Structural correlation of some heterocyclic chalcone analogues and evaluation of their antioxidant potential

Article abstract of DOI:10.3390/molecules181011996

A series of six novel heterocyclic chalcone analogues 4(a-f) has been synthesized by condensing 2-acetyl-5-chlorothiophene with benzaldehyde derivatives in methanol at room temperature using a catalytic amount of sodium hydroxide. The newly synthesized co

Full text of DOI:10.3390/molecules181011996

Molecules 2013, 18, 11996-12011; doi:10.3390/molecules181011996
OPEN ACCESS
molecules
ISSN 1420-3049
www.mdpi.com/journal/molecules
Article
Structural Correlation of Some Heterocyclic Chalcone
Analogues and Evaluation of Their Antioxidant Potential
C. S. Chidan Kumar 1,2, Wan-Sin Loh 1, Chin Wei Ooi 1, Ching Kheng Quah 1 and
Hoong-Kun Fun 1,3,
*
1
X-ray Crystallography Unit, School of Physics, Universiti Sains Malaysia, Penang 11800 USM,
Malaysia; E-Mails: chidankumar@gmail.com (C.S.C.K.); wansin_loh@live.com (W.-S.L.);
ooichinwei88@gmail.com (C.W.O.); ckquah@usm.my (C.K.Q.)
Department of Chemistry, Alva’s Institute of Engineering & Technology, Mijar,
Moodbidri 574225, India
2
3
Department of Pharmaceutical Chemistry, College of Pharmacy, King Saud University,
Riyadh 11451, Saudi Arabia
* Author to whom correspondence should be addressed; E-Mail: hfun.c@ksu.edu.sa or
hkfun@usm.my; Tel.: +966-1-467-5853; Fax: +966-1-467-6220.
Received: 12 August 2013; in revised form: 14 September 2013 / Accepted: 23 September 2013 /
Published: 26 September 2013
Abstract: A series of six novel heterocyclic chalcone analogues 4(a–f) has been synthesized
by condensing 2-acetyl-5-chlorothiophene with benzaldehyde derivatives in methanol at
room temperature using a catalytic amount of sodium hydroxide. The newly synthesized
compounds are characterized by IR, mass spectra, elemental analysis and melting point.
Subsequently; the structures of these compounds were determined using single crystal X-ray
diffraction. All the synthesized compounds were screened for their antioxidant potential by
employing various in vitro models such as DPPH free radical scavenging assay, ABTS
radical scavenging assay, ferric reducing antioxidant power and cupric ion reducing
antioxidant capacity. Results reflect the structural impact on the antioxidant ability of the
compounds. The IC50 values illustrate the mild to good antioxidant activities of the reported
compounds. Among them, 4f with a p-methoxy substituent was found to be more potent as
antioxidant agent.
Keywords: heterocyclic; radical scavenging; reducing; substituent; overlay

Molecules 2013, 18
11997
1. Introduction
Chalcones are important constituents of natural products. They are abundant in edible plants
where they are considered to be the precursors of flavonoids and isoflavonoids. There is a growing
interest in the pharmacological potential of chalcones, which constitute an important group of natural
and synthetic products that have been screened for their wide range of pharmacological activities as
antibacterial [1,2], anti-tumor [3–5], anti-inflammatory [6–9], antifungal [10] and antioxidant
agents [11–15]. Chalcones are well known intermediates for synthesizing various heterocyclic
compounds. Several methods have been reported for the synthesis of chalcones, among which
the aldol condensation and Claisen-Schmidt condensation still occupy prominent positions. The
Claisen–Schmidt condensation between aryl ketones and aromatic aldehydes in acidic or basic media
gives chalcones. Chalcones are characterized by possessing an enone moiety between two aromatic
rings. In an effort to diversify the pharmacological activities of conventional chalcones, a series of
heterocyclic chalcone analogues in which an electron rich thiophene heterocycle replaces the benzene
ring were synthesized.
Based on the above observations, herein we report the synthesis of some novel heterocyclic chalcone
analogues using a conventional base-catalyzed Claisen-Schmidt condensation reaction. The structures of
these compounds are extensively characterized by single crystal X-ray diffraction studies and evaluation
of possible antioxidant activities has been carried out.
2. Results and Discussion
Chalcones, a versatile class of natural and synthetic compounds, attract researchers for their immense
range of biological activities. Chalcones containing thiophene moieties are supposed to further enhance
the scope of these activities. The best known method employed for the synthesis of chalcones is the
Claisen–Schmidt condensation between acetophenone and benzaldehyde in basic media. New chalcones
have been designed and synthesized by the reaction of 2-acetyl-5-chlorothiophene with substituted
benzaldehydes in presence of catalytic amount of NaOH in methanol, as shown in Scheme 1. The
crystallographic data and parameters for structure refinement are listed in Table 1 and hydrogen bonding
interactions are given in Table 2.
Scheme 1. Synthesis of heterocyclic chalcone analogues.
O
S
CH3
Cl
O
R1
R2
R3
S
NaOH / Methanol
RT
Cl
+
O
H
4a: R1 = I, R2 & R3 = H
4b: R2 = I, R1 & R3 = H
4c: R3 = I, R1 & R2 = H
R1
4d: R1 = OCH3, R2 & R3 = H
4e: R2 = OCH3, R1 & R3 = H
4f: R3 = OCH3, R1 & R2 = H
R2
R3

Molecules 2013, 18
11998
Table 1. Crystal data and parameters for structure refinement of compounds 4(a–f).
Compound
4a
939878
C13H8ClIOS
374.60
Monoclinic
Cc
34.308(7)
4.1026(9)
23.367(5)
90
4b
4c
4d
948858
C14H11ClO2S
278.74
Monoclinic
P21
11.9567(15)
17.986(2)
12.2265(15)
90
90.01
90
2629.3(5)
8
1.408
4e
944070
C14H11ClO2S
278.74
Monoclinic
P21/c
5.9343(6)
30.575(3)
7.3758(7)
90
104.623(3)
90
4f
944071
C14H11ClO2S
278.74
Monoclinic
Pc
3.9116(9)
15.867(4)
10.146(2)
90
CCDC deposition numbers
942739
C13H8ClIOS
374.60
Triclinic
P-1
6.0486(8)
8.4673(15)
13.5924(15)
88.616(2)
86.511(2)
70.131(2)
653.48(16)
2
942740
C13H8ClIOS
374.60
Triclinic
P-1
6.1259(4)
8.6600(6)
13.5854(9)
98.330(1)
98.812(1)
110.605(1)
651.06(8)
2
Molecular formula
Molecular weight
Crystal system
Space group
a (Å)
b (Å)
c (Å)
α (°)
β (°)
126.544(10)
90
2642.4(9)
8
95.115(4)
90
627.2(3)
2
γ (°)
V (Å3)
Z
1294.9(2)
4
1.430
D
calc (g cm−3)
1.883
1.904
1.911
1.476
Crystal dimensions (mm)
Colour
0.35 × 0.17 × 0.09 0.52 × 0.22 × 0.09 0.76 × 0.41 × 0.13 0.69 × 0.15 × 0.06 0.56 × 0.40 × 0.07 0.66 × 0.26 × 0.07
Colourless
2.76
Colourless
2.79
Colourless
2.80
Yellow
0.44
Colourless
0.45
Colourless
0.46
μ (mm−1)
Radiation λ (Å)
0.71073
0.448/0.779
19059
−44, 44; −5, 5;
−30, 30
2.4–29.1
5911
4676
308
1.09
0.068, 0.186
0.71073
0.322/0.779
10400
−7, 7; −10, 10;
−17, 17
1.5–26.5
2662
2482
154
1.13
0.039, 0.127
0.71073
0.223/0.705
10227
−7, 7; −11, 11;
−17, 16
1.6–27.5
2972
2735
154
1.08
0.036, 0.098
0.71073
0.753/0.972
19862
0.71073
0.790/0.971
15699
−8, 8; −44, 42;
−9, 10
0.71073
0.753/0.968
6086
−5, 4; −22, 22;
−14, 14
2.6–39.9
3237
2846
163
1.04
0.036, 0.086
T
min/Tmax
Reflections measured
Ranges/indices
−14, 14; −21, 20;
−14, 14
2.8–26.1
8732
(h, k, l)
θ limit (°)
1.3–31.2
4170
2644
163
1.02
Unique reflections
Observed reflections (I > 2σ(I))
Parameters
6388
650
0.97
0.044, 0.113
Goodness of fit on F2
R1, wR2 [I ≥ 2σ(I)]
0.047, 0.144

Molecules 2013, 18
Table 2. Hydrogen bond geometries for compounds 4d, 4e and 4f.
11999
D–H···A
d(D–H) (Å) d(H···A) (Å) d(D···A) (Å) Angle (D–H···A) (°)
4d
C3A—H3AA···O1D i
C3B—H3BA···O1C ii
C3C—H3CA···O1B iii
C3D—H3DA···O1A iv
4e
0.93
0.93
0.93
0.93
2.45
2.43
2.42
2.43
3.319(6)
3.288(6)
3.277(6)
3.275(7)
156
154
154
151
C14—H14B···Cg1
4f
0.96
2.87
3.620(2)
136
C3—H3A···O1 v
0.93
2.54
3.426(3)
160
(iv)
(i)
(ii)
(iii)
Symmetry code: −x, y + 1/2, −z + 1; –x + 1, y + 1/2, −z;
(v) x − 1, −y + 1, z − 1/2.
–x + 1, y − 1/2, −z + 1;
−x, y − 1/2, −z;
2.1. X-ray Crystal Structure Description for Compound 4a, 4b and 4c
The molecular structures of 4a, 4b and 4c are depicted in Figure 1a–c, showing the iodine
substituents at the -ortho (4a), -meta (4b) and -para (4c) positions on the benzene rings. The asymmetric
unit of compound 4a contains two crystallographically independent molecules (molecules A and B,
Figure 2). The compounds crystallized in the monoclinic, space group Cc (4a) and triclinic system,
space group P1 (4b and 4c). The three compounds exist in E configurations with respect to their
C6=C7 double bonds with bond distances of 1.302 (18) Å for molecule A (1.334 (16) Å for molecule B)
in 4a, 1.313 (8) Å in 4b and 1.331 (4) Å in 4c. In all the three compounds, the molecules are almost
planar, except for molecule B in 4a. The essentially planar thiophene (S1/C1—C4) and the benzene
(C8—C13) rings form dihedral angles of 5.3 (7)° for molecule A in 4a, 10.8 (2)° in 4b and 11.78 (15)° in
4c, whereas these two rings are twisted away from each other for molecule B in 4a, making a dihedral
angle of 35.9 (7)°.
Figure 1. Molecular structures of compounds (a) 4a, (b) 4b and (c) 4c, with atom numbering
schemes. Displacement ellipsoids are drawn at the 30% probability level.
(a)
(b)
(c)

Molecules 2013, 18
12000
Molecules A and B in compound 4a are overlaid over all the atoms as illustrated in Figure 2, with the
r.m.s value of 0.426 Å. Figure 3a shows the overlays of all non-H atoms of 4a/4b, 4a/4c and 4b/4c,
calculated using the chlorothiophene moiety where their iodine substituents were excluded, with the
r.m.s values of 0.064, 0.064 and 0.066 Å, respectively. The overlay including the iodine substituents is
displayed in Figure 3b. Molecule A in compound 4a is chosen as the overlay basis. The formation of
short intra-molecular S···O contacts of 2.927(16) and 2.935(10) Å which is 0.39 and 0.38 Å shorter than
the sum of van der Waals radii of the sulfur and oxygen atoms is observed in both the molecules A and
B. This could be due to the charge delocalization into the carbonyl group from the thiophene ring which
helps to stabilize the molecular structure. In the crystal packing of 4a, as illustrated in Figure 4a,
molecules A, which are represented in blue, and molecules B in green, stack one over the other along the
b-axis, forming an inverted head-to-tail arrangement without any significant hydrogen bond
interactions. The packing diagram of 4b and 4c are shown as Figures 4b,c, respectively. The molecules
in 4b and 4c are stacked on each other in an inverse fashion. It is clearly shown in Figure 4b where the
blue molecules are stacked inversely on the green molecule along the b-axis. Similarly, the molecules in
4c are arranged in an inverse manner along the (110) plane. No classical hydrogen bond is found in both
the crystal structures of 4b and 4c.
Figure 2. Overlay of all atoms in compound 4a, calculated using the chlorothiophene moiety.
Figure 3. (a) Overlay of all non-H atoms, calculated using the chlorothiophene moiety.
Molecule B in 4a and the iodine substituents in all three compounds were excluded in the
overlay. (b) Overlays were calculated using the chlorothiophene moiety in compounds 4a,
4b and 4c. Hydrogen atoms have been omitted for clarity.
(a)
(b)

Molecules 2013, 18
12001
Figure 4. (a) Crystal structure of 4a with molecule A represented in blue and molecule B in
green, stacking along the b-axis. Hydrogen bonds have been omitted for clarity. (b) Packing
diagram of 4b, showing the stacking in an inverse fashion. (c) Crystal structure of 4c,
displaying the stacking of molecules along the (110) plane.
(a)
(b)
(c)
2.2. X-ray Crystal Structure Description for Compounds 4d, 4e and 4f
Figures 5a–c display the molecular structures of 4d, 4e and 4f, which differ from each other in the
-ortho, -meta and -para location of their methoxy substituents on the benzene rings. The compounds
crystallized in the monoclinic system, space group P21 (4d), P21/c (4e) and Pc (4f), respectively. Unlike
the other crystal structures, compound 4d comprises four crystallography independent molecules
(molecules A, B, C and D), in its asymmetric unit, with comparable geometrical parameters. The C=C
double bonds of 1.318 (8), 1.317 (8), 1.338 (8) and 1.320 (8) Å with respect to the molecule A, B, C, D
are trans-configured. The same configuration also exists in compound 4e and 4f with C=C double bonds

Molecules 2013, 18
12002
of 1.319 (3) and 1.344 (3) Å. The molecules in these three compounds are almost planar as indicated by
the dihedral angles of the two rings, thiophene (S1/C1—C4) and benzene rings (C8—C13), ranging
from 2.11 (11) to 12.0 (3)° (Figure 5).
Figure 5. Molecular structure of compounds (a) 4d, (b) 4e and (c) 4f, with atom numbering
schemes. Displacement ellipsoids are drawn at the 30% probability level.
(a)
Figure 6. Overlay of all atoms in compound 4d, comprising four independent molecules.
Figure 7. (a) Overlay of all non-H atoms, where the methoxy groups were excluded.
(b) Overlays were calculated using the chlorothiophene moiety in compounds 4d, 4e and 4f.
Hydrogen atoms have been omitted for clarity. Only molecule A in compound 4d is chosen
for the overlay calculation.
(a)
(b)

Molecules 2013, 18
12003
Figure 8. (a) Crystal structure of 4d with the linkage of molecules A and D represented in
blue and the linkage of molecules B and C in green, propagating along the c-axis. Hydrogen
bonds not involved in the interactions have been omitted for clarity. Dashed lines are shown
as intermolecular hydrogen bonds. (b) Packing diagram of 4e, showing the C—H··· π
interactions involving the centroids of the benzene ring. Dashed lines represent the
C—H··· π interactions. (c) Crystal structure of 4f, displaying the infinite chains running
along the (201) plane. Intermolecular hydrogen bonds are represented as dashed lines.
(a)
(b)
(c)

Molecules 2013, 18
12004
The conformations of the four molecules in compound 4d are very analogous, as shown in Figure 6,
with the r.m.s value of 0.060 Å. Compounds 4d (molecule A), 4e and 4f are overlaid over each other
(Figure 7), giving the r.m.s value of 4d/4e being 0.138 Å, of 4d/4f being 0.104 Å and of 4e/4f being
0.084 Å. As presented in Figure 8a, molecules A and D in 4d, highlighted in blue, are linked together via
intermolecular C3A—H3AA···O1D and C3D—H3DA···O1A hydrogen bonds (Table 2) into infinite
chains with the A···D···A···D fashion. Likewise, the same pattern is adopted by molecules B and C where
the intermolecular C3B—H3BA···O1C and C3C—H3CA···O1B hydrogen bonds (Table 2) link
molecules B and C into another type of independent chains which are represented in green. These
chains propagate along the c-axis. The crystal packing of 4e is shown in Figure 8b, without the
significant hydrogen bonds. The crystal structure stability in 4e is provided by the C14—H14B··· π
interactions (Table 2), involving the centroids of the benzene rings, whereas in 4f, intermolecular
C3—H3A···O1 hydrogen bonds (Table 2) bind the molecules to form infinite chains as depicted in
Figure 8c, running along the (201) plane.
2.3. Antioxidant Activity
The synthesized compounds were screened for their antioxidant potential by employing the in-vitro
assays such as DPPH free radical scavenging assay, ABTS radical scavenging assay, ferric ion reducing
antioxidant power (FRAP) assay and cupric ion reducing antioxidant capacity (CUPRAC).
2.3.1. DPPH Radical Scavenging Assay
The DPPH radical scavenging test is a standard and widely used assay for in vitro antioxidant
capacity to trap free radicals. The results of in vitro antioxidant activity (IC50 values) of the synthesized
compounds in comparison with the reference antioxidant BHT are depicted in Figure 9. From the results,
it is noticed that the compounds exhibit mild to good antioxidant activity. At the start with
concentrations of 10–20 μg/mL, no significant change in the radical scavenging was noticed. However,
the radical scavenging ability increased with increasing concentration of the samples (30–50 μg/mL).
The IC50 values indicate that the chalcones 4d, 4e and 4f showed good free radical scavenging activity.
Among those, compound 4f with a p-methoxy substituent was more potent in free radical scavenging
compared to the other test compounds. The compounds 4(a–c) bearing electronegative iodine
substitution displayed mild scavenging activity. Among them, compound 4c exhibited least activity.
2.3.2. ABTS Radical Scavenging Assay
ABTS radical scavenging assay is a widely used method to evaluate the capacity of the test samples to
trap free radicals. ABTS [2,2′-azino-bis(3-ethylbenzothiazoline-6-sulphonic acid)], when reacted
produces the ABTS+ radical cation giving a green color in solution, for which the absorbance is
measured at 730 nm. Antioxidants suppress this reaction by donating electron(s) and hinder the
formation of the green colored ABTS+ radical cation. The amount of antioxidant in the test sample is
thus inversely proportional to the formation of ABTS radical. The samples under test displayed mild to
good ABTS radical scavenging activity and the results are comparable with DPPH radical scavenging
ability. Compounds 4(d–f) bearing methoxy substituents on the phenyl ring exhibited good activity, with

Molecules 2013, 18
12005
4f being dominant, whereas the compounds 4(a–c) bearing an electronegative iodine substituent
displayed mild activity. The IC50 values are compared with the standard and depicted in Figure 9.
Figure 9. IC50 (concentration required for 50% inhibition) values for DPPH and ABTS+
radical scavenging activities of the compounds 4(a–f) in comparison with the standard
antioxidant BHT.
2.3.3. Ferric Reducing Antioxidant Power (FRAP) Assay
In practice, the antioxidant activity of a substance is directly correlated to its reducing ability. An
assay like FRAP provides a reliable method to verify the antioxidant ability of a substance. Substances
having a reduction potential react with potassium ferricyanide forming potassium ferrocyanide. The
formed potassium ferrocyanide further reacts with FeCl3 to form an intensely colored Prussian blue
complex which has a maximum absorbance at 700 nm. The amount of complex formed is directly
proportional to the reducing capacity of the test sample. An increase in absorbance is a measure of the
reducing power of the sample. Results are indicated in Figure 10. From the analysis it is clear that the
compounds 4(d–f) showed good ferric reducing ability, in which 4f was the best compared to the other
tested compounds and the compound 4c showed the least reducing ability.
Figure 10. Comparison of ferric and cupric ion reducing ability of samples 4(a–f) with
reference to standard. Values are expressed as absorbance; high absorbance indicates high
reducing power.

Molecules 2013, 18
12006
2.3.4. Cupric Ion Reducing Antioxidant Capacity (CUPRAC) Assay
In this test the sample under evaluation effectively reduces Cu2+ to Cu+ changing the characteristic
ion absorption. The reduced Cu+ ions combine with the chromogenic reagent neocuproine forming a
stable 2:1 complex which has a maximum absorption at 450 nm. This method operates at pH 7. Results
are shown in Figure 10, which indicates that the test compounds displayed a certain degree of reducing
ability. The compound 4f has a higher reducing ability than the other test compounds. The least reducing
activity was shown by 4c, which may be due to the electronegative iodine substituent at the para position
on the aromatic ring.
3. Experimental
3.1. General Information
Melting points were determined on a Stuart Scientific (city, UK) apparatus. The purity of the
compounds was confirmed by thin layer chromatography using Merck silica gel 60 F254 coated
aluminium plates. The mass spectra were recorded on a Jeol JMS-D 300 mass spectrometer operating at
70 eV. Elemental analysis (CHN) was carried out on a Perkin Elmer Series II, 2400 analyzer. IR spectra
were recorded as KBr pellets on a Perkin Elmer System 2000 FTIR spectrophotometer in the wave
number range of 4,000–400 cm−1.
3.2. X-ray Crystallographic Analysis
X-ray analysis was done using Bruker SMART Apex II DUO CCDC diffractometer. The data were
processed with SAINT and absorption correction was done using SADABS [16]. The structures were
solved by direct method using the program SHELXTL [17], and were refined by full-matrix least
squares technique on F2 using anisotropic displacement parameters. The non-hydrogen atoms were
refined anisotropically. In these compounds, all the H atoms were calculated geometrically with
isotropic displacement parameters set to 1.2 (1.5 for methyl groups) times the equivalent isotropic U
values of the parent carbon atoms. The overlay structures were drawn using Olex2 software [18].
The crystallographic data for the reported compounds are given in Table 1. H-bonding interactions are
listed in Table 2. Crystallographic data have been deposited at the Cambridge Crystallographic Data
Centre. CCDC No: 939878, 942739, 942740, 948858, 944070 and 944071 contain the supplementary
crystallographic data for compounds 4(a–f) respectively. Copies of the data can be obtained
free of charge on application to the CCDC, 12 Union Road, Cambridge CB2 IEZ, UK.
Fax: +44-(0)1223-336033 or E-Mail: deposit@ccdc.cam.ac.uk.
3.3. General Procedure for the Synthesis of Chalcones 4(a–f)
A mixture of 2-acetyl-5-chlorothiophene (0.01 mol) and substituted benzaldehyde (0.01 mol) was
dissolved in methanol (20 mL). A catalytic amount of NaOH was added to the solution dropwise with
vigorous stirring. The reaction mixture was stirred for about 3–4 h at room temperature. The resultant
crude products were filtered, wash successively with distilled water and recrystallized from ethanol to

Molecules 2013, 18
12007
get the corresponding chalcone. Crystals suitable for X-ray diffraction studies were obtained by the slow
evaporation technique using a suitable solvent.
(2E)-1-(5-Chlorothiophen-2-yl)-3-(2-iodophenyl)prop-2-en-1-one (4a): Solvent for growing crystals:
mixture of acetone, ethanol and acetonitrile (1:1:1 v/v); Yield: 64%; M.P.: 104–105 °C; IR (cm−1, KBr):
1645 (C=O), 1588 (HC=CH), 3079 (C-H), 806 (C-I), 795 (C-Cl, thiophene), 759 (C-S); LCMS:
m/z = 375 (M++1); Elemental analysis: Calculated for C13H8ClIOS: C, 41.68%; H, 2.15%; Found: C,
41.65%; H, 2.19%.
(2E)-1-(5-Chlorothiophen-2-yl)-3-(3-iodophenyl)prop-2-en-1-one (4b): Solvent for growing crystals:
mixture of acetone, ethanol and acetonitrile (1:1:1 v/v); Yield: 67%; M.P.: 126–128 °C; IR (cm−1, KBr):
1644 (C=O), 1585 (HC=CH), 3077 (C-H), 802 (C-I), 794 (C-Cl, thiophene), 754 (C-S); LCMS:
m/z = 375 (M++1); Elemental analysis: Calculated for C13H8ClIOS: C, 41.68%; H, 2.15%; Found: C,
41.64%; H, 2.20%.
(2E)-1-(5-Chlorothiophen-2-yl)-3-(4-iodophenyl)prop-2-en-1-one (4c): Solvent for growing crystals:
mixture of acetone, ethanol and acetonitrile (1:1:1 v/v); Yield: 66%; M.P.: 164–166 °C; IR (cm−1, KBr):
1647 (C=O), 1592 (HC=CH), 3078 (C-H), 805 (C-I), 791 (C-Cl, thiophene), 768 (C-S); LCMS:
m/z = 375 (M++1); Elemental analysis: Calculated for C13H8ClIOS: C, 41.68%; H, 2.15%; Found: C,
41.63%; H, 2.18%.
(2E)-1-(5-Chlorothiophen-2-yl)-3-(2-methoxyphenyl)prop-2-en-1-one (4d): Solvent for growing
crystals: N,N-dimethylfomamide; Yield: 61%; M.P.: 134–135 °C; IR (cm−1, KBr): 1645 (C=O), 1579
(HC=CH), 3075 (C-H), 1226 (C-O-C), 795 (C-Cl, thiophene), 723 (C-S); LCMS: m/z = 279 (M++1);
Elemental analysis: Calculated for C14H11ClO2S: C, 60.32%; H, 3.98%; Found: C, 60.27%; H, 4.03%.
(2E)-1-(5-Chlorothiophen-2-yl)-3-(3-methoxyphenyl)prop-2-en-1-one (4e): Solvent for growing
crystals: mixture of acetone, ethanol and acetonitrile (1:1:1 v/v); Yield: 69%; M.P.: 108–110 °C; IR
(cm−1, KBr): 1646 (C=O), 1585 (HC=CH), 3072 (C-H), 1223 (C-O-C), 799 (C-Cl, thiophene), 723
(C-S); LCMS: m/z = 279 (M++1); Elemental analysis: Calculated for C14H11ClO2S: C, 60.32%; H,
3.98%; Found: C, 60.29%; H, 4.01%.
(2E)-1-(5-Chlorothiophen-2-yl)-3-(4-methoxyphenyl)prop-2-en-1-one (4f): Solvent for growing crystals:
mixture of acetone, ethanol and acetonitrile (1:1:1 v/v); Yield: 72%; M.P.: 118–119 °C; IR (cm−1, KBr):
1644 (C=O), 1586 (HC=CH), 3079 (C-H), 1227 (C-O-C), 798 (C-Cl, thiophene), 723 (C-S); LCMS: m/z
= 279 (M++1); Elemental analysis: Calculated for C14H11ClO2S: C, 60.32%; H, 3.98%; Found: C,
60.28%; H, 4.03%.
3.4. In Vitro Antioxidant Activity
3.4.1. DPPH Radical Scavenging Assay
The ability of the test samples in addition to the standard antioxidant butylated hydroxytoluene
(BHT) on DPPH radical scavenging was estimated according to the method described in the
literature [19]. An aliquot (200 μL) of an ethanolic solution of the samples (0–50 μg/mL for samples;
0–5 μg/mL for BHT) was mixed with 100 mM tris-HCl buffer (800 μL, pH 7.4) and then 500 μM DPPH

Molecules 2013, 18
12008
in methanol (1 mL) was added for a final concentration of 250 μM. The mixture was vigorously shaken
and incubated in the dark at room temperature for 20 min. A DPPH blank solution (control) was
prepared as above without the sample, and methanol was used for the baseline correction. The
absorbance of the test solutions were measured spectrophotometrically at 517 nm. The DPPH radical
scavenging activities were calculated using the equation:
DPPH radical scavenging activity (%) = [(Ac − As)/Ac] × 100
(1)
where Ac is the absorbance of the control and As is the absorbance of the test samples. The inhibition
concentration of the samples for 50% (IC50) DPPH radical scavenging was also calculated. Results were
expressed as mean of the three determinations.
3.4.2. ABTS Radical Scavenging Assay
The antioxidant ability of the test compounds were evaluated in terms of the ABTS radical
scavenging activity. The assay is performed according to the reported method [20]. ABTS+ was obtained
by reacting 7 mM ABTS stock solution with 2.45 mM potassium persulfate and the mixture was
incubated in dark at room temperature for about 12–16 h until the reaction was completed or the
absorbance become stable. The solution was then diluted with 5 mM phosphate buffer (pH 7.4) to obtain
an absorbance of 730 nm of 0.70 ± 0.02. One mL of the prepared ABTS+ solution was added to the
different concentration of the test samples 4a–f (0–50 μg/mL) in ethanol and also to the control. After
30 min, the absorbance of the test samples was measured at 730 nm against the blank solution
(without sample). The percentage ABTS+ scavenging is calculated by:
ABTS+ scavenging activity (%) = [(Ac − As)/Ac] × 100
(2)
where Ac is the absorbance of the control and As is the absorbance of the test samples. The inhibition
concentration of the samples for 50% (IC50) ABTS+ radical scavenging was also calculated. Results
were expressed as mean of the three determinations.
3.4.3. Ferric Reducing Antioxidant Power (FRAP) Assay
The synthesized compounds were screened for ferric reducing power antioxidant ability by
the method described by Oyaizu [21]. The method is based on the reduction of ferric (Fe3+) to ferrous
(Fe2+), which is accomplished in the presence of antioxidants. Samples with different concentrations
(0–50 μg/mL) were mixed with an equal volume of 0.2 M phosphate buffer (pH 6.6) and 1% potassium
ferricyanide and the mixture were incubated for 20 min at 50 °C. The mixture was acidified with 10%
trichloroacetic acid (2.5 mL) and then centrifuged at 3,000 rpm for about 15 min. The upper supernatant
liquid was diluted with distilled water and 0.1% ferric chloride was added. The absorbance was
measured at 700 nm. The increase in absorbance is directly proportional to the reducing ability of the
compound. The blank solution was prepared as above without the sample.

Molecules 2013, 18
12009
3.4.4. Cupric Ion Reducing Antioxidant Capacity (CUPRAC) Assay
Furthermore, the synthesized compounds were evaluated for their cupric ion reducing power by the
reported method [22]. CUPRAC is a widely applicable method for evaluating the antioxidant properties
of a substance. A mixture of CuCl2 (1 mL, 0.01 M) solution, ethanolic neocuproine (1 mL, 0.0075 M)
and ammonium acetate (1 mL, 1 M) were dissolved and test samples (1 mL, 10 μM) were added along
with distilled water (0.1 mL). The mixture was incubated for about 30 min and the absorbance was
measured at 450 nm against the blank solution. Blank solution is prepared as above without the sample.
3.4.5. Statistical Analysis
All the assay measurements were performed in triplicates (n = 3) and are expressed as mean of the
three determinations. The amount of compound required to inhibit DPPH and ABTS free radicals by
50% (IC50) was graphically estimated using linear regression algorithm.
4. Conclusions
A series of new 5-chlorothiophene chalcones, 4(a–f), were synthesized in good yield and
characterized by IR, mass, elemental analysis and single crystal X-ray diffraction studies. In addition,
their in-vitro antioxidant activity has been evaluated. The reported compounds displayed mild to good
antioxidant properties, which is due to the presence of electronegative –I and electron donating -OCH3
substituent at different positions on the phenyl ring. The increasing order of antioxidant activity of the
synthesized compounds follows the order 4f > 4d > 4e > 4b > 4a > 4c. The present study revealed that
(1) the influence of the nature of the functional linkage (electron withdrawing and electron donating
groups) and (2) the position of the substituent on the phenyl ring of 5-chlorothiophene chalcones are
crucial for the exhibited antioxidant activities.
Acknowledgments
CSC thanks Universiti Sains Malaysia for a postdoctoral research fellowship. CKQ thanks USM for
APEX DE2012 Grant (No. 1002/PFIZIK/910323). WSL thanks Malaysian Government for MyBrain15
(MyPhD) scholarship. The authors extend their appreciation to The Deanship of Scientific Research at
King Saud University for the funding the work through the research group project No. RGP VPP-207.
Conflicts of Interest
The authors declare no conflict of interest.
References
1. Alcaraz, L.E.; Blanco, S.E.; Puig, O.N.; Tomas, F.; Ferretti, F.H. Antibacterial activity of
Flavonoids against methicillin- resistant Staphylococcus aureus strains. J. Theor. Biol. 2000, 205,
231–240.
2. Baviskar, B.A.; Baviskar, B.; Shiradkar, M.R.; Deokate, U.A.; Khadabadi, S.S. Synthesis and
antimicrobial activity of some novel benzimidazolyl chalcones. J. Chem. 2009, 6, 196–200.

Molecules 2013, 18
12010
3. Echeverria, C.; Santibañez, J.F.; Donoso-Tauda, O.; Escobar, C.A.; Ramirez-Tagle, R. Structural
antitumoral activity relationships of synthetic chalcones. Int. J. Mol. Sci. 2009, 10, 221–231.
4. Modzelewska, A.; Pettit, C.; Achanta, G.; Davidson, N.E.; Huang, P.; Khan, S.R. Anticancer
activities of novel chalcone and bis-chalcone derivatives. Bioorg. Med. Chem. 2006, 14, 3491–3495.
5. Vogel, S.; Heilmann, J. Synthesis, cytotoxicity, and antioxidative activity of minor prenylated
chalcones from Humulus lupulus. J. Nat. Prod. 2008, 71, 1237–1241.
6. Babasaheb, P.B.; Sachin, A.P.; Rajesh, N.G. Synthesis and biological evaluation of
nitrogencontaining chalcones as possible anti-inflammatory and antioxidant agents. Bioorg. Med.
Chem. Lett. 2010, 20, 730–733.
7. Kim, Y.H.; Kim, J.; Park, H.; Kim, H.P. Anti-inflammatory activity of the synthetic chalcone
derivatives: Inhibition of inducible nitric oxide synthase-catalyzed nitric oxide production from
lipopolysaccharide-treated RAW 264.7 cells. Biol. Pharm. Bull. 2007, 30, 1450–1455.
8. Ballesteros, J.F.; Sanz, M.J.; Ubeda, A.; Miranda, M.A.; Iborra, S.; Paya, M.; Alcaraz, M.J.
Synthesis and pharmacological evaluation of 2’-hydroxychalcones and flavones as inhibitors of
inflammatory mediators generation. J. Med. Chem. 1995, 38, 2794–2797.
9. Vogel, S.; Barbic, M.; Jürgenliemk, G.; Heilmann, J. Synthesis, cytotoxicity, anti-oxidative and
anti-inflammatory activity of chalcones and influence of A-ring modifications on the
pharmacological effect. Eur. J. Med. Chem. 2010, 45, 2206–2213.
10. Lopez, S.N.; Castelli, M.V.; Zacchino, S.A.; Domnguez, J.N.; Lobo, G.; Charris-Charris, J.;
Cortes, J.C.G.; Ribas, J.C.; Devia, C.; Rodrguez, A.M.; et al. In vitro antifungal evaluation and
structure–activity relationships of a new series of chalcone derivatives and synthetic analogues,
with inhibitory properties against polymers of the fungal cell wall. Bioorg. Med. Chem. 2001, 9,
1999–2013.
11. Beom-Tae, K.; Kwang-Joong, O.; Jae-Chul, C.; Ki-Jun, H. Synthesis of dihydroxylated chalcone
derivatives with diverse substitution patterns and their radical scavenging ability toward DPPH free
radicals. Bull. Korean Chem. Soc. 2008, 29, 1125–1130.
12. Doan, T.N.; Tran, T.-D. Synthesis, antioxidant and antimicrobial activities of a novel series of
chalcones, pyrazolic chalcones, and allylic chalcones. Pharmacol. Pharm. 2011, 2, 282–288.
13. Go, M.L.; Wu, X.; Liu, X.L. Chalcones: An update on cytotoxic and chemoprotective properties.
Curr. Med. Chem. 2005, 12, 481–99.
14. Sivakumar, P.M.; Prabhakar, P.K.; Doble, M. Synthesis, antioxidant evaluation, and quantitative
structure–activity relationship studies of chalcones. Med. Chem. Res. 2011, 20, 482–492.
15. Vogel, S.; Ohmayer, S.; Brunner, G.; Heilmann, J. Natural and non-natural prenylated chalcones:
Synthesis, cytotoxicity and anti-oxidative activity. Bioorg. Med. Chem. 2008, 16, 4286–4293.
16. Bruker APEX2, SAINT and SADABS. Bruker AXS Inc.: Madison, WI, USA, 2009.
17. Sheldrick, G.M. A short history of SHELX. Acta Cryst. 2008, A64, 112–122.
18. Dolomanov, O.V.; Bourhis, L.J.; Gildea, R.J.; Howard, J.A.K.; Puschmann, H. OLEX2: A
complete structure solution, refinement and analysis program. J. Appl. Cryst. 2009, 42, 339–341.
19. Lai, L.S.; Chou, S.T.; Chao, W.W. Studies on the antioxidant activities of Hsian-tsao (Mesona
procumbens Hemsl) leaf gum. J. Agric. Food Chem. 2001, 49, 963–968.

Molecules 2013, 18
12011
20. Re, R.; Pellergini, N.; Proteggenete, A.; Pannala, A.; Yang, M.; Rice Evans, C. Antioxidant activity
applying an improved ABTS radical cation decolorization assay. Free Radic. Biol. Med. 1999, 26,
1231–1237.
21. Oyaizu, M. Studies on products of browning reaction prepared from glucosamine. Jpn. J. Nutr.
1986, 44, 307–315.
22. Apak, R.; Guclu, K.; Ozyurek, M.; Celik, S.E. Mechanism of antioxidant capacity assays and the
CUPRAC (cupric ion reducing antioxidant capacity) assay. Microchim. Acta 2008, 160, 413–419.
Sample Availability: Samples of the compounds are available from the authors.
© 2013 by the authors; licensee MDPI, Basel, Switzerland. This article is an open access article
distributed under the terms and conditions of the Creative Commons Attribution license
(http://creativecommons.org/licenses/by/3.0/).