Synthesis and biological evaluation of rhodanine derivatives bearing a quinoline moiety as potent antimicrobial agents

Article abstract of DOI:10.1016/j.bmcl.2013.05.082

Three series of rhodanine derivatives bearing a quinoline moiety (6a-h, 7a-g, and 8a-e) have been synthesized, characterized, and evaluated as antibacterial agents. The majority of these compounds showed potent antibacterial activities against several different strains of Gram-positive bacteria, including multidrug-resistant clinical isolates. Of the compounds tested, 6g and 8c were identified as the most effective with minimum inhibitory concentration (MIC) values of 1 μg/mL against multidrug-resistant Gram-positive organisms, including methicillin-resistant and quinolone-resistant Staphylococcus aureus (MRSA and QRSA, respectively). None of the compounds exhibited any activity against the Gram-negative bacteria Escherichia coli 1356 at 64 μg/mL. The cytotoxic activity assay showed that compounds 6g, 7g and 8e exhibited in vitro antibacterial activity at non-cytotoxic concentrations. Thus, these studies suggest that rhodanine derivatives bearing a quinoline moiety are interesting scaffolds for the development of novel Gram-positive antibacterial agents.

Full text of DOI:10.1016/j.bmcl.2013.05.082

Bioorganic & Medicinal Chemistry Letters 23 (2013) 4358–4361
Contents lists available at SciVerse ScienceDirect
Bioorganic & Medicinal Chemistry Letters
journal homepage: www.elsevier.com/locate/bmcl
Synthesis and biological evaluation of rhodanine derivatives bearing
a quinoline moiety as potent antimicrobial agents
⇑
Meng Guo, Chang-Ji Zheng, Ming-Xia Song, Yan Wu, Liang-Peng Sun, Yin-Jing Li, Yi Liu, Hu-Ri Piao
Key Laboratory of Natural Resources and Functional Molecules of the Changbai Mountain, Affiliated Ministry of Education, Yanbian University College of Pharmacy, Yanji 133002,
PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Three series of rhodanine derivatives bearing a quinoline moiety (6a–h, 7a–g, and 8a–e) have been syn-
thesized, characterized, and evaluated as antibacterial agents. The majority of these compounds showed
potent antibacterial activities against several different strains of Gram-positive bacteria, including mul-
tidrug-resistant clinical isolates. Of the compounds tested, 6g and 8c were identified as the most effective
Received 5 March 2013
Revised 8 May 2013
Accepted 24 May 2013
Available online 4 June 2013
with minimum inhibitory concentration (MIC) values of 1 lg/mL against multidrug-resistant Gram-posi-
tive organisms, including methicillin-resistant and quinolone-resistant Staphylococcus aureus (MRSA and
QRSA, respectively). None of the compounds exhibited any activity against the Gram-negative bacteria
Escherichia coli 1356 at 64 lg/mL. The cytotoxic activity assay showed that compounds 6g, 7g and 8e
exhibited in vitro antibacterial activity at non-cytotoxic concentrations. Thus, these studies suggest that
rhodanine derivatives bearing a quinoline moiety are interesting scaffolds for the development of novel
Gram-positive antibacterial agents.
Keywords:
Rhodanine
Quinoline
Antibacterial activity
Methicillin-resistant Staphylococcus aureus
Quinolone-resistant Staphylococcus aureus
Ó 2013 Elsevier Ltd. All rights reserved.
The treatment of bacterial infections capable of causing serious
and widespread diseases still remains a significant and worldwide
problem because of problems associated with the emergence of
new infectious diseases and the increased number of pathogenic
microorganisms that are developing resistance to the existing
drugs.1–5 So development of novel antimicrobical drugs with dif-
ferent mechanisms of action to the currently available antibacterial
drugs is still in demand.6,7
A significant number of compounds bearing a quinoline moiety
have been reported in the literature with a variety of different
pharmacological activities, including antimicrobial,8–10 antituber-
culosis,11 anticancer,12 anti-HIV,13 antimalarial,14 and anti-inflam-
matory activities.15 Furthermore, rhodanine-based molecules have
been reported to be associated with antibiotic activity.16 In our
previous work, we found that several rhodanine derivatives bear-
ing chalcone,17 1,3-diarylpyrazole,6 (2-oxo-2-phenylethoxy)ben-
zylidene,18 and (benzyloxy)benzylidene19 moieties showed
moderate to strong activities against several Gram-positive bacte-
rial strains, including multidrug-resistant clinical isolates.
Although, to date, there have been no reports in the literature pro-
viding a clear understanding of the mechanisms of action associ-
ated with these derivatives, the focus of the present work was to
introduce a (quinolin-3-yl)methylidene moiety at the 5-positon
of the rhodanine ring instead of the benzylidene moiety, with the
aim of modulating the hydrophobicity of the resulting molecule
and affecting its enzyme binding affinity. Moreover, the effects of
different substituents at the 3-position of the rhodanine ring were
simultaneously investigated using aromatic or aliphatic groups to
observe their effects on the inhibitory activities of the compounds.
Thus, as a part of our ongoing studies towards the development of
novel antibacterial agents, herein we report the design, synthesis,
and antimicrobial evaluation of three novel series of rhodanine
derivatives containing a quinoline moiety as efficient antimicrobial
agents.
Nineteen new rhodanine derivatives were synthesized
according to the synthetic route depicted in Scheme 1. William-
son condensation reactions between 6-hydroxy-3,4-dihydroquin-
olin-2(1H)-one (2) and
chloromethylbenzene compounds (1a–1h) afforded the corre-
sponding 6-(substitutedbenzyloxy)-3,4-dihydroquinolin-2(1H)-
a variety of different substituted
ones (3a–3h), which were subsequently reacted under
Vilsmeier–Haack conditions to give the corresponding 6-(substi-
tutedbenzyloxy)-2-chloroquinoline-3-carbaldehydes (4a–4h). The
rhodanine intermediates (5a–c) were prepared according to a
method previously described in the literature.17 The target com-
pounds 6a–h, 7a–g, and 8a–e were synthesized by the Knoeve-
nagel condensation reactions of 4 with compounds 5a–5c in
ethanol in the presence of glacial acid and piperidine. The
structures of the synthesized compounds were confirmed
by Fourier transform infra-red (FTIR), 1H and 13C NMR, and mass
spectroscopy.20
⇑
Corresponding author. Tel.: +86 433 2435003; fax: +86 433 2435004.
E-mail address: piaohuri@yahoo.com.cn (H.-R. Piao).
0960-894X/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.bmcl.2013.05.082

M. Guo et al. / Bioorg. Med. Chem. Lett. 23 (2013) 4358–4361
4359
R
HO
O
POCl3 / DMF
DMF/K2CO3
+
R
Cl
N
H
O
N
H
O
3
1
2
S
R'
S
O
N
R'
COOH
COOH
O
O
CHO
Cl
O
N
S
5a-c
R
S
R
N
Cl
EtOH/Piperidine
acetic acid
N
4
6a-h, 7a-g, 8a-e
R= 6a: 2-Cl, b: 3-Cl, c: 4-Cl, d: 2-F, e: 3-F, f: 4-F, g: 4-CH3., h: 2-Br. R'= 5a: -CH2C6H5
7a: 2-Cl, b: 3-Cl, c: 4-Cl, d: 2-F, e: 3-F, f: 4-F, g: 4-CH3.
8a: 2-Cl, b: 3-Cl, c: 4-Cl, d: 4-F, e: 4-CH3.
R'= 5b: -CH(CH3)CH2CH3
R'= 5c: -CH2CH(CH3)2
Scheme 1. Synthetic route for the construction of target compounds 6a–6h, 7a–7g, and 8a–8e.
The in vitro antimicrobial activities of the newly synthesized
compounds were evaluated against eight different bacterial
strains, including multidrug-resistant clinical isolates, using a 96-
well microtiter plate format and a broth microdilution method to
obtain their minimum inhibitory concentration (MIC) values. Nor-
floxacin, oxacillin, gatifloxacin, and moxifloxacin were used as po-
sitive controls against the different bacterial strains.
The newly synthesized compounds were also screened for their
antibacterial activity against several different Gram-positive
organisms, including Staphylococcus aureus RN4220, S. aureus KCTC
503, and S. aureus KCTC 209, as well as the Gram-negative organ-
ism, Escherichia coli 1356. As shown in Table 1,21 the majority of
the compounds showed potent in vitro antibacterial activities
against the three different types of Gram-positive bacteria, with
MIC values in the range of 1-64
7f, 8a, 8c, and 8e were particularly active against the Gram-posi-
tive strain S. aureus RN4220 with an MIC value of 1 g/mL in both
cases, slightly less active than gatifloxacin and moxifloxacin
(MIC = 1 g/mL). Most of the test compounds presented moderate
inhibitory activities against S. aureus KCTC 503 (MIC = 4–8 g/mL).
lg/mL. Furthermore, compounds
l
l
l
Among the desired compounds, 6c, 6e, 6f, 7b–d, 7f, 7g, and 8c
showed potent inhibitory activities against S. aureus KCTC 503
(MIC = 4
(MIC = 4
l
g/mL), which was comparable to gatifloxacin
lg/mL) but less active than moxifloxacin (MIC = 2 lg/
Table 1
Inhibitory activities (MIC,
l
g/mL) of compounds 6a–6h, 7a–7g, and 8a–8e against bacteria
O
R'
O
N
R
S
COOH
N
Cl
S
Compound
R
R0
Gram-positive strains
S. aureus
Gram-negative strains
E. coli
4220
209
503
1356
6a
6b
6c
6d
6e
6f
6g
6h
7a
7b
7c
7d
7e
7f
7g
8a
2-Cl
3-Cl
4-Cl
2-F
3-F
4-F
4-CH3
2-Br
2-Cl
3-Cl
4-Cl
2-F
–CH2C6H5
–CH2C6H5
–CH2C6H5
–CH2C6H5
–CH2C6H5
–CH2C6H5
–CH2C6H5
–CH2C6H5
–CH(CH3)CH2CH3
–CH(CH3)CH2CH3
–CH(CH3)CH2CH3
–CH(CH3)CH2CH3
–CH(CH3)CH2CH3
–CH(CH3)CH2CH3
–CH(CH3)CH2CH3
–CH2CH(CH3)2
–CH2CH(CH3)2
–CH2CH(CH3)2
–CH2CH(CH3)2
–CH2CH(CH3)2
4
2
4
4
4
4
2
4
4
2
2
2
2
1
2
1
2
1
2
1
2
1
0.25
0.25
64
64
8
64
64
32
8
8
8
4
8
4
4
8
8
8
4
4
4
8
4
4
8
64
4
8
8
2
1
4
2
>64
>64
>64
>64
>64
>64
>64
>64
>64
>64
>64
>64
>64
>64
>64
>64
>64
>64
>64
>64
16
8
64
>64
8
32
32
2
4
4
4
4
8
8
2
1
2
2
3-F
4-F
4-CH3
2-Cl
3-Cl
4-Cl
4-F
8b
8c
8d
8e
4-CH3
Norfloxacin
Oxacillin
Gatifloxacin
Moxifloxacin
>64
16
>64
S. aureus RN 4220, Staphylococcus aureus RN 4220; S. aureus 503, Staphylococcus aureus 503; S. aureus 209, Staphylococcus aureus 209; E. coli 1356, Escherichia coli CCARM 1356.

4360
M. Guo et al. / Bioorg. Med. Chem. Lett. 23 (2013) 4358–4361
mL), norfloxacin (MIC = 2
respectively. With the exception of compound 7b (MIC >64
mL), most of the compounds tested exhibited good levels of inhib-
itory activity against S. aureus KCTC 209 with MIC values in the
l
g/mL), and oxacillin (MIC = 1
l
g/mL),
Table 1. 20 A comparison of the carboxylic acid derivatives at the N-
position of the rhodanine ring indicated that the different carbox-
ylic acids contributed to the antimicrobical activity in the order of
CH2CH(CH3)2 > CH(CH3)CH2CH3 > CH2C6H5. Furthermore, the posi-
tion of the substituent on the phenyl ring influenced the anti-bac-
terial activity with an activity order of 4-F >3-F, 2-F for
fluoro-substituted compounds, and 4-Cl >3-Cl, 2-Cl for chloro-
substituted compounds.
lg/
range of 2–64
(MIC = 1–2 g/mL). Unfortunately, none of the compounds showed
any inhibitory activity against the Gram-negative strain E. coli
1356 at 64 g/mL.
lg/mL, but less active than the control drugs
l
l
As shown in Table 2,21 all of the newly synthesized compounds
were also tested for their inhibitory activities against the clinical
isolates of several different multidrug-resistant Gram-positive bac-
terial strains, including methicillin-resistant Staphylococcus aureus
(MRSA CCARM 3167 and MRSA CCARM 3506) and quinolone-resis-
tant Staphylococcus aureus (QRSA CCARM 3505 and QRSA CCARM
3519). With the exception of compound 6f, compounds 6a–h,
7a–g, and 8a–e showed excellent levels of inhibitory activity
against the different multidrug-resistant Gram-positive bacterial
The cytotoxic properties of compounds 6g, 7g and 8e were also
investigated using MTT colorimetric assay to determine if their ob-
served antibacterial activity was caused by selective toxicity to-
wards the bacterial cells and the results are shown in Table 3.22
Compounds 6g, 7g, and 8e did not affect cell viability on the Hu-
man cervical (HeLa) cells at their MICs but showed cytotoxicity
at much higher concentrations. The inconsistency of compounds
6g, 7g and 8e between their antibacterial activity and cytotoxicity
suggests that there may be an antibacterial mechanism different
from cytotoxicity.
In conclusion, based on our previous work, we synthesized
three novel series of rhodanine derivatives containing a quinoline
moiety and evaluated their antibacterial activities against Gram-
positive and Gram-negative bacteria. The results indicated that
the majority of the target compounds exhibited potent antibacte-
rial activity against Gram-positive bacteria, particularly against
the multidrug-resistant strains of the clinical isolates. Compounds
6g and 8c exhibited a four to eightfold increase in potency relative
to the standard drug norfloxacin against all of the selected multi-
drug-resistant clinical isolates and also a four to eightfold increase
relative to gatifloxacin and moxifloxacin against quinolone-resis-
tant Staphylococcus aureus. These results suggested that the rhoda-
strains with MIC values in the range of 1–2
compounds 6g and 8c exhibited the strongest levels of inhibitory
activity (MIC = 1 g/mL). This value represented a four to eightfold
increase in potency relative to the standard drug norfloxacin
(MIC = 4–8 g/mL) and a four to eightfold increase relative to gati-
floxacin and moxifloxacin against quinolone-resistant Staphylococ-
cus aureus (MIC = 4–8 g/mL). These results suggested that the
lg/mL. Furthermore,
l
l
l
quinoline moiety was an important structural unit and critical to
the levels of antibacterial activity observed in the rhodanine-based
derivatives investigated during the course of the present study. In
addition, some clear structure–activity relationship patterns were
found between the antibacterial activity and the physicochemical
properties of the N-substituted groups on the rhodanine ring from
Table 2
Inhibitory activities (MIC,
lg/mL) of compounds 6a–6h, 7a–7g, and 8a–8e against clinical isolates of multidrug-resistant Gram-positive strains
O
R'
O
N
R
S
COOH
N
Cl
S
Compound
R
R0
Multidrug-resistant Gram-positive strains
MRSA
QRSA
3167
3506
3167
3506
6a
6b
6c
6d
6e
6f
6g
6h
7a
7b
7c
7d
7e
7f
7g
8a
2-Cl
3-Cl
4-Cl
2-F
3-F
4-F
4-CH3
2-Br
2-Cl
3-Cl
4-Cl
2-F
–CH2C6H5
–CH2C6H5
–CH2C6H5
–CH2C6H5
–CH2C6H5
–CH2C6H5
–CH2C6H5
–CH2C6H5
–CH(CH3)CH2CH3
–CH(CH3)CH2CH3
–CH(CH3)CH2CH3
–CH(CH3)CH2CH3
–CH(CH3)CH2CH3
–CH(CH3)CH2CH3
–CH(CH3)CH2CH3
–CH2CH(CH3)2
–CH2CH(CH3)2
–CH2CH(CH3)2
–CH2CH(CH3)2
–CH2CH(CH3)2
2
2
2
1
2
2
1
2
2
2
1
2
2
2
2
1
1
1
2
1
8
>64
2
1
2
2
2
2
1
2
1
2
1
1
1
1
1
1
2
1
2
1
2
2
4
>64
1
1
2
2
2
2
1
2
1
2
2
1
2
2
2
2
2
1
2
1
2
1
>64
1
8
4
2
2
2
2
2
4
1
2
2
2
2
2
2
1
2
2
2
1
1
1
>64
1
4
4
3-F
4-F
4-CH3
2-Cl
3-Cl
4-Cl
4-F
8b
8c
8d
8e
4-CH3
Norfloxacin
Oxacillin
Gatifloxacin
Moxifloxacin
MRSA 3167, methicillin-resistant S. aureus CCARM 3167; MRSA 3506, methicillin-resistant S. aureus CCARM 3506; QRSA 3505, quinolone-resistant S. aureus CCARM 3505;
QRSA 3519, quinolone-resistant S. aureus CCARM 3519.

M. Guo et al. / Bioorg. Med. Chem. Lett. 23 (2013) 4358–4361
4361
Table 3
15. Leatham, P. A.; Bird, H. A.; Wright, V.; Seymour, D.; Gordon, A. Eur. J. Rheumatol.
Inflamm. 1983, 6, 209.
Cytotoxic activity of compounds 6g, 7g and 8e against HeLa cell
16. Hardej, D.; Ashby, C. R., Jr.; Khadtare, N. S.; Kulkatni, S. S.; Singh, S.; Talele, T. T.
Eur. J. Med. Chem. 2010, 45, 5827.
Compound
IC50 (lg/mL)
17. Chen, Z. H.; Zheng, C. J.; Sun, L. P.; Piao, H. R. Eur. J. Med. Chem. 2010, 45, 5739.
18. Song, M. X.; Zheng, C. J.; Deng, X. Q.; Wang, Q.; Hou, S. P.; Liu, T. T.; Xing, X. L.;
Piao, H. R. Eur. J. Med. Chem. 2012, 54, 403.
19. Jin, X.; Zheng, C. J.; Song, M. X.; Wu, Y.; Sun, L. P.; Li, Y. J.; Yu, L. J.; Piao, H. R. Eur.
J. Med. Chem. 2012, 56, 203.
6g
7g
8e
13.29
9.09
8.84
20. Preparation of 8e: A mixture of 4e (2 mmol) and 5c (3 mmol), 10 drops glacial
acetic acid and 10 drops piperidine in ethanol (5 mL) was refluxed for 4 h. After
cooling, the solvent was evaporated in vacuo, followed by the purification of
the resulting residue by silica gel column chromatography (dichloromethane/
nine derivatives bearing a quinoline moiety, which play a critical
role in increasing the antibacterial properties of the compounds,
represented promising lead compounds for the development of no-
vel antibacterial agents. Compounds 6g, 7g and 8e did not exhib-
ited any significant influence on cell viability in the HeLa cells at
their MICs. Further investigations of these compounds are cur-
rently underway in our laboratories, including the study of their
possible mechanism of action.
methanol, 100:1) to get a yellow solid. Yield 35%; mp 96–97 °C. IR (KBr) cmꢀ1
:
3436 (OH), 1736 (C@O). 1H NMR (DMSO-d6, 300 MHz, ppm): d 0.92 (q, 6H,
J = 7 Hz, CH3), 1.55 (m, 1H, CH), 2.05–2.22 (m, 2H, CH2), 2.32 (s, 3H, PhCH3),
5.22 (s, 2H, OCH2), 5.62 (s, 1H, NCH), 7.95 (s, 1H, Ph = CH), 7.22–8.51 (m, 8H,
Ar–H), 13.40 (s, 1H, COOH). MS m/z 541 (M+1). 13C NMR (DMSO-d6, 75 MHz,
ppm): d 193.20, 169.22, 166.34, 157.41, 147.00, 142.61, 137.48, 137.40, 133.20,
129.03, 128.86, 128.06, 127.84, 127.17, 125.65, 125.35, 125.30, 108.07, 69.81,
56.06, 36.44, 24.81, 22.82, 21.92, 20.78.
21. Anti-bacterial activity assay: The micro-organisms used in the present study
were S. aureus (S. aureus RN 4220, S. aureus KCTC 209, S. aureus KCTC 503), and
Escherichia coli (E. coli 1356). The strains of multidrug-resistant clinical isolates
were methicillin-resistant Staphylococcus aureus (MRSA CCARM 3167 and MRSA
CCARM 3506) and quinolone-resistant Staphylococcus aureus (QRSA CCARM
3505 and QRSA CCARM 3519). Clinical isolates were collected from various
patients hospitalized in several clinics. Test bacteria were grown to mid-log
phase in Mueller–Hinton broth (MHB) and diluted 1000-fold in the same
medium. The bacteria of 105 CFU/mL were inoculated into MHB and dispensed
at 0.2 mL/well in a 96-well microtiter plate. As positive controls, oxacillin,
norfloxacin, gatifloxacin, and moxifloxacin were used. Test compounds were
prepared in DMSO, the final concentration of which did not exceed 0.05%. A
twofold serial dilution technique was used to obtain final concentrations of
Acknowledgment
This work was supported by the National Science Foundation of
China (Grant nos. 20962021 and 81260468).
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64–0.5 lg/mL. The MIC was defined as the concentration of a test compound
that completely inhibited bacteria growth during 24 h incubation at 37 °C.
Bacteria growth was determined by measuring the absorption at 650 nm using
a
microtiter enzyme-linked immunosorbent assay (ELISA) reader. All
experiments were carried out three times.
22. Cytotoxicity activity assay: Human cervical (Hela) cell monolayers were used as
an in vitro model of cervicovaginal epithelium for testing the cytotoxicity of
the new compounds. Hela cells were grown in Dulbecco modified Eagle
medium supplemented with fetal bovine serum (10%), and antibiotics
(penicillin–streptomycin mixture [100 U/mL]). Cells at 80–90% confluence
were split by trypsin (0.25% in PBS; pH 7.4), and the medium was changed at
24 h intervals. The cells were cultured at 37 °C in a 5% CO2 incubator. The cells
were grown to 3 passages and approximately 1 ꢁ 104 cells were seeded into
each well of a 96-well plate and allowed to incubate overnight to allow cells to
attach to the substrate. After 24 h, the medium was replaced with DMEM
supplemented with 10% FBS containing various concentrations of test
compounds and incubated for 48 h. Then 10
PBS) was added to each well. After incubation for 4 h, the medium was
removed and the resulting formazan crystals were dissolved with 100
lL of MTT solution (5 mg/ml in
lL
DMSO. After shaking 10 min, the optical density was measured at 570 nm
using a microtiter ELISA reader. The assay was conducted four times. The IC50
values were defined as the concentrations inhibiting 50% of cell growth.
13. Strekowski, L.; Mokrosz, J. L.; Honkan, V. A.; Czarny, A.; Cegla, M. T.; Patterson,
S. E.; Wydra, R. L.; Schinazi, R. F. J. Med. Chem. 1991, 34, 1739.
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