Synthesis of thio-heterocyclic analogues from Baylis-Hillman bromides as potent cyclooxygenase-2 inhibitors

Article abstract of DOI:10.1016/j.bmcl.2014.02.073

A series of thio-substituted pyrimidine, benzoxazole, benzothiazole and triazole analogues were synthesized from Baylis-Hillman bromides in a clean and efficient way. The synthesized twenty new compounds were subjected to in vitro COX-1 and COX-2 inhibito

Full text of DOI:10.1016/j.bmcl.2014.02.073

Bioorganic & Medicinal Chemistry Letters 24 (2014) 1952–1957
Contents lists available at ScienceDirect
Bioorganic & Medicinal Chemistry Letters
journal homepage: www.elsevier.com/locate/bmcl
Synthesis of thio-heterocyclic analogues from Baylis–Hillman
bromides as potent cyclooxygenase-2 inhibitors
Amlipur Santhoshi a, Budde Mahendar a, Saidulu Mattapally c, Partha Sarathi Sadhu a,
Sanjay Kumar Banerjee c, , Vaidya Jayathirtha Rao a,b,
⇑
⇑
a Crop Protection Chemicals Division, CSIR-Indian Institute of Chemical Technology, Uppal Road, Tarnaka, Hyderabad 500007, India
b AcSIR-IICT, CSIR-Indian Institute of Chemical Technology, Uppal Road, Tarnaka, Hyderabad 500007, India
c Medicinal Chemistry and Pharmacology Division, CSIR-Indian Institute of Chemical Technology, Uppal Road, Tarnaka, Hyderabad 500007, India
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of thio-substituted pyrimidine, benzoxazole, benzothiazole and triazole analogues were synthe-
sized from Baylis–Hillman bromides in a clean and efficient way. The synthesized twenty new com-
pounds were subjected to in vitro COX-1 and COX-2 inhibitory activity. Majority of compounds found
to be highly selective COX-2 inhibitor. Seven compounds (16e, 16f, 16k, 16l, 16m, 16r and 16s) displayed
anti-inflammatory activity at micromolar concentrations with IC50 values for COX-2 inhibition ranging
Received 2 October 2013
Revised 20 February 2014
Accepted 28 February 2014
Available online 12 March 2014
from 2.93 to 5.34 lM compared to reference drug whose IC50 is 2.66 lM. All these seven compounds
had very little COX-1 inhibition property and thus are suitable candidates for anti-inflammatory drugs
with less gastrointestinal side effect.
Keywords:
Baylis–Hillman bromide
Thiopyrimidine
Thiobenzoxazole
Thiobenzothiazole
Thiobenzotriazole
Cyclooxygenase
Anti-inflammatory
Ó 2014 Elsevier Ltd. All rights reserved.
Non-steroidal anti-inflammatory drugs (NSAIDS) were pro-
foundly used in the treatment of pain, edema associated with
inflammation and different types of arthritis disorders.1,2 The main
target of these drugs is to inhibit cyclooxygenase (COX) enzyme
which catalyses oxygenation of arachidonic acid (AA) to prosta-
glandins (PGs), prostacyclin (PGI2) and thromboxanes A2 (TXA2)
which are responsible for inflammation.3,4 Cyclooxygenase exists
in mammalian tissues in two isoforms, namely, COX-1 and COX-2.
COX-1 is housekeeping enzyme and expressed constitutively in
many tissues, and plays a physiological role in the production of
prostaglandins which are involved in cytoprotection of gastroin-
testinal tract (lowering the acid secretions), renal blood flow regu-
lation and platelet aggregation. COX-2 is present only in kidney,
brain, endothelial cells and reproductive tissues, and its expres-
sions are induced by various kinds of inflammatory mediators such
physiological processes like inflammation, alzheimer’s disease,
heart failure, hypertension and carcinogenesis.6–8 Therefore, the
main focus is on the inhibition of COX-2 isoform selectively.
NSAIDS such as Indomethacin, Ibuprofen, Diclofenac and Aspi-
rin are nonselective inhibitors of both COX-1 and COX-2. But, these
drugs show adverse effects like damaging the gastrointestinal
tissues and leads to ulcers.9 Rofecoxib (vioxx),10 Valdecoxib
(Bextra),11 and Celecoxib (celebrex)12 drugs are well known selec-
tive COX-2 inhibitors (Fig. 1) in market, but are associated with risk
of cardiovascular symptoms. So there is a need to develop new
molecules which are completely devoid of side effects and are
capable of being selective COX-2 inhibitors without affecting
COX-1 isoform. Very few molecules have been reported to be free
of side effects. DUP69, selective COX-2 inhibitors virtually devoid
of gastrointestinal toxicity and renal damage.13 SC58152 is also
a selective COX-2 inhibitor and has no effect on PG production
in stomach and do not cause gastric toxicity.14 NS-398,15
L-745,33716 and SC-5766617 are selective COX-2 inhibitors
possessing anti-inflammatory activity with very little or no gastric
side effects.
as proinflammatory cytokines (IL-1b, TNFa), mitogens and
oncogens, fibroblast growth factors, luteinizing hormones and
hemostasis disorders, which results in increasing the production
of pain-mediating prostaglandins in neoplastic and inflamed
tissues.5 The excess production of PGs influences a variety of
Heterocyclic compounds comprising of pyrimidine, benzoxazole,
benzothiazole and triazole rings have received special attention due
to their broad range of biological activities. Pyrimidine core with two
nitrogen atoms help to control the lipophilicity of molecule and act
⇑
Corresponding authors. Tel.: +91 40 27193174; fax: +91 40 27160757 (V.J.R.).
E-mail addresses: skbanerjee@iict.res.in (S.K. Banerjee), jrao@iict.res.in
(V. Jayathirtha Rao).
http://dx.doi.org/10.1016/j.bmcl.2014.02.073
0960-894X/Ó 2014 Elsevier Ltd. All rights reserved.

A. Santhoshi et al. / Bioorg. Med. Chem. Lett. 24 (2014) 1952–1957
1953
SO2NH2
N
O
O
N
N
CH3
O
H2NO2S
F3C
O
Valdecoxib 2
Rofecoxib 3
Celecoxib 1
O
S
O
O
H3C
H3C
H3C
NHSO2CH3
S
S
O
S
O
N
N
CF3
Br
F
F
S
O
F
F
F
4
6
7
DUP6971
SC-58125 5
SC-57666
L-745,337
Figure 1. Examples of COX-2 selective anti-inflammatory compounds.
OH
COOR
Br
(i)
COOR
(ii)
Ar
EWG
Ar-CHO
+
Ar
14a-k
15a-k
13a-b
Me, Ar
12a-f
15a
R
=
=
p-fluorophenyl; 15b
R
=
Me, Ar
=
p-Chlorophenyl; 15c
R
=
Me,
Ar = p-methoxyphenyl; 15d R = Me, Ar = 2-napthyl; 15e R = Et, Ar = m- Chlorophenyl; 15f R = Et,
Ar = p-bromophenyl; 15g R = Et, Ar = p-methylphenyl; 15h R = Et, Ar = p-isopropylphenyl;
15i
R = Me, Ar = p-trifluoromethylphenyl; 15j R = Et, Ar = m-fluorophenyl; 15k R = Me,
Ar = p-ethylphenyl.
Scheme 1. Reagents and conditions: (i) DABCO (20–50 mol %), neat, rt, 60–90%; (ii) conc H2SO4, HBr, 0 °C–rt, overnight, 80–90%.
as template to bind COX-2 pocket.18 Aurelio et al. have designed and
synthesized 2-(4-methyl sulponyl phenyl)pyrimidine derivatives
and evaluated as highly potent and specific COX-2 inhibitors.19
New quinazolinone-pyrimide, pyrazolopyimidine compounds have
been found as potent COX-2 inhibitor.20 Many pyrimidine deriva-
tives as anti-inflammatory agents have been patented.21 A number
of highly substituted benzoxazole, benzothiazole and triazole com-
pounds have been reported as selective COX-2 inhibitors.22
In order to gain more insight into selective COX-2 inhibitors and
to minimize the risk of unwanted side effects, we have conducted
exploratory research to find new molecules. In the present study,
we have used Baylis–Hillman (BH) chemistry23,24 to synthesize
allyl phenyl substituted thio derivatives of pyrimidine, benzoxaz-
ole, benzothiazole and triazole (Fig. 2). These four series of newly
synthesized compounds were then evaluated for COX-1 and
COX-2 inhibition property. Majority of compounds found to be
selective COX-2 inhibitors (Fig. 3).
Baylis–Hillman adducts 14a–k were prepared by treating
aromatic aldehydes with acrylates and DABCO as catalyst (20–
50 mol %) under solvent-free condition at room temperature in
very good yields (Scheme 1). Thus prepared BH adducts were then
treated with conc H2SO4 and HBr (48% in acetic acid) at 0 °C in
dichloromethane solution to get the corresponding BH bromides
15a–k in good yields having (Z)-configuration.25 These bromides
served as key intermediates in the synthesis of target compounds.
Treatment of these allyl bromides with various thiol compounds
like 4,6-dimethylpyrimidine-2-thiol 8, 2-mercaptobenzoxazole 9,
2-mercaptobenzothiazole 10, 4-methyl-4H-1,2,4-triazole-3-thiol
11 (Fig. 2) in DMF using K2CO3 as a base at room temperature fur-
nished the final products16a–t in good yields (Scheme 2). All the
products synthesized were characterized by spectral techniques
(Table 1).
calorimetric COX inhibitor screening assay. The in vitro activity
results are reported as a percentage of inhibition of the purified
enzymes at different concentrations 100 lM, 10 lM, 500 nM,
100 nM and 10 nM and their IC50 values were calculated from
the concentration–inhibition curves.
Among the all newly synthesized compounds except 16b, 16o
and 16t showed very good degree of inhibition of COX-2 (more
than 70% as against 82% inhibition showed by the standard). More
than 90% of pyrimidine and benzoxazole derivatives displayed very
N
O
N
S
N
SH
SH
HS
N
9
10
2-mercaptobenzothiazole
4,6-dimethylpyrimidine-2-thiol 8 2-mercaptobenzoxazole
N
N
HS
N
4-methyl-4H-1,2,4-triazole-3-thiol 11
Figure 2. Various thiol compounds.
H
ROOC
S
H
ROOC
S
N
N
N
N
Ar
Ar
8, (i)
N
11, (i)
10, (i)
16s-t
16a-h
COOR
Br
Ar
ROOC
H
ROOC
H
9, (i)
S
N
O
S
Ar
15a-k
S
Ar
16q-r
N
16i-p
All the synthesized compounds were then evaluated for their
inhibition ability and selectivity on both COX isoforms using the
Scheme 2. Reagents and conditions: (i) DMF, K2CO3, rt, 78–93%.

1954
A. Santhoshi et al. / Bioorg. Med. Chem. Lett. 24 (2014) 1952–1957
Table 1
Synthesis of thio derivatives
SI no.
BH allyl bromides
COOMe
Thiol compound
Products
Yield (%)
88
H
COOMe
N
S
F
Br
15a
1
8
N
16a
F
COOMe
Br
H
COOMe
N
N
S
Cl
2
8
91
15b
16b
Cl
COOMe
Br
H
COOMe
N
S
O
15c
3
4
5
8
8
8
85
93
80
N
16c
O
COOMe
Br
H
COOMe
N
S
15d
N
16d
COOEt
Br
H
COOEt
N
N
S
15e
16e
Cl
Cl
H
COOEt
Br
COOEt
S
N
N
Br
15f
6
7
8
8
81
85
16f
Br
COOEt
Br
H
COOEt
N
N
S
15g
16g
COOEt
Br
H
COOEt
N
S
8
8
89
N
15h
16h
COOMe
Br
H
COOMe
O
S
16i
F
9
9
9
9
82
91
80
15a
N
F
COOMe
Br
H
COOMe
O
S
Cl
15b
10
11
N
16j
Cl
COOMe
Br
H
COOMe
O
N
S
F3C
15i
16k
F3C
COOMe
Br
H
COOMe
O
N
S
O
15c
12
13
9
9
78
85
16l
O
COOMe
Br
H
COOMe
O
N
S
15d
16m

A. Santhoshi et al. / Bioorg. Med. Chem. Lett. 24 (2014) 1952–1957
1955
Table 1 (continued)
SI no.
BH allyl bromides
Thiol compound
Products
Yield (%)
COOEt
H
COOEt
O
N
S
14
15
Br
9
78
89
15e
Cl
Br
Cl
16n
COOEt
COOEt
Br
H
O
N
S
Br
9
15f
16o
COOEt
H
COOEt
O
S
Br
15h
16
17
9
84
85
N
16p
COOMe
Br
H
COOMe
S
N
S
F3C
15i
10
16q
F3C
COOMe
Br
COOMe
H
S
N
S
18
19
20
10
11
11
87
83
93
15d
16r
F
COOEt
H
COOEt
N
S
Br
N
15j
N
F
16s
COOMe
Br
H
COOMe
N
S
15k
N
N
16t
Figure 3. COX-2 inhibitory activity of thiopyrimidine, thiobenzoxazole, and thiobenzothiazole derivatives (100
lM in DMSO). Standard Drug: Celecoxib (100
lM in DMSO).
good biological profile. Both the benzothiazole derivatives and one
of the triazole compounds are also active (Fig. 3). When IC50 values
were determined, seven compounds (16e, 16f, 16k, 16l, 16m, 16r
and 16s) exhibited potent activity at micromolar concentration
(Table 2). Benzoxazole derivative 16k (IC50 of 2.95
thiazole derivative 16r (IC50 of 2.93 M) are as active as the stan-
dard Celecoxib (IC50 of 2.66 M). Four compounds that is, 16e,
16f, 16k and 16s have halogens (Cl or Br or CF3) on the Ph ring
lM) and benzo-
l
l

1956
A. Santhoshi et al. / Bioorg. Med. Chem. Lett. 24 (2014) 1952–1957
Figure 4. COX-1 inhibitory activity of thiopyrimidine, thiobenzoxazole, and thiobenzothiazole derivatives (100 lM in DMSO). Standard Drug: Celecoxib (100 lM in DMSO).
Table 2
Acknowledgments
Cox-2 inhibitory activity (IC50) of thiobenzoxazole, thiobenzothiazole derivatives
S. No
Compound
IC50 (lM)
The authors thank the Director, CSIR-Indian Institute of Chem-
ical Technology, for encouragement. A.S. and P.S.S. acknowledge
CSIR-New Delhi, B.M. acknowledge UGC-New Delhi and S.M.
acknowledge ICMR-New Delhi for research fellowship. Origin-
CSC-0108 and SMiLE–CSC112 projects are acknowledged for fund-
ing. S.K.B. is thankful to DBT for providing Ramalingaswami
Fellowship.
1
2
3
4
5
6
7
8
16e
16f
16k
16l
16m
16r
16s
5.88
3.17
2.95
4.76
4.34
2.93
3.88
2.66
Celecoxib
Supplementary data
of BH part. Compound 16l has a p-methoxy substituent on Ph ring.
So there may be some interaction like electronic interaction of
these groups with the enzyme. Compounds 16g, 16h, 16p and
16t have alkyl substituents present on Ph ring and are comparably
less active. Presence of napthyl ring lead to excellent biological
profile for compounds 16m and 16r. As the napthyl ring is highly
rigid it might have a better interaction with the enzyme.
Supplementary data (details experimental procedures, spectral
characterization data and anti-inflammatory activity evaluation
procedures) associated with this article can be found, in the online
version, at http://dx.doi.org/10.1016/j.bmcl.2014.02.073.
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