Synthesis and biological evaluation of novel benzothiazole-2-thiol derivatives as potential anticancer agents

Article abstract of DOI:10.3390/molecules17043933

A series of novel benzothiazole-2-thiol derivatives were synthesized and their structures determined by ¹H-NMR, ¹³C-NMR and HRMS (ESI). The effects of all compounds on a panel of different types of human cancer cell lines were investigated. Among them, pyridinyl-2-amine linked benzothiazole-2-thiol compounds 7d, 7e, 7f and 7i exhibited potent and broad-spectrum inhibitory activities. Compound 7e displayed the most potent anticancer activity on SKRB-3 (IC₅₀ = 1.2 nM), SW620 (IC₅₀ = 4.3 nM), A549 (IC₅₀ = 44 nM) and HepG2 (IC₅₀ = 48 nM) and was found to induce apoptosis in HepG2 cancer cells.

Full text of DOI:10.3390/molecules17043933

Molecules 2012, 17, 3933-3944; doi:10.3390/molecules17043933
OPEN ACCESS
molecules
ISSN 1420-3049
www.mdpi.com/journal/molecules
Article
Synthesis and Biological Evaluation of Novel
Benzothiazole-2-thiol Derivatives as Potential Anticancer Agents
Xuan-Hong Shi, Zhao Wang, Yong Xia, Ting-Hong Ye, Mei Deng, You-Zhi Xu, Yu-Quan Wei
and Luo-Ting Yu *
State Key Laboratory of Biotherapy, West China Hospital, West China Medical School,
Sichuan University, Chengdu 610041, China
* Author to whom correspondence should be addressed; E-Mail: yuluot@scu.edu.cn;
Tel: +86-28-8550-3817; Fax: +86-28-8516-4060.
Received: 22 February 2012; in revised form: 20 March 2012 / Accepted: 26 March 2012 /
Published: 30 March 2012
Abstract: A series of novel benzothiazole-2-thiol derivatives were synthesized and their
1
structures determined by H-NMR, 13C-NMR and HRMS (ESI). The effects of all
compounds on a panel of different types of human cancer cell lines were investigated.
Among them, pyridinyl-2-amine linked benzothiazole-2-thiol compounds 7d, 7e, 7f and 7i
exhibited potent and broad-spectrum inhibitory activities. Compound 7e displayed the most
potent anticancer activity on SKRB-3 (IC50 = 1.2 nM), SW620 (IC50 = 4.3 nM), A549
(IC50 = 44 nM) and HepG2 (IC50 = 48 nM) and was found to induce apoptosis in HepG2
cancer cells.
Keywords: anticancer; synthesis; apoptosis; benzothiazole-2-thiol derivatives
1. Introduction
A number of benzothiazole derivatives have exhibited interesting biological activities [1–3] and
attracted continuing interest for further molecular exploration as useful anticancer agents [4,5]. Our
preceding studies had found that two benzothiazole-2-thiol compounds (compounds 1 and 2) displayed
good anticancer activities and induced HepG2 cell apoptosis in vitro [6]. In order to develop more
potent tumor growth inhibitors as novel anticancer agents, we designed and synthesized a series of
novel benzothiazole-2-thiol derivatives through incorporation of heterocyclic rings (pyridine, pyrimidine
and thiazole) to benzothiazole-2-thiol derivatives with the activity and safety advantages of the

Molecules 2012, 17
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heterocyclic ring structures [7,8]. The effects of all the novel compounds on a panel of different types
of human cancer cell lines were investigated by the MTT assay and compound 7e was selected to
examine apoptosis on HepG2 cell cells by flow cytometry. As a result, the pyridinyl-2-amine linked
benzothiazole-2-thiol compounds exhibited potent anticancer activities and compound 7e inhibited the
proliferation of HepG2 cell via inducing apoptosis.
2. Results and Discussion
2.1. Chemistry
Twenty novel benzothiazole-2-thiol derivatives linked with heterocyclic rings were designed and
synthesized by the route shown in Scheme 1.
Scheme 1. Synthetic route for 7a–t.
Reagents and conditions: (a) 2-chloroacetyl chloride, CH2Cl2, K2CO3, 4 h, rt, 85%–93%;
(b) 6-aminobenzothiazole-2-thiol (compound 5), THF, Et3N, reflux, 4–6 h; (c) 2-methoxybenzoyl
chloride/2-chloroacetyl chloride/3-chloropropanoyl chloride/2-bromoacetyl bromide, Et3N, THF, rt,
4–12 h, 63%–91%.
Commercially available amines (compounds 3a–j) were first reacted with 2-chloroacetyl chloride in
the presence of potassium carbonate as the base in dichloromethane to give crude compounds 4a–j.
The raw products 4a–j were purified by recrystallization from ethyl acetate/petroleum ether. The
compounds 6a–j were prepared by reacting 4a–j with 6-aminobenzothiazole-2-thiol (compound 5) and
triethylamine (TEA) as the base in tetrahydrofuran (THF) under reflux. Compounds 6a–j respectively
were thus obtained and could be used directly for the next step without further purification.
The reaction mixtures of compounds 6a–j were further reacted with 3-chloropropyl chloride,
2-bromoacetyl bromide, 2-chloroacetyl chloride and 2-methoxybenzoyl chloride in the presence of
triethylamine (TEA), respectively. The precipitates were collected by filtration and washed with
water to yield the crude products (compounds 7a–t). Each compound was purified by column
chromatography on silica gel using petroleum ether/ethyl acetate as eluent. The structures of all
compounds were determined by 1H-NMR, 13C-NMR and HRMS (ESI).

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2.2. Biological Activities
The twenty novel synthesized benzothiazole-2-thiol derivatives were investigated for anticancer
activity in vitro on cancer cell lines by the MTT assay with compounds 1 and 2 as positive controls.
Compounds 1 and 2 showed good activity against human colon adenocarcinoma cell line (SW480),
human cervical cancer cell line (HeLa) and human hepatocellular carcinoma cell line (HepG2) in our
preceding studies. Herein, we selected firstly the three human cell lines to test these compounds and
the results were presented in Table 1.
Table 1. The anti-proliferative activities for compounds 1, 2 and 7a–t.
IC50 (µM) a
Compd. R1
R2
HepG2 SW480 HeLa
1
4-chlorobenzyl
2-methoxyphenyl
chloromethyl
chloroethyl
0.7
5.6
4.0
2
benzyl
1.0
5.2
4.6
7a
7b
7c
7d
7e
7f
benzyl
5.6
15.7
14.9
2.3
8.6
benzyl
bromoethyl
6.6
48.1
3.1
2-pyridinyl
chloromethyl
chloromethyl
chloromethyl
chloromethyl
chloromethyl
chloromethyl
0.7
5-chloro-2-pyridinyl
5-bromo-2-pyridinyl
5-methyl-2-pyridinyl
3-pyridinyl
0.26
0.048
0.091
6.6
0.46
0.68
1.0
0.035
0.02
0.03
12.6
1.9
7g
7h
7i
6.4
2-chloro-4-pyridinyl
5.0
2.9
2-chloro-4-methyl-3-pyridinyl chloromethyl
0.4
2.4
0.4
7j
2-pyrimidinyl
chloromethyl
24.3
13.1
>100
>100
23.5
45.6
>100
>100
23.1
27.2
>100
54.8
7.1
24.3
14.5
>100
0.6
7k
7l
2-thiazolyl
chloromethyl
2-pyridinyl
2-methoxyphenyl
2-methoxyphenyl
2-methoxyphenyl
2-methoxyphenyl
2-methoxyphenyl
2-methoxyphenyl
>100
>100
23.8
31.1
>100
>100
>100
>100
>100
7m
7n
7o
7p
7q
7r
7s
7t
5-chloro-2-pyridinyl
5-bromo-2-pyridinyl
5-methyl-2-pyridinyl
3-pyridinyl
23.5
0.8
>100
>100
34.6
>100
>100
2-chloro-4-pyridinyl
2-chloro-4-methyl-3-pyridinyl 2-methoxyphenyl
2-pyrimidyl
2-thiazolyl
2-methoxyphenyl
2-methoxyphenyl
a Values are means of three experiments.
These novel compounds showed great variation of IC50 values on the three cell lines and
compounds 7d, 7e, 7f and 7i exhibited potent inhibitory activities. To further study the cytotoxic
profile, the potent analogues 7d, 7e, 7f and 7i were selected for further evaluation of inhibitory
activities against other eleven types of human cancer cell lines, including colon cancer cell lines
HCT-116 and SW620, lung cancer cell line A549, prostate cancer cell line PC-3, pancreatic cancer cell

Molecules 2012, 17
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line BxPC-3, breast cancer cell line BT474, epidermoid cancer cell line A431, ovarian cancer cell line
SKOV-3, non-small cell lung cancer cell line H460, breast cancer cell line MDA-MB-468 and SKRB-3.
As shown in Table 2, compounds 7d, 7e, 7f and 7i exhibited potent and broad-spectrum anticancer
activities which were much better than compounds 1 and 2. Among them, compound 7d showed the
most potent antitumor activities against A431 (IC50 = 20 nM) and compound 7e displayed the most
potent anticancer activity on SKRB-3 (IC50 = 1.2 nM), SW620 (IC50 = 4.3 nM), A549 (IC50 = 44 nM)
and HepG2 (IC50 = 48 nM). The antitumor activities of compounds 7d and 7e were about 10–1,000 times
greater than compounds 1 and 2 (SW620, A549, SKRB-3 and HepG2). These results suggested that
pyridin-2-amine linking benzothiazole-2-thiol derivatives have potent and broad-spectrum anti-cancer
activities and be worth being further investigated as candidate of anticancer agent.
Table 2. The anti-proliferative activities for compounds 7d, 7e, 7f, 7i, 1 and 2 against
various cancer cell lines.
IC50 (µM) a
MDA-
Compd.
HCT116 BT474 SW620 H460 PC-3 BXPC-3 A431 A549 SKOV-3 MB-
468
SKRB-3
7d
7e
7f
7i
1
0.8
0.3
0.6
2.2
1.6
1.2
4.3
0.033
3.7
7.9
6.0
6.8
8.6
4.8
6.0
0.1
0.3
0.5
9.3
4.7
6.6
0.02
0.2
0.1
4.8
1.0
4.5
0.2
2.7
1.5
0.9
2.5
6.8
2.4
4.0
0.5
0.6
0.0043 0.3
0.044 0.4
0.0012
1.9
5.3
1.5
6.5
4.1
1.3
4.2
0.3
8.3
2.0
4.4
3.5
5.8
6.0
4.6
5.2
14.6
39.7
10.0
11.3
1.5
>100
7.8
2
8.0
a Values are means of three experiments.
In order to investigate the apoptosis effects of the benzothiazole-2-thiol derivatives, we selected
compound 7e to examine apoptosis effects on HepG2 cells by flow cytometry. Flow cytometric
analysis was performed to measure the apoptotic cells and the cell cycle after propidium iodide (PI)
staining [9]. From Figure 1, the percentages of apoptotic cells were 34.2%, 46.2% and 53.3%,
respectively, with 0.625 µM, 1.25 µM, and 2.5 µM compound 7e treatment for 24 h. The results
indicated that compound 7e inhibited the proliferation of HepG2 cell via inducing apoptosis on a
concentration-dependent manner. The exact and further biological mechanism of compound 7e is
under investigation in our laboratory.

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Figure 1. Compound 7e concentration-dependently induced apoptosis in HepG2 cancer cells.
3. Experimental
3.1. General
The human cancer cell lines were purchased from the American Type Culture Collection (ATCC,
Rockville, MD, USA). Dulbecco’s modified Eagle medium (DMEM) and RPMI 1640 were purchased
from Gibco (Grand Island, NY, USA). Fetal bovine serum (FBS) was purchased from Hyclone
(Logan, UT, USA). Column chromatography was carried out on silica gel (200–300 mesh, Qingdao
Marine Chemical Ltd., Qingdao, China). Thin layer chromatography (TLC) was performed on TLC
silica gel 60 F254 plates. Melting points were measured using a on a Kofler hot stage apparatus
1
and were uncorrected. H-NMR spectroscopy was performed using a Varian Unity Inova-400
spectrometer. The chemical shift values are reported in d units (ppm) relative to internal standard
13
tetramethylsilane (TMS). C-NMR spectra were recorded on a Bruker AV II-600 MHz spectrometer.
Mass spectrometry was carried out on a Waters Q-TOF Premier mass spectrometer. All solvents were
dried and freshly distilled prior to use according to standard procedures. All the chemicals used were
of analytical grade and commercially available. The purity of compound screened in biological assays
was determined to be ≥90% by HPLC analysis with a photodiode array detector (Waters, Milford, MA,
USA). An Atlantis C18 (150 mm × 4.6 mm, i.d. 5 μm) (Waters) was used with a gradient elution of
methanol and HPLC-grade water as mobile phase at a flow rate of 1 mL/min.
3.2. Preparation of Compounds 4a–j
Compounds 4a–j were prepared following the literature procedure [10]. 2-Chloroacetyl chloride
(0.75 mol) was added dropwise to a mixture of 3a–j (0.5 mol) and potassium carbonate (1.0 mol) in
dichloromethane (550 mL) at 0 °C with stirring. After removal of the dichloromethane and vacuum
filtration, the solid was washed with water and dried under vacuum for 12 h at 25–30 °C. The title
compounds 4a–j were purified by recrystallization with petroleum ether/ethyl acetate.
3.3. Preparation of Compounds 6a–j
Compounds 6a–j were synthesized according to a literature method [11] with some modifications.
Briefly, 6-aminobenzothiazole-2-thiol (35.0 mmol) was added to a mixture of 4a–j (38.5 mmol) and

Molecules 2012, 17
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triethylamine (TEA, 70.0 mmol) in tetrahydrofuran (THF, 480 mL) at room temperature with stirring
for 5 h. The title compounds 6a–j were obtained, respectively, and could be used directly for the next
step without further purification.
3.4. General Procedure for Preparing Compounds 7a–t
The reaction mixtures of compounds 6a–j (3 mmol) were further reacted with acyl chloride (3.6 mmol)
in the presence of triethylamine (TEA, 4.5 mmol) using tetrahydrofuran (THF, 30 mL) as solvent. The
completion of the reaction was monitored by TLC and took 4–26 h. The precipitate was collected by
filtration and washed with water to yield the crude product. Compounds 7a–t were purified by column
chromatography on silica gel using petroleum ether/ethyl acetate as eluent.
N-(2-(2-(Benzylamino)-2-oxoethylthio)benzo[d]thiazol-6-yl)-3-chloropropanamide (7a). Yellow powder,
yield 72%, purity 99.8%, mp 177.2–178.1 °C; 1H-NMR (DMSO-d6) δ: 2.87 (t, J = 6.2 Hz, 2H), 3.91 (t,
J = 6.2 Hz, 2H), 4.19 (s, 2H), 4.33 (d, J = 6.0 Hz, 2H), 7.22–7.28 (m, 5H), 7.55 (dd, J = 2.0, 4.4 Hz, 1H),
7.77 (d, J = 8.8 Hz, 1H), 8.40 (d, J = 2.0 Hz, 1H), 8.82 (t, J = 5.8 Hz, 1H), 10.33 (s, 1H);
13C-NMR (DMSO-d6) δ: 36.53, 39.49, 40.76, 42.54, 111.35, 118.49, 121.04, 126.76, 127.10, 128.19,
131.70, 135.45, 135.81, 138.80, 148.68, 148.80, 164.35, 166.43, 168.09; HRMS (ESI) m/z: Calcd. for
C19H18ClN3O2S2 420.0602; Found: 420.0592 (M-H+).
N-Benzyl-2-(6-(2-bromoacetamido)benzo[d]thiazol-2-ylthio)acetamide (7b). Yellow powder, yield
1
75%, purity 98.0%, mp 155.6–159.7 °C; H-NMR (DMSO-d6) δ: 4.08 (s, 2H), 4.20 (s, 2H), 4.33 (d,
J = 6.0 Hz, 2H), 7.22–7.28 (m, 5H), 7.55 (d, J = 8.4 Hz, 1H), 7.79 (d, J = 8.8 Hz, 1H), 8.38 (s, 1H),
8.83 (s, 1H), 10.64 (s, 1H); 13C-NMR (DMSO-d6) δ: 28.72, 39.48, 42.54, 111.56, 118.65, 121.10,
126.86, 127.32, 128.29, 131.86, 135.48, 135.59, 138.94, 148.74, 148.81, 165.23, 167.36, 169.70;
HRMS (ESI) m/z: Calcd. for C18H16BrN3O2S2 449.9940; Found: 449.9929 (M-H+).
2-Chloro-N-(2-(2-oxo-2-(pyridin-2-ylamino)ethylthio)benzo[d]thiazol-6-yl)acetamide (7c). Yellow
powder, yield 70%, purity 92.0%, mp 168.4–172.9 °C; 1H-NMR (DMSO-d6) δ: 4.29 (s, 2H), 4.45 (s, 2H),
7.54–7.58 (m, 2H), 7.78 (d, J = 8.8 Hz, 1H), 8.18–8.19 (m, 2H), 8.37 (d, J = 16.0 Hz, 1H), 8.89
13
(s, 1H), 10.35 (s, 1H), 10.54 (s, 1H); C-NMR (DMSO-d6) δ: 39.46, 43.49, 111.69, 113.46, 118.75,
119.69, 121.12, 135.42, 135.57, 138.32, 148.02, 148.91, 151.59, 164.63, 164.79, 166.20; HRMS (ESI)
m/z: Calcd. for C16H13ClN4O2S2 393.0241; Found: 393.0241 (M-H+).
2-Chloro-N-(2-(2-(5-chloropyridin-2-ylamino)-2-oxoethylthio)benzo[d]thiazol-6-yl)acetamide
(7d).
1
Yellow powder, yield 70%, purity 96.8%, mp 217.2–218.8 °C; H-NMR (DMSO-d6) δ: 4.29 (s, 2H),
4.44 (s, 2H), 7.50–7.54 (m, 1H), 7.77 (d, J = 8.8 Hz, 1H), 8.41 (s, 1H), 7.50–8.41 (m, 3H), 10.54 (s, 1H),
13
11.10 (s, 1H); C-NMR (DMSO-d6) δ: 39.48, 43.54, 111.80, 114.52, 118.77, 121.16, 125.38, 135.49,
135.61, 138.01, 148.79, 148.94, 150.27, 164.62, 165.37, 166.35; HRMS (ESI) m/z: Calcd. for
C16H12Cl2N4O2S2 426.9852; Found: 426.9822 (M-H+).
N-(5-Bromopyridin-2-yl)-2-(6-(2-chloroacetamido)benzo[d]thiazol-2-ylthio)acetamide (7e). Yellow
powder, yield 71%, purity 96.0%, mp 221.3–222.5 °C; 1H-NMR (DMSO-d6) δ: 4.29 (s, 2H), 4.43 (s, 2H),
7.53 (d, J = 8.8 Hz, 1H), 7.76 (d, J = 8.8 Hz, 1H), 8.47 (s, 1H), 8.38 (s, 1H), 8.02 (m, 2H), 10.54 (s, 1H),

Molecules 2012, 17
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13
11.09 (s, 1H); C-NMR (DMSO-d6) δ: 39.98, 44.05, 112.29, 114.26, 115.56, 119.26, 121.66, 135.79,
135.99, 141.22, 149.12, 149.44, 151.07, 165.11, 165.23, 166.88; HRMS (ESI) m/z: Calcd. for
C16H12BrClN4O2S2 472.7864; Found: 472.7853 (M-H+).
2-Chloro-N-(2-(2-(5-methylpyridin-2-ylamino)-2-oxoethylthio)benzo[d]thiazol-6-yl)acetamide
(7f).
1
Yellow powder, yield 90%, purity 97.1%, mp 214.0–214.9 °C; H-NMR (DMSO-d6) δ: 2.25 (s, 3H),
4.29 (s, 2H), 4.41 (s, 2H), 7.53 (dd, J = 2.0, 4.4 Hz, 1H), 7.61 (dd, J = 2.0, 4.2 Hz, 1H), 7.77 (d,
J = 9.2 Hz, 1H), 7.94 (d, J = 8.4 Hz, 1H), 8.18 (s, 1H), 8.37 (d, J = 1.0 Hz, 1H), 10.54 (s, 1H), 10.82
(s, 1H); 13C-NMR (DMSO-d6) δ: 17.22, 39.46, 43.53, 111.75, 113.13, 118.74, 121.14, 128.73, 135.30,
135.47, 139.06,147.22, 148.95, 149.20, 164.70, 164.74, 166.02; HRMS (ESI) m/z: Calcd. for
C17H15ClN4O2S2 407.0398; Found: 407.0397 (M-H+).
2-Chloro-N-(2-(2-oxo-2-(pyridin-3-ylamino)ethylthio)benzo[d]thiazol-6-yl)acetamide (7g). Yellow
powder, yield 68%, purity 92.0%, mp 160.6–165.4 °C; 1H-NMR (DMSO-d6) δ: 4.29 (s, 2H), 4.45 (s, 2H),
7.55 (d, J = 8.0 Hz, 2H), 7.78 (d, J = 8.8 Hz, 1H), 8.19 (d, J = 8.4 Hz, 1H), 8.37 (s, 2H), 8.89 (s, 1H),
13
10.66 (s, 1H), 11.20 (s, 1H); C-NMR (DMSO-d6) δ: 39.43, 43.50, 111.67, 118.52, 118.72, 121.18,
130.46, 135.43, 135.51, 140.30, 148.83, 148.89, 164.36, 164.80, 166.47; HRMS (ESI) m/z: Calcd. for
C16H13ClN4O2S2 393.0241; Found: 393.0247 (M-H+).
2-Chloro-N-(2-(2-(2-chloropyridin-4-ylamino)-2-oxoethylthio)benzo[d]thiazol-6-yl)acetamide
(7h).
1
Yellow powder, yield 91%, purity 95.7%, mp 192.5–197.1 °C; H-NMR (DMSO-d6) δ: 4.29 (s, 2H),
4.43 (s, 2H), 7.47–7.54 (m, 2H), 7.75–7.78 (m, 2H), 8.29 (d, J = 5.6 Hz, 1H), 8.39 (s, 1H), 10.55 (s, 1H),
13
11.08 (s, 1H); C-NMR (DMSO-d6) δ: 39.48, 43.53, 111.81, 112.46, 112.62, 118.76, 121.15, 135.34,
135.49, 147.83, 148.85, 150.47, 151.02, 164.34, 164.75, 166.93; HRMS (ESI) m/z: Calcd. for
C16H12Cl2N4O2S2 426.9852; Found: 426.9883 (M-H+).
2-Chloro-N-(2-(2-(2-chloro-4-methylpyridin-3-ylamino)-2-oxoethylthio)benzo[d]thiazol-6-yl)acetamide (7i).
Yellow powder, yield 80%, purity 98.0%, mp 186.4–188.3 °C; 1H-NMR (DMSO-d6) δ: 2.22 (s, 3H), 4.29
(s, 2H), 4.41 (s, 2H), 7.34 (d, J = 4.8 Hz, 1H), 7.67 (t, J = 8.8 Hz, 2H), 8.19 (d, J = 4.8 Hz, 1H), 8.55
13
(s, 1H), 10.24 (s, 1H), 10.37 (s, 1H); C-NMR (DMSO-d6) δ: 17.81, 39.54, 43.53, 111.87, 118.83,
121.08, 125.10, 130.49, 135.30, 135.56, 147.02, 148.67, 148.77, 148.99, 164.43, 164.76, 165.62;
HRMS (ESI) m/z: Calcd. for C17H14Cl2N4O2S2 441.0008; Found: 440.9992 (M-H+).
2-Chloro-N-(2-(2-oxo-2-(pyrimidin-2-ylamino)ethylthio)benzo[d]thiazol-6-yl)acetamide (7j). Brownish
red powder, yield 63%, purity 90.0%, mp 172.6–174.9 °C; 1H-NMR (DMSO-d6) δ: 4.29 (s, 2H), 4.59
(s, 2H), 7.22 (t, J = 4.8 Hz, 1H), 7.54 (d, J = 4.4 Hz, 1H), 7.78 (d, J = 8.8 Hz, 1H), 8.38 (s, 1H), 8.69
(d, J = 2.4 Hz, 2H), 10.58 (s, 1H), 11.04 (s, 1H); 13C-NMR (DMSO-d6) δ: 39.46, 43.52, 109.98,
117.43, 121.60, 136.22, 147.29, 157.33, 157.66, 158.40, 166.22; HRMS (ESI) m/z: Calcd. for
C15H12ClN5O2S2 394.0194; Found: 394.0109 (M-H+).
2-Chloro-N-(2-(2-oxo-2-(thiazol-2-ylamino)ethylthio)benzo[d]thiazol-6-yl)acetamide (7k). Yellow
1
powder, yield 78%, purity 95.0%, mp 218.3–224.7 °C; H-NMR (DMSO-d6) δ: 4.29 (s, 2H), 4.46 (s,
2H), 7.25 (d, J = 3.6 Hz, 1H), 7.50–7.55 (m, 2H), 7.77 (d, J = 8.8 Hz, 1H), 8.38 (s, 1H), 10.55 (s, 1H),

Molecules 2012, 17
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13
12.53 (s, 1H); C-NMR (DMSO-d6) δ: 39.46, 43.54, 111.81, 113.79, 118.77, 121.18, 135.33, 135.52,
137.71, 148.87, 157.75, 164.32, 164.74, 165.64; HRMS (ESI) m/z: Calcd. for C14H11ClN4O2S3
398.9806; Found: 398.9816 (M-H+).
2-Methoxy-N-(2-(2-oxo-2-(pyridin-2-ylamino)ethylthio)benzo[d]thiazol-6-yl)benzamide (7l). Yellow
powder, yield 70%, purity 92.0%, mp 154.2–158.9 °C; 1H-NMR (DMSO-d6) δ: 3.90 (s, 3H), 4.45 (s, 2H),
7.08 (t, J = 7.4 Hz, 1H), 7.14 (t, J = 6.0 Hz, 1H), 7.19 (d, J = 8.4 Hz, 1H), 7.49–7.54 (m, 1H),
7.63–7.69 (m, 2H), 7.80 (dd, J = 7.0, 6.4 Hz, 2H), 8.35 (d, J = 3.6 Hz, 1H), 8.04 (d, J = 8.4 Hz, 1H),
8.54 (s, 1H), 10.35 (s, 1H), 10.91 (s, 1H); 13C-NMR (DMSO-d6) δ: 39.46, 111.69, 118.75, 121.12,
135.41, 135.96, 148.91, 164.19, 164.57, 166.22; HRMS (ESI) m/z: Calcd. for C22H18N4O3S2 451.0893;
Found: 451.0887 (M-H+).
N-(2-(2-(5-Chloropyridin-2-ylamino)-2-oxoethylthio)benzo[d]thiazol-6-yl)-2-methoxybenzamide (7m).
1
Yellow powder, yield 70%, purity 97.0%, mp 148.5–154.1 °C; H-NMR (DMSO-d6) δ: 3.90 (s, 3H),
4.45 (s, 2H), 7.19 (d, J = 8.4 Hz, 1H), 7.2 (t, J = 7.4 Hz, 2H), 7.63–7.75 (m, 2H), 7.77 (d, J = 8.8 Hz, 1H),
7.92 (d, J = 4.4 Hz, 1H), 8.08 (d, J = 8.4 Hz, 1H), 8.41 (d, J = 2.4 Hz, 1H), 8.54 (s, 1H), 10.35 (s, 1H),
13
11.11 (s, 1H); C-NMR (DMSO-d6) δ: 39.46, 55.84, 111.83, 111.93, 114.52, 119.17, 120.46, 120.99,
124.68, 125.38, 129.66, 132,09, 135.41, 135.95, 137.99, 146.43, 148.73, 150.28, 156.46, 164,17,
164.53, 166.37; HRMS (ESI) m/z: Calcd. for C22H17ClN4O3S2 485.0504; Found: 485.0521 (M-H+).
N-(2-(2-(5-Bromopyridin-2-ylamino)-2-oxoethylthio)benzo[d]thiazol-6-yl)-2-methoxybenzamide (7n).
1
Yellow powder, yield 75%, purity 99.0%, mp 178.4–183.5 °C; H-NMR (DMSO-d6) δ: 3.90 (s, 3H),
4.45 (s, 2H), 7.07 (t, J = 7.4 Hz, 1H), 7.19 (d, J = 8.4 Hz, 1H), 7.51 (t, J = 7.8 Hz, 1H), 7.66 (d,
J = 10.8 Hz, 2H), 7.77 (d, J = 8.8 Hz, 1H), 7.95–8.03 (m, 2H), 8.48 (s, 1H), 8.54 (s, 1H), 10.35 (s, 1H),
13
11.10 (s, 1H); C-NMR (DMSO-d6) δ: 39.49, 55.87, 111.86, 111.95, 113.78, 115.08, 119.19, 120.48,
121.01, 124.69, 129.68, 132.11, 135.41, 135.96, 148.63, 148.74, 150.59, 156.48, 164.17, 164.54,
166.41; HRMS (ESI) m/z: Calcd. for C22H17BrN4O3S2 528.9998; Found: 529.0070 (M-H+).
2-Methoxy-N-(2-(2-(5-methylpyridin-2-ylamino)-2-oxoethylthio)benzo[d]thiazol-6-yl)benzamide (7o).
1
Yellow powder, yield 76%, purity 92.0%, mp 182.7–188.5 °C; H-NMR (DMSO-d6) δ: 2.28 (s, 3H),
3.90 (s, 3H), 4.45 (s, 2H), 7.08 (t, J = 7.6 Hz, 1H), 7.19 (d, J = 8.4 Hz, 1H), 7.50–7.62 (m, 2H), 7.68 (t,
J = 10.2 Hz, 2H), 7.78 (d, J = 8.8 Hz, 1H), 7.95 (s, 1H), 8.18 (s, 1H), 8.52 (d, J = 2.0 Hz, 1H), 10.31
13
(s, 1H), 10.76 (s, 1H); C-NMR (DMSO-d6) δ: 17.24, 39.49, 55.86, 111.84, 111.96, 112.99, 119.18,
120.47, 121.00, 124.71, 128.61, 129.67, 132.10, 135.41, 135.95, 138.57, 147.80, 148.79, 149.49,
156.48, 164.33, 164.55, 165.90; HRMS (ESI) m/z: Calcd. for C23H20N4O3S2 465.1050; Found:
465.1071 (M-H+).
2-Methoxy-N-(2-(2-oxo-2-(pyridin-3-ylamino)ethylthio)benzo[d]thiazol-6-yl)benzamide (7p). Yellow
powder, yield 75%, purity 95.0%, mp 154.8–159.5 °C; 1H-NMR (DMSO-d6) δ: 3.90 (s, 3H), 4.45 (s, 2H),
7.08 (t, J = 7.6 Hz, 1H), 7.19 (d, J = 8.4 Hz, 1H), 7.50–7.62 (m, 2H), 7.55(d, J = 8.0 Hz, 1H), 7.70 (t,
J = 6.6 Hz, 1H), 7.78 (d, J = 8.8 Hz, 1H), 8.29 (d, J = 8.4 Hz, 1H), 8.46 (d, J = 4.4 Hz, 1H), 8.39 (s, 1H),
8.95 (s, 1H), 10.36 (s, 1H), 11.12 (s, 1H); 13C-NMR (DMSO-d6) δ: 39.44, 55.62, 111.90, 112.33,
119.21, 119.95, 120.99, 121.24, 124.69, 126.23, 129.67, 132.97, 135.41, 135.96, 140.75, 144.51,

Molecules 2012, 17
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148.75, 156.47, 158.03, 164.19, 164.57, 167.32; HRMS (ESI) m/z: Calcd. for C22H18N4O3S2 451.0893;
Found: 451.0833 (M-H+).
N-(2-(2-(2-Chloropyridin-4-ylamino)-2-oxoethylthio)benzo[d]thiazol-6-yl)-2-methoxybenzamide (7q).
1
Yellow powder, yield 70%, purity 90.0%, mp 177.9–180.8 °C; H-NMR (DMSO-d6) δ: 3.90 (s, 3H),
4.44 (s, 2H), 7.07 (t, J = 7.4 Hz, 1H), 7.19 (d, J = 8.0 Hz, 1H), 7.47–7.53 (m, 2H), 7.53–7.62 (m, 2H),
7.76 (t, J = 5.8 Hz, 1H), 7.78 (s, 1H), 8.29 (d, J = 5.6 Hz, 1H), 8.55 (s, 1H), 10.35 (s, 1H), 11.06 (s, 1H);
13C-NMR (DMSO-d6) δ: 39.44, 55.84, 111.90, 111.93, 112.47, 112.62, 119.20, 120.46, 120.99,
124.67, 129.65, 132.11, 135.41, 135.97, 148.65, 150.47, 151.03, 156.45, 163.91, 164.57, 166.95;
HRMS (ESI) m/z: Calcd. for C22H17ClN4O3S2 485.0504; Found: 485.0491 (M-H+).
N-(2-(2-(2-Chloro-4-methylpyridin-3-ylamino)-2-oxoethylthio)benzo[d]thiazol-6-yl)-2-methoxybenzamide
(7r). Yellow powder, yield 77%, purity 90.0%, mp 166.5–168.1 °C; 1H-NMR (DMSO-d6) δ: 2.22 (s, 3H),
3.90 (s, 3H), 4.41 (s, 2H), 7.08 (t, J = 7.2 Hz, 1H), 7.19 (d, J = 8.4 Hz, 1H), 7.34 (d, J = 4.8 Hz, 1H),
7.52 (t, J = 7.8 Hz, 1H), 7.64 (d, J = 7.2 Hz, 1H), 7.69 (d, J = 8.8 Hz, 1H), 7.79 (d, J = 8.8 Hz, 1H),
13
8.19 (d, J = 4.8 Hz, 1H), 8.55 (s, 1H), 10.24 (s, 1H), 10.37 (s, 1H); C-NMR (DMSO-d6) δ: 17.84,
39.51, 55.87, 111.91, 111.97, 119.22,120.49, 120.92, 124.76, 129.65, 130.51, 132.11, 135.47, 135.96,
147.05, 148.70, 148.76, 156.48, 164.03, 164.61, 165.67; HRMS (ESI) m/z: Calcd. for C23H19ClN4O3S2
500.0123; Found: 500.0124 (M-H+).
2-Methoxy-N-(2-(2-oxo-2-(pyrimidin-2-ylamino)ethylthio)benzo[d]thiazol-6-yl)benzamide (7s). Yellow
powder, yield 70%, purity 90.0%, mp 169.2–171.8 °C; 1H-NMR (DMSO-d6) δ: 3.90 (s, 3H), 4.46 (s, 2H),
7.08 (t, J = 7.4 Hz, 1H), 7.18–7.24 (m, 2H), 7.52 (t, J = 7.6 Hz, 1H), 7.64–7.69 (m, 2H), 7.78 (d,
J = 8.8 Hz, 1H), 8.54 (s, 1H), 8.69 (d, J = 8.4 Hz, 2H), 10.35 (s, 1H), 11.04 (s, 1H); 13C-NMR
(DMSO-d6) δ: 39.46, 55.86, 111.84, 111.95, 119.17, 120.48, 121.00, 124.67, 129.68, 132.12, 135.38,
135.90, 148.80, 156.48, 157.37, 158.41, 164.45, 164.55, 166.18; HRMS (ESI) m/z: Calcd. for
C21H17N5O3S2 452.0846; Found: 452.0821 (M-H+).
2-Methoxy-N-(2-(2-oxo-2-(thiazol-2-ylamino)ethylthio)benzo[d]thiazol-6-yl)benzamide (7t). Yellow
powder, yield 68%, purity 92.0%, mp 237.7–239.1 °C; 1H-NMR (DMSO-d6) δ: 3.90 (s, 3H), 4.47 (s, 2H),
7.08 (t, J = 7.4 Hz, 1H), 7.19 (d, J = 8.4 Hz, 1H), 7.25 (d, J = 3.6 Hz, 1H), 7.52 (t, J = 6.4 Hz, 2H),
7.64–7.69 (m, 2H), 7.77 (d, J = 8.8 Hz, 1H), 8.54 (s, 1H), 10.36 (s, 1H), 12.55 (s, 1H); 13C-NMR
(DMSO-d6) δ: 39.37, 56.37, 112.39, 112.47, 114.32, 119.71, 120.98, 121.54, 125.24, 130.15, 132.60,
135.95, 136.50, 138.24, 149.19, 156.97, 158.27, 164.38, 165.07, 166.18; HRMS (ESI) m/z: Calcd. for
C20H16N4O3S3 457.0458; Found: 457.0457 (M-H+).
3.5. Cell Culture
Cell lines HepG2, HeLa, HCT116, SW620, SKOV-3 and MDA-MB-468 were maintained in
Dulbecco’s modified Eagle medium (DMEM) containing 10% fetal bovine serum (FBS), penicillin
(100 U/mL) and streptomycin (10 mg/L). Cell lines BT474, A431 and SKRB-3 were maintained in
Dulbecco’s modified Eagle medium (DMEM) containing 20% fetal bovine serum (FBS), penicillin
(100 U/mL) and streptomycin (10 mg/L). Cell lines H460, PC-3, A549, SW480, and BxPC-3 were

Molecules 2012, 17
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maintained in RPMI 1640 containing 10% FBS, penicillin (100 U/mL) and streptomycin (10 mg/L).
Cells were grown in a 5% CO2 incubator at 37 °C.
3.6. Cell Proliferation Assay (MTT Assay)
Cells (3–5 × 103/well) were seeded in 200 µL of medium/well in 96-well plates (Costar Corning,
Rochester, NY, USA) and cultured for 24 h. The compounds dissolved in dimethylsulfoxide (DMSO)
were added to final concentration of 40 μM, 20 μM, 10 μM, 5 μM, 2.5 μM, 1.25 μM, respectively.
Compounds 1 and 2 were used as positive control. After 48 h exposure, a volume of 10 μL of 10 mg/mL
3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) was added per well and incubated
for another 4 h at 37 °C, then the supernatant fluid was removed and DMSO (150 μL) was added
150 μL/well to dissolve formazan crystals for 15–20 min. The light absorptions (OD) were measured
at 570 nm with SpectraMAXM5 microplate spectrophotometer (Molecular Devices). The effect of
compounds on tumor cells viability was expressed by IC50 of each cell line. Values shown are the %
viability vs. ctrl + SD, n = six independent experiments in triplicate.
3.7. Apoptosis Analysis by Flow Cytometry (FCM)
Cells were seeded into 1 mL of medium/well cell in 6-well plates (Costar Corning) culture bottles at
10 × 106 cells 24 h before treatment. Then cells were treated with compound 7e for 24 h: 0 µM
(control), 0.625 µM, 1.25 µM, 2.5 µM. After 24 h incubation, floating and adherent cells were
collected, washed three times with PBS (pH 7.4) and fixed for 24 h with cool alcohol at 4 °C.
1 mL cell suspension (106 /mL) was washed three times with cooled PBS, treated with RNase for
30 min at 37 °C, stained it with PI for 30 min at 37 °C in a dark environment, and taken for flow
cytometry analysis.
4. Conclusions
In conclusion, a series of novel benzothiazole-2-thiol derivatives were synthesized and their
anti-proliferative activities were evaluated in vitro. The results showed that the pyridinyl-2-amine
linked benzothiazole-2-thiol compounds 7d, 7e, 7f and 7i exhibited increased anticancer activities
compared with compounds 1 and 2 on the three human cancer cells (SW480, HeLa and HepG2).
Further studies showed that they displayed potent and broad-spectrum anti-proliferative activities
against other human cancer cell lines. Among these compounds, compound 7e displayed the most
potent anticancer activity on SKRB-3 (IC50 = 1.2 nM), SW620 (IC50 = 4.3 nM), A549 (IC50 = 44 nM)
and HepG2 (IC50 = 48 nM). The results of flow cytometry analysis indicated that compound 7e induce
apoptosis in HepG2 cancer cells on a concentration-dependent manner. These data suggested that
compound 7e may be powerful tumor growth inhibitors via apoptosis as novel anticancer agents and be
worth being further investigated as a potential of anticancer agent.

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Acknowledgements
This work was supported by National Science and Technology Major Project of China
(2009ZX09103-132). The authors gratefully thank Yong-Qiu Mao (State Key Laboratory of
Biotherapy, Huaxi Hospital, Sichuan University) for helpful technical assistance and discussion in the
flow cytometry analysis.
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Sample Availability: Samples of the compounds 7a–t are available from the authors.
© 2012 by the authors; licensee MDPI, Basel, Switzerland. This article is an open access article
distributed under the terms and conditions of the Creative Commons Attribution license
(http://creativecommons.org/licenses/by/3.0/).