Synthesis and biological evaluation of a series of benzoxazole/ benzothiazole-containing 2,3-dihydrobenzo[b][1,4]dioxine derivatives as potential antidepressants

Article abstract of DOI:10.1016/j.bmcl.2014.02.031

A series of benzoxazole/benzothiazole-2,3-dihydrobenzo[b][1,4]dioxine derivatives (5a-5d and 8a-8j) was synthesized. Compounds were evaluated for binding affinities at the 5-HT_1A and 5-HT_2A receptors. Antidepressant activities of the compounds were screened using the forced swimming test (FST) and the tail suspension test (TST). The results indicated that the compounds exhibited high affinities for the 5-HT_1A and 5-HT_2A receptors and showed a marked antidepressant-like activity. Compound 8g exhibited high affinities for the 5-HT_1A (K_i = 17 nM) and 5-HT_2A (K_i = 0.71 nM) receptors; it also produced a decrease of the immobility time and exhibited potent antidepressant-like effects in the FST and TST in mice.

Full text of DOI:10.1016/j.bmcl.2014.02.031

Bioorganic & Medicinal Chemistry Letters 24 (2014) 1766–1770
Contents lists available at ScienceDirect
Bioorganic & Medicinal Chemistry Letters
journal homepage: www.elsevier.com/locate/bmcl
Synthesis and biological evaluation of a series
of benzoxazole/benzothiazole-containing
2,3-dihydrobenzo[b][1,4]dioxine derivatives
as potential antidepressants
Songlin Wang a, Yin Chen b, Song Zhao b, Xiangqing Xu b, Xin Liu a, Bi-Feng Liu a, Guisen Zhang a,b,
⇑
a Systems Biology Theme, Department of Biomedical Engineering, College of Life Science and Technology, Huazhong University of Science and Technology, 1037 Luoyu Road,
Wuhan 430074, China
b Jiangsu Nhwa Pharmaceutical Co., Ltd, 69 Democratic South Road, Xuzhou 221116, China
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of benzoxazole/benzothiazole-2,3-dihydrobenzo[b][1,4]dioxine derivatives (5a–5d and 8a–8j)
was synthesized. Compounds were evaluated for binding affinities at the 5-HT1A and 5-HT2A receptors.
Antidepressant activities of the compounds were screened using the forced swimming test (FST) and
the tail suspension test (TST). The results indicated that the compounds exhibited high affinities for
the 5-HT1A and 5-HT2A receptors and showed a marked antidepressant-like activity. Compound 8g exhib-
ited high affinities for the 5-HT1A (Ki = 17 nM) and 5-HT2A (Ki = 0.71 nM) receptors; it also produced a
decrease of the immobility time and exhibited potent antidepressant-like effects in the FST and TST in
mice.
Received 27 November 2013
Revised 8 February 2014
Accepted 12 February 2014
Available online 20 February 2014
Keywords:
Antidepressant
Benzoxazole/benzothiazole
2,3-Dihydrobenzo[b][1,4]dioxine
5-HT1A
Ó 2014 Elsevier Ltd. All rights reserved.
5-HT2A
Depression is a severe, recurring, chronic, and potentially life-
threatening illness characterized by loss of interest in pleasurable
activities, feelings of worthlessness, loss of energy, weight gain or
loss, insomnia or hypersomnia, and recurrent thoughts of suicide.
Tricyclic antidepressants and monoamine oxidase inhibitors were
the first generation of antidepressant drugs prescribed for the
treatment of depression.1 Since the introduction of these drugs,
the treatment of depression has been dominated by the mono-
amine hypothesis. Serotonin (5-HT) and norepinephrine are known
as important drug targets in the treatment of depression. Selective
serotonin reuptake inhibitors, noradrenaline reuptake inhibitors,
and serotonin and noradrenaline reuptake inhibitors were devel-
oped as the next generation of antidepressants. Selective serotonin
reuptake inhibitors have achieved great success in treating depres-
sion, showing better safety than tricyclic antidepressants and
monoamine oxidase inhibitors.2,3 However, several limitations
are associated with the use of selective serotonin reuptake inhibi-
tors, including a long onset of action, moderate patient response,
and adverse effects such as sexual dysfunction.4–6 Thus, novel
antidepressants are needed to improve efficacy in treatment-
resistant patients, shorten the onset of action, and reduce adverse
effects.
5-HT1A and 5-HT2A subtypes are important drug targets for the
treatment of depression.7,8 It has been suggested that agents that
interact with the 5-HT1A and 5-HT2A receptors may have a benefi-
cial effect on depression. For example, nefazodone, with potent
binding affinity for the 5-HT2A receptor and moderate binding
affinity for the 5-HT1A receptor, could treat some patients who
did not respond to other antidepressant drugs and reduce the pos-
sibility of sexual dysfunction.9 Adatanserin and flibanserin, inter-
acting with the 5-HT1A and 5-HT2A receptors, showed potent
activity in animal models of depression.10,11 Thus, we wish to
describe the synthesis and behavioral investigation of a series of
novel and potent antidepressant-like activities of compounds,
characterized by potent affinities for the 5-HT1A and 5-HT2A
receptors.
In previous studies, our group evaluated several psychotropic
agents with benzoxazole or benzothiazole moieties that exhibited
moderate binding affinity for the 5-HT1A and 5-HT2A receptors.12
Recently, we reported that a series of derivatives containing
2,3-dihydrobenzo[b][1,4]dioxine moiety and a straight chain ami-
noalkyl indole moiety, which exhibited potent binding affinity for
the 5-HT1A receptor and serotonin transporter, showed potential
⇑
Corresponding author. Tel.: +86 27 8779 2235; fax: +86 27 8779 2170.
E-mail address: gszhang@mail.hust.edu.cn (G. Zhang).
http://dx.doi.org/10.1016/j.bmcl.2014.02.031
0960-894X/Ó 2014 Elsevier Ltd. All rights reserved.

S. Wang et al. / Bioorg. Med. Chem. Lett. 24 (2014) 1766–1770
1767
antidepressant-like activities.13 Aminoalkyl indole moiety has
been used successfully to produce serotonin transporter pharma-
cophore moiety,14 while 2,3-dihydrobenzo[b][1,4]dioxine moiety
have served as 5-HT1A pharmacophore moiety. In the present
study, we describe the synthesis and biological evaluation of a ser-
ies of novel benzoxazole/benzothiazole-containing 2,3-dihydro-
benzo[b][1,4]dioxine derivatives. The general structure of target
compounds I and II is shown in Figure 1. Compounds were evalu-
ated for binding affinities at both receptors in vitro and antidepres-
sant-like activities in vivo. Preliminary results indicated the target
compounds exhibited high binding affinities at the 5-HT1A and 5-
HT2A receptors and produced potent antidepressant-like effects.
The synthesis of target compounds 5a–5d and 8a–8j was carried
out as depicted in Scheme 1 The condensation of methyl 2,3-
dibromo- ester with catechol afforded the corresponding
methyl 3-substituted 1,4-benzodioxan-2-carboxylate 1a–1c.15
2,3-Dihydrobenzo[b][1,4]dioxine-2-carboxylic acid (2) was ob-
tained by hydrolysis of methyl 1,4-benzodioxan-2-carboxylate (1a)
in the presence of lithium hydroxide. 2,3-Dihydrobenzo[b][1,4]
dioxine-2-carboxylic acid (2) was reacted with thionyl chloride to
give 2,3-dihydrobenzo[b][1,4]dioxine-2-carbonyl chloride (3).16
The intermediates 4a–4b were obtained by treating 2,3-dihydro-
benzo[b][1,4]dioxine-2-carbonyl chloride (3) with 2-bromo-ethyla-
mine or 3-chloropropan-1-amine in methylene chloride at 0 °C. The
target compounds 5a–5d were synthesized from 4a–4b, anhydrous
potassium carbonate with 2-(piperidin-4-yl)-benzo[d]oxazole or
2-(piperidin-4-yl)benzo[d]thiazole. The reagent LiAlH4 was used
to reduce methyl 3-substituted 1,4-benzodioxan-2-carboxylate
1a–1c to alcohols 6a–6c.17 The intermediates (7a–7e) were obtained
by treating 6a–6c, sodium hydroxide, benzyltriethylammonium
chloride with dibromoalkyl in a mixture of water and dimethyl sulf-
oxide at 60 °C. The target compounds 8a–8j were synthesized from
Figure 1. Chemical structures of representative antidepressant compounds and title compounds.
Scheme 1. Reagents and conditions: (a) bromine, methylene chloride, room temperature; (b) catechol, anhydrous potassium carbonate, anhydrous acetone, refluxed; (c)
lithium hydroxide hydrate, tetrahydrofuran, water, hydrochloric acid, room temperature; (d) thionyl chloride, refluxed; (e) 2-bromoethanamine or 3-chloropropan-1-amine,
triethylamine, methylene chloride, 0 °C; (f) 2-(piperidin-4-yl)-benzo[d]oxazole or 2-(piperidin-4-yl)benzo[d]thiazole, anhydrous potassium carbonate, acetone, refluxed; (g)
lithium aluminum hydride, anhydrous tetrahydrofuran, 0 °C; (h) 1,3-dibromopropane (1,4-dibromobutane or 1,5-dibromopentane), benzyltriethylammonium chloride,
sodium hydroxide, dimethylsulfoxide, methanol, 60 °C; (i) anhydrous potassium carbonate, anhydrous acetone, refluxed.

1768
S. Wang et al. / Bioorg. Med. Chem. Lett. 24 (2014) 1766–1770
7a–7e, anhydrous potassium carbonate with 2-(piperidin-4-yl)-
benzo[d]oxazole or 2-(piperidin-4-yl)benzo[d]thiazole.
We investigated the length of the linker of the derivatives 8e–
8j. The benzoxazole derivative 8g increased the binding affinities
for both the 5-HT1A (Ki = 17 nM) and 5-HT2A (Ki = 0.71 nM) recep-
tors when the length of the linker extended from three to four car-
bons (8g vs 8e). The benzothiazole derivative 8h increased the
affinity for the 5-HT1A receptor (Ki = 31 nM) with potent affinity
for the 5-HT2A receptor (Ki = 24 nM) when the linker was elongated
from three to four carbons. When tethered by a five-carbon chain,
8i and 8j showed moderate binding affinities for the 5-HT1A recep-
tor (8i, Ki = 106 nM; 8j, Ki = 131 nM). The benzothiazole derivative
8i exhibited high binding for the 5-HT2A receptor (Ki = 0.74 nM).
Unfortunately, the benzothiazole derivative 8j was found to have
decreased affinity for the 5-HT2A receptor (8j vs 8f).
Antidepressant-like activity of the test compounds was
screened using the forced swimming test19,20 and tail suspension
test.21,22 The results are shown in Figure 2. Imipramine was used
as a reference compound for comparison. Compared to the control
group, compounds 5c, 5d, 8e, 8f, 8g, and 8h were found to have po-
tent antidepressant activity in the FST and TST. Compounds 5c, 5d,
8e, 8f, and 8h promoted a decrease in immobility time, and their
effects were similar to those of imipramine. Compared with imip-
ramine, 8g showed more potent efficacy in the FST and TST. Dose-
response studies on the active compound 8g revealed that the
compound had an ED50 of 39 mg/kg in the forced swim test (Fig. 3).
To check for possible occurrence of drug-induced changes in
locomotor activity, which may have contributed to behavior in
the TST and the FST, the influence of the compounds of interest
on locomotor activity was examined. A locomotor activity test
was conducted to distinguish false-positive effects in the FST and
TST.23 The influence of compound 8g on locomotor activity was
examined to check for the possible occurrence of drug-induced
changes. As shown in Table 2, compared to the control group, these
responses were unaffected with 8g (po, 10, 40 and 100 mg/kg)
when locomotor activity was recorded for 10 min.
Compounds were evaluated in vitro to determine their binding
affinities for the 5-HT1A and 5-HT2A receptors. The binding affinity
for the 5-HT1A receptor was determined by investigating the dis-
placement of the binding of [3H]-8-hydroxy-2-(di-n-propylami-
no)tetralin to rat cerebral cortex homogenates, and the affinity
for the 5-HT2A receptor was determined by displacement binding
to rat cerebral cortex homogenates using [3H]ketanserine accord-
ing to previously reported procedures.18 The results are shown in
Table 1. Compounds 5a–5d exhibited moderate binding affinities
for the 5-HT1A receptor (76–94 nM) and the 5-HT2A receptor (87–
113 nM), which was tethered by a carbon chain linked through
an amide bond. The benzoxazole derivatives (5a, 5c) and benzothi-
azole derivatives (5b, 5d) displayed similar affinities for both
receptors. The benzoxazole derivatives (5a, 5c) had little effect
on the binding affinities for both receptors, when tethered by a
two or three-carbon side chain. The benzothiazole derivative 5d
had slightly higher affinity for the 5-HT2A receptor (Ki of 90 nM
for 5d vs 113 nM for 5b), when tethered by a two carbon side
chain.
We investigated the effect of 8a and 8b where they were linked
through an oxygen atom. 8a and 8b exhibited higher affinity for
the 5-HT2A receptor than that of compounds 5c and 5d where they
were tethered to an amide bond. Compounds 8c–8f exhibited sig-
nificant affinities for both the 5-HT1A receptor (51–152 nM) and
the 5-HT2A receptor (4.1–131 nM). The benzoxazole derivatives
8a and 8c exhibited moderate binding affinities for the 5-HT1A
receptor, and 8c with a methyl on the 1,4-dioxan ring increased
the binding affinity for the 5-HT2A receptor (Ki of 8.2 nM for 8c
vs Ki of 30 nM for 8a). The benzothiazole derivatives 8b and 8d dis-
played moderate binding affinities for the 5-HT1A receptor, and 8d
with a methyl on the 1,4-dioxan ring decreased the binding affinity
for the 5-HT2A receptor (Ki of 28 nM for 8b vs Ki of 131 nM for 8d).
With the introduction of a phenyl group on the ring, the benzoxaz-
ole derivative 8e (5-HT1A, Ki = 51 nM; 5-HT2A, Ki = 4.1 nM) had
good affinities at both receptors, and the benzothiazole derivative
8f had increased binding affinity at the 5-HT2A receptor (Ki = 19 -
nM) with moderate affinity for the 5-HT1A receptor (Ki = 134 nM).
Among these derivatives, 8e exhibited high binding affinities at
the 5-HT1A and 5-HT2A receptors.
In summary, benzothiazole/benzoxazole-2,3-dihydrobenzo[b]
[1,4]dioxine derivatives 5a–5d and 8a–8j were synthesized. These
compounds were considered to possess potent to moderate bind-
ing affinities at the 5-HT1A and 5-HT2A receptors, and exhibited
marked antidepressant-like activity in the FST and TST. Compound
8g, with high binding affinities at the 5-HT1A and 5-HT2A receptors,
showed more potent efficacy in the animal tests, and had an ED50
Table 1
Binding affinities for the 5-HT1A and 5-HT2A receptors for 5a–5d and 8a–8j
Compounds
n
R
X
Binding data of compounds, Ki (mean SEM), nM
5-HT1A
5-HT2A
5a
5b
5c
5d
8a
8b
8c
8d
8e
8f
1
1
2
2
1
1
1
1
1
1
2
2
3
3
—
—
—
—
H
H
CH3
CH3
Ph
Ph
Ph
Ph
Ph
Ph
O
S
O
S
O
S
O
S
O
S
O
S
O
S
76
94
84
85
120
92
152 11
103
51
134 24
17
31
5
87
113 11
88
90
30
28
8.2
131
4.1 0.5
19
0.71 0.2
24
0.74 0.2
148 21
7
6
9
5
9
7
4
3
5
5
2
8
7
8
2
8g
8h
8i
3
4
9
6
106
131 16
8j

S. Wang et al. / Bioorg. Med. Chem. Lett. 24 (2014) 1766–1770
1769
Figure 2. The effects of the test compounds and imipramine on immobility in the forced swimming test and tail suspension test in mice. Results are represented as
mean SEM. with n = 10 in each group. Values are significant at ⁄P < 0.05, ⁄⁄P < 0.01 when compared with vehicle group.
pressant. The preliminary results are promising enough to warrant
further detailed studies and antidepressant research around this
scaffold.
Acknowledgments
The authors gratefully acknowledge the financial support from
the Jiangsu Nhwa Pharmaceutical and the Huazhong University of
Science and Technology Analytical and Testing Center.
Supplementary data
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.bmcl.2014.02.
031.
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