Design, synthesis and biological evaluation of benzimidazole/benzothiazole and benzoxazole derivatives as cyclooxygenase inhibitors

Article abstract of DOI:10.1016/S0960-894X(02)01006-5

We have synthesised a series of 2-[[2-alkoxy-6-pentadecylphenyl)methyl]thio]-1H-benzimidazoles/benzothiazoles and benzoxazoles from anacardic acid and investigated their ability to inhibit human cyclooxygenase-2 enzyme (COX-2). The active compounds were s

Full text of DOI:10.1016/S0960-894X(02)01006-5

Bioorganic & Medicinal Chemistry Letters 13 (2003) 657–660
Design, Synthesis and Biological Evaluation of
Benzimidazole/Benzothiazole and Benzoxazole
Derivatives as Cyclooxygenase Inhibitors
R. Paramashivappa, P. Phani Kumar, P. V. Subba Rao and A. Srinivasa Rao*
Vittal Mallya Scientific Research Foundation, PO Box #406, K. R. Road, Bangalore 560 004, India
Received 11 October 2002; accepted 19 November 2002
Abstract—We have synthesised a series of 2-[[2-alkoxy-6-pentadecylphenyl)methyl]thio]-1H-benzimidazoles/benzothiazoles and
benzoxazoles from anacardic acid and investigated their ability to inhibit human cyclooxygenase-2 enzyme (COX-2). The active
compounds were screened for cyclooxygenase-1 (COX-1) inhibition. Compound 13 is 384-fold and 19 is more than 470-fold selec-
tive towards COX-2 compared to COX-1. Thus, this class of compounds may serve as excellent candidates for selective COX-2
inhibition.
# 2003 Elsevier Science Ltd. All rights reserved.
Cyclooxygenase (COX) is the key enzyme which cata-
lyses the conversion of arachidonic acid to pros-
taglandins and thromboxanes.1,2 There are two types of
cyclooxygenase enzymes, COX-1 and COX-2. COX-1 is
a constitutive enzyme, produced in many tissues such as
the kidney and the gastrointestinal tract, while COX-2 is
inducible and is expressed during inflammation at a site
of injury.3ꢀ5 Prostaglandins made by COX-1 enzyme are
protective prostaglandins, the presence of which leads to
normal renal function in the kidneys,6 whereas, pros-
taglandins made by COX-2 cause inflammation.7 Cur-
of anacardic acid has stimulated many researchers to
derive drug analogues for different application.13,14
Recently, we reported a novel method for isolation of
anacardic acid from cashew nut shell liquid (CNSL).15
More recently, we reported dialkyl 1,4-dihydro(20-
alkoxy-60 -pentadecylphenyl)-2,6-dimethyl-3,5-pyridine
dicarboxylates as selective T-type calcium channel
blockers.16 More over, Ringbom et al. reported long
chain fatty acids are potent and selective cyclooxygen-
ase inhibitors.17 This discovery made us to turn our
interest towards synthesis of new class of cyclooxygen-
ase inhibitors from anacardic acid. Hence, we have
designed a new series of tricyclic compounds, which
belongs to group of benzimidazole, benzoxazole and
benzothiazole derivatives, which are known to possess
anti-inflammatory activity.18ꢀ21 Here, we report synth-
esis and biological evaluation of above said compounds
from anacardic acid.
rently
available
NSAIDs
(Nonsteroidal
anti-
inflammatory drugs) inhibit both COX-1 and COX-2
enzymes.8 Inhibition of COX-1 reduces the basal pro-
duction of cytoprotective PGE2 and PGI2 and hence
causes ulceration. Therefore complete inhibition of
COX-1 is not preferred and drugs that inhibit the COX-
2 enzyme are better anti-inflammatory agents.
Anacardic acid (pentadecyl salicylic acid) is a phenolic
constituent present in cashew (Anacardium occidentale
L.) nut shells and is reported to exhibit variety of biolo-
gical activities.9 It is known to inhibit medicinally
important enzymes such as, prostaglandinsynthase.10
tyrosinase11 and 5-lipoxygenase.12 The biological activity
Title compounds (8–22) were synthesised from anacar-
dic acid as shown in Scheme 1. Saturated anacardic acid
1 was obtained by hydrogenation of the ene mixture of
anacardic acid.15 This was converted to dialkylated
compound by reacting with dimethyl sulfate/diethyl
sulfate in acetone. Dialkylated anacardic acid was
reduced to corresponding alcohol by treatment with
lithium aluminium hydride in tetrahydrofuran22 and
then converted to chloro compound by reacting with
*Corresponding author. Fax: +91-80-661-2806; e-mail: alapati61@
hotmail.com
0960-894X/03/$ - see front matter # 2003 Elsevier Science Ltd. All rights reserved.
PII: S0960-894X(02)01006-5

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R. Paramashivappa et al. / Bioorg. Med. Chem. Lett. 13 (2003) 657–660
Scheme 1. Synthesis of cyclooxygenase inhibitor from anacardic acid. Reagents and conditions: (a) (R)sSO4 (R=Me/Et), K2CO3, acetone; (b)
LiAlH4, THF; (c) SOCl2, dichloromethane, 20% NaOH, (C4H9)4NBr, substituted 2-mercapto benzimidazole; (d) SOCl2, dichloromethane, 20%
NaOH, (C4H9)4NBr, 2-mercapto benzothiazole; (e) SOCl2, dichloromethane, 20% NaOH, (C4H9)4NBr, 2-mercapto benzoxazole.
thionyl chloride in dichloromethane. Resultant chloro
compound was condensed with substituted 2-mercapto-
benzimidazole/benzoxazole/benzothiazoles in dichloro-
methane solution containing 20% aqueous sodium
hydroxide and tetrabutyl ammonium bromide as phase
transfer catalyst.23 Obtained compounds were recrys-
tallised in ethanol to yield title compounds. All the
compounds were characterised by IR, 1H NMR, 13C
NMR and mass spectroscopy.
free phenolic function on the phenyl ring and methoxy
group on benzimidazole exhibited highest activity
(IC50=1mM) among tested compounds. Remaining
compounds are less active with an IC50 more than 10 mM.
The compound which shown IC50 less than 10 mM con-
centration were tested for COX-1 inhibition. Interest-
ingly two compounds, namely 13 and 19 shown good
activity with high selectivity towards COX-2 inhibition
when compared to rest of the compounds. Compound
13 is 384 times more selective towards COX-2 when
compared to COX-1 (COX-1 IC50=384 mM; COX-2
IC50=1 mM). Surprisingly compound 19 is 470 times
more selective towards COX-2 inhibition than COX-1
(COX-1 IC50=>500; COX-2 IC50=1.06 mM), interest-
ingly 100 fold more selective than 13. However they are
more selective and less potent than rofecoxib in human
whole blood assay. Although compounds 11, 12, 14, 15
and 21 possess good selectivity, they have shown mod-
erate activity towards COX-2. In conclusion, these
classes of compounds may serve as excellent candidates
for selective COX-2 inhibition (Table 1).
The synthesized compounds were tested for their ability
to inhibit human cyclooxgenase-2 (COX-2) enzyme and
the more active compounds were tested for cyclooxgen-
ase-1 (COX-1) inhibition in human whole blood
assay.24,25 Rofecoxib was used as active control in
cyclooxygenase inhibition assay.
In the case of the methoxy series (R=Me) compound 11
having a methyl group and compound 12 having nitro
group on 5-position of benzimidazole ring showed
moderate activity towards COX-2 inhibition. Com-
pound 19 possessing benzothiazole in place of benzimi-
dazole exhibited more inhibition (IC50=1.06 mM) when
compare to 11. In the case of 21, which bears benzox-
azole ring exhibited 2.5-fold less inhibition compared to
11. In the case of ethoxy series, compound 14 had no
substitution on benzimidazole showed 1.8-fold more
inhibition with reference to 11. Compound 15 bearing
methoxy substituent on benzimidazole ring exhibited
very weak inhibition (IC50=6.25 mM). Compound 18
having nitro group on the benzimidazole same as 12
exhibited similar inhibition. Compound 13 possessing
Preparation of 5-(methoxy)-2-[(2-hydrox-6-pentadecyl-
phenyl)-methyl]-thio]-1H-benzimidazole 13. 2-Hydroxy-
6-pentadecylbenzyl alcohol 6 was prepared by the mod-
ified procedure.22 To a solution of compound 6 (1.0 g,
2.9 mmol) in dichloromethane (25 mL) thionyl chloride
(0.51 g, 4.3 mmol) was added slowly at 15–20 ꢁC under
stirring. After the addition was complete, the solution
was heated to 30–35 ꢁC for 2 h, cooled to 10 ꢁC and
distilled water (0.5 mL) was added to decompose excess

R. Paramashivappa et al. / Bioorg. Med. Chem. Lett. 13 (2003) 657–660
Table 1. Inhibitory effect on COX-2 and COX-1 activity in human whole blood assay
659
Compd
R
R2
X
COX-2a IC50 mM
COX-1b IC50 mM
COX-1/COX-2
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
CH3
CH3
CH3
CH3
CH3
H
CH2CH3
CH2CH3
CH2CH3
CH2CH3
CH2CH3
CH3
CH2CH3
CH3
CH3CH2
—
H
OCH3
OCHF2
CH3
NO2
OCH3
H
OCH3
OCHF2
CH3
NO2
H
N
H
>10
>10
>10
2.63
2.27
1
1.47
6.25
>10
>10
3.84
1.06
>10
2.77
>10
0.057
nt*
nt*
nt*
>500
>500
384
>500
>500
nt*
—
—
—
>190
>220
384
>340
>80
—
NH
NH
NH
NH
NH
NH
NH
NH
NH
NH
S
nt*
—
>500
>500
nt*
>500
nt*
11.4
>130
>470
—
H
H
H
—
S
O
O
—
>220
Rofecoxib
200
aCOX-2 activity was evaluated in human whole blood as LPS induced PGE2 generation.
bCOX-1 activity was measured in Human whole blood as TXB2 generation. IC50 values were estimated from dose–response curve analysed by
nonlinear regression using GraphPad software and values are average of three determinations, nt* samples those have IC50>10 mM for COX-2
inhibition are not tested for COX-1 inhibition.
thionyl chloride. To the resultant chloro compound, 2-
mercapto-5-methoxybenzimidazole (0.59 g, 3.2 mmol),
and tetrabutyl ammonium bromide (0.1 g, 0.31 mmol)
were added. The pH of the solution was adjusted to 10.5
using 20% sodium hydroxide solution. The reaction
mixture was stirred at room temperature for 5 h. The
dichloromethane layer was separated, and washed with
distilled water. The organic layer was dried over anhy-
drous sodium sulfate, concentrated under reduced pres-
sure to yield a residue of 2.5 g. This product was purified
on silica gel column by eluting with mixture of hexane/
ethylacetate (9:1) to yield the title compound, which was
further recrystallised in ethyl alcohol to give colourless
solid (0.50 g, 32%), mp: 122–123 ꢁC; IR (KBr): 3300,
2910, 2850, 2798, 1584, 1450, 1402, 1359, 1262, 1065,
J=7.9 Hz), 7.88 (1H, d, ArH, J=7.4); mass: 497 (M+),
464, 331, 280, 180, 161, 135 (BP), 108, 105, 91, 69, 55;
Anal: C 72.38%, H 8.70%, N2.81%, calcd for
C30H43NOS2, C 72.76%, H 8.75%, N2.78%.
Acknowledgements
We thank Dr. C. S. Ramadoss and Dr. M. Srinivas for
helpful discussions. P. Phani Kumar thanks the Council
of Scientific and Industrial Research (India) for a senior
research fellowship.
1
988, 745 cmꢀ1; H NMR (200 MHz, CDCl3): d 0.84–
References and Notes
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Hz), 3.80 (3H, s, OMe), 4.65 (2H, s, CH2S), 6.60–6.8
(3H, m, ArH), 6.9 (1H, s, ArH), 7.1 (1H, t, ArH).
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