amino)benzoate: synthesis, crystal structure

Z. Kristallogr. 2018; 233(2): 125–134

Li Yee Then, C.S. Chidan Kumar*, Huey Chong Kwong, Siau Hui Mah, Wan-Sin Loh, Ching Kheng Quah*, Yip-Foo Win and Siddegowda Chandraju

Radical scavenging potency of 2-(benzofuran-2-yl)-2-oxoethyl 3-(methyl/amino)benzoate: synthesis, crystal structure and Hirshfeld surface analysis https://doi.org/10.1515/zkri-2017-2083 Received June 23, 2017; accepted August 19, 2017; published online October 9, 2017

Abstract: Two novel 2-(benzofuran-2-yl)-2-oxoethyl 3-(methyl/amino)benzoates 2(a–b), were synthesized under mild conditions and characterized by spectroscopic analysis. The three-dimensional (3D) crystal structures of these compounds were further determined using single crystal X-ray diffraction technique and revealed that both compounds tend to twist away with respect to their attached carbonyl groups at the C(=O)–C–O–C(=O) connecting bridge, exhibiting a nearly perpendicular conformation with torsion angle in a range of 75–94°. In both compounds, each of the benzofuran and substituted phenyl rings are individually planar while almost perpendicular to each other. Their molecular conformation are comparable with related structures from the Cambridge Structural Database (CSD). Furthermore, the intermolecular interactions in these crystal structures were quantified and analyzed using Hirshfeld surfaces computational method. The quantitative data on the percentage contributions of overall interactions on the molecules 2a and 2b (A & B) is calculated by the 2-D fingerprint plots from the HS analysis. These structures were evaluated for their antioxidant properties by diphenyl-2-picrylhydrazyl (DPPH) radical scavenging, *Corresponding authors: Ching Kheng Quah, X-ray Crystallography Unit, School of Physics, Universiti Sains Malaysia, Penang 11800 USM, Malaysia, E-mail: [email protected]; and C.S. Chidan Kumar, Department of Engineering Chemistry, Vidya Vikas Institute of Engineering and Technology, Visvesvaraya Technological University, Alanahally, Mysuru 570028, Karnataka, India, Tel.: +604 653 3888 Ext. 3690; Fax: +6046579150, E-mail: [email protected] Li Yee Then and Wan-Sin Loh: X-ray Crystallography Unit, School of Physics, Universiti Sains Malaysia, Penang 11800 USM, Malaysia Huey Chong Kwong: School of Chemical Sciences, Universiti Sains Malaysia, Penang 11800 USM, Malaysia Siau Hui Mah: School of Biosciences, Taylor’s University, Lakeside Campus, 47500 Subang Jaya, Selangor, Malaysia Yip-Foo Win: Department of Chemical Science, Faculty of Science, Universiti Tunku Abdul Rahman, Perak Campus, Jalan Universiti, Bandar Barat, 31900 Kampar, Perak, Malaysia Siddegowda Chandraju: Department of Sugar Technology and Chemistry, Sir M. V. PG Centre, University of Mysore, Tubinakere, Mandya 571401, Karnataka, India

ferrous ion chelating (FIC) and hydrogen peroxide radical scavenging assays (H2O2). Noteworthy, compound 2b with an amino substituent attached to the phenyl ring exhibited an IC50 value of 45.37 μg/mL in H2O2 scavenging assay, which showed better scavenging effect than the standard drug, ascorbic acid (IC50 = 81.02 μg/mL). Keywords: antioxidant; benzofuran; crystal structure; electron donating group; Hirshfeld surface analysis.

Introduction Heterocyclic compounds [1] have been a popular topic among researchers through decades due to their availability and wide range of applications in pharmacy [2], agriculture [3], industry [4] and so on. Benzofuran is a class of heterocyclic compounds consisting of fused benzene and furan rings. It is an essential building block of various biologically active compounds. Benzo[b]furan nucleus is widespread in plants and often the natural products possessing benzofuran are useful for their immense pharmacological properties [5, 6]. Both natural and synthetic benzofuran compounds were revealed to possess a wide range of profound biological activities, such as antioxidant [7], anti-acetylcholinesterase [8], antimicrobial [9] and anticancer [10]. Iron (Fe) is an essential but potentially toxic metal in human body, as with the ability to donate and accept electrons, its can catalyze the conversion of hydrogen peroxide (H2O2) into free radicals. Free radicals can harm and kill a wide variety of cellular structures [11]. Similarly, free radical such as diphenyl-2-picrylhydrazyl (DPPH) are readily attack and promote oxidative damage to biomolecules such as lipids, proteins and DNA, leading to many human pathological conditions [12, 13]. On account of the significant outcome of the previous structures [14, 15], herein we report the synthesis, structural determinations and antioxidant activities of two promising benzofuran ester derivatives, namely 2-(Benzofuran-2-yl)-2-oxoethyl 3-methylbenzoate (2a) and 2-(Benzofuran-2-yl)-2-oxoethyl 3-aminobenzoate (2b). Although, both compounds showed weak antioxidant properties in diphenyl-2-picrylhydrazyl (DPPH) radical scavenging and Authenticated | [email protected] author's copy Download Date | 5/23/18 8:32 AM

126 L.Y. Then et al.: Benzofuranyl esters as radical scavengers ferrous ion chelating assay (FIC), but both compounds showed a comparable (2a) and better (2b) H2O2 radical scavenging effect than the standard drug, ascorbic acid.

Materials and methods Instrumentation The Fourier transform infrared spectroscopy (FTIR) spectra were recorded on a Perkin Elmer Frontier FTIR spectrometer equipped with attenuated total reflection (ATR) from 4000 to 650 cm−1. The 1H and 13C nuclear magnetic resonance (NMR) spectra were determined in CDCl3 at 500 MHz and 125 MHz, respectively using Bruker Avance III 500 spectrometer. Besides, the melting points of all compounds were determined by Stuart SMP10 digital melting point apparatus. The antioxidant activities of synthesized products were evaluated by using Biotek Epoch 2 Microplate Spectrophotometer. The crystal structures of the synthesized compounds were determined using Bruker APEX II DUO CCD or Bruker SMART APEX II area-detector diffractometer. Data collection process was carried out at room temperature by applying MoKα radiation (λ = 0.71073 Å) and employing ϕ and ω scans. SAINT [16] program was used to integrate the raw files to obtain useful crystal data, which was further solved and refined by SHELXTL [17] software. Lastly, the data was enhanced by applying the absorption correction process through SADABS [16]. Crystallographic data of the reported structures have been deposited at the Cambridge Crystallographic Data Centre with CCDC deposition numbers of 1,037,761 and 1,449,590, respectively. Copies of available materials can be obtained free of charge, on application to CCDC, 12 Union Road, Cambridge CB2 1EZ, UK, (Fax: +44-(0)1223-336033 or E-mail: [email protected]).

Synthesis

temperature (Scheme 1). The reaction progress was monitored by thin-layer chromatography (TLC). Once the reaction has completed, the reaction mixture was poured into ice-cooled water to obtain its precipitate. The resulted precipitate was then filtered, air-dried and finally recrystallized with suitable solvent to obtain pure crystal. 2-(Benzofuran-2-yl)-2-oxoethyl 3-methylbenzoate (2a): 2 1

3

O

O 8

4

6 5

18

9

7

10

13

O 11

14

12

O

15 17

16

Solvent used to grow crystal: acetone; Yield: 82%; M.P.: 117–119°C; FT-IR (ATR (solid) cm−1): 3099 (Ar C–H, ν), 2942 (C–H, ν), 1722, 1683 (C=O, ν), 1612, 1476 (Ar C=C, ν), 1276, 1112 (C–O, ν); 1H-NMR (500 MHz, CDCl3): δ ppm 7.999–7.975 (m, 2H, 13CH and 17CH overlapped), 7.775–7.759 (d, 1H, J = 8.0 Hz, 2CH), 7.670 (s, 1H, 7CH), 7.639–7.622 (d, 1H, J = 8.5 Hz, 15 CH), 7.564–7.531 (t, 1H, J = 8.5 Hz, 16CH), 7.455–7.439 (d, 1H, J = 8.0 Hz, 5 CH), 7.412–7.358 (m, 2H, 3CH and 4CH overlapped), 5.572 (s, 2H, 10CH2), 2.453 (s, 3H, 18CH3); 13C-NMR (125 MHz, CDCl3): δ ppm 183.86 (C9), 166.16 (C11), 155.71 (C1), 150.56 (C8), 138.40 (C14), 134.39 (C15), 130.56 (C17), 129.16 (C12), 128.75 (C13), 128.46 (C16), 127.19 (C3), 126.78 (C6), 124.23 (C5), 123.53 (C4), 113.47 (C7), 112.53 (C2), 66.33 (C10), 21.41 (C18). 2-(Benzofuran-2-yl)-2-oxoethyl 3-aminobenzoate (2b): 2 1

3

8

4

The target compounds (2) were synthesized by reacting 2-bromoacetyl benzofuran (0.5 mmol) and substituted benzoic acid (0.6 mmol) in the presence of anhydrous potassium carbonate (0.5 g) in dimethylformamide (8 mL). The solution was stirred for 2 h under room

O

O

6 5

7

NH2

9 10

11 O

14

13

O

12

15 17

16

Scheme 1: Reaction scheme of the synthesis of target compounds 2(a–b).

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L.Y. Then et al.: Benzofuranyl esters as radical scavengers 127

Solvent used to grow crystal: acetone + dimethylformamide (1:1 v/v); Yield: 75%; M.P.: 129–131°C; FT-IR (ATR (solid) cm−1): 3452, 3356 (N–H, ν), 3067 (Ar C–H, ν), 2928 (C–H, ν), 1722, 1686 (C=O, ν), 1608, 1476 (Ar C=C, ν), 1230, 1108 (C–O, ν); 1H-NMR (500 MHz, CDCl3): δ ppm 7.768–7.752 (d, 1H, J = 8.0 Hz, 17CH), 7.660 (s, 1H, 7CH), 7.632–7.616 (d, 1H, J = 8.2 Hz, 2CH), 7.573–7.525 (m, 2H, 3CH and 5CH overlapped), 7.474 (s, 1H, 13CH), 7.383–7.351 (t, 1H, J = 8.2 Hz, 4CH), 7.294–7.262 (t, 1H, J = 8.0 Hz, 16CH), 6.939–6.923 (d, 1H, J = 8.0 Hz, 15CH), 5.544 (s, 2H, 10CH2); 13C-NMR (125 MHz, CDCl3): δ ppm 183.84 (C9), 166.12 (C11), 155.67 (C1), 150.51 (C14), 146.57 (C8), 130.16 (C12), 129.42 (C16), 128.73 (C3), 126.74 (C6), 124.19 (C5), 123.51 (C4), 120.20 (C17), 119.94 (C15), 116.11 (C13), 113.49 (C7), 112.51 (C2), 66.22 (C10).

Hirshfeld surface analysis A 3D picture of intermolecular interactions can be obtained from Hirshfeld surface (HS) analysis. It is a graphical tool for visualizing and understanding various intermolecular interactions. Molecular Hirshfeld surface analysis and the related 2-D fingerprint plots for the investigated compound were realized using CrystalExplorer 3.1 program [18]. Hirshfeld surfaces were generated using standard high surface resolution [19]. The HS of the crystal structures under investigation are unique. The asymmetric unit of 2b contains two molecules, A and B, in which molecule B is disordered and the disordered/ minor part is labeled as ‘Y’. Hence, the crystal data (CIF) was separated for molecules A and B, the major part of the molecule labeled as B was considered for the HS analysis.

DPPH radical scavenging assay: The DPPH (2,2-diphenyl-1-picrylhydrazyl) radical scavenging assay was used to evaluate the antioxidant properties of the compounds. This assay was carried out following the method of Farasat et al. [20]. The DPPH solution (0.04 mM) was prepared in ethanol. The compounds were prepared in ethanol with the concentrations of 100 and 500 μg/mL. Approximately 100 μL of each compound was mixed with 100 μL of DPPH solution in a 96-well plate. Each sample and the control was prepared in triplicates. The plate was then incubated in a dark room at room temperature for 30 min and the absorbance was measured at 517 nm with a microplate reader. Ascorbic acid was used as a standard drug. The percentage of DPPH free radical scavenging activity was calculated by using the formula below:

Percentage of chelation =

Ac − As × 100 Ac

(2)

where Ac is the absorbance of the control and As is the absorbance of the sample. Hydrogen peroxide (H2O2) radical scavenging assay: The abilities of benzofuranyl-based compounds to scavenge hydroxyl radical were determined by the method of Ruch et al. [22] with some modifications. The solution of hydrogen peroxide (0.4 mM) was prepared in PBS. The compounds with the concentration of 100 μg/mL were prepared in PBS and added to 0.6 mL of H2O2 solution. The mixture was mixed well and incubated at room temperature for 10 min. The absorbance was determined at 230 nm by using spectrophotometer. Ascorbic acid was used as a standard drug while PBS as the blank. The percentage of hydrogen peroxide scavenging activity was calculated by using the following formula: Percentage of scavenging activity =

Ac − As × 100 Ac

(3)

where Ac is the absorbance of the control and As is the absorbance of the sample.

Results and discussion

Antioxidant studies

Percentage of scavenging activity =

measured at 517 nm with a microplate reader. EDTA was used as a reference drug while ethanol as the control. The percentage of inhibition was calculated by using the formula below:

A b − As × 100 Ab

(1)

where Ab is the absorbance of the blank and As is the absorbance of the sample. Ferrous ion chelating (FIC) assay: The FIC assay was conducted according to the methods reported by Che Othman et al. [21] with some minor modifications. Briefly, the compounds with the concentration of 2 mg/mL were added into a 96 well plate and followed by 10 μL of 2 mM FeCl2. Then, the mixture was incubated for 5 min in dark at room temperature. After the incubation, 20 μL of 5 mM ferrozine was added to the mixture and incubated for another 10 min in dark at room temperature. After the incubation, the absorbance was

Chemistry Generally, the FT-IR spectra of 2a and 2b showed absorption bands around 3099–2942 cm−1 indicating the presence of C–H stretching vibrations (above 3000 cm−1 for aromatic rings and below 3000 cm−1 for methyl group as well as methylene group). Two strong absorption peaks were observed for carbonyl groups in the range of 1722– 1683 cm−1. In addition, a moderate peak for ν(C=C) in the benzene ring was revealed around 1612 and 1476 cm−1 whereas two distinct peaks were discovered for ν(C–O) around 1276–1108 cm−1. For 2b, the presence of primary amino group in the benzene ring was confirmed by two ν(N–H) absorption bands around 3452–3356 cm−1. The 1H NMR of compounds 2a and 2b shows similarities to each other in their spectra. Two sharp singlets were observed around δ≈7.6 ppm and δ≈5.5 ppm indicating the presence of methine proton (at furan ring) and methylene proton (at carbonyl bridge). The –CH– protons of the benzofuran group and the benzene group were resolved by their different J-coupling values. Exceptionally, the spectrum of 2a revealed an extra singlet located in the upfield region around δ≈2.453 ppm, representing the methyl Authenticated | [email protected] author's copy Download Date | 5/23/18 8:32 AM

128 L.Y. Then et al.: Benzofuranyl esters as radical scavengers group substituting at its benzene ring. However, the peak for –NH2 in 2b did not appear in the spectrum. In general, the integration values of protons for both compounds are in agreement with the number of proposed protons. Appearance of carbonyl carbon peaks were spotted in the most downfield region of 13C NMR spectra for compounds 2a and 2b at around 184 and 166 ppm. In the upfield region, saturated –CH2– carbon signals were located approximately at δ≈66 ppm. Meanwhile, the sets of carbon signals for benzofuran group and benzene group were found in the range of δ≈112–156 ppm. Remarkably, an extra primary carbon signal was observed at δ≈21 ppm in 2a due to its methyl group substitution.

X-ray crystal structure description Both compounds in present study are meta-substituted with methyl and amino functional group respectively at their phenyl rings. The molecular structures of compounds 2a and 2b with 30% ellipsoid probability are depicted in Figure 1. Table 1 shows the crystal data and the parameters of structure refinement for compounds 2a and 2b. Basically, the molecules of these compounds can be considered as three individual parts, which are the benzofuran ring, the phenyl ring, and the C(=O)–C– O–C(=O) carbonyl bridge, that can be described by three

Tab. 1: Crystal data and parameters of structure refinement for compound 2a and 2b. Compound

2a

CCDC deposition number Molecular formula Molecular weight Temperature Crystal system Space group a (Å) b (Å) c (Å) α (°) β (°) γ (°) V (Å3) Z Dcalc (g cm−3) Crystal dimensions (mm) Colour Shape μ (mm−1) Ranges/indices (h,k,l)

θ limit (°) Reflections measured Unique reflections Observed reflections [I > 2σ(I)] Parameters Goodness of fit on F2 R1, wR2 [I > 2σ(I)]

2b

1037761 C18 H14O4 294.29 297 Monoclinic P21/n 8.7461 (8) 6.7795 (6) 25.252 (2) 90 98.868 (2) 90 1479.4 (2) 4 1.321 0.08 × 0.24 × 0.53 Colourless Plate 0.09 –11, 11; –8, 8; –32, 29 2.7–25.8 12,888 3404 2375 200 1.03 0.042, 0.118

1449590 C17H13NO4 · C3H7NO 368.38 297 Triclinic P1̅ 9.0825 (4) 11.1062 (5) 18.7811 (8) 89.6954 (15) 83.1431 (15) 82.5057 (14) 1864.77 (14) 4 1.312 0.11 × 0.32 × 0.54 Bronze Block 0.10 –11, 11; –13, 13; –22, 22 2.4–28.4° 47,892 6937 3971 690 1.07 0.085, 0.250

degree-of-freedom, which are the torsion angles between benzofuran and carbonyl group O1–C8–C9–O3 (τ1), between two carbonyl groups C9–C10–O2–C11 (τ2) and between carbonyl group and phenyl ring O2–C11–C12– C13 (τ3), respectively (Figure 2). Nevertheless, torsion angles τ1 and τ3 are approximately 0° or ±180°, showing that the benzofuran and phenyl rings are nearly planar with the C9–C10–O2–C11 connecting bridge. In contrast, the torsion angles of the interconnecting carbonyl bridge are almost perpendicular to each other (75–94°). In both

Fig. 1: Molecular structure of compounds 2a and 2b drawn with 30% ellipsoid probability and atomic labelling scheme.

Fig. 2: General chemical diagram showing torsion angles τ1, τ2 and τ3 in compound 2(a–b).



Tab. 2: Torsion angles τ1, τ2 and τ3 for present and previous compounds. Compound 2a 2b ITAXUY ITAYAF ITAYEJ ITAYIN ITAYOT

τ1 (°)

τ2 (°)

τ3 (°)

–3 –1, –3, 8 –177 5, –175 –4 –3 7, 3

75 –83, 94, –84 75 163, –70 78 80 178, 180

–175 177, –170, –172 172 –177, –172 –171 –169 –175, –176

compounds, their benzofuran and phenyl rings are individually planar and nearly normal to each other with dihedral angles of 62–87°. A search of Cambridge Structural Database (CSD) was performed using 2-(benzofuran-2-yl)-2-oxoethyl benzoate as basic skeleton. As a result, five benzofuran derivatives are revealed to have similar structures with compounds 2a and 2b, with differently substituted phenyl rings, which including ITAXUY, ITAYAF, ITAYEJ, ITAYIN and ITAYOT [15]. By comparison, it was observed that torsion angles τ1 and τ3 of both studied and previous compounds are

almost identical, with values close to 0° or ±180°. Whereas, the torsion angle τ2 for present structures only similar to 3 out of 5 related structures (except ITAYAF (molecule B) and ITAYOT) as shown in Table 2. In overall, the torsion angles of both studied and previously reported structures are generally comparable. Compound 2a is crystallized in monoclinic system with space group of P21/n, consisting of one crystallographically unique molecule in its asymmetric unit. In its crystal, two adjacent inversion-related molecules are bounded into dimers by intermolecular C4–H4A · · · O4 hydrogen bonds and furthered connected by a pairs of C5–H5A · · · O1 and C5–H5A · · · O3 hydrogen bonds, forming a columns extending along [010] direction. Here the atom C5 served as a bifurcate donor with a R12(5) ring motif (Figure 3, Table 3). Furthermore, those dimers within the columns are stabilized by π · · · π interaction (3.5552 Å; –x + 1, –y + 2, –z) between the benzofuran rings whereas neighboring columns were connected by extensive C–H · · · π interactions, into a three dimensional network (Figure 4, Table 4). Compound 2b crystallizes as a 1:1 solvate with Z′ = 2. The host molecules are denoted as molecule A and molecule B, respectively, while molecule B experiences a whole

Fig. 3: Bifurcated hydrogen bonds join molecules into chain in compound 2a. Tab. 3: Hydrogen bond geometries (D–H · · · A; Å, °) for compounds 2a–b.a D–H · · · A 2a C4–H4A · · · O4 C5–H5A · · · O1 C5–H5A · · · O3 C15–H15A · · · Cg3 C16–H16A · · · Cg2 C17–H17A · · · Cg1 2b N1A–H1A · · · O1 N1B–H1D · · · O2 N1Y–H1E · · · O3A N1Y–H1F · · · O2

D–H (Å)

H · · · A (Å)

D · · · A (Å)

D–H · · · A (°)

0.93 0.93 0.93 0.93 0.93 0.93

2.56 2.51 2.57 2.84 2.87 2.89

3.301(2) 3.3782(17) 3.325(2) 3.6633(19) 3.6277(17) 3.6592(16)

137 156 138 148 140 141

–x + 1, –y + 2, –z x, y + 1, z x, y + 1, z –x, y–1/2, –z + 1/2 –x, –y + 1, –z –x, –y + 1, –z

0.86 0.86 0.86 0.86

2.17 2.51 2.22 2.19

3.011(5) 3.324(10) 2.875(8) 3.02(1)

164 157 133 163

x + 1, y, z x–1, y–1, z x + 1, y, z x + 1, y + 1, z

Symmetry code

Cg1, Cg2 and Cg3 are the centroids of O1/C1/C6/C7/C8, C1–C6 and C12–C17 rings, respectively.

a


130 L.Y. Then et al.: Benzofuranyl esters as radical scavengers π · · · π interactions formed between their benzofuran rings (Figure 6, Tables 3 and 4).

Hirshfeld surface analysis

Fig. 4: Extensive interactions in compound 2a, including hydrogen bondings (cyan dotted lines), C–H · · · π interactions (blue dotted lines) and π · · · π interactions (red dotted lines).

molecule disorder in which the disordered molecule (molecule Y) is twisted 180° at its benzoate ring as compared to molecule B with refined site occupancy ratio of 0.41:0.59 (Figure 5). In the crystal structures of 2b, each host molecule (molecule A, B and Y) are related to adjacent dimethylformamide solvate by N–H · · · O hydrogen bonds between their amino and carbonyl group. Among the main molecules, N1Y–H1E · · · O3A hydrogen bonds are observed to join molecules A and molecules Y together. Meanwhile, neighboring molecules B and molecules Y are linked by

The dnorm is a normalized contact value [18] which is defined in turns of di, de and Van der Waals radii of the atoms, which were mapped into the HS by employing a red-blue-white color scheme. The red region represents shorter contacts with a negative dnorm value, blue color region represents longer contacts with a positive dnorm value and white color region represents the contacts around the sum of Van der Waals separation with a dnorm value of zero. The di and de represents the distance from the HS to the nearest atoms outside and inside the surface, respectively [23, 24]. The blue color region also refers to the low frequency of occurrence of (di, de) pair and the gray color is the outline of the full fingerprint [25]. The 3-D dnorm surface can be resolved into 2-D fingerprint plots, which quantitatively summarize the nature and type of all intermolecular contacts experienced by the molecules in the crystal. The dnorm mapped on Hirshfeld surface (Figure 7) of the title compounds 2a and 2b showing blue region corresponds to positive electrostatic potential and red region to negative electrostatic potential. The analysis of intermolecular interactions through the mapping of dnorm is permitted by the contact distances di and de from the HS to the nearest atom inside and outside, respectively.

Tab. 4: π · · · π interactions in title compounds.a Compound

Centroid 1

Centroid 2

2a 2b

Cg1 Cg4 Cg5 Cg5

Cg2 Cg5 Cg5 Cg6

Centroid-to-centroid distance (Å) 3.5552(9) 3.612(11) 3.629(10) 3.745(12)

Symmetry code −x + 1, −y + 2, −z −x + 1, −y + 1, −z −x + 1, −y + 1, −z −x + 1, −y + 1, −z

Cg1 and Cg2 are the centroids of O1/C1/C6/C7/C8 and C1–C6 rings respectively; Cg4 is the centroids of C1B–C6B ring; Cg5 and Cg6 are the centroids of O1Y/C1Y/C6Y/C7Y/C8Y and C1Y–C6Y rings respectively.

a

Fig. 5: Molecular disorder in compound 2b, molecule Y (purple colour) is the disorder part of molecule B.



Fig. 6: Packing diagram of compound 2b, showing disordered molecules (purple colour) and solvent molecules (green colour), with hydrogen bonds in cyan dotted lines and π · · · π interactions in red dotted lines.

In the present work, HS analysis of the compounds 2a and 2b was performed to visualize various C–H · · · O, N–H · · · O, C–H · · · π and π · · · π interactions. The intermolecular hydrogen bonding interactions are visualized as bright-red spots on the Hirshfeld surface mapped over dnorm (Figure 7). The quantitative data on the percentage contributions of overall interactions on the molecules 2a and 2b (A & B) have been calculated by the 2-D fingerprint plots from the HS analysis and depicted in Figure 8a. The significance of intermolecular interaction patterns in the investigated compounds are delineated into H/H, O · · · H/H · · · O, C · · · H/H · · · C, C · · · C and N · · · H/H · · · N (for 2b) contacts as illustrated in Figure 8. The H · · · H interactions appear as the largest region on the fingerprint plot with a high concentration of 38% for 2a, 44.6%/44.1% for 2b (A & B) in the middle region shown in light blue on the Hirshfeld surface (Figure 8b).

27.3% (2a), and 28.3%/28.4% (2b) interaction contributions are from O · · · H/H · · · O contacts which corresponding to C–H · · · O interactions in compound 2a and N–H · · · O interactions in compound 2b, it was represented by a pair of sharp spikes characterized by a strong hydrogen-bond interaction having de + di ~ 2.8 Å and 2.4 Å as shown in Figure 8c. In compound 2a, there are four O atoms that can serve as a potential acceptors for hydrogen bond, but only three of O atoms establish intermolecular hydrogen bonds as depicted in Figure 9. Whereas, in compound 2b, there are five O atoms in each of the molecules (A & B) (four on the ester molecule and one in the solvent molecule) can act as potential acceptors for hydrogen bond, however only one O atoms from each solvent molecules and molecule A ester are involved in the establishment of intermolecular hydrogen bonds. The pair of characteristics “wings” with the edge at de + di ~3.2 Å are delineated by C · · · H/H · · · C contacts and contributed around 28.3% in compound 2a, 20.5%/21.3% for compound 2b (A & B), respectively. The C · · · C contacts appear near de = di ~1.8 Å for both 2a and 2b corresponds to intermolecular π · · · π interactions (Figure 8e) with 3.8% and 1.6% contributions, respectively. Although the N · · · H/H · · · N interactions were observed in 2b (A & B), however, it contributes only 2.2%/2.3% of the overall interaction with de + di ~3.4 Å. The shape index (Figure 10a) is most sensitive to very subtle changes in the surface shape. The red triangles concave regions indicate atoms of the π-stacked molecule above them, while the blue triangles convex regions indicate the ring atoms of the molecule inside the surface. The curvedness (Figure 10b) measured the shape of the molecule surface area in a crystal, where the sharp curvature areas correspond to high values of curvedness and usually tend to divide the surface into patches hence indicating interactions between neighboring molecules.

Fig. 7: (a) and (b) view of dnorm mapped on Hirshfeld surface of title compounds 2a and 2b showing blue region corresponds to positive electrostatic potential and red region to negative electrostatic potential.


132 L.Y. Then et al.: Benzofuranyl esters as radical scavengers

Fig. 8: Fingerprint plots for title compounds, broken down into contributions from specific pairs of atom types. For each plot, the gray shadow is an outline of the complete fingerprint plot. Surfaces to the right highlight the relevant surface patches associated with the specific contacts.



Fig. 9: dnorm mapped on Hirshfeld surfaces for visualizing the intermolecular interactions of the 2a compound.

Antioxidant activities The antioxidant properties of benzofuranyl-based compounds were evaluated by measuring the scavenging effects of DPPH, hydrogen peroxide (H2O2), as well as chelating effect of ferrous ion and their results were summarized in Table 5. Overall, compounds 2a and 2b are able to inhibit H2O2 radical significantly if compared to DPPH radical and ferrous ion. For H2O2 radical scavenging assay, compounds 2a and 2b exhibited excellent inhibitory abilities at the concentration of 100 μg/mL compared to the standard drug, ascorbic acid. Thus, IC50 of these compounds in scavenging

H2O2 were obtained and presented in Table 6. Referring to the results, the IC50 value of compound 2b was much lower than ascorbic acid, revealing its significant inhibition effect against H2O2 radical. By looking at the molecular structures of compounds 2a and 2b, both compounds have an electron donating moiety (methyl and amino group, respectively) substituted at their meta-position. It was observed that compound with stronger electron donating group exhibits higher H2O2 inhibition (better antioxidant agent) [26]. Hence, it is persuasive to deduce that a more electropositive substituent can enhance the antioxidant ability of a compound.

Conclusion Two novel 2-(benzofuran-2-yl)-2-oxoethyl 3-(methyl/ amino)benzoates 2(a–b), were synthesized and characterized by spectroscopic analysis. The 3D crystal structures of these compounds was confirmed by single crystal X-ray diffraction studies. From the studies, it was observed that the present compounds are closely related to previous structures from the Cambridge Structural Database (CSD) owing to their nearly identical τ1 and τ3 torsion angles

Fig. 10: (a) Hirshfeld surfaces mapped over the shape index (b) Hirshfeld surfaces mapped over curvedness of title compounds 2a and 2b.


134 L.Y. Then et al.: Benzofuranyl esters as radical scavengers Tab. 5: Antioxidant properties of compounds 2a and 2b. Compound

2a 2b Ascorbic acid (100 μg/mL) EDTA (100 μg/mL)

DPPH (% of inhibition) 100 μg/mL 500 μg/mL 1.8 ± 0.3 5.9 ± 0.2

8.5 ± 0.4 27.1 ± 0.2 96.7 ± 0.1a n/a

FIC (% of Inhibition) 2 mg/mL

H2O2 (% of Inhibition)

22.4 ± 0.9 49.8 ± 0.4 n/a 93.1 ± 1.00a

91.5 ± 0.9 99.0 ± 0.2 68.60 ± 0.7a n/a

100 μg/mL

Percentage of inhibition for standard drug in 100 μg/mL.

a

Tab. 6: IC50 value of each compound for H2O2 radical scavenging assay. Compound

IC50 (μg/mL)

2a 2b Ascorbic acid

89.90 ± 0.4 45.4 ± 0.1 81.0 ± 1.7

which close to 0° or ±180°. Instead, their τ2 torsion angle only similar to 3 out of 5 related structures, exhibiting a nearly perpendicular conformation with respect to their C(=O)–C–O–C(=O) connecting bridge. The intermolecular interactions in each compound were quantified and analyzed using Hirshfeld surfaces analysis. HS analysis of the compounds 2a and 2b was performed visualizing various C–H · · · O, N–H · · · O, C–H · · · π and π · · · π interactions contributing for the crystal stability. Both compounds showed good antioxidant properties in H2O2 radical scavenging assay, implying electron donating substituent(s) in the compounds may be beneficial to their scavenging activities. Acknowledgements: LYT thanks Universiti Sains Malaysia for USM Fellowship Scheme and Malaysian Government for MyBrain15 (MyMaster) scholarship. HCK thanks Malaysian Government for MyBrain15 (MyPhD) scholarship. The authors thank the Malaysian Government and Universiti Sains Malaysia (USM) for the Fundamental Research Grant Scheme (FRGS) (203/PFIZIK/6711563). Authors extend their appreciation to Vidya Vikas Research & Development Center for the facilities and encouragement.

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Supplemental Material: The online version of this article offers supplementary material (https://doi.org/10.1515/zkri-2017-2083).
