The cinnamoyloxyacetophenone (3a) was established by the presence of α and β unsaturated proton resonances in the ¹H NMR spectrum at 6.76 and 8.00 as two doublets with large coupling constants of 16.2 Hz, typical of trans olefinic protons, a methyl singlet at 2.55, and aromatic protons between 7.11 and 7.85. The structure of 3a was further supported by two carbonyl resonances in the ¹³C NMR spectrum at 197.7 for the ketone and 165.1 for the ester carbonyl. The aromatic carbon to which fluorine was attached was detected at 161.8 (J = 252.6 Hz). The fluorine NMR resonance at δ −113.57 was used to confirm the presence of fluorine on the aromatic ring, and the structure was confirmed by the detection of the molecular ion at m/z 284 in the EIMS. The other intermediates 3b–j had similar NMR data, and the structures were elucidated in the same manner as 3a. The aromatic oxygenated carbon resonance in 3h was recorded at 161.91 with similar resonances occurring in 3i-j.

Conversion to the ketoenol (4a) was indicated by the disappearance of the methyl singlet resonance and the appearance of a singlet proton resonance at 6.32 for the olefinic α proton. This was supported by the enol carbon resonance at approximately 173.6 and the keto resonance at 196.3. The fluorinated carbon resonance could be seen at 164.9 with a coupling constant of 247.2 Hz and the olefinic C–O resonance at 162.7. The ¹⁹F NMR resonance at δ −112.32 and the molecular ion at m/z 284 in the MS spectrum further confirmed the structure. The other intermediates, 4b–j, were elucidated in a similar manner.

Cyclisation to the 2-styrylchromones was indicated by a marked shift in the H-5 resonance from 7.69 in 4a to 8.17 in 5a. Further to this, only a single pyran carbonyl resonance could be seen at 178.4 in the ¹³C NMR spectrum. The C-2 resonance was evident at 161.5 and the doublet C–F resonance at 161.2 (J = 253.3), which was supported by the ¹⁹F NMR resonance at δ −115.39. The structures of 5b–j were confirmed similarly. All intermediates and final products were further confirmed by 2D NMR spectroscopy and by the presence of the molecular ion using mass spectrometry.

In addition, the crystal structure of 5a was carried out to determine the extent to which the molecule was planar. As can be seen in Figure 2 and from the data in Table 1, the molecule is almost planar with bond angles between 116 and 124°. The compound crystallizes with four planar molecules in the symmetric unit and contains four molecules per unit cell. The molecule has a C–F bond distance of 1.336 Å and a C=O distance of 1.237 Å (Table 2). The fluorine atom is 0.392 Å from the plane, and the Hα and Hβ hydrogen atoms are 0.958 and 1.082 Å from the plane, respectively.

3.2. Antibacterial Activity

The fluorinated derivatives (5a–g) were most effective against Gram-positive bacteria, particularly B. subtilis and S. aureus, with that against B. subtilis being more predominant (Table 2). The two methoxy derivatives (5h and 5i) were only effective against B. subtilis, with the dimethoxy derivative (5i) also being active against a strain of S. aureus (ATCC 29212), while the dioxole derivative (5j) displayed no antibacterial activity against both Gram-negative and Gram-positive bacteria. Thus, in comparing the methoxy (5h-i) and fluoro derivatives (5a–g), the fluoro derivatives were far superior in their activity to the methoxy compounds. Limited antibacterial activity was observed with Gram-negative bacteria (Table 2), with K. pneumoniae and P. aeruginosa being completely resistant to all the tested compounds. Although the addition of fluorine to the benzene ring resulted in antibacterial action against . coli ATCC 25922, it was not effective against the E. coli ATCC 35218 strain (Table 2) and the activity appeared to be strain-specific. The difluorinated styrylchromones showed a broader spectrum, with only 5d and 5e being effective against both E. coli strains tested (Table 2), indicating that multiple fluorinations on the 2-styrylchromone backbone could lead to enhanced activity against E. coli. However, fluorination on the chromone ring only resulted in no activity against E. coli.

The 3′,5′ derivative (5d) showed the greatest activity from all the compounds substituted on the phenyl ring. This compound also showed activity against both E. coli strains tested. This could therefore indicate that the activity of the 2-styrylchromones increases with increased fluorine substitution on the phenyl ring. Fluorination at position 7 on the chromone ring resulted in the compound being active against B. subtilis alone. This activity increased slightly with additional fluorine substitution at the 4′ position, as activity was now experienced with S. sciuri and both the E. coli strains with 5e. However, both compounds with fluorine substitution at position 7 did not show activity against S. aureus. In contrast, the 5g derivative, with fluorine at position 6 of the chromone ring, was the most effective of all the tested compounds, with an observable inhibitory effect against 100% of the Gram-positive bacteria (Table 2). In fact, it was the only compound that showed any activity against E. faecium. This compound however did not show any activity to any of the Gram-negative bacteria. In another study, the 5g derivative (6-fluorinated) also showed antirhinovirus activity by interfering with the replication of the HRV serotype 14 and serotype 1B [27].

Although an activity index of greater than or equal to 1 relative to tetracycline susceptibility is ideal, in the present study activity indices ranged from 0 (no activity) to 0.75 (Table 3). Activity indices ranging from 0.27–0.56 were obtained following testing of the 6-F derivative (5g) against Gram-positive bacteria. Gram-positive organisms appeared to be more susceptible to the fluorine and methoxy derivatives compared to Gram-negative bacteria.

This may be related to their mode of antimicrobial action, which remains to be elucidated. The low activity indices do not preclude the use of these derivatives with antibacterial activity, and these compounds may have the potential to be used as mild antibiotics.

4. Conclusion

Several new fluorinated 2-styrylchromones (5a–5f) were synthesized along with a known fluorinated compound (5g), two methoxylated compounds (5h-i), and a methylenedioxy derivative (5j). The compounds were characterized and screened for their antibacterial activity. In general, the fluorinated compounds displayed antibacterial activity against Gram-positive bacteria more than Gram-negative bacteria, with the fluorinated styrylchromones being most active against B. subtilis followed by S. aureus and then a single strain of E. coli (ATCC 25922), but not the E. coli (ATCC 35218) strain, indicating that their activity toward E. coli is strain specific. However, the styrylchromones with two fluorine substitutions showed activity against both E. coli strains, indicating that a broader spectrum could be obtained with multiple fluorinations on the styrylchromone backbone. Furthermore, the 3′,5′-difluorostyrylchromone (5d) showed the best activity from all the compounds fluorinated on the phenyl ring, also indicating that more fluorine substitutions on the styrylchromone could lead to enhanced activity.

Activity of the styrylchromones substituted on the chromone ring was specific to fluorination at position 6, which showed the best activity amongst all the compounds tested. Fluorination at position 7 was only active against one bacterial strain, B. subtilis. Thus, the position and number of fluorine substituents on either the phenyl or the chromone ring have an effect on the antibacterial activity of the 2-styrylchromones. It is worthwhile exploring the effect of hydroxy, methoxy, and fluorine substitutions on the phenyl ring together with fluorine substitution at position 6, as these compounds may show enhanced activity.

Conflict of Interests

The authors declare that they do not have any financial relations with any of the commercial entities mentioned in the paper that could lead to a conflict of interests.

Acknowledgments

This research was supported by grants from the National Research Foundation (NRF), South Africa, and was supported by the South African Research Chairs Initiative of the Department of Science and Technology.

References

B. K. Park, N. R. Kitteringham, and P. M. O'Neill, “Metabolism of fluorine-containing drugs,” Annual Review of Pharmacology and Toxicology, vol. 41, pp. 443–470, 2001.
View at: Publisher Site | Google Scholar
K. L. Kirk and R. Filler, “Recent advances in the biomedicinal chemistry of fluorine-containing compounds,” in Biomedical Frontiers of Fluorine Chemistry, I. Ojima, J. R. McCarthy, and T. Welch, Eds., vol. 639 of Symposium Series, pp. 1–24, American Chemical Society, Washington, DC, USA, 1996.
View at: Google Scholar
D. O'Hagan and H. S. Rzepa, “Some influences of fluorine in bioorganic chemistry,” Chemical Communications, no. 7, pp. 645–652, 1997.
View at: Google Scholar
A. Gomes, M. Freitas, E. Fernandes, and J. L. F. C. Lima, “Biological activities of 2-styrylchromones,” Mini-Reviews in Medicinal Chemistry, vol. 10, no. 1, pp. 1–7, 2010.
View at: Publisher Site | Google Scholar
Y. Karton, J.-L. Jiang, X.-D. Ji et al., “Synthesis and biological activities of flavonoid derivatives as A₃ adenosine receptor antagonists,” Journal of Medicinal Chemistry, vol. 39, no. 12, pp. 2293–2301, 1996.
View at: Publisher Site | Google Scholar
E. Fernandes, M. Carvalho, F. Carvalho et al., “Hepatoprotective activity of polyhydroxylated 2-styrylchromones against tert-butylhydroperoxide induced toxicity in freshly isolated rat hepatocytes,” Archives of Toxicology, vol. 77, no. 9, pp. 500–505, 2003.
View at: Publisher Site | Google Scholar
P. Filipe, A. M. S. Silva, P. Morlière et al., “Polyhydroxylated 2-styrylchromones as potent antioxidants,” Biochemical Pharmacology, vol. 67, no. 12, pp. 2207–2218, 2004.
View at: Publisher Site | Google Scholar
G. Doria, C. Romeo, and A. Forgione, “Antiallergic agents. III. Substituted trans-2-etheneyl-4-oxo-4H-1-benzopyran-6-carboxylic acids,” European Journal of Medicinal Chemistry, vol. 14, no. 4, pp. 347–351, 1979.
View at: Google Scholar
N. Desideri, C. Conti, P. Mastromarino, and F. Mastropaolo, “Synthesis and anti-rhinovirus activity of 2-styrylchromones,” Antiviral Chemistry and Chemotherapy, vol. 11, no. 6, pp. 373–381, 2000.
View at: Google Scholar
J. Marinho, M. Pedro, D. C. G. A. Pinto et al., “4′-Methoxy-2-styrylchromone a novel microtubule-stabilizing antimitotic agent,” Biochemical Pharmacology, vol. 75, no. 4, pp. 826–835, 2008.
View at: Publisher Site | Google Scholar
K. Momoi, Y. Sugita, M. Ishihara et al., “Cytotoxic activity of styrylchromones against human tumor cell lines,” In Vivo, vol. 19, no. 1, pp. 157–164, 2005.
View at: Google Scholar
W. H. Gerwick, “Cytotoxic substances from the marine cyanophyte Hormothamnion enteromorphoides Grunow,” European patent 237166, 1987.
View at: Google Scholar
E. Fernandes, F. Carvalho, A. M. S. Silva et al., “2-Styrylchromones as novel inhibitors of xanthine oxidase. A structure-activity study,” Journal of Enzyme Inhibition and Medicinal Chemistry, vol. 17, no. 1, pp. 45–48, 2002.
View at: Publisher Site | Google Scholar
A. M. S. Silva, D. C. G. A. Pinto, J. A. S. Cavaleiro, A. Levai, and T. Patonay, “Synthesis and reactivity of styrylchromones,” Arkivoc, vol. 2004, no. 7, pp. 106–123, 2004.
View at: Google Scholar
A. M. S. Silva, D. C. G. A. Pinto, H. R. Tavares, J. A. S. Cavaleiro, M. L. Jimeno, and J. Elguero, “Novel (E)- and (Z)-2-styrylchromones from (E,E)-2′-hydroxycinnamylideneacetophenones—xanthones from daylight photooxidative cyclization of (E)-2-styrylchromones,” European Journal of Organic Chemistry, no. 9, pp. 2031–2038, 1998.
View at: Google Scholar
D. C. G. A. Pinto, A. M. S. Silva, and J. A. S. Cavaleiro, “A convenient synthesis of new (E)-5-hydroxy-2-styrylchromones by modifications of the Baker-Venkataraman method,” New Journal of Chemistry, vol. 24, no. 2, pp. 85–92, 2000.
View at: Publisher Site | Google Scholar
B. P. Reddy and G. L. David Krupadanam, “The synthesis of 8-allyl-2-styrylchromones by the modified Baker-Venkataraman transformation,” Journal of Heterocyclic Chemistry, vol. 33, no. 6, pp. 1561–1565, 1996.
View at: Google Scholar
D. C. G. A. Pinto, A. M. S. Silva, C. M. Brito et al., “Reactivity of 3-styrylchromones as dienes in Diels-Alder reactions under microwave irradiation: a new synthesis of xanthones,” European Journal of Organic Chemistry, no. 14, pp. 2973–2986, 2005.
View at: Publisher Site | Google Scholar
A. M. S. Silva, A. M. G. Silva, A. C. Tomé, and J. A. S. Cavaleiro, “New syntheses of flavones from Diels-Alder reactions of 2- styrylchromones with ortho-benzoquinodimethanes,” European Journal of Organic Chemistry, no. 1, pp. 135–139, 1999.
View at: Google Scholar
D. C. G. A. Pinto, A. M. S. Silva, L. M. P. M. Almeida, J. A. S. Cavaleiro, A. Lévai, and T. Patonay, “Synthesis of 4-aryl-3-(2-chromonyl)-2-pyrazolines by the 1,3-dipolar cycloaddition of 2-styrylchromones with diazomethane,” Journal of Heterocyclic Chemistry, vol. 35, no. 1, pp. 217–224, 1998.
View at: Google Scholar
G. Toth, A. Levai, A. Szollosy, and H. Duddeck, “Synthesis and conformational analysis of some spiropyrazoline isomers,” Tetrahedron, vol. 49, no. 4, pp. 863–880, 1993.
View at: Google Scholar
A. M. S. Silva, J. S. Vieira, J. A. S. Cavaleiro, T. Patonay, A. Lévai, and J. Elguero, “New syntheses of 4(5)-aryl-5(4)-(2-chromonyl)-1,2,3-triazoles from 2- styrylchromones and sodium azide,” Heterocycles, vol. 51, no. 3, pp. 481–487, 1999.
View at: Google Scholar
K. Takagi, M. Tanaka, and T. Murakami, “Synthesis of new 3(5)-(2-hydroxyphenyl)pyrazoles as potential analgesic agents and platelet aggregation inhibitors,” European Journal of Medicinal Chemistry, vol. 21, no. 1, pp. 65–69, 1986.
View at: Google Scholar
D. C. G. A. Pinto, A. M. S. Silva, and J. A. S. Cavaleiro, “Novel (E)-3-(2′-benzyloxy-6′-hydroxyphenyl)-5-styrylpyrazoles from (E)-2-styrylchromones,” Heterocyclic Communications, vol. 3, no. 5, pp. 433–436, 1997.
View at: Google Scholar
D. C. G. A. Pinto, A. M. S. Silva, and J. A. S. Cavaleiro, “Synthesis of 3-(2-benzyloxy-6-hydroxyphenyl)-1-methylpyrazoles by the reaction of chromones with methylhydrazine,” Journal of Heterocyclic Chemistry, vol. 37, no. 6, pp. 1629–1634, 2000.
View at: Google Scholar
B. K. Karale, C. H. Gill, and M. S. Shingare, “Synthesis of styrylpyrimidines,” Indian Journal of Heterocyclic Chemistry, vol. 12, no. 3, pp. 267–270, 2003.
View at: Google Scholar
C. Conti, P. Mastromarino, P. Goldoni, G. Portalone, and N. Desideri, “Synthesis and anti-rhinovirus properties of fluoro-substituted flavonoids,” Antiviral Chemistry and Chemotherapy, vol. 16, no. 4, pp. 267–276, 2005.
View at: Google Scholar
A. Y. Shaw, C.-Y. Chang, H.-H. Liau et al., “Synthesis of 2-styrylchromones as a novel class of antiproliferative agents targeting carcinoma cells,” European Journal of Medicinal Chemistry, vol. 44, no. 6, pp. 2552–2562, 2009.
View at: Publisher Site | Google Scholar
Y. Qian, H.-J. Zhang, H. Zhang, C. Xu, J. Zhao, and H.-L. Zhu, “Synthesis, molecular modeling, and biological evaluation of cinnamic acid metronidazole ester derivatives as novel anticancer agents,” Bioorganic and Medicinal Chemistry, vol. 18, no. 14, pp. 4991–4996, 2010.
View at: Publisher Site | Google Scholar
W. F. Bailey and S. C. Longstaff, “Generation and cyclization of a benzyne-tethered alkyllithium: preparation of 4-substituted indans,” Journal of Organic Chemistry, vol. 63, no. 3, pp. 432–433, 1998.
View at: Google Scholar
W. Szymanski, B. Wu, B. Weiner, S. De Wildeman, B. L. Feringa, and D. B. Janssen, “Phenylalanine aminomutase-catalyzed addition of ammonia to substituted cinnamic acids: a route to enantiopure α- and β-amino acids,” Journal of Organic Chemistry, vol. 74, no. 23, pp. 9152–9157, 2009.
View at: Publisher Site | Google Scholar
T. Fukuyama, M. Arai, H. Matsubara, and I. Ryu, “Mizoroki-Heck arylation of α,β-unsaturated acids with a hybrid fluorous ether, F-626: facile filtrative separation of products and efficient recycling of a reaction medium containing a catalyst,” Journal of Organic Chemistry, vol. 69, no. 23, pp. 8105–8107, 2004.
View at: Publisher Site | Google Scholar
C. D. M. Duarte, H. Verli, J. X. D. Araújo-Júnior, I. A. D. Medeiros, E. J. Barreiro, and C. A. M. Fraga, “New optimized piperamide analogues with potent in vivo hypotensive properties,” European Journal of Pharmaceutical Sciences, vol. 23, no. 4-5, pp. 363–369, 2004.
View at: Publisher Site | Google Scholar
D. R. Brittelli, “Phosphite-mediated in situ carboxyvinylation: a new general acrylic acid synthesis,” Journal of Organic Chemistry, vol. 46, no. 12, pp. 2514–2520, 1981.
View at: Google Scholar
M. Sharma, M. C. Joshi, V. Kumar, S. V. Malhotra, and D. S. Rawat, “Synthesis and anticancer activity of 13-membered cyclic enediynes,” Archiv der Pharmazie, vol. 344, no. 9, pp. 564–571, 2011.
View at: Google Scholar
M. Gupta, R. Gupta, and M. Anand, “Hydroxyapatite supported caesium carbonate as a new recyclable solid base catalyst for the Knoevenagel condensation in water,” Beilstein Journal of Organic Chemistry, vol. 5, article A68, 2009.
View at: Publisher Site | Google Scholar
C. M. M. Santos, A. M. S. Silva, and J. A. S. Cavaleiro, “Efficient syntheses of new polyhydroxylated 2,3-diaryl-9H-xanthen-9-ones,” European Journal of Organic Chemistry, no. 16, pp. 2642–2660, 2009.
View at: Publisher Site | Google Scholar
N. Desideri, R. Fioravanti, L. P. Monaco et al., “1, 5-Diphenylpenta-2, 4-dien-1-ones as potent and selective monoamine oxidase-B inhibitors,” European Journal of Medicinal Chemistry, vol. 59, pp. 91–100, 2013.
View at: Google Scholar
“SAINT Version 7.60a,” Bruker AXS Inc., Madison, Wis, USA, 2006.
View at: Google Scholar
G. M. Sheldrick, “SHELXS-97, SHELXL-97 and SADABS version 2.05,” University of Göttingen, Germany, 1997.
View at: Google Scholar
L. J. Barbour, “X-seed—a software tool for supramolecular crystallography,” Journal of Supramolecular Chemistry, vol. 1, no. 4–6, pp. 189–191, 2001.
View at: Google Scholar
J. L. Atwood and L. J. Barbour, “Molecular graphics: from science to art,” Crystal Growth and Design, vol. 3, no. 1, pp. 3–8, 2003.
View at: Publisher Site | Google Scholar
A. W. Bauer, W. M. Kirby, J. C. Sherris, and M. Turck, “Antibiotic susceptibility testing by a standardized single disk method,” American Journal of Clinical Pathology, vol. 45, no. 4, pp. 493–496, 1966.
View at: Google Scholar

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Copyright © 2013 Mehbub Momin et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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