Synthesis and characterization of (Z)-5-arylmethylidenerhodanines with photosynthesis-inhibiting properties

Article abstract of DOI:10.3390/molecules16065207

A series of rhodanine derivatives was prepared. The synthetic approach, analytical and spectroscopic data of all synthesized compounds are presented. Lipophilicity of all the discussed rhodanine derivatives was analyzed using the RP-HPLC method. The compo

Full text of DOI:10.3390/molecules16065207

Molecules 2011, 16, 5207-5227; doi:10.3390/molecules16065207
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molecules
ISSN 1420-3049
www.mdpi.com/journal/molecules
Article
Synthesis and Characterization of (Z)-5-Arylmethylidene-
rhodanines with Photosynthesis-Inhibiting Properties †
Veronika Opletalova 1,*, Jan Dolezel 1, Katarina Kralova 2, Matus Pesko 3, Jiri Kunes 4 and
Josef Jampilek 5,*
1
Department of Pharmaceutical Chemistry and Drug Control, Faculty of Pharmacy in Hradec
Kralove, Charles University in Prague, Heyrovskeho 1203, 500 05 Hradec Kralove, Czech Republic
2
Institute of Chemistry, Faculty of Natural Sciences, Comenius University, Mlynska dolina Ch-2,
842 15 Bratislava, Slovakia
3
Department of Ecosozology and Physiotactics, Faculty of Natural Sciences, Comenius University,
Mlynska dolina Ch-2, 84215 Bratislava, Slovakia
4
Department of Inorganic and Organic Chemistry, Faculty of Pharmacy in Hradec Kralove, Charles
University in Prague, Heyrovskeho 1203, 500 05 Hradec Kralove, Czech Republic
5
Department of Chemical Drugs, Faculty of Pharmacy, University of Veterinary and Pharmaceutical
Sciences, Palackeho 1/3, 612 42 Brno, Czech Republic
†
Preliminary results related to the topic of this article were presented at The Eleventh Electronic
Conference on Synthetic Organic Chemistry (ECSOC-11, http://www.usc.es/congresos/
ecsoc/11/hall_aGOS/a012/index.htm), November 1-30, 2007.
* Authors to whom correspondence should be addressed; E-Mails: veronika.opletalova@faf.cuni.cz
(V.O.); josef.jampilek@gmail.com (J.J.); Tel.: +420495067372 (V.O.); Tel.: +420776722508 (J.J.).
Received: 16 May 2011; in revised form: 15 June 2011 / Accepted: 21 June 2011 /
Published: 22 June 2011
Abstract: A series of rhodanine derivatives was prepared. The synthetic approach,
analytical and spectroscopic data of all synthesized compounds are presented. Lipophilicity
of all the discussed rhodanine derivatives was analyzed using the RP-HPLC method. The
compounds were tested for their ability to inhibit photosynthetic electron transport (PET)
in spinach (Spinacia oleracea L.) chloroplasts and reduce chlorophyll content in freshwater
alga Chlorella vulgaris. Structure-activity relationships between the chemical structure,
physical properties and biological activities of the evaluated compounds are discussed. For
majority of the tested compounds the lipophilicity of the compound and not electronic
properties of the R1 substituent were decisive for PET-inhibiting activity. The most

Molecules 2011, 16
potent PET inhibitor was (5Z)-5-(4-bromobenzylidene)-2-thioxo-1,3-thiazolidin-4-one
5208
(IC50 = 3.0 μmol/L) and the highest antialgal activity was exhibited by (5Z)-5-(4-
chlorobenzylidene)-2-thioxo-1,3-thiazolidin-4-one (IC50 = 1.3 μmol/L).
Keywords: rhodanine derivatives; synthesis; lipophilicity; photosynthesis inhibition;
spinach chloroplasts; Chlorella vulgaris; structure-activity relationships
1. Introduction
Pesticides are used to control pests. About 700 pesticides, including insecticides, herbicides and
fungicides, act on perhaps some 95 biochemical targets in insects, weeds and destructive fungi. They
must be effective without human or crop injury and safe relative to humans and the environment.
Herbicides act mostly in plant-specific pathways, e.g., by blocking photosynthesis. Green plant
pigments absorb light and with the coupled system of chloroplasts convert light energy to the chemical
energy of adenosine triphosphate. Herbicides disrupting some of the processes unique to plants are of
low toxicity to mammals, which lack analogous targets. Photosystem (PS) II was an early target for
herbicides and is still highly important as the mode of action for about 50 commercial compounds.
More than one target is involved since resistance to one PS II inhibitor does not confer cross-resistance
to all others. The targets are denoted as the triazine, urea, and nitrile sites [1].
PS II electron transport inhibitors bind to the D1 protein of the PS II reaction centre, thus blocking
electron transfer to plastoquinone. The inhibition of PS II electron transport prevents the conversion of
absorbed light energy into electrochemical energy and results in production of triplet chlorophyll and
singlet oxygen, which induces the peroxidation of membrane lipids [2]. The interaction of herbicides
with the photosynthetic apparatus and a model for orientation of the herbicides within the
three-dimensional structure of their target, the D1 protein of PS II, were reported by Draber et al. [3].
Many QSAR studies of PS II inhibitors with diverse chemical structures have emphasized the
hydrophobic nature of the binding domain, with lipophilicity being the dominant determinant of Hill
inhibition activity [4].
Rhodanine represents an important scaffold in drug discovery [5], and the influence of its
derivatives on plant physiology has been well documented, too [6-16]. 5-Arylalkylidenerhodanines [7]
and 3-arylrhodanines [9] were patented as potential herbicides, and 5-(5-barbiturilidene)rhodanine
inhibited growth of algae in water at relatively low concentrations [8]. Herbicidal activity of
complexes of transition metals with rhodanine [12] was reported as well. Rhodanine derivatives also
inhibit diaminopimelate aminotransferase, an enzyme catalyzing L-lysine synthesis in plants and bacteria
but not in mammals that acquire this essential amino acid in their diet. Specific inhibitors of this
enzyme could thus potentially serve as herbicides and antibiotics that are non-toxic to mammals [16].
The inhibition of photosynthetic electron transport by rhodanine (IC50 ≈ 1 mmol/L) was observed
by Muro et al. [14]. In another study, rhodanine and rhodanine-N-acetic acid strongly inhibited
growth of Daucus carota L. var. sativa DC even at low concentration 0.3 mmol/L. Both compounds
also inhibited germination of seeds of Daucus carota L. var. sativa DC and Sesamum indicum
(at 1 mmol/L) [13]. Chlorophyll synthesis in the cotyledons of Brassica rapa L. was strongly inhibited

Molecules 2011, 16
5209
with rhodanine at the concentration of 0.3 mmol/L [11]. Similar results were later observed with
N–aminorhodanine [13,15]. It was found that the free amino group at N–3 was essential for the greater
inhibitory activity of rhodanine derivatives. It was also confirmed that the plant-growth inhibition by
these derivatives was related to the chlorophyll content in treated plants [13].
Many low molecular weight drugs cross biological membranes through passive transport, which
strongly depends on their lipophilicity. This property has a major effect on absorption, distribution,
metabolism, excretion and toxicity (ADME/Tox) properties as well as biological activity. Lipophilicity
has been studied and applied as an important drug property for decades [17]. This paper is a follow-up
work to previous papers [18-23] aimed at studying relationships between the structure and lipophilicity
of various compounds and their biological effects.
2. Results and Discussion
2.1. Chemistry
The synthesis of compounds is indicated in Scheme 1, and the compounds are listed in Table 1.
Either commercially available rhodanines or prepared 3-(2-hydroxyethyl)rhodanine [24] and pyrazine-
2–carbaldehyde [25] were used as starting materials. Most of the compounds were reported previously
[26–34]. (5Z)-3-(2-Hydroxyethyl)-5-(2-nitrobenzylidene)-2-thioxo-1,3-thiazolidin-4-one (11a), (5Z)-
3-(2-hydroxyethyl)-5-(3-nitrobenzylidene)-2-thioxo-1,3-thiazolidin-4-one (11b) and (5Z)-5-(pyrazin-
2-ylmethylidene)-2-thioxo-1,3-thiazolidin-4-one (15) are novel compounds.
Scheme 1. Synthesis of target rhodanine derivatives 1–16.
O
O
O
a
H
N R2
R1
N R2
R1
X
X
+
S
S
S
S
1-16
Reagents and conditions a) NH4Cl/NH4OH, EtOH
Arylmethylidenerhodanines can form two isomers. According to references [35-39], syntheses of
these compounds results in the Z–izomer. Configuration on the exocyclic double bond can be
1
determined on the basis of NMR spectra where H-NMR signals of the methine-group hydrogens for
Z–isomers are more downfield compared to those of the E–isomers The experimental signals of
methine-group hydrogens in the rhodanine derivatives studied in the present paper were compared
with the values reported previously and the values predicted in silico (Table 1). It can be concluded
that all arylmethylidenerhodanines reported in the present paper were obtained as single (Z)–isomers.
In most cases experimental values are between the values predicted with CS ChemOffice 7.0 and those
predicted with CS ChemOffice 10.0.
2.2. Lipophilicity
Hydrophobicities (log P/Clog P) of compounds 1–16 were calculated using two commercially
available programs (ChemDraw Ultra 10.0 and ACD/LogP), and also measured by means of RP-HPLC

Molecules 2011, 16
5210
determination of capacity factors k with a subsequent calculation of log k. The procedure was
performed under isocratic conditions with methanol as an organic modifier in the mobile phase using
an end-capped non-polar C18 stationary RP column. The ChemDraw program did not resolve various
lipophilicity values of individual positional isomers, that is, the same log P/Clog P data were
calculated for isomers a–c, respectively for positional isomers 13 and 14. Due to high functionalization
of these small molecules, the program ACD did not resolve various lipophilicity values of individual
positional isomers 7–11 as well as 12 and 13. Therefore it can be assumed that the determined log k
data specify lipophilicity within the individual series of compounds. The results are summarized in
Table 1.
Table 1. Comparison of 1H-NMR signals of methine-group for Z-isomers and comparison
of calculated lipophilicities (log P, Clog P) with determined log k values of compounds.
O
2
1
N R
X
R
S
S
Exp. values
reported
previously
7.63c,7.65d,
7.62g
Predicted
values
Exp.
log P b
Clog P b
Comp.
X
R1
R2
log k
log P j
values
2.04
1.802
1.65
2.94
± 0.76
2.21
1
2a
2b
2c
3
C
C
C
C
C
C
C
C
C
C
C
H
H
H
H
H
H
H
H
H
H
H
H
7.42a,7.80b
7.69a, 8.07b
7.42a,7.80b
7.42a,7.80b
7.69a, 8.07b
7.69a, 8.07b
7.42a, 7.80b
7.42a, 7.80b
7.42a, 7.80b
7.42a, 7.80b
7.98a, 8.36b
7.63
7.84
7.53
7.55
7.73
7.78
7.60
7.59
7.56
7.49
7.86
0.5122
0.4664
0.2744
0.2641
0.2776
0.5867
0.5713
0.5425
0.3553
0.6466
0.2254
2-OH
7.83c, 7.86e
1.135
1.65
± 0.76
2.86
3-OH
7.54e
1.135
1.65
± 0.77
2.96
4-OH
7.56e, 7.56g
7.79f
1.135
1.26
± 0.77
2.23
2,4-OH
2-OCH3
3-OCH3
4-OCH3
0.468
1.91
± 0.78
2.95
4a
4b
4c
5
7.79e
1.721
1.91
± 0.77
2.92
7.59e
1.721
1.91
± 0.77
2.89
7.59c,7.52d,
7.45e,
1.721
1.52
± 0.77
2.72
3-OCH3-
4-OH
7.94g
7.47c
7.82c
0.984
2.32
± 0.78
3.05
4-
6
N(CH3)2
1.967
2.49
± 0.77
2.67
7a
2-NO2
1.545
± 0.77

Molecules 2011, 16
5211
Table 1. Cont.
2.49
1.545
2.49
2.67
7b
7c
C
C
3-NO2
4-NO2
2-F
H
H
H
H
H
H
H
H
H
H
H
7.53a, 7.91b
7.70
7.70
7.59
7.63
7.64
7.74
7.68
7.62
7.70
7.61
7.60
7.88
7.94
7.88
7.67
7.66
7.55
7.73
7.47
7.79h
7.73h
0.2399
0.2436
0.8108
0.8204
0.7909
0.9019
1.0270
0.5936
0.9368
1.0820
0.6940
0.4751
0.6077
0.6349
0.4864
0.4539
0.1878
0.2359
0.4656
± 0.77
2.67
7.56a, 7.94b
7.69a, 8.07b
7.42a,7.80b
7.42a, 7.80b
7.69a, 8.07b
7.42a, 7.80b
1.545
2.19
± 0.77
2.99
8a
C
7.59d, 7.48e
7.83e
1.945
2.19
± 0.81
2.99
8b
C
3-F
1.945
2.19
± 0.81
2.99
8c
C
4-F
7.65d
1.945
2.59
± 0.81
3.54
9a
C
2-Cl
3-Cl
4-Cl
2-Br
3-Br
4-Br
2-NO2
3-NO2
4-NO2
H
NR
2.515
2.59
± 0.77
3.54
9b
C
NR
2.515
2.59
± 0.77
3.54
7.61c,7.55d,
7.61i
9c
C
7.42a 7.80b
,
2.515
2.86
± 0.77
3.91
10a
10b
10c
11a
11b
11c
12
C
7.69a, 8.07b
7.42a, 7.80b
7.42a, 7.80b
NR
NR
2.665
2.86
± 0.81
3.71
C
2.665
2.86
± 0.81
3.71
C
7.61d
NR
2.665
2.04
± 0.81
1.81
C
C2H4OH 7.98a, 8.36b
C2H4OH 7.53a, 7.91b
C2H4OH 7.56a, 7.94b
0.993
2.04
± 0.80
1.81
C
NR
0.993
2.04
± 0.80
1.81
C
NR
0.993
1.12
± 0.80
1.45
2-N
3-N
4-N
H
H
H
H
H
7.63a, 7.63b
7.42a, 7,68b
7.40a, 7.78b
7.42a, 7.42b
7.42a, 7.37b
7.65d
7.60d
7.58d
NR
0.305
0.70
± 0.76
1.70
13
H
0.305
0.70
± 0.77
1.45
14
H
0.305
-0.22
-0.652
0.65
± 0.76
0.69
15
2,4-N
H
± 0.77
2.10
O
16
H
8.09g
0.978
± 0.77
a CS ChemOffice 7.0, b CS ChemOffice 10.0 (CambridgeSoft, Cambridge, MA, U.S.A.); c ref. [16];
d
e
f
g
h
i
j
ref. [26]; ref. [27]; ref. [28]; ref. [35]; ref. [38]; ref. [39]; NR = not reported; ACD/LogP
1.0 (Advanced Chemistry Development, Toronto, Canada).
Compounds 14 (pyridin-4-ylmethylidene) and 7a (2-nitrobenzylidene) showed the lowest
lipophilicity, while compound 10b (3-bromobenzylidene) exhibited the highest. Generally, it can be
concluded that ring-substituted 5-benzylidene derivatives are more lipophilic than their

Molecules 2011, 16
5212
5-heteroarylmethylidene congeners. Unsubstituted (5Z)-5-benzylidene-2-thioxo-1,3-thiazolidin-4-one
(1) is situated approx. in the middle of the lipophilicity range of the compound series.
When the lipophilicity of 5-heteroarylmethylidene-2-thioxo-1,3-thiazolidin-4-ones 12–16 and
compound 1 was compared, the lowest lipophilicity was surprisingly shown by compound 14 (pyridin-
4-ylmethylidene) contrary to expected compound 15 (pyrazin-2-ylmethylidene), as it was predicted by
all calculated log P/Clog P data. Lipophilicity within 5-heteroarylmethylidene series increased in the
following order: pyridin-4-yl (14) < pyrazin-2-yl (15) < pyridin-3-yl (13) < furan-2-yl (16) < pyridin-
2-yl (12); it is lower than the lipophilicity of unsubstituted benzylidene (1).
In 5-benzylidene series 1–11 nitro (7a–c) and hydroxyl (2a–c) substituted compounds possessed
lower lipophilicity than unsubstituted compound 1. Compound 2a (2-OH) showed higher experimental
lipophilicity value in comparison with the results calculated by the software and in comparison with
other two isomers 2b (3-OH) and 2c (4-OH). Surprisingly, disubstituted derivatives 11a–c showed
dramatically higher lipophilicity than monosubstituted nitro derivatives 7a–c. Lipophilicity in both
cases increased in the order: 2-NO2 (a) < 3-NO2 (b) < 4-NO2 (c). Methoxy (4a–c) and halogeno (8–10)
derivatives were more lipophilic than unsubstituted parent compound 1. Experimental lipophilicity
values of methoxy derivatives 4a–c were higher than it was expected on the basis of log P values
calculated in silico and similarly as those of hydroxyl derivatives 2a–c increased in the following
order: 4-OCH3/4-OH (c) < 3-OCH3/3-OH (b) < 2-OCH3/2-OH (a). Generally, fluoro derivatives 8a–c
showed lower lipophilicity than chloro derivatives 9a–c, and both showed lower lipophilicity than
bromo derivatives 10a–c. Depending on the substituent position, lipophilicity within substituted
5-benzylidene series increased in the following order: 4- (c) < 2- (a) < 3- (b).
In general, experimentally-determined log k values correlated relatively poorly with the calculated
log P/Clog P. These facts are possibly caused by limitations of the software used and intramolecular
interactions between heteroatoms and substituents. Based on the facts discussed above, it can be stated
that the lipophilicity of the discussed compounds is significantly influenced by intramolecular
interactions.
2.3. Study of PET Inhibition in Spinach Chloroplasts
The inhibitory activity (IC50 values) of rhodanine derivatives related to inhibition of photosynthetic
electron transport (PET) in spinach (Spinacia oleracea L.) chloroplasts is summarized in Table 2. IC50
values of compounds 2c, 3, 4a, 5, 6, 11c, 12 and 14 could not be determined due to low solubility of
the compounds in the chloroplast suspension or due to very weak activity of the compounds;
compound 11b interacted with the artificial electron acceptor 2,6-dichlorophenol-indophenol.
Replacement of H by C2H4OH group in R2 substituent (compounds 7a and 11a, respectively)
causing the increase of log k from 0.2254 to 0.4751 led to a slight activity increase. Similarly, more
lipophilic compound 2a with R1: 2-OH (log k = 0.4664) exhibited higher inhibitory activity than 4-OH
substituted compound 2c (log k = 0.2641), see Tables 1 and 2.
The dependence of inhibitory activity on compound lipophilicity expressed by log k for compounds
with X=C (benzylidene derivatives) is shown in Figure 1. It is evident that particularly high inhibitory
activity was exhibited by halogen substituted compounds 10c (4-Br) and 9c (4-Cl), and also with 7b
(3-NO2). High PET-inhibiting potency was also observed for 10b (3-Br) and 9b (3-Cl), and for 7c

Molecules 2011, 16
5213
(4-NO2). Hence, electron-withdrawing substituents with higher values of Hammett's constants
(σ constants for 4-Br: 0.232, 3-Br: 0.390, 4-Cl: 0.227, 4-NO2: 1.238, 3-NO2: 0.710 [40]) seem to
contribute to the PET-inhibitory activity which is in a good agreement with the results obtained in the
experiment with Chlorella vulgaris, see below. The superior activities of 9c and 10c in comparison
with those of other compounds with similar lipophilicity could be connected with the favourable steric
and electronic properties of 4-halogeno substituted compounds with respect to the site of inhibitory
action in PS II of spinach chloroplasts.
Table 2. The inhibitory activity of the selected rhodanine derivatives related to the
inhibition of photosynthetic electron transport (PET inhibition) in spinach chloroplasts
(Spinacia oleracea L.) as well as their activity related to the reduction of chlorophyll
content in Chlorella vulgaris (expressed as IC50 values or as reduction of chlorophyll
content [%] caused by application of 100 μmol/L of the studied compound) in comparison
with standard 3-(3,4-dichlorophenyl)-1,1-dimethylurea DCMU.
Spinach chloroplasts
(PET)
Chlorella vulgaris
Comp.
IC50 [μmol/L]
reduction of Chl. cont. [%]
IC50 [μmol/L]
374.7
1
2a
13.7
59.4
–
88.2
59.0
9.6
29.8
48.5
–
368.6
2b
2c
444.0
a
–
a
3
108.2
a
4b
4c
220.6
a
173.8
–
a
5
–
19.4
12.6
–
a
6
–
a
7a
427.6
16.9
20.1
99.5
23.8
63.5
53.3
17.0
6.0
7b
7c
4.4
85.7
87.1
–
21.9
a
8a
a
a
a
a
8b
8c
–
–
9a
–
9b
9c
–
1.3
a
84.8
–
10a
10b
10c
11a
13
18.1
5.2
a
a
a
a
a
–
3.0
–
127.4
310.7
216.5
1.9
–
–
15
1.8
–
DCMU
7.3
a interaction with DCPIP or precipitation during the experiment.

Molecules 2011, 16
5214
Halogen as well as nitro substituents contributed to enhanced PET-inhibiting activity of
2,6-disubstituted 4-amidopyridines and 4-thioamidopyridines [41], pyrazine-2-carboxanilides [42,43],
derivatives of 3-nitro-2,4,6-trihydroxybenzamide [44], substituted benzanilides and thiobenzanilides
[45-47], or antialgal/PET-inhibiting activity of quinoline derivatives [47-52] and substituted
salicylanilides [53]. Nonetheless, it is evident from the results of statistical analysis that the inhibitory
activity of 12 compounds with X=C for which IC50 in mol/L could be determined depended
predominantly on compound lipophilicity expressed as log k, see Figure 1.
log (1/IC50) = 2.411 (± 0.211) + 2.356 (± 0.276) log k
r = 0.938, s = 0.229, F = 73.0, n = 12
The IC50 value of the unsubstituted rhodanine related to PET inhibition in spinach chloroplasts was
previously determined by Muro et al. [14] using spinach chloroplasts. However, the IC50 value of
approximately 1 mmol/L which was obtained for rhodanine during the normal 1 min assay decreased
to 0.1 mmol/L when the assay was done after illumination for 3 min, indicating possible chemical
modification of the compound.
Figure 1. Dependence of log (1/IC50 [mol/L]) related to PET inhibition in spinach
chloroplasts on the compound lipophilicity expressed by log k.
6.0
10c
5.5
5.0
4.5
4.0
3.5
3.0
10b
9b
9c
10a
9a
7b
7c
8b
8c
11a
8a
4c
1
4b
2a
7a
2b
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
1.1
log k
2.4. Reduction of Chlorophyll Content in Chlorella vulgaris
The inhibitory activities (IC50 values) of rhodanine derivatives related to the reduction of
chlorophyll content in Chlorella vulgaris algae are summarized in Table 2. Ten compounds were
tested for their inhibitory potency to reduce chlorophyll content in C. vulgaris suspension. Only for six
of them (1, 2a, 3, 7b, 7c, 9c) the IC50 value, i.e., concentration causing 50% reduction of chlorophyll
concentration, could be determined, see Table 2. Therefore, the extent of chlorophyll content
reduction in C. vulgaris suspension treated with equimolar concentration (100 μmol/L) of the studied
compounds (1-3, 5, 6, 7b, 7c, 9c) was compared as well, see Table 2.

Molecules 2011, 16
5215
Based on dependencies of IC50 on the lipophilicity expressed by log k, the compounds could be
divided into 2 groups. For compounds with lower values of log k, ranging from 0.2399 (7b) to 0.2776
(3), the inhibitory activity decreased linearly with increasing log k value, whereas for compounds with
log k raging from 0.4664 (2a) to 0.5936 (9c) the reverse relationship was observed.
Similar results were obtained also for the dependence of the reduction of chlorophyll content in
C. vulgaris suspension treated with equimolar concentration (100 μmol/L) of the compounds on the
log k values of the compounds. However, for the most lipophilic compound in the set (6, log k: 0.6466)
strong decrease in potency was observed (Table 2). Thus, it could be assumed that in the investigated
set of compounds higher values of Hammett's σ constants of the R1 (4-Cl: 0.227, 4-NO2: 1.238, 3-NO2:
0.710 [40]) substituent contributed significantly to the increase of biological activity. Muro et al. [14]
found that rhodanine applied at 1 mmol/L concentration completely inhibited growth of immature cells
of Marchantia polymorpha within 90 hours and caused decrease of chlorophyll content. Algicidal
properties of 5-(5-barbiturilidene)rhodanine against the algae species Scenedesmus, Plectonema,
Anabena, Ankistrodesmus, Oscillatoria, Coccochloris, Chlamydomonas, Lyngbya, Synura and
Chlorella were reported by Kerst et al. [8].
3. Experimental
3.1. General
Commercially available rhodanine and aldehydes were used as starting materials. Methods reported
previously were employed for preparation of 3-(2-hydroxyethyl)rhodanine [24] and pyrazine-
2–carbaldehyde [25]. For analysis, the samples of compounds were dried for 24 hours in a dessicator at
1.33 kPa. The melting points were determined on a Boëtius apparatus HMK 73/4615 (VEB Analytik,
Dresden, Germany) and are uncorrected. Elemental analyses were performed with an EA 1110 CHNS
Analyzer (Carlo Erba). UV spectra (λ, nm) were determined on a Waters Photodiode Array Detector
2996 (Waters Corp., Milford, MA, USA) in ca. 6 × 10−4 M methanolic solution and log ε (the
logarithm of molar absorption coefficient ε) was calculated for the absolute maximum λmax of
individual target compounds. Infrared spectra were recorded using KBr pellets on the FT-IR
spectrometer Nicolet 6700 (Nicolet–Thermo Scientific, USA). Wavenumbers are given in cm−1. All
1H-NMR and 13C-NMR spectra were recorded with a Varian Mercury-VxBB 300 spectrometer
(299.95 MHz for 1H and 75.43 MHz for 13C; Varian Corp., Palo Alto, CA, USA). Chemical shifts were
recorded as δ values in ppm and were indirectly referenced to tetramethylsilane (TMS) via the solvent
signal (2.49 for 1H, 39.7 for 13C in DMSO-d6).
3.2. Synthesis
3.2.1. General procedure for synthesis of arylmethylidenerhodanines 1–16
An equimolar amounts of an aldehyde and rhodanine or 3-(2-hydroxyethyl)rhodanine (0.015 mol)
were heated under a reflux condenser with ethanol (15 mL) and concentrated ammonia solution
(1.1 mL) until all solid components dissolved. The solution of ammonium chloride (1.00 g) in 2 mL of
hot (80 °C) distilled water was then added, and the reaction mixture was refluxed for 2 hours. After

Molecules 2011, 16
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cooling, the separated solid was filtered through a sintered glass, washed with distilled water (50 mL)
and then with 50% ethanol (50 mL). The product was crystallized from anhydrous ethanol.
(5Z)-5-Benzylidene-2-thioxo-1,3-thiazolidin-4-one (1). Yellow crystalline compound; Yield 65%; Mp
206–207 °C (205 [26], 203–205 °C [27]); Anal. Calcd. for C10H7NOS2 (221.30): C 54.27%, H 3.19%,
N 6.33%, S 28.98%; found: C 54.03%, H 3.13%, N 6.35%, S 26.42%; UV (nm), λmax/log ε:
1
376.0/3.31; IR (KBr, cm−1): 3154 (NH), 1700 (C=O); H-NMR (DMSO-d6), δ: 7.63 (1H, s, CH),
13
7.61–7.45 (5H, m, H2, H3, H4, H5, H6); C-NMR (DMSO-d6), δ: 195.9, 169.6, 133.2, 131.9, 131.0,
130.7, 129.7, 125.7.
(5Z)-5-(2-Hydroxybenzylidene)-2-thioxo-1,3-thiazolidin-4-one (2a). Yellow crystalline compound;
Yield 57%; Mp 219–226 °C (224–225 °C [27]); Anal. Calcd. for C10H7NO2S2 (237.30): C 50.61%,
H 2.97%, N 5.90%, S 27.03%; found: C 50.81%, H 2.94%, N 5.83%, S 26.93%; UV (nm), λmax/log ε:
1
394.1/3.38; IR (KBr, cm−1): 3153 (NH), 1700 (C=O); H-NMR (DMSO-d6), δ: 13.73 (1H, bs, NH),
10.66 (1H, bs, OH), 7.84 (1H, s, CH), 7.37–7.26 (2H, m, H4 and H6), 6.99–6.90 (2H, m, H3 and H5);
13C-NMR (DMSO-d6), δ: 196.2, 169.8, 157.8, 133.0, 129.5, 127.5, 124.0, 120.2, 120.1, 116.4.
(5Z)-5-(3-Hydroxybenzylidene)-2-thioxo-1,3-thiazolidin-4-one (2b). Yellow crystalline compound;
Yield 64%; Mp 244–251 °C (237–239 °C [27]); Anal. Calcd. for C10H7NO2S2 (237.30): C 50.61%,
H 2.97%, N 5.90%, S 27.03%; found: C 50.45%, H 2.80%, N 5.93%, S 25.39%; UV (nm), λmax/log ε:
1
365.0/3.38; IR (KBr, cm−1): 3343 (OH), 3169 (NH), 1699 (C=O); H-NMR (DMSO-d6), δ: 9.86 (1H,
bs, OH), 7.53 (1H, s, CH), 7.32 (1H, t, J = 8.0, H5), 7.06–7.01 (1H, m, H6), 6.96 (1H, t, J = 1.9, H2),
6.91–6.86 (1H, m, H4); 13C-NMR (DMSO-d6), δ: 196.0, 169.6, 158.2, 134.3, 132.1, 130.8, 125.5,
122.1, 118.3, 116.4.
(5Z)-5-(4-Hydroxybenzylidene)-2-thioxo-1,3-thiazolidin-4-one (2c). Orange crystalline compound;
Yield 78%; Mp 294–295 °C (279–280 °C [27]). Anal. Calcd. for C10H7NO2S2 (237.30): C 50.61%,
H 2.97%, N 5.90%, S 27.03%; found: C 50.42%, H 2.91%, N 5.82%, S 25.94%; UV (nm), λmax/log ε:
392.9/3.35; IR (KBr, cm−1): 3393 (OH), 3145 (NH), 1688 (C=O); 1H-NMR (DMSO-d6), δ: 10.42 (1H,
bs, OH), 7.55 (1H, s, CH), 7.50–7.41 (2H, m, AA´, BB´, H2 and H6), 6.95–6.87 (2H, m, AA´, BB´, H3
and H5); 13C-NMR (DMSO-d6), δ: 195.7, 169.7, 130.6, 133.3, 132.7, 124.2, 121.1, 116.8.
(5Z)-5-(2,4-Hydroxybenzylidene)-2-thioxo-1,3-thiazolidin-4-one (3). Orange crystalline compound;
Yield 81%; Mp 272–276 °C (>300 [28]). Anal. Calcd. for C10H7NO3S2 (253.30): C 47.42%, H 2.79%,
N 5.53%, S 25.32%; found: C 46.27%, H 3.40%, N 6.45%, S 24.03%; UV (nm), λmax/log ε:
1
407.3/3.41; IR (KBr, cm−1): 3197, 3139 (NH), 1683 (C=O); H-NMR (DMSO-d6), δ: 10.48 (2H, bs,
OH), 7.73 (1H, s, CH), 7.13 (1H, d, J = 9.1 Hz, H6´), 6.44–6.34 (2H, m, H3´, H5´); 13C-NMR
(DMSO-d6), δ: 197.2, 172.4, 162.1, 159.8, 131.1, 126.7, 120.9, 112.4, 108.9, 102.7.
(5Z)-5-(2-Methoxybenzylidene)-2-thioxo-1,3-thiazolidin-4-one (4a). Yellow crystalline compound;
Yield 95%; Mp 257–258 °C (205–206 [28]). Anal. Calcd. for C11H9NO2S2 (251.32): C 52.57%,
H 3.61%, N 5.57%, S 25.52%; found: C 52.87%, H 3.34%, N 5.67%, S 25.09%; UV (nm), λmax/log ε:

Molecules 2011, 16
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1
389.2/3.46; IR (KBr, cm−1): 3141(NH), 1705 (C=O); H-NMR (DMSO-d6), δ: 13.76 (1H, bs, NH),
7.78 (1H, s, CH), 7.53–7.44 (1H, m, H4´), 7.37 (1H, dd, J = 7.7 Hz, J = 1.7 Hz, H6´), 7.14 (1H, d,
13
J = 7.7 Hz, H3´), 7.08 (1H, t, J = 7.7 Hz, H5´), 3.88 (3H, s, OCH3); C-NMR (DMSO-d6), δ: 196.3,
169.6, 158.3, 133.2, 129.9, 126.9, 125.5, 121.5, 121.4, 112.2, 56.0.
(5Z)-5-(3-Methoxybenzylidene)-2-thioxo-1,3-thiazolidin-4-one (4b). Yellow crystalline compound;
Yield 81%; Mp 236–237 °C (232 °C [29]); Anal. Calcd. for C11H9NO2S2 (251.32): C 52.57%,
H 3.61%, N 5.57%, S 25.52%; found: C 52.73%, H 5.43%, N 5.65%, S 25.87%; UV (nm), λmax/log ε:
1
379.6/3.41; IR (KBr, cm−1): 3151 (NH), 1698 (C=O); H-NMR (DMSO-d6), δ: 13.82 (1H, bs, NH),
7.60 (1H, s, CH), 7.44 (1H, t, J = 8.1 Hz, H5´), 7.16–7.03 (3H, m, H2´, H4´, H6´), 3.79 (3H, s, OCH3);
13C-NMR (DMSO-d6), δ: 195.8, 169.5, 159.9, 134.5, 131.8, 130.7, 126.0, 122.6, 116.9, 115.8, 55.5.
(5Z)-5-(4-Methoxybenzylidene)-2-thioxo-1,3-thiazolidin-4-one (4c). Yellow crystalline compound;
Yield 90%; Mp 260–261 °C (261–262 °C [30]); Anal. Calcd. for C11H9NO2S2 (251.32): C 52.57%,
H 3.61%, N 5.57%, S 25.52%; found: C 52.25%, H 3.60%, N 5.75%, S. 27.94%; UV (nm), λmax/log ε:
1
385.6/3.48; IR (KBr, cm−1): 3137 (NH), 1687 (C=O); H-NMR (DMSO-d6), δ: 13.72 (1H, bs, NH),
7.59 (1H, s, CH), 7.58–7.50 (2H, m, AA´, BB´, H2´, H6´), 7.13–7.05 (2H, m, AA´, BB´, H3´, H5´),
3.82 (3H, s, OCH3); 13C-NMR (DMSO-d6), δ: 195.7, 169.6, 161.5, 132.9, 132.1, 125.7, 122.4,
115.3, 55.8.
(5Z)-5-(4-Hydroxy-3-methoxybenzylidene)-2-thioxo-1,3-thiazolidin-4-one (5). Yellow crystalline
compound; Yield 75%; Mp 233–234 °C (227–228 °C [31]); Anal. Calcd. for C11H9NO3S2 (267.32):
C 49.42%, H 3.39%, N 5.24%, S 23.99%; found: C 49.46%, H 3.17%, N 5.24%, S 22.56%; UV (nm),
λmax/log ε: 406.1/3.33; IR (KBr, cm−1): 3340 (OH), 3269 (NH), 1714 (C=O); 1H-NMR (DMSO-d6), δ:
10.09 (1H, bs, OH), 7.56 (1H, s, CH), 7.14 (1H, d, J = 2.1, H2), 7.07 (1H, dd, J = 8.4 and 2.1, H6),
13
6.92 (1H, d, J = 8.4, H5), 3.82 (3H, s, OCH3); C-NMR (DMSO-d6), δ: 195.7, 169.7, 150.2, 148.3,
133.0, 125.3, 124.6, 121.3, 116.6, 114.5, 55.8.
(5Z)-5-[(4-Dimethylamino)benzylidene]-2-thioxo-1,3-thiazolidin-4-one (6). Orange crystalline
compound; Yield 90%; Mp 283–286 °C (283–284 °C [30]); Anal. Calcd. for C12H12N2OS2 (264.37):
C 54.52%, H 4.58%, N 10.60%, S 24.26%; found: C 54.38%, H 4.75%, N 10.68%, S 22.70%;
1
UV (nm), λmax/log ε: 463.0/3.39; IR (KBr, cm−1): 3138 (NH), 1683 (C=O); H-NMR (DMSO-d6), δ:
13.55 (1H, bs, NH), 7.49 (1H, s, CH), 7.45–7.36 (2H, m, AA´, BB´, H2´, H6´), 6.84–6.76 (2H, m,
AA´, BB´, H3´, H5´), 3.01 (6H, s, NCH3); 13C-NMR (DMSO-d6), δ: 195.2, 169.6, 151.9, 133.5, 133.1,
120.0, 117.5, 112.4, 39.8.
(5Z)-5-(2-Nitrobenzylidene)-2-thioxo-1,3-thiazolidin-4-one (7a). Yellow crystalline compound; Yield
68%; Mp 190–195 °C (204–205 °C [32]); Anal. Calcd. for C10H6N2O3S2 (266.30): C 45.10%,
H 2.27%, N 10.52%, S 24.08%; found: C 43.78%, H 1.53%, N 10.14%, S 26.31%; UV (nm),
1
λmax/log ε: 360.1/3.43; IR (KBr, cm−1): 3098 (NH), 1735 (C=O); H-NMR (DMSO-d6), δ: 13.93 (1H,
bs, NH), 8.19 (1H, d, J = 8.2 Hz, H3´), 7.92–7.84 (1H, m, H5´), 7.86 (1H, s, CH), 7.76–7.66 (2H, m,

Molecules 2011, 16
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H4´, H6´); 13C-NMR (DMSO-d6), δ: 196.0, 168.8, 148.1, 134.8, 131.5, 130.5, 129.6, 129.0,
128.1, 125.8.
(5Z)-5-(3-Nitrobenzylidene)-2-thioxo-1,3-thiazolidin-4-one (7b). Yellow crystalline compound; Yield
71%; Mp 257–263 °C (263–264 °C [33]); Anal. Calcd. for C10H6N2O3S2 (266.30): C 45.10%,
H 2.27%, N 10.52%, S 24.08%; found: C 44.64%, H 2.41%, N 11.07%, S 24.32%; UV (nm),
1
λ
max/log ε: 367.6/3.38; IR (KBr, cm−1): 3255, 3182 (NH), 1728 (C=O); H-NMR (DMSO-d6), δ: 8.39
(1H, t, J = 2.0 Hz, H2´), 8.26 (1H, ddd, J = 8.1 Hz, J = 2.0 Hz, J = 0.8 Hz, H4´), 7.96 (1H, d,
13
J = 8.1 Hz, H6´), 7.79 (1H, t, J = 8.1 Hz, H5´), 7.70 (1H, s, CH); C-NMR (DMSO-d6), δ: 196.2,
171.2, 148.5, 135.9, 135.2, 131.1, 130.1, 128.1, 124.8, 124.6.
(5Z)-5-(4-Nitrobenzylidene)-2-thioxo-1,3-thiazolidin-4-one (7c). Orange crystalline compound; Yield
58%; Mp 272–276 °C (273–274 °C [32]); Anal. Calcd. for C10H6N2O3S2 (266.30): C 45.10%,
H 2.27%, N 10.52%, S 24.08%; found: C 45.15%, H 2.28%, N 10.52%, S 24.31%; UV (nm),
1
λ
max/log ε: 394.1/3.37; IR (KBr, cm−1): 3274, 3105 (NH), 1728 (C=O); H-NMR (DMSO-d6), δ:
8.35–8.26 (2H, m, AA´, BB´, H3´, H5´), 7.86–7.77 (2H, m, AA´, BB´, H2´, H6´), 7.70 (1H, s, CH);
13C-NMR (DMSO-d6), δ: 195.5, 169.5, 147.7, 139.4, 131.5, 130.1, 128.8, 124.5.
(5Z)-5-(2-Fluorobenzylidene)-2-thioxo-1,3-thiazolidin-4-one (8a). Yellow crystalline compound;
Yield 67%; Mp 201–203 °C (201–203 °C [27]). Anal. Calcd. for C10H6FNOS2 (239.29): C 50.19%,
H 2.53%, N 5.85%, S 26.80%; found: C 50.27%, H 2.70%, N 6.11%, S 26.42%; UV (nm),
λmax/log ε: 370.0/3.48; IR (KBr, cm−1): 3159 (NH), 1698 (C=O); 1H-NMR (DMSO-d6),
δ: 13.91 (1H, bs, NH), 7.59 (1H, s, CH), 7.58–7.32 (4H, m, H3´, H4´, H5´, H6´); 13C-NMR
(DMSO-d6), δ: 195.6, 169.4, 160.8 (d, J = 252.3 Hz), 133.3 (d, J = 8.6 Hz), 129.6, 128.3, 125.8 (d,
J = 3.5 Hz), 122.5 (d, J = 6.3 Hz), 121.1 (d, J = 11.5 Hz), 116.5 (d, J = 21.3 Hz).
(5Z)-5-(3-Fluorobenzylidene)-2-thioxo-1,3-thiazolidin-4-one (8b). Yellow crystalline compound;
Yield 46%; Mp 199–200 °C (201–202 °C [27]). Anal. Calcd. for C10H6FNOS2 (239.29): C 50.19%,
H 2.53%, N 5.85%, S 26.80%; found: C 50.36%, H 2.72%, N 6.05%, S 26.31%; UV (nm), λmax/log ε:
1
384.2/3.47; IR (KBr, cm−1): 3184 (NH), 1705 (C=O); H-NMR (DMSO-d6), δ: 13.88 (1H, bs, NH),
13
7.63 (1H, s, CH), 7.62–7.52 (1H, m, H6´), 7.48–7.29 (3H, m, H2´, H4´, H5´); C-NMR (DMSO-d6),
δ: 195.6, 169.5, 162.5 (d, J = 245.3 Hz), 135.5 (d, J = 8.1 Hz), 131.7 (d, J = 8.7 Hz), 130.2 (d,
J = 2.3 Hz), 127.4, 126.1 (d, J = 8.9 Hz), 117.7 (d, J = 21.4 Hz), 117.3 (d, J = 22.5 Hz).
(5Z)-5-(4-Fluorobenzylidene)-2-thioxo-1,3-thiazolidin-4-one (8c). Yellow crystalline compound; Yield
78%; Mp 225–227 °C (226–227 °C [27]). Anal. Calcd. for C10H6FNOS2 (239.29): C 50.19%,
H 2.53%, N 5.85%, S 26.80%; found: C 50.00%, H 2.51%, N 5.87%, S 26.74%; UV (nm),
1
λmax/log ε: 386.7/3.45; IR (KBr, cm−1): 3103 (NH), 1724 (C=O); H-NMR (DMSO-d6), δ: 13.83 (1H,
bs, NH), 7.70–7.60 (2H, m, H2´, H6´), 7.64 (1H, s overlapped, CH), 7.58–7.32 (2H, m, H3´. H5´);
13C-NMR (DMSO-d6), δ: 195.8, 169.6, 163.2 (d, J = 251.7 Hz), 133.2 (d, J = 8.7 Hz), 130.7, 129.9
(d, J = 3.4 Hz), 125.4 (d, J = 2.9 Hz), 116.8 (d, J = 21.9 Hz).

Molecules 2011, 16
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(5Z)-5-(2-Chlorobenzylidene)-2-thioxo-1,3-thiazolidin-4-one (9a). Yellow crystalline compound;
Yield 48%; Mp 191–193 °C (192 °C [34]); Anal. Calcd. for C10H6ClNOS2 (255.74): C 46.96%, H
2.36%, N 5.48%, S 25.08%; found: C 47.06%, H 2.38%, N 5.41%, S 25.49%; UV (nm), λmax/log ε:
1
365.0/3.29; IR (KBr, cm−1): 3069 (NH), 1734, 1698 (C=O); H-NMR (DMSO-d6), δ: 13.93 (1H, bs,
NH), 7.74 (1H, s, CH), 7.66–7.60 (1H, m, H3´), 7.54–7.47 (3H, m, H4´, H5´, H6´); 13C-NMR
(DMSO-d6), δ: 195.7, 169.3, 135.0, 132.3, 131.0, 130.7, 129.5, 129.3, 128.5, 126.3.
(5Z)-5-(3-Chlorobenzylidene)-2-thioxo-1,3-thiazolidin-4-one (9b). Orange crystalline compound;
Yield 76%; Mp 234–235 °C (233 °C [34]); Anal. Calcd. for C10H6ClNOS2 (255.74): C 46.96%,
H 2.36%, N 5.48%, S 25.08%; found: C 46.86%, H 2.28%, N 5.48%, S 25.71%; UV (nm), λmax/log ε:
1
376.2/3.31; IR (KBr, cm−1): 3109 (NH), 1718 (C=O); H-NMR (DMSO-d6), δ: 13.90 (1H, bs, NH),
7.68 (1H, s, CH), 7.62–7.47 (4H, m, H2´, H4´, H5´, H6´); 13C-NMR (DMSO-d6), δ: 195.5, 169.4,
135.3, 134.2, 131.4, 130.5, 130.4, 130.0, 128.3, 127.5.
(5Z)-5-(4-Chlorobenzylidene)-2-thioxo-1,3-thiazolidin-4-one (9c). Yellow crystalline compound;
Yield 88%; Mp 232–233 °C (230–231 °C [33]); Anal. Calcd. for C10H6ClNOS2 (255.74): C 46.96%,
H, 2.36%, N 5.48%, S 25.08%; found: C 46.87%, H 2.78%, N 5.60%, S 24.02%. UV (nm), λmax/log ε:
1
379.6/3.36; IR (KBr, cm−1): 3150 (NH), 1709 (C=O). H-NMR (DMSO-d6), δ: 13.87 (1H, bs, NH),
7.62 (1H, s, CH), 7.60–7.58 (4H, m, H2´, H3´, H5´, H6´); 13C-NMR (DMSO-d6), δ: 195.6, 169.5,
135.6, 132.3, 132.1, 130.4, 129.7, 126.5.
(5Z)-5-(2-Bromobenzylidene)-2-thioxo-1,3-thiazolidin-4-one (10a). Yellow crystalline compound;
Yield 59%; Mp 185–186 °C (183.5 °C [34]); Anal. Calcd. for C10H6BrNOS2 (300.19): C 40.01%,
H, 2.01%, N 4.67%, S 21.36%; found: C 39.90%, H 1.89%, N 4.62%, S 22.01%; UV (nm), λmax/log ε:
1
365.0/3.46; IR (KBr, cm−1): 3150 (NH), 1709 (C=O). H-NMR (DMSO-d6), δ: 13.97 (1H, bs, NH),
7.82–7.78 (1H, m, H3´), 7.70 (1H, s, CH), 7.61–7.47 (2H, m, H4´, H6´), 7.45–7.37 (1H, m, H5´);
13C-NMR (DMSO-d6), δ: 195.8, 169.3, 133.9, 132.7, 132.4, 129.6, 129.3, 129.1, 129.0, 125.9.
(5Z)-5-(3-Bromobenzylidene)-2-thioxo-1,3-thiazolidin-4-one (10b). Yellow crystalline compound;
Yield 45%; Mp 244–246 °C (238 °C [34]); Anal. Calcd. for C10H6BrNOS2 (300.19): C 40.01%,
H 2.01%, N 4.67%, S 21.36%; found: C 40.14%, H 1.98%, N 4.53%, S 22.19%; UV (nm), λmax/log ε:
1
379.9/3.46; IR (KBr, cm−1): 3111 (NH), 1717 (C=O). H-NMR (DMSO-d6), δ: 13.88 (1H, bs, NH),
7.80 (1H, s, H2´), 7.71–7.64 (1H, m, H4´), 7.61 (1H, s, CH), 7.55 (1H, d, J = 7.6 Hz, H6´), 7.48 (1H, t,
J = 7.6 Hz, H5´); 13C-NMR (DMSO-d6), δ: 195.6, 169.5, 135.6, 133.4, 133.3, 131.6, 129.9, 128.7,
127.5, 122.7.
(5Z)-5-(4-Bromobenzylidene)-2-thioxo-1,3-thiazolidin-4-one (10c). Yellow crystalline compound;
Yield 92%; Mp 236–238 °C (238–239 °C [27]); Anal. Calcd. for C10H6BrNOS2 (300.19): C 40.01%,
H 2.01%, N 4.67%, S 21.36%; found: C 39.95%, H 1.87%, N 4.64%, S 21.78%; UV (nm), λmax/log ε:
1
382.0/3.39; IR (KBr, cm−1): 3150 (NH); 1708 (C=O); H-NMR (DMSO-d6), δ: 7.76–7.69 (2H, m,
AA´, BB´, H2´, H6´), 7.60 (1H, s, CH), 7.55–7.49 (2H, m, AA´, BB´, H3´, H5´); 13C-NMR
(DMSO-d6), δ: 195.6, 169.5, 132.6, 132.4, 132.4, 130.5, 126.5, 124.5.

Molecules 2011, 16
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(5Z)-3-(2-Hydroxyethyl)-5-(2-nitrobenzylidene)-2-thioxo-1,3-thiazolidin-4-one (11a). The product was
separated from the reaction mixture by means of column chromatography using Silicagel 60 Fluka
(0.040–0.063 mm) as adsorbent and light petroleum/ethyl acetate 6:4 as mobile phase. After
crystallization from ethanol a yellow crystalline compound was obtained. Yield 5%; Mp 105–107 °C;
Anal. Calcd. for C10H6BrNOS2 (310.35): C 46.44%, H 3.25%, N 9.03%, S 20.66%; found: C 46.60%,
H 3.25%, N 8.96%, S 20.23%; UV (nm), λmax/log ε: 361.0/3.46; IR (KBr, cm−1): 3458 (OH), 1716
1
(C=O); H-NMR (DMSO-d6), δ: 8.39–8.26 (2H, m, AA´, BB´, H3´, H5´), 7.91–7.84 (2H, m, AA´,
BB´, H2´, H6´), 7.88 (1H, s, overlapped, CH), 4.94 (1H, bs, OH), 4.11 (2H, t, J = 5.9 Hz, NCH2),
13
3.73–3.59 (2H, m, OCH2); C-NMR (DMSO-d6), δ: 193.6, 167.2, 147.8, 139.3, 131.7, 129.8, 127.1,
124.6, 56.9, 46.9.
(5Z)-3-(2-Hydroxyethyl)-5-(3-nitrobenzylidene)-2-thioxo-1,3-thiazolidin-4-one (11b). The product was
separated from the reaction mixture by means of column chromatography using Silicagel 60 Fluka
(0.040–0.063 mm) as adsorbent and light petroleum/ethyl acetate 6:4 as mobile phase. After
crystallization from ethanol a yellow crystalline compound was obtained. Yield 14%; Mp 217–220 °C;
Anal. Calcd. for C10H6BrNOS2 (310.35): C 46.44%, H, 3.25%, N 9.03%, S 20.66%; found: C 46.79%,
H 2.97%, N 9.12%, S 20.46%; UV (nm), λmax/log ε: 366.4/3.38; IR (KBr) 3448 (OH); 1716 (C=O);
1H-NMR (DMSO-d6), δ: 8.47 (1H, t, J = 1.9 Hz, H2´), 8.33–8.27 (1H, m, H4´), 8.02 (1H, d,
J = 8.0 Hz, H6´), 7.94 (1H, s, CH), 7.82 (1H, t, J = 8.0 Hz, H5´), 4.93 (1H, t, J = 6.0 Hz, OH), 4.12
13
(2H, t, J = 6.0 Hz, NCH2), 3.65 (2H, q, J = 6.0 Hz, OCH2); C-NMR (DMSO-d6), δ: 193.4, 167.1,
148.5, 135.9, 134.8, 131.3, 130.2, 125.7, 125.2, 125.0, 56.9, 46.9.
(5Z)-3-(2-Hydroxyethyl)-5-(4-nitrobenzylidene)-2-thioxo-1,3-thiazolidin-4-one (11c). The product was
separated from the reaction mixture by means of column chromatography using Silicagel 60 Fluka
(0.040–0.063 mm) as adsorbent and light petroleum/ethyl acetate 6:4 as mobile phase. After
crystallization from ethanol a red crystalline compound was obtained. Yield 16%; Mp 202–205 °C
(204–205 °C [24]); Anal. Calcd. for C10H6BrNOS2 (310.35): C 46.44%, H 3.25%, N 9.03%,
S 20.66%; found: C 46.66%, H 3.37%, N 8.87%, S 20.15%; UV (nm), λmax/log ε: 386.8/3.39; IR (KBr,
1
cm−1): 3421 (OH), 1713 (C=O); H-NMR (DMSO-d6), δ: 8.39–8.26 (2H, m, AA´, BB´, H3´, H5´),
7.91–7.84 (2H, m, AA´, BB´, H2´, H6´), 7.88 (1H, s, overlapped, CH), 4.94 (1H, bs, OH), 4.11 (2H, t,
13
J = 5.9 Hz, NCH2), 3.73–3.59 (2H, m, OCH2); C-NMR (DMSO-d6), δ: 193.6, 167.2, 147.8, 139.3,
131.7, 129.8, 127.1, 124.6, 56.9, 46.9.
(5Z)-5-(Pyridin-2-ylmethylidene)-2-thioxo-1,3-thiazolidin-4-one (12). Yellow crystalline compound;
Yield 70%; Mp 269–272 °C (268 °C [26]); Anal. Calcd. for C9H6N2OS2 (222.29): C 48.63%,
H 2.72%, N 12.60%, S 28.85%; found: C 48.26%, H 2.65%, N 12.82%, S 28.90%; UV (nm),
1
λmax/log ε: 349.9/3.44; IR (KBr, cm−1): 3096 (NH), 1726 (C=O); H-NMR (DMSO-d6), δ: 13.66 (1H,
bs, NH), 8.77 (1H, d, J = 4.7 Hz, H6´), 7.94 (1H, dt, J = 7.6 Hz, J = 1.8 Hz, H4´), 7.88 (1H, d,
13
J = 7.6 Hz, H3´), 7.67 (1H, s, CH), 7.45–7.39 (1H, m, H5´); C-NMR (DMSO-d6), δ: 202.2, 169.5,
151.3, 149.7, 137.8, 129.9, 128.3, 127.6, 124.2.

Molecules 2011, 16
5221
(5Z)-5-(Pyridin-3-ylmethylidene)-2-thioxo-1,3-thiazolidin-4-one (13). Yellow crystalline compound;
Yield 68%; Mp 309–313 °C (295 °C [26]); Anal. Calcd. for C9H6N2OS2 (222.29): C 48.63%, H
2.72%, N 12.60%, S 28.85%; found: C 48.49%, H 2.81%, N 12.41%, S 28.45%; UV (nm), λmax/log ε:
356.7/3.46; IR (KBr, cm−1): 3431 (NH), 1709 (C=O); 1H-NMR (DMSO-d6), δ: 8.82 (1H, d, J = 1.9 Hz,
H2), 8.62 (1H, dd, J = 4.8 Hz, J = 1.9 Hz, H6), 7.96–7.88 (1H, m, H4), 7.66 (1H, s, CH), 8.62 (1H, dd,
J = 4.8 Hz, J = 1.9 Hz, H5); 13C-NMR (DMSO-d6), δ: 195.5, 169.4, 151.9, 150.9, 136.5, 129.3, 128.3,
128.0, 124.5.
(5Z)-5-(Pyridin-4-ylmethylidene)-2-thioxo-1,3-thiazolidin-4-one (14). Orange crystalline compound;
Yield 87%; Mp 320–322 °C (295 °C [26]); Anal. Calcd. for C9H6N2OS2 (222.29): C 48.63%, H
2.72%, N 12.60%, S 28.85%; found: C 48.53%, H 3.22%, N 12.68%, S 29.12%; UV (nm), λmax/log ε:
358.1/3.46; IR (KBr, cm−1): 3420 (NH), 1701 (C=O); 1H-NMR (DMSO-d6), δ: 8.74–8.68 (2H, m, H2´,
H6´), 7.55 (1H, s, CH), 7.54–7.50 (2H, m, H3´, H5´); the 13C-NMR spectrum could not been recorded
due to the poor solubility of the compound.
(5Z)-5-(Pyrazin-2-ylmethylidene)-2-thioxo-1,3-thiazolidin-4-one (15). Orange crystalline compound;
Yield 57%; Mp 310 °C; Anal. Calcd. for C8H5N3OS2 (223.27): C 43.03%, H 2.26%, N 18.82%,
S 28.72%; found: C 43.12%, H 1.96%, N 18.72%, S 28.29%; UV (nm), λmax/log ε: 374.8/3.39;
1
IR (KBr, cm−1): 3198 (NH), 1715, 1704 (C=O); H-NMR (DMSO-d6), δ: 13.78 (1H, bs, NH), 9.09
(1H, d, J = 1.4 Hz, H3´), 8.85–8.80 (1H, m, H5´), 8.63 (1H, d, J = 2.7 Hz, H6´), 7.73 (1H, s, CH);
13C-NMR (DMSO-d6), δ: 200.9, 169.3, 148.6, 147.4, 144.6, 144.4, 132.2, 124.2.
(5Z)-5-(Furan-2-ylmethylidene)-2-thioxo-1,3-thiazolidin-4-one (16). Orange crystalline compound;
Yield 55%; Mp 235–237 °C (230–231°C [33]); Anal. Calcd. for C8H5NO2S2 (211.26): C 45.48%,
H 2.39%, N 6.63%, S 30.36%; found: C 45.44%, H 2.48%, N 6.43%, S 30.44%; UV (nm), λmax/log ε:
1
395.1/3.29; IR (KBr, cm−1): 3141 (NH), 1689 (C=O); H-NMR (DMSO-d6), δ: 13.67 (1H, bs, NH),
8.09 (1H, dd, J = 1.9 Hz, J = 0.69 Hz, H5), 7.47 (1H, s, CH), 7.16 (1H, dd, J = 3.6 Hz, J = 0.6 Hz, H3),
6.75 (1H, dd, J = 3.6 Hz, J = 1.9 Hz, H4); 13C-NMR (DMSO-d6), δ: 196.7, 169.2, 149.6, 148.5, 122.6,
120.1, 117.9, 114.1.
3.2. Lipophilicity HPLC Determination (capacity factor k/calculated log k)
A Waters Alliance 2695 XE HPLC separation module, a Waters Photodiode Array Detector Waters
Alliance 2695 XE HPLC separation module and a Waters Photodiode Array Detector 2996 (Waters
Corp., Milford, MA, USA) were used. Waters Symmetry® C18 5 μm, 4.6 × 250 mm, Part No.
WAT054275 (Waters Corp., Milford, MA, USA) chromatographic column was used. The HPLC
separation process was monitored by Empower™ 2 Chromatography Data Software, Waters 2009
(Waters Corp., Milford, MA, USA). The mixture of MeOH (HPLC grade, 70%) and H2O
(HPLC – Mili-Q Grade, 30%) was used as a mobile phase. The total flow rate of the column was
0.9 mL/min; injection volume 30 μL, column temperature 30 °C and sample temperature 10 °C were
used. The detection wavelength of 210 nm was chosen. The KI methanolic solution was used for the
dead time (tD) determination. Retention times (tR) were measured in minutes. The capacity factors k

Molecules 2011, 16
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were calculated using the Empower™ 2 Chromatography Data Software according to formula
k = (tR − tD)/tD, where tR is the retention time of the solute, whereas tD denotes the dead time obtained
using an unretained analyte. Log k, calculated from the capacity factor k, is used as the lipophilicity
index converted to log P scale. The log k values of the individual compounds are shown in Table 1.
3.3. Study of Photosynthetic Electron Transport (PET) Inhibition in Spinach Chloroplasts
Chloroplasts were prepared from spinach (Spinacia oleracea L.) according to ref. [54]. The
inhibition of photosynthetic electron transport (PET) in spinach chloroplasts was determined
spectrophotometrically (Genesys 6, Thermo Scientific, USA) using an artificial electron acceptor
2,6-dichlorophenol-indophenol (DCIPP) according to ref. [55], and the rate of photosynthetic electron
transport was monitored as a photoreduction of DCPIP. The measurements were carried out in
phosphate buffer (0.02 mol/L, pH 7.2) containing sucrose (0.4 mol/L), MgCl2 (0.005 mol/L) and NaCl
(0.015 mol/L). The chlorophyll content was 30 mg/L in these experiments, and the samples were
irradiated (~100 W/m2) from 10 cm distance with a halogen lamp (250 W) using a 4 cm water filter
to prevent warming of the samples (suspension temperature 22 °C). The studied compounds were
dissolved in DMSO due to their limited water solubility. The applied DMSO concentration (up to 4%)
did not affect the photochemical activity in spinach chloroplasts. The inhibitory efficiency of the
studied compounds was expressed by IC50 values, i.e., by molar concentration of the compounds
causing 50% decrease in the oxygen evolution rate relative to the untreated control. The comparable
IC50 value for the selective herbicide 3-(3,4-dichlorophenyl)-1,1-dimethylurea, DCMU (Diuron®) was
about 1.9 μmol/L [56]. The results are summarized in Table 2.
3.4. Reduction of Chlorophyll Content in Green Algae Chlorella vulgaris Beij.
Green algae Chlorella vulgaris Beij. were cultivated statically at room temperature according to
ref. [57] (photoperiod 16 h light/8 h dark; photosynthetically active radiation (PAR) 80 μmol/m2s;
pH 7.2). The effect of rhodanine compounds on algal chlorophyll (Chl) content was determined after
7-day cultivation in the presence of the compounds tested, expressing the response as percentage of the
corresponding values obtained for control. The Chl content in the algal suspension was determined
spectrophotometrically (Genesys 6, Thermo Scientific, USA) after extraction into methanol according
to Wellburn [58]. The Chl content in the suspensions at the beginning of cultivation was 0.01 mg/L.
Because of their low water solubility, the tested compounds were dissolved in DMSO. DMSO
concentration in the algal suspensions did not exceed 0.25% and at the end the control samples
contained the same DMSO amount as the suspensions treated with the tested compounds. The antialgal
activity of most effective compounds was expressed as IC50 value (the concentration of the inhibitor
causing a 50% decrease in the content of Chl as compared with the control sample) or by percentual
reduction of chlorophyll content (with respect to the control) after treatment with equimolar
concentration of the studied compounds (100 μmol/L). The IC50 value for the standard, the selective
herbicide 3-(3,4-dichlorophenyl)-1,1-dimethylurea, DCMU (Diuron®) was about 7.3 µmol/L. The
results are summarized in Table 2.

Molecules 2011, 16
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4. Conclusions
The series of thirty rhodanine derivatives is presented. Their lipophilicity was determined using a
well established RP-HPLC method. The compounds were tested for their activity related to inhibition
of photosynthetic electron transport (PET) in spinach (Spinacia oleracea L.) chloroplasts and
reduction of chlorophyll content in freshwater alga Chlorella vulgaris. Structure-activity relationships
between the chemical structure, physical properties and biological activities of the evaluated
compounds are discussed. (5Z)-5-(4-Bromobenzylidene)-2-thioxo-1,3-thiazolidin-4-one (10c) showed
the highest PET-inhibition activity among the discussed compounds. (5Z)-5-(4-Chlorobenzylidene)-2-
thioxo-1,3-thiazolidin-4-one (9c) expressed the highest reduction of chlorophyll content in freshwater
alga Chlorella vulgaris. The results of the present study confirm previous observations regarding the
influence of rhodanine and its derivatives on plants and show that both lipophilicity and character of
substituents are important for their potency. These noteworthy compounds surely deserve further
attention as potential pesticides and/or drugs.
Acknowledgements
The study was supported by the Ministry of Education, Youth and Sports (Research Project MSM
0021620822) and by Sanofi-Aventis Pharma Slovakia.
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Sample Availability: Samples of the compounds are available from the authors.
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