Synthesis, structure and structure-activity relationship analysis of 3-tert-butoxycarbonyl-2-arylthiazolidine-4-carboxylic acid derivatives as potential antibacterial agents

Article abstract of DOI:10.1016/j.ejmech.2009.04.014

Nine 2-arylthiazolidine-4-carboxylic acid derivatives and nine 3-tert-butoxycarbonyl-2-arylthiazolidine-4-carboxylic acid derivatives were synthesized to screen for their antibacterial activities. Compounds 5, 14-18 were first reported. Their chemical str

Full text of DOI:10.1016/j.ejmech.2009.04.014

European Journal of Medicinal Chemistry 44 (2009) 3903–3908
Contents lists available at ScienceDirect
European Journal of Medicinal Chemistry
journal homepage: http://www.elsevier.com/locate/ejmech
Original article
Synthesis, structure and structure–activity relationship analysis
of 3-tert-butoxycarbonyl-2-arylthiazolidine-4-carboxylic acid
derivatives as potential antibacterial agents
*
Zhong-Cheng Song, Gao-Yuan Ma, Peng-Cheng Lv, Huan-Qiu Li, Zhu-Ping Xiao, Hai-Liang Zhu
Institute of Functional Biomolecules, State Key Laboratory of Pharmaceutical Biotechnology, Nanjing University, Nanjing 210093, People’s Republic of China
a r t i c l e i n f o
a b s t r a c t
Article history:
Nine 2-arylthiazolidine-4-carboxylic acid derivatives and nine 3-tert-butoxycarbonyl-2-arylthiazolidine-
4-carboxylic acid derivatives were synthesized to screen for their antibacterial activities. Compounds 5,
14–18 were first reported. Their chemical structures were clearly determined by 1H NMR, 13C NMR, ESI
mass spectra and elemental analyses, coupled with one selected single-crystal structure. All the
compounds were assayed for antibacterial activities against two Gram-positive bacterial strains (Bacillus
subtilis ATCC 6633 and Staphylococcus aureus ATCC 6538) and two Gram-negative bacterial strains
(Escherichia coli ATCC 35218 and Pseudomonas aeruginosa ATCC 13525) by MTT method. Most of the 3-
tert-butoxycarbonyl-2-arylthiazolidine-4-carboxylic acid derivatives exhibited better antibacterial
activities against the four bacterial strains than relative 2-arylthiazolidine-4-carboxylic acid derivatives.
Received 3 June 2008
Received in revised form
28 March 2009
Accepted 2 April 2009
Available online 14 April 2009
Keywords:
Thiazolidine
Antibacterial activity
P. aeruginosa
Structure–activity relationship (SAR)
Compound
(2RS,4R)-3-(tert-butoxycarbonyl)-2-(5-fluoro-2-hydroxyphenyl)thiazolidine-4-carboxylic
g/mL,
acid (14) showed powerful antibacterial activities against P. aeruginosa with IC50 value of 0.195
m
which was superior to the positive controls Penicillin G and Kanamycin B, respectively. On the basis of
the biological results, structure–activity relationships were discussed.
Ó 2009 Elsevier Masson SAS. All rights reserved.
1. Introduction
ATCC 6538) and Gram-negative (Pseudomonas aeruginosa ATCC
13525 and Escherichia coli ATCC 35218), which often caused
Currently, heterocyclic compounds have been extensively
studied due to their important properties and applications.
Among these compounds, thiazole and thiazolidine derivatives
have become especially noteworthy in recent years [1,2]. Addi-
tionally, thiazole and thiazolidine moieties have played an
increasingly important role in many different types of chemical
infection in burned patients. Herein, various TBTCA derivatives
with modifications were synthesized in order to find new anti-
bacterial agents. Our current work highlights antibacterial activity
evaluation of a series of synthesized TBTCA against B. subtilis as
well as similarity comparison of antibacterial activities against
S. aureus, P. aeruginosa and E. coli strains. Furthermore, the
minimal inhibitory concentration (MIC) of these TBTCA deriva-
tives with antibacterial activities against the above four bacteria
and structure–activity relationship (SAR) were also discussed.
structures, generating by structure modification
a series of
compounds with broad-spectrum bioactivities [3]. Moreover,
thiazolidine derivatives can be antibacterial agents to cure
patients infected by some pathogenic bacteria [4–7]. In previous
studies [8,9], it was reported that 2-substituted 3-tert-butox-
ycarbonyl-thiazolidine-(4R)-carboxylic acid (TBTCA) can protect
against the hepatotoxicity elicited by high doses of acetamino-
phen in mice. Thus, it is very significant to study on these types of
thiazolidine derivatives. However, to the best of our knowledge, it
has not been reported that thiazolidine derivatives like TBTCA had
better antibacterial activities against the four bacteria, Gram-
positive (Bacillus subtilis ATCC 6633 and Staphylococcus aureus
2. Results and discussion
2.1. Chemistry
Nine 2-arylthiazolidine-4-carboxylic acid derivatives were
obtained by reaction of
L-cysteine with various aldehydes, as
described in Scheme 1 [3,10,11].
Initial SAR studies were performed by modification of the parent
compound to determine if any of the subunits displayed antibac-
terial activity. Based on this consideration, we made related
modifications to TCA as shown in Fig. 1.
* Corresponding author. Tel.: þ86 25 8359 2572; fax: þ86 25 8359 2672.
E-mail address: zhuhl@nju.edu.cn (H.-L. Zhu).
0223-5234/$ – see front matter Ó 2009 Elsevier Masson SAS. All rights reserved.
doi:10.1016/j.ejmech.2009.04.014

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Z.-C. Song et al. / European Journal of Medicinal Chemistry 44 (2009) 3903–3908
CHO
R3
R2 R1
R2
R4
R1
O
R1
+
R2
R5
R4
S
EtOH / r.t.
S
R3
OH
HS
R3
N
H
N
H
COOH
NH2
COOH
R4 R5
R5
1-9
1-9
Scheme 1. Syntheses of substituted 2-arylthiazolidine-4-carboxylic acid (TCA) 1–9.
R1
H
R2
R3
R4
H
R5
In order to improve the solubility, we have also carried out the
next step 3-tert-butoxycarbonyl reaction. The corresponding nine
TBTCA were obtained by the reaction of tert-butyloxycarbonyl
anhydride (BOC2O) with compounds 1–9, as described in Scheme 2
[12] and Fig. 2.
H
1
2
3
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
OH
H
H
OMe
Br
H
4
5
6
H
H
2.2. Crystal structure of ‘2R, 4R’ isomer of two TBTCA (12)
OH
OH
OH
H
F
The ‘2R, 4R’ isomer of TBTCA (12) was characterized by single-
crystal X-ray determinations. It crystallizes in the monoclinic space
group P21, and the crystallographic data are presented in Table 1. The
selected bond distances, bond angles and torsion angles are given in
Table 2. As shown in Table 2, the bond lengths of O2]C5 (1.233(6) Å)
and O3]C6 (1.193(6) Å) conform to the values for carbon–oxygen
double bonds. Generally, a typical C–O single bond is around 1.40–
1.42 Å. However, the short bond C6–O4 (1.315(7) Å) in the carboxyl
group comes from the resonance with C6–O3 (1.193(6) Å) and the
very long bond C4–O1 (1.482(7) Å) might come from a steric
hindrance with the bulky tert-butyl-group. The C–S (1.802(6)–
1.844(5) Å) single bonds are in the normal ranges. A normal C–N-
single bond is around 1.45 Å, so N–C7 (1.446(6) Å) and N–C9
(1.474(6) Å) might be considered as normal single bonds; however,
N–C5 (1.360(6) Å) is significantly shortened due to the interaction
with the two C5–O bonds. The C5–N–C7, C5–N–C9 and C7–N–C9
bond angles are close to 120ꢀ, while the N–C(9)–S and C(8)–S–C(9)
bond angles are 103.0(3) and 90.7(3)ꢀ. The C(5)–N–C(9)–C(10), C(5)–
N–C(9)–S and C(8)–S–C(9)–C(10) torsion angles in the molecule are
58.4(6), 179.3(4) and 155.1(4)ꢀ, respectively. The presence of the S
atom allowed the unambiguous assignment of the absolute config-
uration on the basis of the refined Flack parameter value (ꢁ0.09 (19))
[13–15]. Molecular structure (thermal ellipsoids at the 35% proba-
bility level) of ‘2R, 4R’ isomer of compound 12 shown in Fig. 3
revealed the 2R, 4R absolute configuration. In the crystal structure of
the compound, molecules are linked through intermolecular
hydrogen bonds of O4–H4A/O2 and C9–H9A/O3 (Table 3), form-
ing chains running along the a axis. The chains are further linked
through intermolecular hydrogen bonds of C16–H16A/O3 at the
b axis, forming layers parallel to the ab plane, as shown in Fig. 4.
H
H
Cl
Br
7
8
9
OH
H
H
Br
H
H
Br
C(CH3)3
OH
C(CH3)3
Fig. 1. Chemical structures of TCA 1–9.
It can be seen from Table 4 that many of these TCA derivatives
were very effective and selective for antibacterial activities in vitro
against the four bacteria used in the biological experiments. As
shown in Table 4, the synthesized thiazolidine derivative
(compound 5) exhibited excellent high selective antibacterial
activity against P. aeruginosa with IC50 as low as 1.5625 mg/mL,
however, exhibited low antibacterial activities against other three
bacteria (B. subtilis, S. aureus and E. coli). Furthermore, inspection
of the chemical structure of the synthesized TCA derivatives
(Fig. 1) with phenyl functional group part suggested that they
could be divided into three types by the number of substituents:
a single substituent, two substituents and three substituents.
Initial SAR studies were performed by modification of the parent
compound to determine how the substituents of the subunits
affected the antibacterial activities. Compounds bearing two
substituents exhibited higher antibacterial activities than the
compounds containing a single substituent or three substituents,
as shown in Table 4.
It can be found that the results of antibacterial activities of TCA
derivatives bearing two substituents against the four bacteria were
very selective and complicated. According to the experiment results
(Table 4), the antibacterial activity against the four bacteria is in
accordance with the order F > Cl > Br this order at the 5-position on
the phenyl group.
2.3. Biological activity
All the TCA and TBTCA derivatives were screened for antibac-
terial activities in vitro against two Gram-positive bacterial strains
B. subtilis and S. aureus, and two Gram-negative bacterial strains
E. coli and P. aeruginosa by MTT method [16,17]. The IC50s of the TCA
derivatives against the four bacteria are presented in Tables 4 and 5.
It can be found that all TBTCA derivatives exhibited excellent
results of antibacterial activities (Table 5) compared with those of
R2
R1
R2
R4
R1
S
S
O
O
1,4-dioxane
NaOH
A
R3
C
R3
+
N
O
O
O
COOH
N
H
COOH
R4
R5
R5
O
O
1-9
10-18
Scheme 2. Syntheses of TBTCA 10–18.

Z.-C. Song et al. / European Journal of Medicinal Chemistry 44 (2009) 3903–3908
3905
Table 2
R2 R1
Selected bond lengths (Å), bond angles (ꢀ) and torsion angles(ꢀ) for ‘2R, 4R’ isomer of
compound 12.
S
N
R3
COOH
S1–C(8)
S1–C(9)
N–C(5)
N–C(7)
N–C(9)
O(1)–C(5)
O(1)–C(4)
O(2)–C(5)
1.802(6)
1.844(5)
1.360(6)
1.446(6)
1.474(6)
1.321(7)
1.482(7)
1.233(6)
O(3)–C(6)
O(4)–C(6)
O(5)–C(13)
O(5)–C(16)
C(6)–C(7)
C(7)–C(8)
C(9)–C(10)
1.193(6)
1.315(7)
1.355(8)
1.400(9)
1.515(7)
1.525(7)
1.499(8)
R5
R4
BOC
10-18
R1
R2
R3
R4
R5
H
H
H
H
H
H
H
H
H
Br
H
OH
OMe
Br
H
H
H
H
H
H
H
H
H
10
11
12
13
14
15
16
C(8)–S–C(9)
C(5)–N–C(7)
C(5)–N–C(9)
C(7)–N–C(9)
C(5)–O(1)–C(4)
O(1)–C(4)–C(2)
O(1)–C(4)–C(1)
O(1)–C(4)–C(3)
C(13)–O(5)–C(16)
O(2)–C(5)–O(1)
O(2)–C(5)–N
90.7(3)
122.8(4)
118.8(4)
117.8(4)
121.5(5)
109.0(6)
110.8(5)
101.2(5)
118.0(6)
126.0(5)
123.3(5)
O(1)–C(5)–N
O(3)–C(6)–O(4)
O(3)–C(6)–C(7)
O(4)–C(6)–C(7)
N–C(7)–C(6)
N–C(7)–C(8)
C(6)–C(7)–C(8)
C(7)–C(8)–S
110.7(4)
123.2(5)
122.4(5)
114.3(4)
114.8(4)
105.6(4)
110.6(4)
104.0(4)
117.8(4)
103.0(3)
109.7(3)
H
H
H
H
N–C(9)–C(10)
N–C(9)–S
C(10)–C(9)–S
OH
OH
OH
H
F
H
Cl
Br
C(4)–O(1)–C(5)–N
C(7)–N–C(5)–O(2)
C(9)–N–C(5)–O(2)
C(7)–N–C(5)–O(1)
C(9)–N–C(5)–O(1)
C(5)–N–C(7)–C(6)
C(9)–N–C(7)–C(6)
C(5)–N–C(7)–C(8)
C(9)–N–C(7)–C(8)
O(3)–C(6)–C(7)–N
O(4)–C(6)–C(7)–N
N–C(7)–C(8)–S
177.1(5)
ꢁ175.7(5)
ꢁ5.0(8)
4.7(7)
175.4(4)
ꢁ86.8(6)
102.4(5)
151.1(5)
ꢁ19.8(6)
ꢁ169.7(5)
10.9(6)
C(6)–C(7)–C(8)–S
C(9)–S–C(8)–C(7)
C(5)–N–C(9)–C(10)
C(7)–N–C(9)–C(10)
C(5)–N–C(9)–S
ꢁ84.8(5)
ꢁ40.5(4)
58.4(6)
H
ꢁ130.4(5)
179.3(4)
ꢁ9.6(5)
OH
H
H
Br
H
H
17
18
C(7)–N–C(9)–S
C(8)–S–C(9)–N
28.8(4)
C(CH )
OH
C(CH )
3 3
3 3
C(8)–S–C(9)–C(10)
N–C(9)–C(10)–C(15)
S–C(9)–C(10)–C(15)
N–C(9)–C(10)–C(11)
S–C(9)–C(10)–C(11)
155.1(4)
ꢁ141.6(5)
101.1(5)
42.6(7)
Fig. 2. Chemical structures of TBTCA 10–18.
TCA derivatives (Table 4). This may be due to the introduction of
tert-butyloxycarbonyl, destructing the intermolecular hydrogen
bond of the original carboxylic acid, making an increasing solubility
of the compounds and resulting in the increasing antibacterial
activity. Compound 14 exhibited higher antibacterial activity
against P. aeruginosa with IC50 value of 0.195 mg/mL, which may be
attributed to the electron-withdrawing inductive effect and the
unique biological activity of F atom substituent. Because of the
40.0(5)
ꢁ74.8(5)
stronger bond energy of C–F (477 KJ/mol) compared with C–H
(398 KJ/mol) and smaller radius (van der Waals radius nearly to H
atom radius), F atom often links the vulnerable attacked metabo-
lism site to prevent the metabolism of the bacteria.
It is well known that P. aeruginosa is a Gram-negative, aerobic,
rod-shaped bacterium with unipolar motility [18,19] that grows in
soil, marshes, and coastal marine habitats, as well as on plant and
animal tissues [20], and is commonly isolated from clinical speci-
mens. An opportunistic human pathogen [21–24], P. aeruginosa
infection occurring in any part of the human body and organiza-
tions, is common in parts of burns or trauma, ear, cornea, urethra
Table 1
Crystal structure data for ‘2R, 4R’ isomer of compound 12.
Compound
‘2R, 4R’ isomer of 12
Formula
Mr
C16H21NO5S
339.40
0.40 ꢂ 0.30 ꢂ 0.20
Monoclinic
P21
6.490(2)
11.019(2)
12.296(3)
90.00
95.53(3)
90.00
Crystal size (mm3)
Crystal system
Space group
a (Å)
b (Å)
c (Å)
a
b
g
(ꢀC)
(ꢀC)
(ꢀC)
V (Å3)
Z
875.2(3)
2
Dc (g/cm3)
1.288
0.208
m
(mmꢁ1
)
F (000)
Range (ꢀ)
Index range (h, k, l)
Reflections collected/unique
R(int)
360
q
1.66/25.96
ꢁ7/7, 0/13, 0/15
1961/1798
0.0144
Observed reflections I > 2
s(I)
1498
Min. and max. transmission
Data/restraints/parameters
Goodness-of-fit on F2
0.9595 and 0.9213
1798/1/202
1.029
R1, wR2 [I > 2
R1, wR2
Absolute structure parameter
Large diff. peak and hole (e Åꢁ3
s
(I)]
0.0549/0.1329
0.0691/0.1483
ꢁ0.09(19)
0.226/ꢁ0.244
Fig. 3. Molecular structure (thermal ellipsoids at the 35% probability level) of ‘2R, 4R’
isomer of compound 12.
)

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Z.-C. Song et al. / European Journal of Medicinal Chemistry 44 (2009) 3903–3908
Table 3
Table 4
Hydrogen Bonds for ‘2R, 4R’ isomer of compound 12 ((Å) and (ꢀ)).
IC50 values of the synthesized TCA 1–9 against the four bacteria.
D–H/A
d(D–H)
d(H/A)
d(D/A)
<(DHA)
Compound
IC50s (mg/mL)
O(4)–H(4A)/O(2)a
C(1)–H(1B)/O(2)
C(2)–H(2A)/O(2)
C(8)–H(8B)/O(3)
C(9)–H(9A)/O(3)b
C(11)–H(11A)/O(4)
C(16)–H(16A)/O(3)c
0.82
0.96
0.96
0.97
0.98
0.93
0.96
1.81
2.39
2.49
2.53
2.50
2.55
2.48
2.629(5)
2.976(9)
3.060(8)
2.908(7)
3.303(7)
3.421(7)
3.031(10)
178.9
119.3
117.7
102.9
139.2
155.5
116.7
Gram-positive
Gram-negative
B. subtilis
S. aureus
P. aeruginosa
E. coli
1
2
3
4
5
6
7
8
3.125
25
12.5
25
3.125
12.5
12.5
12.5
25
12.5
>50
25
>50
25
25
>50
>50
12.5
12.5
12.5
25
12.5
25
3.125
>50
6.25
1.5625
3.125
6.25
25
25
3.125
6.25
6.25
6.25
6.25
12.5
>50
12.5
1.562
1.562
Symmetry transformations used to generate equivalent atoms:
a
x þ 1, y, z.
b
x ꢁ 1, y, z.
c
x ꢁ 1, y ꢁ 1, z.
9
Kanamycin G
Penicillin B
0.39
1.562
6.25
Table 5
IC50 values of the synthesized TBTCA 10–18 against the four bacteria.
Compound
IC50s (mg/mL)
Gram-positive
Gram-negative
B. subtilis
S. aureus
P. aeruginosa
E. coli
10
11
12
13
14
15
16
17
3.125
0.39
3.125
3.125
0.39
1.562
1.562
3.125
0.39
0.78
0.78
0.39
0.39
0.39
0.195
0.39
0.78
0.39
0.39
3.125
6.25
0.39
0.78
0.78
0.39
0.78
1.562
1.562
0.78
0.69
3.125
6.25
3.125
1.562
3.125
0.39
1.562
1.562
1.562
1.562
1.562
1.562
18
Kanamycin G
Penicillin B
0.39
1.562
and respiratory tract and can cause endocarditis, gastroenteritis,
septicemia or empyema. The most widely used medicines to treat
this infection are broad-spectrum cephalosporins such as cefepime
(CFPM) [25], cefozopran (CZOP) [26], and ceftazidime (CAZ) [27,28]
or quinolones such as ciprofloxacin [29].
However, there are few reports showing that thiazolidine
derivatives may be used as antibacterial agents to cure patients
who were infected by P. aeruginosa. It is very interesting to find that
compound 14 exhibited the highest antibacterial activity against
P. aeruginosa with the lowest IC50 value of 0.195 mg/mL. Therefore, it
is suggested that one of our synthesized TBTCA derivatives
(compound 14) could be potential antibacterial agents against
P. aeruginosa.
3. Conclusions
In summary, a series of TCA and TBTCA derivatives were
synthesized and their antibacterial activities were also tested
against B. subtilis, S. aureus, P. aeruginosa, and E. coli. Based on the
analysis and evaluation of antibacterial activities, compound
(2RS,4R)-3-(tert-butoxycarbonyl)-2-(5-fluoro-2-hydroxyphenyl)
thiazolidine-4-carboxylic acid (14) exhibited higher selective anti-
bacterial activities against pathogenic bacterium P. aeruginosa than
positive controls Kanamycin G and Penicillin B did. Hence, it may be
as potential therapeutic utilities if further structural optimization
and reorganization are performed. Future efforts shall be aimed at
synthesis and evaluation of pure individual stereoisomer of the
most promising thiazolidine derivatives discussed above. Our
understanding of the antibacterial activity of thiazolidine deriva-
tives against P. aeruginosa is still limited, and further investigation
is in progress.
Fig. 4. Packing diagram of ‘2R, 4R’ isomer of compound 12, viewed along from a axis,
showing the intermolecular hydrogen-bonding scheme (dashed lines).

Z.-C. Song et al. / European Journal of Medicinal Chemistry 44 (2009) 3903–3908
3907
4. Experiments
MS (ESI) m/z 208 (M-1). Anal. Calcd. for C10H11NO2S: C, 57.39; H,
5.30; N, 6.69. Found: C, 57.62; H, 5.28; N, 6.71.
4.1. Crystallographic studies
4.3.1.2. (2RS,4R)-2-(4-Hydroxyphenyl)thiazolidine-4-carboxylic acid
X-ray single-crystal diffraction data for ‘2R, 4R’ isomer of
compound 12 were collected on a Nonius CAD4 diffractometer
(2). Obtained as white solids (0.209 g, 93%). 1H NMR (DMSO-d6)
d
6.71–6.77 (m, 2H), 7.24–7.33(m, 2H), 5.53 (s, 0.5H), 5.40 (s, 0.5H),
equipped with graphite-monochromatized Mo K
a
(
l
¼ 0.71073 Å)
4.27 (dd, J ¼ 4.02 Hz, 4.02 Hz, 0.5H), 3.85 (dd, J ¼ 7.32 Hz, 7.32 Hz,
0.5H), 3.26–3.43 (m,1H), 2.96–3.18 (m,1H); MS (ESI) m/z 224 (M-1).
Anal. Calcd. for C10H11NO3S: C, 53.32; H, 4.92; N, 6.22. Found: C,
53.50; H, 4.94; N, 6.20.
radiation. The program CAD4 software was used for data collection
and cell refinement. Data reduction was solved by XCAD4 program.
Structure was solved by direct methods using the SHELXS program
of the SHELXTL package and refined by full-matrix least-squares
methods with SHELXL [30]. All non-hydrogen atoms of ‘2R, 4R’
isomer of compound 12 were refined with anisotropic thermal
parameters. All hydrogen atoms were placed in geometrically
idealized positions and constrained to ride on their parent atoms.
4.3.1.3. (2RS,4R)-2-(4-Methoxyphenyl)thiazolidine-4-carboxylic acid
(3). Obtained as white solids (0.220 g, 92%). 1H NMR (DMSO-d6)
d
7.35–7.45 (m, 2H), 6.87–6.94(m, 2H), 5.60 (s, 0.5H), 5.45 (s, 0.5H),
4.24 (dd, J ¼ 4.05 Hz, 4.38 Hz, 0.5H), 3.86 (dd, J ¼ 8.40 Hz, 7.32 Hz,
0.5H), 3.28–3.42 (m, 1H), 2.77–2.92 (m, 1H); MS (ESI) m/z 238
(M-1). Anal. Calcd. for C11H13NO3S: C, 55.21; H, 5.48; N, 5.85. Found:
C, 55.43; H, 5.50; N, 5.83.
4.2. Biological assay
The antibacterial activities of the synthesized compounds were
tested against B. subtilis, E. coli, P. aeruginosa and S. aureus using MH
medium (Muellere–Hinton medium: casein hydrolysate 17.5 g,
soluble starch 1.5 g, beef extract 1000 mL). The IC50s of the tested
compounds were determined by a colorimetric method using the
4.3.1.4. (2RS,4R)-2-(4-Bromophenyl)thiazolidine-4-carboxylic acid
(4). Obtained as white solids (0.245 g, 85%). 1H NMR (DMSO-d6)
d
7.53–7.58 (m, 2H), 7.38–7.50(m, 2H), 5.68 (s, 0.6H), 5.50 (s, 0.4H),
4.17 (dd, J ¼ 5.19 Hz, 6.75 Hz, 0.5H), 3.90 (dd, J ¼ 6.99 Hz, 8.70 Hz,
0.5H), 3.10–3.12 (m, 1H), 3.05–3.08 (m, 1H); MS (ESI) m/z 286 (M-1).
Anal. Calcd. for C10H10BrNO2S: C, 41.68; H, 3.50; N, 4.86. Found: C,
41.51; H, 3.52; N, 4.88.
dye MTT. A stock solution of the synthesized compound (50 mg/mL)
in DMSO was prepared and graded quantities of the tested
compounds were incorporated in specified quantity of sterilized
liquid medium (MH medium for antibacterial activity). A specified
quantity of the medium containing the compound was poured into
microtitration plates. Suspension of the microorganism was
prepared to contain approximately 105 cfu/mL and applied to
microtitration plates with serially diluted compounds in DMSO to
be tested and incubated at 37 ꢀC for 24 h. After the IC50s were
visually determined on each of the microtitration plates, 50 mL of
PBS phosphate buffered saline 0.01 mol/L, pH 7.4, Na2HPO4$12H2O
(2.9 g), KH2PO4 (0.2 g), NaCl (8.0 g), KCl (0.2 g), distilled water
(1000 mL) containing 2 mg of MTT/mL was added to each well.
Incubation was continued at room temperature for 4–5 h. The
content of each well was removed, and 100 mL of isopropanol
containing 5% 1 mol/L HCl was added to extract the dye. After 12 h
of incubation at room temperature, the optical density (OD) was
4.3.1.5. (2RS,4R)-2-(5-Fluoro-2-hydroxyphenyl)thiazolidine-4-carbox-
ylic acid (5). Obtained as white solids (0.218 g, 90%). 1H NMR
(DMSO-d6)
d 6.74–6.83 (m, 1H), 6.87–7.01 (m, 1H), d 7.10–7.41
(m, 1H), 5.80 (s, 0.6H), 5.63 (s, 0.4H), 4.20 (dd, J ¼ 5.85 Hz, 6.21 Hz,
0.6H), 3.86 (dd, J ¼ 6.75 Hz, 6.78 Hz, 0.4H), 3.19–3.24 (m, 1H), 2.96–
3.04 (m, 1H); MS (ESI) m/z 242 (M-1). Anal. Calcd for C10H10FNO3S:
C, 49.37; H, 4.14; N, 5.76. Found: C, 49.56; H, 4.12; N, 5.74.
4.3.1.6. (2RS,4R)-2-(5-Chloro-2-hydroxyphenyl)thiazolidine-4-carbox-
ylic acid (6). Obtained as white solids (0.230 g, 88%). 1H NMR
(DMSO-d6)
d 7.33–7.50 (m, 1H), d 7.10–7.20 (m, 1H), d 6.79–6.85
(m, 1H), 5.80 (s, 0.6H), 5.63 (s, 0.4H), 4.18 (dd, J ¼ 6.03 Hz, 6.03 Hz,
0.6H), 3.87 (dd, J ¼ 6.93 Hz, 6.78 Hz, 0.4H), 3.19–3.25 (m, 1H), 2.97–
3.03 (m, 1H); MS (ESI) m/z 258 (M-1). Anal. Calcd. for C10H10ClNO3S:
C, 46.25; H, 3.88; N, 5.39. Found: C, 46.43; H, 3.86; N, 5.41.
measured with a microplate reader at 570 nm5. The observed IC50
are presented in Tables 4 and 5.
s
4.3.1.7. (2RS,4R)-2-(5-Bromo-2-hydroxyphenyl)thiazolidine-4-carbox-
4.3. Chemistry syntheses
ylic acid (7). Obtained as white solids (0.278 g, 91%). 1H NMR
(DMSO-d6) d 7.44–7.60 (m, 1H), d 7.20–7.30 (m, 1H), d 6.72–6.78 (m,
All the reagents and solvents used were of analytical reagent
grade or were purified by standard methods before use. Melting
points (uncorrected) were determined on an XT4 MP apparatus
(Taike Corp., Beijing, China). ESI mass spectra were obtained on
a Mariner System 5304 mass spectrometer, and 1H NMR spectra
were recorded at DPX300 in DMSO-d6 on a Bruker spectrometer.
Elemental analyses were performed on a CHN-O-Rapid instrument
and were within ꢃ 0.4% of the theoretical values.
1H), 5.80 (s, 0.6H), 5.63 (s, 0.4H), 4.16 (dd, J ¼ 6.03 Hz, 6.21 Hz, 0.6H),
3.85 (dd, J ¼ 6.96 Hz, 6.78 Hz, 0.4H), 3.18–3.24 (m, 1H), 2.96–3.02
(m, 1H); MS (ESI) m/z 302 (M-1). Anal. Calcd. for C10H10BrNO3S: C,
39.49; H, 3.31; N, 4.61. Found: C, 39.65; H, 3.33; N, 4.59.
4.3.1.8. (2RS,4R)-2-(3,5-Dibromo-2-hydroxyphenyl)thiazolidine-4-
carboxylic acid (8). Obtained as white solids (0.623 g, 75%). 1H NMR
(DMSO-d6)
d 7.41–7.68 (m, 2H), 5.88 (s, 0.6H), 5.70 (s, 0.4H), 4.12–
4.17 (m, 0.4H), 3.96–4.05 (m, 0.6H), 2.94–3.03 (m, 1H), 2.73–2.82
(m, 1H); MS (ESI) m/z 380 (M-1). Anal. Calcd. for C10H9Br2NO3S: C,
31.35; H, 2.37; N, 3.66. Found: C, 31.48; H, 2.38; N, 3.64.
4.3.1. General procedure for the preparation of TCA derivatives 1–9
A mixture of L-cysteine (0.121 g, 1.0 mmol) and appropriate
aldehyde (1.0 mmol) in ethanol (25 mL) was stirred at room
temperature for 8 h, and the solid separated was collected, washed
with diethyl ether and dried to obtain 1–9.
4.3.1.9. (2RS,4R)-2-(3,5-Di-tert-butyl-4-hydroxyphenyl)thiazolidine-
4-carboxylic acid (9). Obtained as white solids (0.273 g, 81%). 1H
NMR (DMSO-d6)
d 6.93–7.25 (m, 2H), 5.51 (s, 0.6H), 5.40 (s, 0.4H),
4.3.1.1. (2RS,4R)-2-Phenylthiazolidine-4-carboxylic acid (1). Obtained
4.29 (dd, J ¼ 3.90 Hz, 3.75 Hz, 0.5H), 3.84 (dd, J ¼ 7.53 Hz, 8.16 Hz,
0.5H), 3.14–3.19 (m, 1H), 2.73–2.92 (m, 1H), 1.41 (s, 18H); MS (ESI)
m/z 336 (M-1). Anal. Calcd. for C18H27NO3S: C, 64.06; H, 8.06; N,
4.15. Found: C, 64.27; H, 8.09; N, 4.13.
as white solids (0.196 g, 94%). 1H NMR (DMSO-d6)
d 7.24–7.53
(m, 5H), 5.67 (s, 0.6H), 5.50 (s, 0.4H), 4.22 (dd, J ¼ 6.9, 4.5 Hz, 0.6H),
3.90 (dd, J ¼ 8.7, 7.2 Hz, 0.4H), 3.27–3.40 (m, 1H), 3.04–3.16 (m, 1H);

3908
Z.-C. Song et al. / European Journal of Medicinal Chemistry 44 (2009) 3903–3908
4.3.2. General procedure for the preparation of TBTCA derivatives
10–18
(s, 4.5H); MS (ESI) m/z 402 (M-1). Anal. Calcd. for C15H18BrNO5S: C,
44.56; H, 4.49; N, 3.46. Found: C, 44.38; H, 4.47; N, 3.49.
A mixture of TCA (1.0 mmol) and appropriate NaOH (10%,
1.0 mmol) in dioxane (25 mL) was stirred at ice-water temperature
for 2 h. BOC2O (1.0 mmol) was added and stirred at ice-water
temperature for 1 h and then room temperature for 5 h. Most of the
solvent was extracted and appropriate amount of water was added
to adjust to neutral pH values. Ethyl acetate was added and extracted
(50 mL ꢂ 3), and washed with appropriate saturated aqueous solu-
tion of common salt, and dried with anhydrous magnesium
sulphate. Solvent was extracted to dry to obtain whiter solids 10–18.
4.3.2.8. (2RS,4R)-3-(tert-Butoxycarbonyl)-2-(3,5-dibromo-4-hydrox-
yphenyl)thiazolidine-4-carboxylic acid (17). Obtained as white
solids (0.429 g, 89%). 1H NMR (DMSO-d6)
d 7.38–7.51 (m, 2H), 5.72–
5.88 (m, 1H), 4.01–4.15 (m, 1H), 2.85–3.10 (m, 1H), 2.65–2.79 (m,
1H); 1.27 (s, 4.5H), 1.10 (s, 4.5H); MS (ESI) m/z 480 (M-1). Anal.
Calcd. for C15H17Br2NO5S: C, 37.29; H, 3.55; N, 2.90. Found: C, 37.02;
H, 3.57; N, 2.87.
4.3.2.9. (2RS,4R)-3-(tert-Butoxycarbonyl)-2-(3,5-di-tert-butyl-4-hy-
droxyphenyl)thiazolidine-4-carboxylic acid (18). Obtained as white
4.3.2.1. (2RS,4R)-3-(tert-Butoxycarbonyl)-2-phenylthiazolidine-4-
carboxylic acid (10). Obtained as white solids (0.283 g, 92%). 1H
solids (0.381 g, 87%). 1H NMR (DMSO-d6)
d 6.85–7.52 (m, 2H), 5.41–
NMR (DMSO-d6)
d
7.64–7.66 (m, 2H),
d
7.26–7.35 (m, 3H), 6.13 (s,
5.56 (m, 1H), 3.97–4.26 (m, 2H), 3.07–3.25 (m, 1H), 2.58–2.79 (m,
1H), 1.32 (s, 18H); 1.25 (s, 4.5H), 1.19 (s, 4.5H); MS (ESI) m/z 436 (M-
1). Anal. Calcd. for C23H35NO5S: C, 63.13; H, 8.06; N, 3.20. Found: C,
63.41; H, 8.04; N, 3.23.
0.5H), 5.97 (s, 0.5H), 4.73 (s, 0.5H), 4.59 (s, 0.5H), 3.44–3.50 (m, 1H),
2.09–2.21 (m, 1H), 1.35 (s, 4.5H), 1.12 (s, 4.5H); MS (ESI) m/z 308 (M-
1). Anal. Calcd. for C15H19NO4S: C, 58.23; H, 6.19; N, 4.53. Found: C,
58.05; H, 6.15; N, 4.57.
Acknowledgements
4.3.2.2. (2RS,4R)-3-(tert-Butoxycarbonyl)-2-(4-hydroxyphenyl)th-
iazolidine-4-carboxylic acid (11). Obtained as white solids (0.301 g,
The work was financed by grants (Project 30772627) from
National Natural Science Foundation of China.
86%). 1H NMR (DMSO-d6)
d 9.36 (s, 1H) 7.43 (d, 2H), 6.70 (d, 2H),
5.87–6.02 (m, 1H), 4.56–4.69 (m, 1H), 3.44 (dd, J ¼ 6.90 Hz, 6.90 Hz,
1H), 3.12–3.17 (m, 1H), 1.39 (s, 4.5H), 1.15 (s, 4.5H); MS (ESI) m/z 324
(M-1). Anal. Calcd. for C15H19NO5S: C, 55.37; H, 5.89; N, 4.30. Found:
C, 55.62; H, 6.85; N, 4.32.
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4.90–4.92 (m, 1H), 3.31–3.37 (m, 2H), 1.25 (s, 9H); MS (ESI) m/z 338
(M-1). Anal. Calcd. for C15H18BrNO4S: C, 46.40; H, 4.67; N, 3.61.
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nyl)thiazolidine-4-carboxylic acid (14). Obtained as white solids
(0.319 g, 93%). 1H NMR (DMSO-d6)
d 9.83–9.87 (b, 1H), 7.75–7.79
(m, 1H), 6.85–6.92 (m, 1H), 6.73–6.77 (m, 1H), 6.09 (s,1H), 4.48–4.59
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d 10.11 (b, 1H), 7.96 (s, 1H), 7.12
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(dd, J ¼ 2.7 Hz, 2.4 Hz,1H), 6.80 (s, 1H), 4.50–4.63 (m, 1H), 3.41–3.47
(m, 1H), 2.01–2.13 (m, 1H), 1.37 (s, 4.5H); 1.16 (s, 4.5H); MS (ESI) m/z
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nyl)thiazolidine-4-carboxylic acid (16). Obtained as white solids
(0.370 g, 92%). 1H NMR (DMSO-d6)
d 7.95–8.09 (m, 1H), 7.21 (dd,
J ¼ 2.70 Hz,2.40 Hz,1H), 6.70(s,1H), 6.02–6.05(m,1H), 4.46–4.54(m,
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