Synthesis and free radical scavenging activity of coumarin derivatives containing a 2-methylbenzothiazoline motif

Article abstract of DOI:10.1002/ardp.201000271

Coumarin and benzothiazole scaffolds can be found in a number of natural or synthetic antioxidants. In an effort to develop a novel radical scavenger and potential antioxidant, a series of coumarin derivatives containing 2-methylbenzothiazoline motif and related compounds was synthesized and evaluated for their DPPH (1,1-diphenyl-2-picrylhydrazyl) and ABTS ^?+ (2,2′-azinobis(3-ethylbenzothiazoline-6-sulfonic acid) radicals scavenging activities. Among them, 7-hydroxy-3-(2-methyl-2,3- dihydrobenzo[d]thiazol-2-yl)-2H-chromen-2-one (3e) has shown a significant free radical scavenging activity. From the structure-activity point of view, it was found that phenolic coumarin ring and benzothiazoline moiety in target compounds may contribute to the scavenging activity against free radicals. A series of coumarin derivatives containing a 2-methylbenzothiazoline motif and related compounds were synthesized and evaluated for their DPPH and ABTS ^?+ radicals scavenging activities. 7-Hydroxy-coumarin derivative 3e showed a significant free radical scavenging activity. Copyright

Full text of DOI:10.1002/ardp.201000271

588
Arch. Pharm. Chem. Life Sci. 2011, 344, 588–594
Full Paper
Synthesis and Free Radical Scavenging Activity of Coumarin
Derivatives Containing a 2-Methylbenzothiazoline Motif
Mehdi Khoobi1, Saeed Emami2, Gholamreza Dehghan3, Alireza Foroumadi4,5, Ali Ramazani1, and
Abbas Shafiee5
1 Chemistry Department, Zanjan University, Zanjan, Iran
2 Department of Medicinal Chemistry and Pharmaceutical Sciences Research Center, Faculty of Pharmacy,
Mazandaran University of Medical Sciences, Sari, Iran
3 Department of Biology, Faculty of Natural Science, University of Tabriz, Tabriz, Iran
4 Drug Design & Development Research Center, Tehran University of Medical Sciences, Tehran, Iran
5 Department of Medicinal Chemistry, Faculty of Pharmacy and Pharmaceutical Sciences Research Center,
Tehran University of Medical Sciences, Tehran, Iran
Coumarin and benzothiazole scaffolds can be found in a number of natural or synthetic antioxidants.
In an effort to develop a novel radical scavenger and potential antioxidant, a series of coumarin
derivatives containing 2-methylbenzothiazoline motif and related compounds was synthesized and
evaluated for their DPPH (1,1-diphenyl-2-picrylhydrazyl) and ABTSꢀþ (2,20-azinobis(3-ethylbenzothiazoline-
6-sulfonic acid) radicals scavenging activities. Among them, 7-hydroxy-3-(2-methyl-2,3-dihydrobenzo[d]-
thiazol-2-yl)-2H-chromen-2-one (3e) has shown a significant free radical scavenging activity. From the
structure–activity point of view, it was found that phenolic coumarin ring and benzothiazoline
moiety in target compounds may contribute to the scavenging activity against free radicals.
Keywords: ABTS / Antioxidants / Benzothiazoline / Coumarin / DPPH / Radical scavenging activity
Received: September 19, 2010; Revised: October 24, 2010; Accepted: October 29, 2010
DOI 10.1002/ardp.201000271
Introduction
possesses innate defense mechanisms to counter free radicals
in the form of enzymes such as superoxide dismutase, catalase,
and glutathione peroxidase. The unbalance between formation
and detoxification of free radical species results in the pro-
gression of oxidative stress and leads to the development of
chronic and degenerative diseases. Therefore, inhibition of
oxidative damage by supplementation with antioxidant and/
or free radical scavengers might reduce the risk of these dis-
eases [5, 6]. In the past decade, medicinal chemists, food chem-
ists and biologists have increasingly focused their attention on
researching and testing for new and efficient natural
or synthetic antioxidants as a protective strategy against these
diseases by reducing and/or inhibiting free-radical reactions.
Many synthetic and natural coumarin derivatives have
special ability to scavenge free radicals, such as hydroxyl
radicals and superoxide anion radicals, and to influence
processes involving free radical-injury [7]. On the other hand,
a large number of benzothiazole derivatives have been pre-
pared because of their wide biological activity, i.e. significant
anticancer and antioxidant properties [8–10].
Free radicals and reactive oxygen species (ROS) including super-
oxide anion, hydrogen peroxide, and hydroxyl radical, are
being generated during normal cellular metabolism and bio-
organic redox processes. Generation of free radicals is not
limited to normal cellular process but also occurs upon
exposure to certain chemicals, radiation, cigarette smoke,
and high fat diet. Furthermore, radical reactions play a signifi-
cant role in the development of life-limiting chronic diseases
such as cancer, hypertension, cardiac infarction, stroke, arte-
riosclerosis, rheumatoid arthritis, Alzheimer’s and Parkinson’s
diseases, cataracts and others [1–4]. Exposure of a normal cell to
free radical is known to damage structures and consequently
interfere with functions of enzymes and critical macromol-
ecules (e.g., lipids, proteins and nucleic acids). The human body
Correspondence: Abbas Shafiee, Department of Medicinal Chemistry,
Faculty of Pharmacy and Pharmaceutical Sciences Research Center,
Tehran University of Medical Sciences, Tehran14176, Iran.
E-mail: ashafiee@ams.ac.ir
In continuation of our research program and in an effort to
develop a novel radical scavenger and potential antioxidant
Fax: þ98 21 66461178
ß 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Arch. Pharm. Chem. Life Sci. 2011, 344, 588–594
Free Radical Scavenging Activity of Coumarin Derivatives
589
Scheme 1. Synthesis of compounds 3a–e
and 4.
[11–13], we synthesized a series of coumarin derivatives con-
taining 2-methylbenzothiazoline motif and evaluated their
radical scavenging properties. Thus, we report here, the syn-
Knoevenagel condensation of salicylaldehyde derivatives 1
with ethyl acetoacetate in piperidine furnished 3-acetyl-
coumarins 2 [14]. The reaction of 3-acetylcoumarins 2 with
2-aminothiophenol derivatives in the presence of acetic acid
under reflux afforded benzothiazoline derivatives 3a–e.
Similarly, 3-acetylcoumarin 2a was reacted with 2-amino-
ethanethiol hydrochloride in refluxing acetic acid to yield
thiazolidine derivative 4. N-methyl analog 3f was obtained
by refluxing compound 3a and methyl iodide in butan-2-one,
in the presence of K2CO3. Alkylation of compound 3e resulted
in N,O-dialkylation. Thus, the reaction of compound 3e with
appropriate alkyl halide in refluxing butan-2-one and in the
presence of K2CO3 yielded compounds 3g and 3h (Scheme 2).
For the synthesis of 3-(benzothiazol-2-yl) coumarin 6, differ-
ent strategy was used starting from salicylaldehyde 1a.
Treatment of salicylaldehyde 1a with malononitrile in aque-
ous solution of NaOH afforded 3-cyano-2-iminocoumarin
intermediate which on subsequent hydrolysis yielded corre-
sponding 3-cyanocoumarin 5 [15]. The desired benzothiazole
derivative 6 was synthesized by reaction of 3-cyanocoumarin
5 with 2-aminothiophenol as shown in Scheme 3.
þ
thesis and DPPH (1,1-diphenyl-2-picrylhydrazyl) and ABTSꢀ
(2,20-azinobis(3-ethylbenzothiazoline-6-sulfonic acid) radicals
scavenging activities of 3-(2-methyl-2,3-dihydrobenzo[d]thia-
zol-2-yl)-2H-chromen-2-ones and some related compounds.
Results and discussion
Chemistry
The synthetic pathway to target compounds was started
from salicylaldehyde derivatives as shown in Scheme 1.
Antioxidant activity
In the present study, the radical scavenging activity of the
newly synthesized compounds were assessed using the differ-
ent assay systems; DPPH (1,1-diphenyl-2-picrylhydrazyl) and
þ
ABTSꢀ (2,20-azinobis(3-ethylbenzothiazoline-6-sulfonic acid).
These synthetic nitrogen-centered radicals are not biologi-
cally relevant, but are often used as indicator compounds in
testing hydrogen transfer capacity that is related to the
Scheme 2. Synthesis of compounds 3f–h.
Scheme 3. Synthesis of compound 6.
ß 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
www.archpharm.com

590
M. Khoobi et al.
Arch. Pharm. Chem. Life Sci. 2011, 344, 588–594
antioxidant activity. Antioxidant activity can be considered
as an index of pharmacological usefulness [16]. Quercetin was
used as a standard drug.
As shown in Table 1, compounds 3d and 3e exhibited high
ABTSꢀþ scavenging activity with percentage over than 50% at
the concentration of 15 mg/mL.
DPPH radical scavenging activity
Conclusions
At the first, free radical scavenging activity of compounds
was evaluated by the colorimetrical decrease in absorbance of
DPPH due to the chemical trapping of unpaired electron.
Primarily, each compound was evaluated at a final concen-
tration of 100 mg/mL in the assay mixture. All experiments
were carried out in triplicate. Results are expressed as per-
centage decrease with respect to the control values. Several
dilutions of primary solution were made and assayed accord-
ingly to obtain concentration of the sample required to
scavenge 50% (IC50) of DPPH free radical applying suitable
regression analysis of the mean values. The results are given
in Table 1. Generally, compounds 3a, 3c, 3d and 3e showed
good scavenging against DPPH. Indeed, DPPH radicals were
scavenged almost 80% at 100 mg/mL of these compounds. The
best result was obtained by phenolic compound 3e with IC50
value of 7.6 mg/mL. The IC50 values of compounds 3a, 3c and
3d were 38.3, 37.2 and 34.6 mg/mL, demonstrating the same
activity for these compounds.
In summary, a series of coumarin derivatives containing 2-
methylbenzothiazoline motif and related compounds were
þ
synthesized and evaluated their DPPH and ABTSꢀ radicals
scavenging activities. Among them, 7-hydroxy-3-(2-methyl-
2,3-dihydrobenzo[d]thiazol-2-yl)-2H-chromen-2-one (3e) has
shown a significant free radical scavenging activity. From
the structure–activity point of view, it was found that phe-
nolic coumarin ring and benzothiazoline moiety in target
compounds may contribute to the scavenging activity against
free radicals. These results demonstrate that title compounds
have potential application in various physiopathological
processes and diseases such as inflammation, aging, cancer
and atherosclerosis.
Experimental
Chemistry
All chemicals and reagents were obtained from Merck
Chemical Company (Darmstadt, Germany). 1H-NMR spectra
From the results of compounds 3a and 3b, it is evident
that substitution of Cl at the 6-position of benzothiazoline ring
reduces the scavenging activity. The comparison of scavenging
capability of unsubstituted compound 3a with those of
6-bromo analog 3c and 8-methoxy derivative 3d indicate that
the introduction of 6-bromo or 8-methoxy substituent on cou-
marin ring is well tolerated in term of DPPH radical scavenging
activity. Introduction of methyl group at the N-3 position of
benzothiazoline ring, significantly reduces the activity (com-
pound 3f versus 3a). It is found that incorporation of hydroxy
group at the 7-position of coumarin ring leads to the most
active compound 3e. Also, it is obvious from the results that
N,O-dialkylation (e.g., in compounds 3g and 3h) decreases the
activity compared to the unsubstituted compound 3e. By com-
parison of compounds 4 and 6 with 3a, it can be concluded that
the alteration of benzothiazoline ring to the thiazolidine or
benzothiazole dramatically diminishes the activity.
were recorded using
a Bruker 500 spectrometer (Bruker,
Rheinstatten, Germany), and chemical shifts are expressed as
d (ppm) related to tetramethylsilane (TMS) as internal standard.
The IR spectra were taken using a Nicolet FT-IR Magna 550
spectrographs (KBr disks) (Nicolet, Madison, WI, USA). The mass
spectra were run on a Finnigan MAT TSQ-70 spectrometer
(Finnigan Mat, Bremen, Germany) at 70 eV. The results of
elemental analyses (C, H, N) were within ꢁ0.4% of the theoretical
values for C, H, and N. Merck silica gel 60 F254 plates were
applied for analytical TLC. Column chromatography was per-
formed on Merck silica gel (70–230 mesh) for purification of
compounds.
General procedure for the synthesis of 3-acetylcoumarin
derivatives 2a–d
To a stirring cold mixture of salicylaldehyde derivatives 1a–d
(0.2 mol) and ethyl acetoacetate (0.22 mol), 2 mL of piperidine
were added. After 20 min, the yellowish solid separated was
filtered off, subsequently washed with ethanol and recrystallized
from water/ethanol (3:7) to afford compounds 2a–d (yields were
84–95%) [14].
ABTSꢀþ radical scavenging activity
ABTSꢀþ is also a free and stable radical cation which is able to
react with antioxidants that can give a hydrogen atom or an
electron. ABTS radical cation shows a strong absorption at
723 nm and becomes colorless on reduction. Thus, the ABTSꢀþ
radical scavenging activity of each compound was determined
by the decolorization of the ABTSꢀþ and measured as a quench-
ing of the absorbance at 723 nm. Each compound was tested at
the final concentration of 15 mg/mL in the assay mixture. The
results are expressed in Table 1 as percentage decrease with
respect to the control values.
General procedure for the reaction of 3-acetylcoumarins
2a–d and 2-aminothiophenol
Appropriate 3-acetylcoumarin 2a–d (1 mmol) and 2-aminothio-
phenole or 2-amino-4-chlorobenzenethiol (1.2 mmol) was mixed
with catalytic amount of acetic acid. Then, the mixture was
refluxed for 5–8 h. The progress of reaction was followed by
TLC. After completion of the reaction, the mixture was cooled
and the solid was filtered off and washed with Et2O. The crude
product was recrystallized from EtOH to give pure compounds
3a–e.
ß 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
www.archpharm.com

Arch. Pharm. Chem. Life Sci. 2011, 344, 588–594
Free Radical Scavenging Activity of Coumarin Derivatives
591
Table 1. Radical scavenging activity of compounds 3a-h, 4 and 6.
Entry
Compound
DPPH method
ABTS method
% Scavenging
at 100 mg/mL
IC50
% Scavenging
at 15 mg/mL
S
3a
3b
79.8
31.9
38.3
nd a
31.4
N
H
O
O
Cl
153.6
S
N
H
O
O
S
Br
3c
81.6
83.2
37.2
34.6
nd
N
H
O
O
S
3d
50.1
N
H
O
O
O
S
3e
3f
83.9
9.5
7.6
78.6
5.7
10
N
H
HO
O
O
S
893.5
808.5
N
O
O
S
3g
7.6
N
O
O
O
O
S
3h
19.8
6.6
250.1
632.4
7.1
N
O
O
S
N
H
4
6
21.4
O
O
N
S
4.5
153.6
–
0
O
O
Quercetin
38 b
34.3 b
a
b
Not determined. Quercetin was tested at 10 mM.
ß 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
www.archpharm.com

592
M. Khoobi et al.
Arch. Pharm. Chem. Life Sci. 2011, 344, 588–594
(t, J ¼ 7.5 Hz, 1H, H6 benzothiazoline), 6.70 (d, J ¼ 7.7 Hz,
1H, H4 benzothiazoline), 6.73 (s, 1H, H8 coumarin), 6.77
(d, J ¼ 8.5 Hz, 1H, H6 coumarin), 6.87 (t, J ¼ 7.7 Hz, 1H, H5
benzothiazoline), 6.92 (d, J ¼ 7.5 Hz, 1H, H7 benzothiazoline),
7.00 (s, 1H, H4 coumarin), 7.56 (d, J ¼ 8.5 Hz, 1H, H5 coumarin),
10.59 (s, 1H, OH); 13C-NMR (125 MHz, DMSO-d6); 19.0, 76.2, 102.1,
110.0, 111.2, 113.9, 119.4, 121.4, 125.6, 126.5, 128.2, 130.5, 137.5,
146.2, 154.9, 160.0, 161.6þ; IR (KBr, cmꢂ1) nmax: 3343 (NH), 1704
3-(2-Methyl-2,3-dihydrobenzo[d]thiazol-2-yl)-2H-chromen-
2-one (3a)
Yellow solid (2.87 g, 97%); m.p. 159–1618C; 1H-NMR (CDCl3)
d: 2.19 (s, 3H, 2-CH3 benzothiazoline), 5.03 (s, 1H, NH), 6.76
(m, 2H, H4 benzothiazoline and H6 coumarin), 6.95
(t, J ¼ 7.5 Hz, 1H, H5 benzothiazoline), 7.07 (d, J ¼ 7.5 Hz,
1H, H7 benzothiazoline), 7.27 (t, J ¼ 7.5 Hz, 1H, H6 benzothiazo-
line), 7.33 (d, J ¼ 8.5 Hz, 1H, H8 coumarin), 7.50 (m, 2H, H5,7
coumarin), 8.10 (s, 1H, H4 coumarin); 13C-NMR (125 MHz, CDCl3):
28.2, 76.9, 111.3, 116.2, 118.9, 121.1, 121.8, 124.5, 125.4, 127.3,
128.4, 131.5, 138.4, 145.1, 153.1, 160.2; IR (KBr, cmꢂ1) nmax: 3344
–
(C O); MS, m/z (%) 311 (M , 25), 296 (100), 150 (8), 109 (7). Anal.
–
calcd. for C17H13NO3S: C, 65.58; H, 4.21; N, 4.50. Found: C, 65.46;
H, 4.13; N, 4.69.
–
(NH), 1705 (C O). Anal. calcd. for C17H13NO2S: C, 69.13; H, 4.44;
–
N, 4.74. Found: C, 69.26; H, 4.23; N, 4.62.
Preparation of 3-(2,3-dimethyl-2,3-dihydrobenzo[d]thiazol-
2-yl)-2H-chromen-2-one (3f)
To a mixture of compound 3a (1 mmol) and K2CO3 (1.5 mmol) in
butan-2-one (10 mL) was added methyl iodide (1.2 mmol). Then,
the mixture was refluxed for 2–3 h. After completion of the
reaction, the solvent was evaporated under reduced pressure.
The residue was washed with water and crystallized from EtOH/
H2O to give compound 3f.
3-(6-Chloro-2-methyl-2,3-dihydrobenzo[d]thiazol-2-yl)-2H-
chromen-2-one (3b)
Yellow solid (2.40 g, 73%); m.p. 190–1928C; 1H NMR (CDCl3)
d: 2.16 (s, 3H, 2-CH3 benzothiazoline), 5.21 (s, 1H, NH), 6.69
(m, 2H, H7 benzothiazoline and H6 coumarin), 6.92
(d, J ¼ 8.5 Hz, 1H, H5 benzothiazoline), 7.26 (t, J ¼ 8.0 Hz,
1H, H7 coumarin), 7.33 (d, J ¼ 8.5 Hz, 1H, H4 benzothiazoline),
7.51 (m, 2H, H5,8 coumarin), 8.07 (s, 1H, H4 coumarin); 13C-NMR
(125 MHz, CDCl3): 28.4, 77.8, 110.8, 116.3, 118.8, 120.5, 122.3,
124.6, 125.0, 128.4, 131.0, 131.2, 131.7, 138.6, 146.3, 153.1, 160.2.
Yellow solid (0.285 g, 92%); m.p. 118–1208C; 1H-NMR (CDCl3)
d: 2.14 (s, 3H, 2-CH3 benzothiazoline), 2.46 (s, 3H, N-CH3), 6.59
(d, J ¼ 7.4 Hz, 1H, H4 benzothiazoline), 7.13 (m, 2H, H6 benzo-
thiazoline and H6 coumarin), 7.23 (d, J ¼ 7.4 Hz, 1H, H7 benzo-
thiazoline), 7.32 (t, J ¼ 8.0 Hz, 1H, H5 benzothiazoline), 7.37
(d, J ¼ 7.4 Hz, 1H, H8 coumarin), 7.58 (t, J ¼ 8.0 Hz, 1H, H7 cou-
marin), 7.64 (d, J ¼ 7.4 Hz, 1H, H5 coumarin), 8.49 (s, 1H, H4
coumarin); 13C-NMR (125 MHz, CDCl3): 14.8, 19.9, 116.5, 118.5,
119.0, 124.5, 124.6, 125.0, 125.2, 128.0, 129.2, 132.6, 143.4, 147.1,
IR (KBr, cmꢂ1) nmax: 3355 (NH), 1704 (C O). Anal. calcd. for
–
–
C17H12ClNO2S: C, 61.91; H, 3.67; N, 4.25. Found: C, 61.76;
H, 3.83; N, 4.02.
6-Bromo-3-(2-methyl-2,3-dihydrobenzo[d]thiazol-2-yl)-2H-
154.4, 159.8, 166.5; IR (KBr, cmꢂ1) nmax: 1712 (C O). Anal. calcd.
–
–
for C18H15NO2S: C, 69.88; H, 4.89; N, 4.53. Found: C, 69.60; H,
5.01; N, 4.29.
chromen-2-one (3c)
Yellow solid (3.48 g, 93%); m.p. 109–1118C; 1H-NMR (CDCl3)
d: 2.17 (s, 3H, 2-CH3 benzothiazoline), 5.20 (s, 1H, NH), 6.77
(m, 2H, H4,7 benzothiazoline), 6.94 (t, J ¼ 7.5 Hz, 1H, H6 benzo-
thiazoline), 7.04 (t, J ¼ 7.5 Hz, 1H, H5 benzothiazoline), 7.19
(d, J ¼ 8.5 Hz, 1H, H8 coumarin), 7.56 (dd, J ¼ 8.5 Hz and
J ¼ 2.2 Hz, 1H, H7 coumarin), 7.62 (d, J ¼ 2.2 Hz, 1H, H5 cou-
marin), 7.98 (s, 1H, H4 coumarin); 13C-NMR (125 MHz, CDCl3):
27.8, 76.5, 111.8, 117.0, 117.9, 120.5, 121.6, 121.8, 125.5, 127.9,
General procedure for the reaction of compound 3e with
alkyl halides
To a mixture of compound 3e (1 mmol) and K2CO3 (1.5 mmol) in
butan-2-one (10 mL) was added appropriate alkyl halide
(1.2 mmol). The mixture was refluxed for 2–3 h. After completion
of the reaction, the solvent was evaporated under reduced pres-
sure. The residue was washed with water and crystallized from
EtOH/H2O to give corresponding N,O-dialkylated compound.
130.6, 132.7, 134.2, 136.9, 144.9, 151.9, 159.6; IR (KBr, cmꢂ1) nmax
:
þ
–
3351 (NH), 1697 (C O); MS, (m/z, %) 375 (M þ 2, 17), 373 (M , 17),
–
360 (100), 358 (89), 251 (17), 150 (20), 109 (17). Anal. calcd.
for C17H12BrNO2S: C, 54.56; H, 3.23; N, 3.74. Found: C, 54.50;
H, 3.03; N, 3.86.
3-(2,3-Dimethyl-2,3-dihydrobenzo[d]thiazol-2-yl)-7-
methoxy-2H-chromen-2-one (3g)
Yellow solid (0.303 g, 89%); m.p. 98–1008C; 1H-NMR (CDCl3)
d: 2.28 (s, 3H, 2-–CH3 benzothiazoline), 2.49 (s, 3H, N–CH3),
3.94 (s, 3H, O–CH3), 6.74 (dd, J ¼ 7.5 Hz, J ¼ 1.5 Hz, 1H, H4
benzothiazoline), 6.87 (d, J ¼ 2.3 Hz, 1H, H8 coumarin), 6.90
(dd, J ¼ 8.6 Hz, J ¼ 2.3 Hz, 1H, H6 coumarin), 7.12–7.19
(m, 2H, H5,6 benzothiazoline), 7.25 (dd, J ¼ 8.6 Hz, J ¼ 1.5 Hz,
1H, H7 benzothiazoline), 7.55 (d, J ¼ 8.6 Hz, 1H, H5 coumarin),
8.50 (s, 1H, H4 coumarin); 13C-NMR (125 MHz, CDCl3): 14.8, 19.9,
29.6, 55.8, 100.3, 112.9, 113.2, 118.6, 124.3, 125.0, 125.2, 128.7,
8-Methoxy-3-(2-methyl-2,3-dihydrobenzo[d]thiazol-2-yl)-
2H-chromen-2-one (3d)
Yellow solid (3.1 g, 96%); m.p. 187–1898C; 1H-NMR (CDCl3) d: 2.18
(s, 3H, 2-CH3 benzothiazoline), 3.95 (s, 3H, OCH3), 5.10 (s, 1H, NH),
6.74 (m, 2H, H4,6 benzothiazoline), 6.94 (t, J ¼ 7.5 Hz, 1H, H6
coumarin), 7.04 (m, 3H, H7 benzothiazoline and H5,7 coumarin),
7.18 (t, J ¼ 8.0 Hz, 1H, H5 benzothiazoline), 8.08 (s, 1H, H4 cou-
marin); 13C-NMR (125 MHz, CDCl3): 28.3, 56.2, 76.7, 111.1, 113.3,
119.6, 119.8, 120.9, 121.8, 124.3, 125.4, 127.0, 131.7, 138.6, 142.8,
130.3, 143.7, 147.4, 156.5, 160.2, 163.7, 166.5; IR (KBr, cmꢂ1) nmax
:
145.2, 146.9, 159.7; IR (KBr, cmꢂ1) nmax: 3348 (NH), 1700 (C O).
–
–
Anal. calcd. for C18H15NO3S: C, 66.44; H, 4.65; N, 4.30. Found: C,
66.36; H, 4.43; N, 4.49.
–
1701 (C O). Anal. calcd. for C19H17NO3S: C, 67.24; H, 5.05; N, 4.13.
–
Found: C, 67.01; H, 5.31; N, 4.00.
7-Hydroxy-3-(2-methyl-2,3-dihydrobenzo[d]thiazol-2-yl)-
2H-chromen-2-one (3e)
7-Ethoxy-3-(3-ethyl-2-methyl-2,3-dihydrobenzo[d]thiazol-
2-yl)-2H-chromen-2-one (3h)
Yellow solid (2.89 g, 93%); m.p. 101–1038C; 1H-NMR (DMSO-d6)
d; 1.95 (s, 3H, 2-CH3 benzothiazoline), 4.36 (s, 1H, NH), 6.57
Yellow solid (0.313 g, 85%); m.p. 76–788C; 1H-NMR (CDCl3) d: 1.36
(t, J ¼ 7.4 Hz, 3H, N–CH2CH3), 1.46 (t, J ¼ 7.0 Hz, 3H, O–CH2CH3),
ß 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
www.archpharm.com

Arch. Pharm. Chem. Life Sci. 2011, 344, 588–594
Free Radical Scavenging Activity of Coumarin Derivatives
593
2.26 (s, 3H, 2–CH3 benzothiazoline), 2.95 (q, J ¼ 7.4 Hz, 2H,
N–CH2CH3), 4.12 (q, J ¼ 7.0 Hz, 2H, O–CH2CH3), 6.74
(dd, J ¼ 7.7 Hz, J ¼ 1.3 Hz, 1H, H4 benzothiazoline), 6.84 (d,
J ¼ 2.4 Hz, 1H, H8 coumarin), 6.88 (dd, J ¼ 8.5 Hz, J ¼ 2.4 Hz,
1H, H6 coumarin), 7.10 (dt, J ¼ 7.7 Hz, J ¼ 1.3 Hz, 1H, H6 benzo-
thiazoline), 7.18 (dt, J ¼ 7.7 Hz, J ¼ 1.3 Hz, 1H, H5 benzothiazo-
line), 7.30 (dd, J ¼ 7.7 Hz, J ¼ 1.3 Hz, 1H, H7 benzothiazoline),
7.58 (d, J ¼ 8.5 Hz, 1H, H5 coumarin), 8.43 (s, 1H, H4 coumarin);
13C-NMR (125 MHz, CDCl3): 14.0, 14.5, 20.0, 25.9, 28.4, 64.3,
100.7, 112.5, 113.5, 119.0, 124.2, 125.8, 127.0, 127.2, 130.3,
C16H9NO2S: C, 68.80; H, 3.25; N, 5.01. Found: C, 68.65; H, 3.49;
N, 4.82.
Antioxidant activity
DPPH radical scavenging assay
The DPPH radical scavenging activity of the synthesized com-
pounds was determined by using the Brand-Williams et al.
method with some modifications [17]. Several concentrations
of test compounds in DMSO were prepared. The compound
solution (1.0 mL) was added to the methanolic DPPH solution
(2.0 mL, 0.1 mM) and the mixture was kept in the dark for
15 min. The absorbance at 517 nm was then measured by an
UV/visible spectrophotometer. The percent scavenging activity
was calculated using the following formula:
143.8, 148.4, 156.5, 160.3, 163.1, 166.2; IR (KBr, cmꢂ1) nmax
:
–
1728 (C O). Anal. calcd. for C21H21NO3S: C, 68.64; H, 5.76; N,
–
3.81. Found: C, 68.85; H, 5.59; N, 4.02.
Preparation of 3-(2-methylthiazolidin-2-yl)-2H-chromen-2-
one (4)
Inhibition ð%Þ ¼ 100 ꢃ ðAbscontrolꢂAbscompoundÞ=Abscontrol
The DPPH radical scavenging activity of compounds was
expressed in term of IC50, which obtained from linear regression
plot between concentrations of test compound and percent
inhibitions.
A mixture of 3-acetylcoumarin 2a (1 mmol) and 2-aminoethane-
thiol hydrochloride (5 mmol) in acetic acid (5 mL) was refluxed
for 5 h. The progress of the reaction was followed by TLC. After
completion of the reaction, the solvent was evaporated under
reduced pressure and the residue was purified by silica gel
column chromatography (hexane/ethyl acetate, 9:1) to give the
pure product 4.
ABTSꢀþ radical scavenging assay
Red solid (0.185 g, 75%); m.p. 102–1058C; 1H-NMR (CDCl3) d:
1.99 (s, 3H, 2–CH3 benzothiazoline), 2.92–3.04 (m, 3H, S–CH2 and
NH), 3.65 (m, 2H, N–CH2), 7.26 (t, J ¼ 8.0 Hz, 1H, H6 coumarin),
7.32 (d, J ¼ 8.0 Hz, 1H, H8 coumarin), 7.48 (m, 2H, H5,7 cou-
marin), 7.89 (s, 1H, H4 coumarin); 13C-NMR (125 MHz, CDCl3):
29.2, 36.7, 51.6, 77.6, 116.3, 119.3, 124.2, 128.1, 130.9, 133.0,
ABTS radical scavenging activity was assayed using the method of
Pennycooke et al. [18]. Following this method, 54.2 mg of ABTS
powder was dissolved in phosphate buffer (pH 7.0; 5 mM) and
mixed thoroughly with 1.0 g of MnO2 and incubated at room
temperature for 30 min for generation of green colored ABTS
radicals. The prepared solution was centrifuged for 5 min and
after filtration; the filtrate was diluted with phosphate buffer
until the absorbance of solution equals with 0.700 ꢁ 0.01 in
723 nm wavelengths. Absorbance reduction of prepared ABTS
reagent using 15 mg/mL of test compounds in 2 mL of reagent
was recorded by UV/VIS spectrometer for comparing of their
abilities.
134.5, 153.1, 160.3; IR (KBr, cmꢂ1) nmax: 3104 (NH), 1712 (C O),
–
–
1611. Anal. calcd. for C13H13NO2S: C, 63.13; H, 5.30; N, 5.66.
Found: C, 63.35; H, 5.55; N, 5.32.
Preparation of 3-cyanocoumarin (5)
A mixture of salicylaldehyde 1a (10 mmol), malononitrile
(0.80 g, 12.5 mmol) and 0.2 M aq NaOH solution (50 mL) was
vigorously stirred at r.t. The progress of the reaction was followed
by TLC. After completion of the reaction, the intermediate
3-cyano-2-iminocoumarin was separated by filtration. Then, con-
centrated HCl (1.25–2.0 mL) was added and heterogeneous mix-
ture was stirred at 908C for 1–3 h. After cooling, the precipitated
solid was separated by filtration and recrystallized from EtOH to
give pure 3-cyanocoumarin (5) [15].
This work was supported by grants from the Research Council of Tehran
University of Medical Sciences and Iran National Science Foundation
(INSF).
The authors have declared no conflict of interest.
References
Preparation of 3-(benzo[d]thiazol-2-yl)-2H-chromen-2-one
(6)
A mixture of 3-cyanocoumarin 5 (1 mmol), 2-aminothiophenol
(1.2 mmol) and catalytic amount of acetic acid was refluxed
overnight. The progress of the reaction was followed by TLC.
After cooling, the precipitated solid was filtered off and recrystal-
lized from EtOH to afford pure compound 6.
[1] T. Finkel, N. J. Holbrook, Nature 2000, 408, 239–247.
´
[2] C. Soler-Rivas, J. C. Espın, H. J. Wichers, Phytochem. Anal. 2000,
11, 1–9.
[3] S. P. Hussain, L. J. Hofseth, C. C. Harris, Nat. Rev. Cancer 2003,
3, 276–285.
[4] M. S. Cooke, M. D. Evans, M. Dizdaroglu, J. Lunec, FASEB J.
2003, 17, 1195–1214.
Yellow solid (0.245 g, 88%); m.p. 215–2178C; 1H-NMR (CDCl3)
d: 7.40 (dt, J ¼ 8.0 Hz, J ¼ 1.4 Hz, 1H, H6 coumarin), 7.45–7.47
(m, 2H, H4,6 benzothiazine), 7.55 (dt, J ¼ 8.0 Hz, J ¼ 1.4 Hz,
1H, H7 coumarin), 7.66 (dt, J ¼ 8.0 Hz, J ¼ 1.4 Hz, 1H, H5
benzothiazine), 7.75 (dd, J ¼ 8.5 Hz, J ¼ 2.4 Hz, 1H, H7 benzo-
thiazine), 7.99 (d, J ¼ 8.0 Hz, 1H, H8 coumarin), 8.11
(d, J ¼ 8.0 Hz, 1H, H5 coumarin), 9.10 (s, 1H, H4 coumarin);
13C-NMR (125 MHz, CDCl3): 116.7, 118.6, 120.3, 121.7, 122.9,
125.2, 125.4, 126.5, 129.3, 133.2, 136.8, 141.4, 152.4, 153.8,
[5] M. A. Babizhayev, A. I. Deyev, V. N. Yermakova, I. V. Brikman,
J. Bours, Drugs 2004, 5, 125–139.
[6] L. Liu, M. Meydani, Nutr. Rev. 2002, 60, 368–371.
[7] A. Mora, M. Paya, J. L. Rios, M. J. Alcaraz, Biochem. Pharmacol.
1990, 40, 793–797.
´
[8] I. Caleta, M. Kralj, M. Marjanovic, B. Bertosa, S. Tomic,
´
G. Pavlovic, K. Pavelic, G. Karminski-Zamola, J. Med. Chem.
ˇ
´
´
2009, 52, 1744–1756.
´
159.8, 159.9; IR (KBr, cmꢂ1) nmax: 1716 (C O). Anal. calcd. for
–
–
ß 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
www.archpharm.com

594
M. Khoobi et al.
Arch. Pharm. Chem. Life Sci. 2011, 344, 588–594
[9] M. J. de Brabander, A. S. Josephine, J. E. M. F. Leysen, PCT Int.
Appl. (1996), WO 9625931 A2 19960829.
[14] S. Khode, V. Maddi, P. Aragade, M. Palkar, P. K. Ronad, S.
Mamledesai, A. H. M. Thippeswamy, D. Satyanarayana, Eur. J.
Med. Chem. 2009, 44, 1682–1688.
[10] J. Singh, M. E. Gurney, B. Alex, A. Kiselyov, M. Rao, US Patent
Appl. (2009), WO 275164 A1 20090130076.
[15] F. Fringuelli, O. Piermatti, F. Pizzo, Synthesis 2003, 2331–
2334.
[11] N. Yassa, F. Sharififar, A. Shafiee, Pharm. Biol. 2005, 43, 33–
38.
[16] M. Esteves, C. Siquet, A. Gaspar, V. Rio, J. B. Sousa, S. Reis,
M. P. M. Marques, F. Borges, Arch. Pharm. Chem. Life Sci. 2008,
341, 164–173.
[12] A. Foroumadi, G. Dehghan, A. Samzadeh-Kermani, F.
Arabsorkhi, M. Sorkhi, A. Shafiee, M. Abodllahi, Asian J.
Chem. 2007, 19, 1391–1396.
[17] W. Brand-Williams, M. E. Cuvelier, C. Berset, LWT-Food Sci.
Technol. 1995, 28, 25–30.
[13] A. Foroumadi, A. Samzade-Kermani, S. Emami, G. Dehghan,
M. Sorkhi, F. Arabsorkhi, M. Hediari, M. Abdollahi, A.
Shafiee, Bioorg. Med. Chem. Lett. 2007, 17, 6764–6769.
[18] C. Joyce Pennycooke, S. Cox, C. Stushnoff, Environ. Exp. Bot.
2005, 53, 225–232.
ß 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
www.archpharm.com