Synthesis and biological evaluation of (E)-1-(substituted)-3-phenylprop-2-en-1-ones bearing rhodanines as potent anti-microbial agents

Article abstract of DOI:10.3109/14756366.2013.837899

Herein, we report the design, syntheses and in vitro anti-microbial activity of two series of rhodanines with chalcone moiety. Anti-microbial tests showed that some of the synthesized compounds exhibited good inhibition (MIC=1-8μg/mL) against multi-drug-r

Full text of DOI:10.3109/14756366.2013.837899

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ISSN: 1475-6366 (print), 1475-6374 (electronic)
J Enzyme Inhib Med Chem, 2014; 29(5): 647–653
2014 Informa UK Ltd. DOI: 10.3109/14756366.2013.837899
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ORIGINAL ARTICLE
Synthesis and biological evaluation of (E)-1-(substituted)-3-phenylprop-
2-en-1-ones bearing rhodanines as potent anti-microbial agents
Ming-Xia Song1, Xian-Qing Deng1, Ya-Ru Li2, Chang-Ji Zheng2, Lan Hong3, and Hu-Ri Piao2
2
1Department of Pharmacy, Jing Gangshan University College of Medicine, Ji’an, People’s Republic of China, Key Laboratory of Natural Resources
and Functional Molecules of the Changbai Mountain, Affiliated Ministry of Education, Yanbian University College of Pharmacy, Yanji, People’s
3
Republic of China, and Department of Physiology, Yanbian University College of Medicine, Yanji, People’s Republic of China
Abstract
Keywords
Herein, we report the design, syntheses and in vitro anti-microbial activity of two series of
rhodanines with chalcone moiety. Anti-microbial tests showed that some of the synthesized
compounds exhibited good inhibition (MIC ¼ 1–8 mg/mL) against multi-drug-resistant Gram-
positive organisms, including methicillin resistant and quinolone-resistant Staphylococcus
aureus, in which the compound 4g was found to be the most potent with minimum inhibitory
concentration (MIC) value of 1 mg/mL against two methicillin-resistant S. aureus.
Anti-bacterial, methicillin-resistant
Staphylococcus aureus, rhodanine
History
Received 17 June 2013
Revised 20 August 2013
Accepted 21 August 2013
Published online 8 October 2013
Introduction
inhibitory activity against the Gram-positive microorganisms
especially for the multi-drug-resistant clinical isolates such as
MRSA and quinolone-resistant S. aureus (QRSA). Especially in
the series of I, compound 6q (A moiety was a naphthalene
nucleus) showed the strongest activity with a minimum inhibitory
concentration (MIC) value of 2 mg/mL against the multi-drug-
resistant clinical isolates23. In the present work, as a part of our
ongoing research, a new series of rhodanine derivatives (4a–i)
were designed using 6q as the lead compound, in which the
modification of 6q was focused on reserving the naphthalene
nucleus, substituting the acetic acid group on the three-position of
the rhodanine with different amino acid side chains (including D-
or L-phenylalanine, D- or L-tyrosine, D- or L-valine, D- or L-
leucine and L-isoleucine) as shown in Figure 2. Moreover,
another series of rhodanine derivatives (5a–i) were also
been designed, in which the naphthalene nucleus was replaced
by a biphenyl group in order to discuss the structure–activity
relationship better. Both R- and S-configuration were involved
in the two series of compounds so as to investigate the
contribution of configuration on their anti-bacterial activity
(Figure 2). Thus, two series of rhodanine derivatives, including
17 new compounds were synthesized and screened for their anti-
bacterial activities.
The target compounds (4a–i and 5a–i) were synthesized
according to the route described in Scheme 1. The intermediates 1
and 2 were prepared by reacting acetophenones with terephtha-
laldehyde in the ethanol–NaOH/H2O condition24. Compounds
4a–i and 5a–i were obtained in good yields via a Knoevenagel
condensation of compounds 1 (or 2) and N-substituted rhodanines
using the reported procedure25. The newly synthesized com-
pounds were characterized by IR, 1H-NMR and mass spectra.
Taking compound 4b as an example, the mass spectroscopy of 4b
displayed an M þ H signal at m/z 550, which was corresponding
The severe infections caused by multi-drug-resistant microorgan-
isms such as methicillin-resistant Staphylococcus aureus (MRSA)
and glycopeptide-resistant Staphylococci (GRSA) have substan-
tially increased over the last few years. However, the number of
effective anti-bacterial agents has decreased gradually with the
appearance of new resistance mechanisms of microorganisms,
which is a major concern in medicine, especially for hospitals
now1,2
.
Rhodanine scaffold is a powerful device for medicinal chemist
and have a broad substrate scope for the synthesis of various
heterocyclic moieties with wide range of pharmacological
activities such as anti-bacterial3–5, anti-fungal6, anti-diabetic7,
anti-tubercular8,9, anti-HIV10,11 and anthelmintic agents12,13
.
Recently, this scaffold showed important therapeutic targets
against plasmodium fatty acid synthesis pathway (FAS-II) in the
parasites which is distinct from type I fatty acid synthesis pathway
(FAS-I) in human14. Hardej et al.15 reported some rhodanine
derivatives as anti-microbial agents against MRSA strains.
In our previous study, we designed and synthesized three series
of rhodanine derivatives bearing chalcone (Figure 1, I), 4-(2-oxo-
2-phenylethoxy)benzene (Figure 1, II) and 5-aryloxy pyrazole
moieties (Figure 1, III), respectively16–23. Their anti-bacterial
tests in vitro showed that these compounds all exhibited good
Address for correspondence: 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, People’s Republic of China. Tel: + 86-433-
2435003. Fax: + 86-433-2435026. E-mail: piaohuri@aliyun.com.cn;
piaohuri@yahoo.com.cn

648 M.-X. Song et al.
J Enzyme Inhib Med Chem, 2014; 29(5): 647–653
O
R
R
S
R
2
S
2
2
N
N
N
S
S
S
O
S
O
OR
1
N
N
R
R
1
B
1
O
II
O
O
I
III
Figure 1. The structures of compounds I, II and III.
R'
S
S
COOH
N
N
S
S
O
O
A
A
4a-i
5a-i
O
O
6q (the lead compound)
R'
S
N
S
O
A
O
Figure 2. Lead compound and structure-based design of the target compounds.
R'
S
N
R'
S
S
O
O
a
CHO
CHO
N
4a-i:
5a-i:
R=
CHO
3
S
H C
R
3
O
R
R
b
O
1 or 2
O
NH
COOH
NH
e:
R'= a:
g:
d:
b:
h:
c:
f:
COOH
COOH
COOH
COOH
COOH
COOH
i:
COOH
COOH
Scheme 1. Synthetic scheme for the synthesis of compounds 4a–i and 5a–i.
to its molecular weight of 549. In the 1H-NMR spectrum of were only in (Z)-configuration as thermodynamically favored
compound 4b, in addition to the aromatic protons of benzene ring structures.
protons (ꢀ ¼ 7.15–8.97 ppm), a sharp singlet due to vinylic
The physicochemical properties of them are presented in
hydrogen was observed at 8.08 ppm, and a broad singlet due to the ‘‘Chemistry’’ section. Their anti-bacterial activities were
methyne hydrogen linked to nitrogen-atoms was observed at all evaluated by a serial dilution method to obtain the MIC
5.90 ppm. Meanwhile, it has been reported that the (Z)- or with different strains, including multi-drug-resistant clinical
1
(E)-geometry was readily identified by H-NMR, as the vinylic isolates.
proton is more de-shielded in the (Z)-isomer than the (E)-isomer.
In (Z)-form, vinylic proton appeared at 7.21 ppm due to the Experimental protocols
magnetic anisotropy effects of the carbonyl group on the vinylic
proton, while in (E)-form the resonance should be around
Chemistry
6.50 ppm26,27. In our 1H-NMR spectra, only a set of signals Melting points were determined in open capillary tubes and were
around 8.08 ppm was appeared, which confirmed our products uncorrected. Reaction courses were monitored by TLC on silica

DOI: 10.3109/14756366.2013.837899
Rhodanines as antimicrobial agents 649
gel-precoated F254 Merck plates. Developed plates were (R)-2-((Z)-5-(4-((E)-3-(naphthalen-2-yl)-3-oxoprop-1-en-1-
examined with ultra violet lamps (254 nm). IR spectra were yl)benzylidene)-4-oxo-2-thioxothiazolidin-3-yl)-3-phenylpropa-
recorded (in KBr) on a FTIR1730. 1H NMR spectra were noic acid (4b)
measured on a Bruker AV-300 spectrometer using tetramethylsi-
20
Yellow solid; yield 15%; m.p. 204–206 ꢀC. ½ꢁꢁD : 211.9 (c ¼ 0.62,
lane as the internal standard. Mass spectra were measured on an
acetone). IR (KBr) cmꢂ1: 3424 (OH), 1709 (C¼O). 1H NMR
matrix assisted laser desorption ionization-time of flight/time of
(DMSO-d6, 300 MHz, ppm): ꢀ 3.52 (d, J ¼ 5.5 Hz, 2H, CH2), 5.90
flight (MALDI-TOF/TOF) mass spectrometer (Bruker Daltonik,
(br.s, 1H, NCH), 7.82 (d, 1H, J ¼ 15.7 Hz, CH¼CH), 8.08 (s, 1H,
Bremen, Germany). Specific optical rotation was measured on a
C¼CH), 8.25 (d, 1H, J ¼ 15.7 Hz, CH¼CH), 7.15–8.97 (m, 16H,
Digital automatic polariscope JASCO P-1020 (JASCO, Tokyo,
Ar–H), 13.44 (s, 1H, COOH). MS m/z 550 (M þ H). ESI-HRMS
Japan). The major chemicals were purchased from Sigma-Aldrich
calcd for C32H23NNaO4S2þ ([M þ Na]þ): 572.0961; found:
(St. Louis, MO) and Fluka Companies (Milwaukee, MI).
572.0960.
(S)-3-(1H-indol-3-yl)-2-((Z)-5-(4-((E)-3-(naphthalen-2-yl)-3-oxo-
prop-1-en-1-yl)benzylidene)-4-oxo-2-thioxothiazolidin-3-yl)pro-
panoic acid (4c)
The general synthesis route of compound 1 and 2
To a stirred solution of terephthalaldehyde (12 mmol) in 15 mL
ethanol and 15 mL 5% NaOH, the solution of 1-(naphthalen-2-
yl)ethanone or 1-([1,10-biphenyl]-4-yl)ethanone (10 mmol) in
15 mL ethanol was added drop-wise about 30 min in an ice-
bath. The mixture was stirred at room temperature for 8 h and the
pH value of the mixture was adjusted to 3 using dilute HCl. After
adding 25 mL water into the mixture, the resulting precipitate was
filtered, washed with water and recrystallized with ethanol to get
the yellow solid.
20
Yellow solid; yield 23%; m.p. 130–132 ꢀC. ½ꢁꢁD : ꢂ237.9
(c ¼ 0.30, acetone). IR (KBr) cmꢂ1: 3386 (OH), 2889 (NH),
1
1711(C¼O). H NMR (DMSO-d6, 300 MHz, ppm): ꢀ 3.57–3.75
(m, 2H, CH2), 5.90 (br.s, 1H, N–CH), 7.82 (d, 1H, J ¼ 15.5 Hz,
CH¼CH), 8.07 (s, 1H, C¼CH), 8.24 (d, 1H, J ¼ 15.5 Hz,
CH¼CH), 7.07–8.96 (m, 16H, Ar–H), 10.81 (s, 1H, NH),
13.21 (s, 1H, COOH). MS m/z 589 (M þ H). ESI-HRMS
calcd for C34H24N2NaO4S2þ ([M þ Na]þ): 611.1070; found:
611.1073.
Synthesis of rhodanine-3-acids (3)
To a solution of appropriate amino acid (30.3 mmol) and sodium
hydroxide (30.3 mmol) in 25 mL of water, carbon disulfide
(30.3 mmol) was added, and the resulting mixture was stirred
vigorously overnight. An aqueous solution of sodium chloroace-
tate (30.3 mmol) was added to the mixture and stirring was
continued at 23 ꢀC for 3 h. Then, the reaction mixture was
acidified with dilute HCl until pH 1.0 and refluxed overnight. The
reaction mixture was neutralized with saturated NaHCO3 solution.
The resultant solution was acidified again with dilute HCl. The
cyclized product was extracted with ethyl acetate, dried over
anhydrous sodium sulfate, evaporated under vacuum, and the
residue was purified by column chromatography (dichloro-
methane/methanol ¼ 95:05) to afford a brown liquid.
(R)-3-(1H-indol-3-yl)-2-((Z)-5-(4-((E)-3-(naphthalen-2-yl)-3-oxo-
prop-1-en-1-yl)benzylidene)-4-oxo-2-thioxothiazolidin-3-yl)pro-
panoic acid (4d)
20
Yellow solid; yield 17%; m.p. 120–122 ꢀC. ½ꢁꢁD : 227.6 (c ¼ 0.63,
CH3CO2C2H5). IR (KBr) cmꢂ1: 3406 (OH), 2909 (NH), 1713
1
(C¼O). H NMR (DMSO-d6, 300 MHz, ppm): ꢀ 3.55–3.78 (m,
2H, CH2), 5.89 (br.s, 1H, N–CH), 7.84 (d, 1H, J ¼ 15.6 Hz,
CH¼CH), 8.09 (s, 1H, C¼CH), 8.27 (d, 1H, J ¼ 15.6 Hz,
CH¼CH), 6.89–8.99 (m, 16H, Ar–H), 10.83 (s, 1H, NH), 13.37
(s, 1H, COOH). MS m/z 589 (M þ H). ESI-HRMS calcd for
C34H24N2NaO4S2þ ([M þ H]þ): 589.1250; found: 589.1244.
(S)-3-methyl-2-((Z)-5-(4-((E)-3-(naphthalen-2-yl)-3-oxoprop-1-
en-1-yl)benzylidene)-4-oxo-2-thioxothiazolidin-3-yl)butanoic
acid (4e)
General procedure for the preparation of
compounds 4a–i
20
Yellow solid; yield 28%; m.p. 206–208 ꢀC. ½ꢁꢁD : 45.2 (c ¼ 0.59,
To a suspension of 6 (2 mmol) in dry ethanol (10 mL),
(E)-4-(3-(naphthalen-2-yl)-3-oxoprop-1-en-1-yl)benzaldehyde
CHCl3). IR (KBr) cmꢂ1: 3406 (OH), 1713 (C¼O). 1H NMR
(DMSO-d6, 300 MHz, ppm): ꢀ 0.75 (d, 3H, J ¼ 6.8 Hz, CH–CH3),
1.20 (d, 3H, J ¼ 6.5 Hz, CH–CH3), 2.69–2.75 (m, 1H, CH(CH3)2),
5.19 (br.s, 1H, N–CH), 7.84 (d, 1H, J ¼ 15.6 Hz, CH¼CH), 7.92
(s, 1H, C¼CH), 8.26 (d, 1H, J ¼ 15.6 Hz, CH¼CH), 7.63–8.97
(m, 11H, Ar–H), 13.22 (s, 1H, COOH). MS m/z 502 (M þ H).
ESI-HRMS calcd for C28H23NNaO4S2þ ([M þ Na]þ):
524.0961; found: 524.0947.
2
(2.1 mmol), catalytic amounts of piperidine (0.1 mmol) and glacial
acetic acid (0.1 mmol) were added. The resulting mixture was
stirred and refluxed overnight. After cooling, the solvent was
evaporated in vacuo, dried, and purified by silica gel column
chromatography (dichloromethane/methanol ¼ 100:1). The yield,
melting point and spectral data of each compound are given below.
(S)-2-((Z)-5-(4-((E)-3-(naphthalen-2-yl)-3-oxoprop-1-en-1-
yl)benzylidene)-4-oxo-2-thioxothiazolidin-3-yl)-3-phenylpropa-
noic acid (4a)
(R)-3-methyl-2-((Z)-5-(4-((E)-3-(naphthalen-2-yl)-3-oxoprop-1-
en-1-yl)benzylidene)-4-oxo-2-thioxothiazolidin-3-yl)butanoic
acid (4f)
20
20
Yellow solid; yield 62%; m.p. 218–219 ꢀC. ½ꢁꢁD : ꢂ202.3 Yellow solid; yield 15%; m.p. 136–139 ꢀC. ½ꢁꢁD : 45.2 (c ¼ 0.59,
(c ¼ 0.54, acetone). IR (KBr) cmꢂ1: 3449 (OH), 1687 (C¼O). CHCl3). IR (KBr) cmꢂ1: 3437 (OH), 1714 (C¼O). 1H NMR
1H NMR (DMSO-d6, 300 MHz, ppm): ꢀ 3.55 (d, J ¼ 5.7 Hz, 2H, (DMSO-d6, 300 MHz, ppm): ꢀ 0.77 (d, 3H, J ¼ 6.5 Hz, CHCH3),
CH2), 5.76 (br.s, 1H, NCH), 7.82 (d, 1H, J ¼ 15.7 Hz, CH¼CH), 1.21 (d, 3H, J ¼ 6.1 Hz, CHCH3), 2.70–2.77 (m, 1H, CH(CH3)2),
8.09 (s, 1H, C¼CH), 8.23 (d, 1H, J ¼ 15.7 Hz, CH¼CH), 7.16– 5.18 (br.s, 1H, N–CH), 7.86 (d, 1H, J ¼ 15.7 Hz, CH¼CH), 7.94
8.29 (m, 16H, Ar–H), 13.44 (s, 1H, COOH). MS m/z 572 (s, 1H, C¼CH), 8.28 (d, 1H, J ¼ 15.7 Hz, CH¼CH), 7.35–8.99
([M þ Na]þ). Electrospray ionization high resolution mass (m, 11H, Ar–H), 13.18 (s, 1H, COOH). MS m/z 502 (M þ H).
spectrometry (ESI-HRMS) calcd for C32H23NNaO4S2þ ESI-HRMS calcd for C28H23NNaO4S2þ ([M þ Na]þ):
([M þ Na]þ): 572.0961; found: 572.0979.
524.0961; found: 524.0966.

650 M.-X. Song et al.
J Enzyme Inhib Med Chem, 2014; 29(5): 647–653
(S)-4-methyl-2-((Z)-5-(4-((E)-3-(naphthalen-2-yl)-3-oxoprop-1-
en-1-yl)benzylidene)-4-oxo-2-thioxothiazolidin-3-yl)pentanoic
acid (4g)
(R)-2-((Z)-5-(4-((E)-3-([1,10-biphenyl]-4-yl)-3-oxoprop-1-en-1-
yl)benzylidene)-4-oxo-2-thioxothiazolidin-3-yl)-3-phenylpropa-
noic acid (5b)
20
20
Yellow solid; yield 31%; m.p. 150–152 ꢀC. ½ꢁꢁD : ꢂ5.1 Yellow solid; yield 21%; m.p. 98–100 ꢀC. ½ꢁꢁD : 187.2 (c ¼ 0.63,
(c ¼ 0.58, CHCl3). IR (KBr) cmꢂ1: 3260 (OH), 1721 (C¼O). CH3CO2C2H5). IR (KBr) cmꢂ1: 3028 (OH), 1713 (C¼O). 1H
1H NMR (DMSO-d6, 300 MHz, ppm): ꢀ 0.87 (d, 3H, J ¼ 6.8 Hz, NMR (DMSO-d6, 300 MHz, ppm): ꢀ 3.51 (s, 2H, J ¼ 5.5 Hz,
CHCH3), 0.92 (d, 3H, J ¼ 6.3 Hz, CHCH3), 1.47–1.53 (m, 1H, CH2), 5.83 (br.s, 1H, NCH), 7.10–8.21 (m, 20H, Ar–H), 7.77 (s,
CH(CH3)2), 2.02–2.23 (m, 2H, CH2), 5.61 (br.s, 1H, NCH), 1H, C¼CH), 13.32 (s, 1H, COOH). MS m/z 576 (M þ H). ESI-
7.84 (d, 1H, J ¼ 15.7 Hz, CH¼CH), 8.13 (s, 1H, C¼CH), 8.27 HRMS calcd for C34H25NNaO4S2þ ([M þ Na]þ): 598.1117;
(d, 1H, J ¼ 15.7 Hz, CH¼CH), 7.63–8.98 (m 11H, Ar–H), found: 598.1122.
13.37 (s, 1H, COOH). MS m/z 516 (M þ H). ESI-HRMS
calcd for C29H25NNaO4S2þ ([M þ Na]þ): 538.1117; found: (S)-2-((Z)-5-(4-((E)-3-([1,10-biphenyl]-4-yl)-3-oxoprop-1-en-1-
538.1130.
yl)benzylidene)-4-oxo-2-thioxothiazolidin-3-yl)-3-(1H-indol-3-
yl)propanoic acid (5c)
(R)-4-methyl-2-((Z)-5-(4-((E)-3-(naphthalen-2-yl)-3-oxoprop-1-
en-1-yl)benzylidene)-4-oxo-2-thioxothiazolidin-3-yl)pentanoic
acid (4h)
20
Yellow solid; yield 35%; m.p. 108–112 ꢀC. ½ꢁꢁD : ꢂ269.1
(c ¼ 0.61, CH3CO2C2H5). IR (KBr) cmꢂ1: 3398 (OH), 2890
(NH), 1706 (C¼O). 1H NMR (DMSO-d6, 300 MHz, ppm): ꢀ
20
Yellow solid; yield 16%; m.p. 198–200 ꢀC. ½ꢁꢁD : 6.0 3.55–3.80 (m, 2H, CH2), 5.89 (br.s, 1H, NCH), 7.08–8.29 (m,
(c ¼ 0.63, CHCl3). IR (KBr) cmꢂ1: 3441 (OH), 1721 (C¼O). 20H, Ar–H), 7.78 (s, 1H, C¼CH), 10.83 (s, 1H, NH), 13.53 (s,
1H NMR (DMSO-d6, 300 MHz, ppm): ꢀ 0.86 (d, 3H, J ¼ 6.8 Hz, 1H, COOH). MS m/z 615 (M þ H). ESI-HRMS calcd for
CHCH3), 0.91 (d, 3H, J ¼ 6.3 Hz, CHCH3), 1.46–1.52 (m, 1H, C36H26N2NaO4S2þ ([M þ Na]þ): 637.1226; found: 637.1246.
CH(CH3)2), 2.01–2.20 (m, 2H, CH2), 5.59 (br.s, 1H,
NCH), 7.83 (d, 1H, J ¼ 15.6 Hz, CH¼CH), 8.12 (s, (R)-2-((Z)-5-(4-((E)-3-([1,10-biphenyl]-4-yl)-3-oxoprop-1-en-1-
1H, C¼CH), 8.25 (d, 1H, J ¼ 15.6 Hz, CH¼CH), 7.64–8.97 (m, yl)benzylidene)-4-oxo-2-thioxothiazolidin-3-yl)-3-(1H-indol-3-
11H, Ar–H), 13.37 (s, 1H, COOH). MS m/z 516 (M þ H). ESI- yl)propanoic acid (5d)
HRMS calcd for C29H25NNaO4S2þ ([M þ Na]þ): 538.1117;
20
Yellow solid; yield 19%; m.p. 126–128 ꢀC. ½ꢁꢁD : 271.1 (c ¼ 0.60,
found: 538.1118.
CHCl3). IR (KBr) cmꢂ1: 3410 (OH), 2897 (NH), 1709 (C¼O). 1H
NMR (DMSO-d6, 300 MHz, ppm): ꢀ 3.55–3.76 (m, 2H, CH2),
(2S)-3-methyl-2-((Z)-5-(4-((E)-3-(naphthalen-2-yl)-3-oxoprop-1-
5.90 (br.s, 1H, NCH), 7.49–8.07 (m, 20H, Ar–H), 7.77 (s, 1H,
en-1-yl)benzylidene)-4-oxo-2-thioxothiazolidin-3-yl)pentanoic
C¼CH), 10.83 (s, 1H, NH), 13.47 (s, 1H, COOH). MS m/z 615
acid (4i)
(M þ H).
ESI-HRMS
calcd
for
C36H26N2NaO4S2þ
20
Yellow solid; yield 36%; m.p. 160–162 ꢀC. ½ꢁꢁD : ꢂ45.8 ([M þ Na]þ): 637.1226; found: 637.1236.
(c ¼ 0.63, CHCl3). IR (KBr) cmꢂ1: 2967 (OH), 1717 (C¼O).
1H NMR (DMSO-d6, 300 MHz, ppm): ꢀ 0.79 (t, 3H, J ¼ 7.2 Hz, (S)-2-((Z)-5-(4-((E)-3-([1,10-biphenyl]-4-yl)-3-oxoprop-1-en-1-
CH2CH3), 0.92–0.97 (m, 2H, CH2CH3), 1.15 (d, 3H, J ¼ 6.4 Hz, yl)benzylidene)-4-oxo-2-thioxothiazolidin-3-yl)-3-methylbutanoic
CHCH3), 1.20–1.24 (m, 1H, CHCH3), 5.23 (br.s, 1H, NCH), acid (5e)
7.83 (d, 1H, J ¼ 15.8 Hz, CH¼CH), 8.15 (s, 1H, C¼CH), 8.26
20
Yellow solid; yield 26%; m.p. 120–123 ꢀC. ½ꢁꢁD : ꢂ31.4 (c ¼ 0.57,
(d, 1H, J ¼ 15.8 Hz, CH¼CH), 7.62–8.97 (m, 11H, Ar–H),
CHCl3). IR (KBr) cmꢂ1: 2942 (OH), 1719 (C¼O). 1H NMR
12.95 (s, 1H, COOH). MS m/z 538 (M þ Naþ). ESI-HRMS
(DMSO-d6, 300 MHz, ppm): ꢀ 0.75 (d, 3H, J ¼ 6.8 Hz, CHCH3),
calcd for C29H25NNaO4S2þ ([M þ H]þ): 516.1298; found:
1.19 (d, 3H, J ¼ 6.4 Hz, CHCH3), 2.70–2.76 (m, 1H, CH(CH3)2),
516.1298.
5.17 (br.s, 1H, NCH), 7.49–8.29 (m, 15H, Ar–H), 7.77 (s, 1H,
C¼CH), 13.14 (s, 1H, COOH). MS m/z 528 (M þ H). ESI-HRMS
General procedure for the preparation of compounds 5a–i
calcd for C30H25NNaO4S2þ ([M þ H]þ): 528.1298; found:
To a suspension of 6 (2 mmol) in dry ethanol (10 mL), 528.1310.
(E)-4-(3-([1,10-biphenyl]-4-yl)-3-oxoprop-1-en-1-yl)benzaldehyde
3 (2.1 mmol), catalytic amounts of piperidine (0.1 mmol) and (R)-2-((Z)-5-(4-((E)-3-([1,10-biphenyl]-4-yl)-3-oxoprop-1-en-1-
glacial acetic acid (0.1 mmol) were added. The resulting mixture yl)benzylidene)-4-oxo-2-thioxothiazolidin-3-yl)-3-methylbutanoic
was stirred and refluxed overnight. After cooling, the solvent was acid (5f)
evaporated in vacuo, dried and purified by silica gel column
20
Yellow solid; yield 20%; m.p. 89–92 ꢀC. ½ꢁꢁD : 28.5 (c ¼ 0.38,
chromatography (dichloromethane/methanol ¼ 100:1). The yield,
CHCl3). IR (KBr) cmꢂ1: 2959 (OH), 1721 (C¼O). 1H NMR
melting point and spectral data of each compound are given
(DMSO-d6, 300 MHz, ppm): ꢀ 0.90 (d, 3H, J ¼ 7.3 Hz, CHCH3),
below.
1.19 (d, 3H, J ¼ 6.5 Hz, CHCH3), 2.69–2.74 (m, 1H, CH(CH3)2),
5.17 (br.s, 1H, NCH), 7.43–8.29 (m, 15H, Ar–H), 7.77 (s, 1H,
C¼CH), 13.03 (s, 1H, COOH). MS m/z 528 (M þ H). ESI-HRMS
(S)-2-((Z)-5-(4-((E)-3-([1,10-biphenyl]-4-yl)-3-oxoprop-1-en-1-
yl)benzylidene)-4-oxo-2-thioxothiazolidin-3-yl)-3-phenylpropa-
noic acid (5a)
calcd for C30H25NNaO4S2þ ([M þ H]þ): 528.1298; found:
528.1294.
20
Yellow solid; yield 38%; m.p. 123–125 ꢀC. ½ꢁꢁD : ꢂ206.5
(c ¼ 0.61, CH3CO2C2H5). IR (KBr) cmꢂ1: 3005 (OH), 1711
(C¼O). 1H NMR (DMSO-d6, 300 MHz, ppm): ꢀ 3.54 (s, 2H,
J ¼ 5.6 Hz, CH2), 5.91 (br.s, 1H, NCH), 7.16–8.30 (m, 20H, Ar–
H), 7.78 (s, 1H, C¼CH), 13.53 (s, 1H, COOH). MS m/z 576
(M þ H). ESI-HRMS calcd for C34H25NNaO4S2þ ([M þ Na]þ):
598.1117; found: 598.1126.
(S)-2-((Z)-5-(4-((E)-3-([1,10-biphenyl]-4-yl)-3-oxoprop-1-en-1-
yl)benzylidene)-4-oxo-2-thioxothiazolidin-3-yl)-4-methylpenta-
noic acid (5g)
20
Yellow solid; yield 32%; m.p. 110–112 ꢀC. ½ꢁꢁD : ꢂ16.9 (c ¼ 0.59,
CH3CO2C2H5). IR (KBr) cmꢂ1: 3028 (OH), 1713 (C¼O). 1H
NMR (DMSO-d6, 300 MHz, ppm): ꢀ 0.87 (d, 3H, J ¼ 6.5 Hz,

DOI: 10.3109/14756366.2013.837899
Rhodanines as antimicrobial agents 651
CHCH3), 0.92 (d, 3H, J ¼ 6.5 Hz, CHCH3), 1.47–1.53 (m, 1H, approximately 1 ꢃ 104 cells were seeded into each well of a 96-
CH(CH3)2), 2.02–2.21 (m, 2H, CH2), 5.60 (br.s, 1H, N–CH), well plate and allowed to incubate overnight to allow cells to
7.42-8.29 (m, 15H, Ar–H), 7.78 (s, 1H, C¼CH), 13.37 (s, 1H, attach to the substrate. After 24 h, the medium was replaced with
COOH). MS m/z 542 (M þ H). ESI-HRMS calcd for DMEM supplemented with 10% fetal bovine serum (FBS)
C31H27NNaO4S2þ ([M þ Na]þ): 564.1274; found: 564.1291.
(R)-2-((Z)-5-(4-((E)-3-([1,10-biphenyl]-4-yl)-3-oxoprop-1-en-1-
yl)benzylidene)-4-oxo-2-thioxothiazolidin-3-yl)-4-methylpenta-
noic acid (5h)
containing various concentrations of test compounds and
incubated for 48 h. Then, 10 ml of methyl thiazolyl tetrazolium
(MTT) solution (5 mg/mL in PBS) was added to each well. After
incubation for 4 h, the medium was removed and the resulting
formazan crystals were dissolved with 100 ml 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.
20
Yellow solid; yield 13%; m.p. 96–99 ꢀC. ½ꢁꢁD : 19.3 (c ¼ 0.45,
CH3CO2C2H5). IR (KBr) cmꢂ1: 3045 (OH), 1712 (C¼O). 1H
NMR (DMSO-d6, 300 MHz, ppm): ꢀ 0.86 (d, 3H, J ¼ 6.5 Hz,
CHCH3), 0.89 (d, 3H, J ¼ 6.4 Hz, CHCH3), 1.46–1.52 (m, 1H,
CH(CH3)2), 2.01–2.23 (m, 2H, CH2), 5.60 (br.s, 1H, NCH), 7.45–
8.59 (m, 15H, Ar–H), 7.76 (s, 1H, C¼CH), 13.51(s, 1H, COOH).
Results and discussion
MS m/z 564
([M þ Na]þ). ESI-HRMS
calcd for Anti-bacterial activity
C30H25NNaO4S2þ ([M þ Na]þ): 564.1274; found: 564.1279.
The anti-microbial assay was carried out using the following
bacterial strains S. aureus RN 4220, S. aureus KCTC 209, S.
aureus KCTC 503, MRSA CCARM 3167 and 3506, QRSA
CCARM 3505 and 3519 and E. coli 1356. The in vitro anti-
bacterial activity was evaluated using a 96-well microtiter plate
(2S)-2-((Z)-5-(4-((E)-3-([1,10-biphenyl]-4-yl)-3-oxoprop-1-en-1-
yl)benzylidene)-4-oxo-2-thioxothiazolidin-3-yl)-3-methylpenta-
noic acid (5i)
20
Yellow solid; yield 25%; m.p. 108–111 ꢀC. ½ꢁꢁD : ꢂ27.2 (c ¼ 0.61, and a serial dilution method to obtain the MIC. Oxacillin,
CH3CO2C2H5). IR (KBr) cmꢂ1: 3426 (OH), 1717 (C¼O). 1H norfloxacin, gatifloxacin and moxifloxacin were used as positive
NMR (DMSO-d6, 300 MHz, ppm): ꢀ 0.80 (t, 3H, J ¼ 7.0 Hz, controls.
CH2CH3), 0.94–0.97 (m, 2H, CH2CH3), 1.16 (d, 3H, J ¼ 6.3 Hz,
Table 1 summarized the results obtained for the MICs of the
CHCH3), 1.27–1.33 (m, 1H, CHCH3), 5.24 (br.s, 1H, NCH), 18 target compounds (4a–i and 5a–i) against three Gram-positive
7.42–8.30 (m, 15H, Ar–H), 7.78 (s, 1H, C¼CH), 13.29 (s, 1H, strains (S. aureus RN 4220, S. aureus KCTC 209 and S. aureus
COOH). MS m/z 542 (M þ H). ESI-HRMS calcd for KCTC 503) and one Gram-negative strain (E. coli 1356). For
C30H25NNaO4S2þ ([M þ Na]þ): 564.1274; found: 564.1284.
S. aureus RN 4220, it could be found that most of the compounds
(except 4b, 4c, 4d, 4i, 5c and 5d) showed good inhibition
with MIC values in the range of 1–16 mg/mL. In particular,
compounds 4g and 4h (MIC ¼ 1 mg/mL) had a twofold more
potent activity than the positive control norfloxacin (MIC ¼ 2 mg/
mL), and comparable activity to the positive control oxacillin
(MIC ¼ 1 mg/mL). For S. aureus KCTC 209, most of the
Pharmacology
Evaluation of anti-bacterial activity in vitro
The micro-organisms used in the present study were S. aureus (S.
aureus RN 4220, S. aureus KCTC 209 and S. aureus KCTC 503)
and Escherichia coli (E. coli 1356). The strains of multi-drug-
resistant clinical isolates were MRSA (MRSA CCARM 3167 and
MRSA CCARM 3506) and QRSA (QRSA CCARM 3505 and QRSA
CCARM 3519). Clinical isolates were collected from various
patients hospitalized in several clinics.
Table 1. Inhibitory activity (MIC*, mg/mL) of compounds 4a–i and 5a–i
against bacteria.
Gram-positive strains
Gram-negative strains
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 and norfloxacin were used. Test compounds were
prepared in DMSO, the final concentration of which did not
exceed 0.05%. A twofold serial dilution technique28 was used to
obtain final concentrations. 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.
Staphylococcus aureus
Escherichia coli
Compound
4220
209
503
1356
4a
4b
4c
4d
4e
4f
4g
4h
4i
5a
5b
5c
5d
5e
8
464
464
464
4
16
464
464
464
8
32
4
8
464
464
464
464
8
464
464
464
464
464
464
464
464
464
464
464
464
464
464
464
464
464
464
16
4
1
1
8
2
2
464
16
8
464
464
8
16
8
16
8
464
8
464
8
8
8
464
464
8
32
8
64
8
2
464
464
8
16
2
64
8
2
Evaluation of cytotoxicity in vitro
5f
5g
5h
5i
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 (DMEM) 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
phosphate buffered saline (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 three passages and
Norfloxacin
Oxacillin
Gatifloxacin
Moxifloxacin
2
1
0.25
0.25
1
2
2
1
4
2
464
16
464
*The anti-bacterial testing was carried out three times, and the MICs are
average of them.

652 M.-X. Song et al.
J Enzyme Inhib Med Chem, 2014; 29(5): 647–653
compounds (except 4b, 4c, 4d, 4i, 5c and 5d) displayed poor all of the selected bacteria at 64 mg/mL. While the compounds
inhibition with MICs ranging from 4 to 32 mg/mL, while these bearing leucine-derived rodanines (4g, 4h, 5g and 5h) exhibited a
compounds showed better inhibitory activity against S. aureus strong inhibitory activity with MICs of 1–4 mg/mL against the
KCTC 503, whose MICs were mostly in the range of 2–8 mg/mL. four drug-resistance bacterial strains. The cytotoxicity of the
Unfortunately, all of the test compounds did not exhibit any selected compound (4g) was also evaluated. The IC50 value (HeLa
inhibition against the Gram-negative strain E. coli 1356 cells) of 4g is 2.41, which is comparable with its MIC value.
(MICs464 mg/mL).
The compounds 4a–i and 5a–i were also evaluated for their
inhibitory activity against MRSA and QRSA. As shown in
Table 2, compounds 4a–i and 5a–i (except 4b, 4c, 4d, 4i, 5c and
5d) presented the high levels of activity with MIC values of
2–8 mg/mL. For MRSA 3167 and 3506, most compounds presented
comparable or much more potent activities than norfloxacin.
Among them, compound 4g, with MIC value of 1 mg/mL, showed
eightfold more potent than norfloxacin (MIC ¼ 8 mg/mL) and 64-
fold more potent than oxacillin (MIC464 mg/mL), twofold more
potent than gatifloxacin (MIC ¼ 2 mg/mL), and comparable with
moxifloxacin (MIC ¼ 1 mg/mL). For QRSA 3505 and 3519,
compounds 4e, 4g, 4h and 5h also presented higher levels of
activity (MIC ¼ 2 mg/mL), which were slightly less active than
oxacillin (MIC ¼ 1 mg/mL) but much more potent than norfloxacin
(MIC464 mg/mL), gatifloxacin (MIC ¼ 8 or 4 mg/mL) and moxi-
floxacin (MIC ¼ 4 mg/mL).
Conclusion
Based on our previous work, we synthesized two new series of
rhodanine derivatives (4a–i and 5a–i) and evaluated their anti-
bacterial activities. Most of the compounds showed good anti-
bacterial activities against Gram-positive bacteria as well as
multi-drug-resistant strains of clinical isolates. Among them,
compound 4g, with MIC value of 1 mg/mL, showed eightfold
more potent than norfloxacin (MIC ¼ 8 and 4 mg/mL) and 64-fold
more potent than oxacillin (MIC464 mg/mL). Efforts to deter-
mine the reason for its anti-bacterial activity are ongoing and will
be reported in due course.
Declaration of interest
This work was supported by the National Science Foundation of China
(grant number 81260468). The authors report no conflicts of interest. The
authors alone are responsible for the content and writing of this article.
Most of synthesized compounds (4a–i and 5a–i) exhibit
strong anti-bacterial activity, although the mechanism of action is
not yet clearly understood. However, in this study, some
preliminary remarks on the structure–activity relationship can
be drawn from the results of bioactivities. (1) There is no
remarkable difference of anti-bacterial activity between 4a–i and
5a–i. (2) The activity of S-configuration compound is a little
better than the R-configuration compound. For example, the
activity of compound 4g (S-configuration) is higher than that of
compound 4h (R-configuration) generally. (3) The MIC values of
the compounds derived from different amino acids differed
greatly. For example, the compounds bearing tryptophan-derived
rhodanines (4c, 4d, 5c and 5d) did not show any inhibition against
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*The anti-bacterial testing was carried out three times, and the MICs are
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a