Synthesis of benzothiophenones and naphthothiophenone as anticholinesterases

Article abstract of DOI:10.14233/ajchem.2013.14168

Since AChE and BuChE belong to the same serine hydrolazes, we designed and synthesized compounds 1-6 as anticholinesterases and measured their inhibition poteucies on ChEs. 7-Methoxy-3H-benzo[c]thiophen-1-one 4, 4,6-dibromo-5,7- methoxy-3H-benzo[c]thiophe

Full text of DOI:10.14233/ajchem.2013.14168

Asian Journal of Chemistry; Vol. 25, No. 10 (2013), 5325-5327
http://dx.doi.org/10.14233/ajchem.2013.14168
Synthesis of Benzothiophenones and Naphthothiophenone as Anticholinesterases
D.I. JUNG
Department of Chemistry, Dong-A University, Busan 604-714, South Korea
Corresponding author: Fax: +82 51 2007259; Tel: +82 51 2007249; E-mail: dijung@dau.ac.kr
(Received: 7 June 2012;
Accepted: 1 April 2013)
AJC-13197
Since AChE and BuChE belong to the same serine hydrolazes, we designed and synthesized compounds 1-6 as anticholinesterases and
measured their inhibition poteucies on ChEs. 7-Methoxy-3H-benzo[c]thiophen-1-one 4, 4,6-dibromo-5,7-methoxy-3H-benzo[c]thiophe-
1-one 5 and 9-methoxy-3H-naphtho-[2,3-c]thiophen-1-one 6 were synthesized by using ethyl 2-methoxy-6-methylbenzoate, ethyl 3,4-
dibromo-2,4-dimethoxy-6-methylbenzoate and ethyl 1-methoxy-3-methyl-2-naphthoate.
Key Words: Cholinesterase, Inhibition, Benzothiophenone, Naphthothiophenone.
NO2
O
O
O
INTRODUCTION
The physiological role ofAChE is hydrolysis of the neuro-
transmitter, acetylcholine (Ach) the acetate and choline. AChE
inhibitors are used as chemical warfare agents, insecticides
and drugs againstAlzheimer's disease (AD). Numerous evidences
suggest that the neurobiological basis of senile dementia in
Alzheimer's disease and related dementias is a loss of cholin-
ergic neurons and a resultant decline in acetylcholine (Ach)
in brain regions which regulate behavioural and emotional
responses1-5. Well knownAChE inhibitors such as tacrine (THA,
Cognex), E2020 (Donepezil, Aricept), rivastigmine (Exelon)
and galantamine (Raminyl) are approved by FDA as drugs
against Alzheimer's disease6-14. In the healthy brain AChE
predominates (80 %) and BuChE is considered to play a minor
role in regulating vrain Ach levels. In the Alzheimer's disease
brain, BuChE activity rises while AChE acitivity remains
unchanged or declines. Therefore, both enzymes are likely to
have involvement in regulating Ach levels and represent
legitimate therapeutic targets to ameliorate the cholinergic deficit.
Powers et al., showed that serine protease such as elastase and
chymotrypsin is inactivated by isobenzofuranone and
benzopyrandiones15-19. If a compound has a functionality similar
to the thioester of acetylthiocholine and hydrophobic region,
it is expected that the compound would bind to theAChE active
site. Most insecticides are irreversible inhibitors of AChE and
thus specific irreversible inhibitors are also required to develop.
The benzothiophenone compounds have hydrophobic group
and the carbonyl group and expected to inhibit ChEs. Since
AChE and BuChE belong to the same serine hydrolases, we
designed and synthesized compounds 1-6 (Fig. 1) as anticholi-
nestrases and measured their inhibition potencies on ChEs.
S
S
S
MeO
1
2
3
OMe
OMe
OMe
O
O
O
Br
S
S
S
MeO
Br
4
6
5
Fig. 1. Synthesized products
EXPERIMENTAL
Electic eelAChE, type V-S lyophilized powder and horse
serum BuChE were purchased from Sigma Chemical Co and
was used as received. Prior to use they were dissolved in 0.1
M, pH 7.3 sodium phosphate buffer, containing 0.1 M NaCl.
Acetylthiocholine (ATCh), butyrylthiocholine (BuTCh), 5,5'-
dithio-bis(2-nitrobenzoic acid) (DTNB) and buffer components
were also purchased from Sigma Chemical Co. Water used in
experiments was distilled and deionized by passage through a
mixed bead ion-exchange column. NMR spectra were obtained
with a Bruker AC200 spectometer (200 MHz) using
tetramethylsilane or D2O as internal standards.
Synthesis of 3H-benzo[c]thiophen-1-one (1): To a
solution of ethyl o-methylbenzoate (8.21 g. 50 mmol) and
benzoyl peroxide (10 mg, 41.3 mmol) in carbon tetrachloride
(125 mL) was added N-bromosuccinimide (8.90 g, 50 mmol)

5326 Jung
Asian J. Chem.
and refluxed for 6 h. After completion of the reaction the
mixture was filtered and evaporated. The residue in acetone
(50 mL) was mixed with thiourea (4.19 g, 55 mmol) and
refluxed for 5.5 h. After removal of acetone by evaporation,
the obtained thiouronium salt was warmed to 80-90 ºC in 20 %
NaHCO3 solution (50 mL) under nitrogen for 2 h. Then, the
solution was acidified with 20 % HCl solution. The oily residue
was subjected to silica gel column chromatography (eluent;
hexane:ethyl acetate = 10:1, v/v) to give 1 (4.81 g, 64 %):
m.p. 58-60 ºC (Ref.17 m.p. 58-60 ºC): IR (KBr, nmax, cm-1) 3030,
1686, 1595, 1237; 1H NMR (200 MHz, CDCl3) δ 4.45 (s, 2H),
7.38-7.97 (m, 4H); anal. calcd. (%) for C8H6OS: C, 63.97; H,
4.03; S, 21.35. Found (%): C, 63.84; H, 4.02; S, 21.30.
Synthesis of 4-nitro-3H-benzo[c]thiophen-1-one (2): To
a solution of ethyl 2-methyl-3-nitrobenzoate (10.45 g, 50
mmol) and benzoyl peroxide (10 mg, 41.3 mmol) in carbon
tetrachloride (125 mL) was added N-bromosuccinimide (8.90
g, 50 mmol) and refluxed fo 7 h. After completion of the
reaction the mixture was filtered and evaporated. The residue
in acetone (50 mL) was mixed with thiourea (4.19 g, 55 mmol)
and refluxed for 5.5 h.After removal of acetone by evaporation,
the obtained thiouronium salt was warmed to 80-90 ºC in 20 %
NaHCO3 solution (50 mL) under nitrogen for 2 h. Then, the
solution was acidified with 20 % HCl solution. The oily residue
was subjected to silica gel column chromatography (eluent;
hexane:ethyl acetate = 10:1, v/v) to give 2 (4.39 g, 45 %):
m.p. 105 ºC; IR (KBr, νmax, cm-1) 3030, 1685, 1595, 1530; 1H
NMR (200 MHz, CDCl3): δ 4.40 (s, 2H), 7.04 (d, 1H), 7.29
(d, 1H), 7.72 (t, 1H); anal. calcd. (%) for C8H5NO3S: C, 49.23;
H, 2.58; N, 7.18; s, 16.43. Found (%): C, 49.28; H, 2.53; N,
7.19; s, 16.32.
8 Hz, 1H); anal. calcd. (%) for C9H8O2S: C, 59.98; H, 4.47; S,
17.79. Found (%): C, 59.97; H, 4.46; S, 17.80.
4,6-Dibromo-5,7-methoxy-3H-benzo[c]thiophen-1-one
(5): yield: 65 %; m.p. 142-143 ºC; IR (KBr, νmax, cm-1) 3030,
1
1687, 1561, 1385; H NMR (200 MHz, CDCl3): δ 3.95 (s,
3H), 4.30 (s, 2H); anal. calcd. (%) for C10H8O3SBr2: C, 32.63;
H, 2.19; S, 8.72. Found (%): C, 32.62; H, 2.19; S, 8.71.
9-Methoxy-3H-naphto[2,3-c]thiophen-1-one (6):Yield:
52 %; m.p. 129-130 ºC; IR (KBr, νmax, cm-1) 3035, 1690, 1054;
1H NMR (200 MHz, CDCl3): δ 4.10 (s, 3H), 4.50 (s, 2H), 7.63
(m, 4H), 8.30 (m, 1H); anal. calcd. (%) for C13H10O2S: C, 67.80;
H, 4.38; S, 13.93. Found (%): C, 67.81; H, 4.37; S, 13.94.
RESULTS AND DISCUSSION
The designed compounds were synthesized and tested as
anticholinesterases. They turned out to be ChE inhibitors with
the inhibition constant shown in Table-1. The inhibition
constant obtained by a replot of Km/Vmax versus inhibitor
concentration ranges from µM to nM. The boronic acid and
its protected form are effective inhibitors ofAChE and BuChE.
The inhibition constant for AChE.
TABLE-1
ANTICHOLINESTREASE ACTIVITIES OF THE COMPONDS
Ki for AChE
(µM)
Ki for BuChE
(µM)
Ki(BuChE)/
Ki(AChE)
Compound
59.8 3.1
11.8 1.2
4894 358
341 30
82
29
1
2
78 nM is close to that of the most potent AChE inhibitors
such as tacrine and huperzineA. Since the inhibitors increased
the Michaelis constant while they have little effect on the
maximal velocities, the inhibition mechanism is mixed (data
not shown).
Synthesis of 5-methoxy-3H-benzo[c]thiopen-1-one (3):
To a solution of ethyl 2-methyl-3-nitrobenzoate (10.45 g, 50
mmol) and benzoyl peroxide (10 mg, 41.3 mmol) in carbon
tetrachloride (125 mL) was added N-bromosuccinimide
(8.90 g, 50 mmol) and refluxed fo 7 h. After completion of
the reaction the mixture was filtered and evaporated. The
residue in acetone (50 mL) was mixed with thiourea (4.19 g,
55 mmol) and refluxed for 5.5 h. After removal of acetone by
evaporation, the obtained thiouronium salt was warmed to
80-90 ºC in 20 % NaHCO3 solution (50 mL) under nitrogen
for 2 h. Then, the solution was acidified with 20 % HCl solution.
The oily residue was subjected to silica gel column chroma-
tography (eluent; hexane:ethyl acetate = 10:1, v/v) to give 3
(4.09 g, 45 %): m.p. 103-104 ºC; IR (KBr, νmax, cm-1) 3030,
The benzothiophenone 1 and the nitro substituted one 2
inhibit ChE with much less potency. The nitro group in 2
increased the inhibition potency by 5 fold toward AChE
compared to 1. These compounds are not good inhibitors for
BuChE, either. Compound 1 is the least potent BuChE inhibitor
with the inhibition constant of 4.9 mM among the tested
compounds. This may due to the increased hydrophobic site
of BuChE and the absence of any electronic effect by the nitro
group. As the inhibition potency decrease, the Ki(BuChE)/
Ki(AChE) also decreases, meaning the selectivity decreases.
7-Methosxy-3H-benzo[c]thiophen-1-one (4), 4,6-dibromo-
5,7-methoxy-3H-benzo- [c]thiophen-1-one (5) and 9-methoxy-
3H-naphtho[2,3-c]thiophen-1-one (6) were synthesized by
using ethyl 2-methoxy-6-methylbenzoate, ethyl 3,5-dibromo-
2,4-dimethoxy-6-methylbenzoate and ethyl 1-methoxy-3-
methyl-2-naphthoate in same method.
1
1660, 1592, 1247; H NMR (200 MHz, CDCl3): δ 3.96 (s,
3H), 4.40 (s, 2H), 7.0 (m, 2H), 7.72 (m, 1H); anal. calcd. (%)
for C9H8O2S: C, 59.98; H, 4.47; S, 17.79. Found (%): C, 59.97;
H, 4.46; S, 17.80.
Similar method was adopted to synthesize, 7-methoxy-
3H-benzo[c]thiophen-1-one (4), 4,6-dibromo-5,7-methoxy-
3H-benzo[c]thiophen-1-one 5 and 9-methoxy-3H-naphtho-
[2,3-c]thiophen-1-one (6) by using ethyl 2-methoxy-6-
methylbenzoate, ethyl 3,4-dibromo-2,4-dimethoxy-6-
methylbenzoate and ethyl 1-methoxy-3-methyl-2-naphthoate.
7-Methoxy-3H-benzo[c]thiophen-1-one (4): Yield:
61 %; m.p. 70-71 ºC; IR (KBr, νmax, cm-1) 3030, 1679, 1590,
ACKNOWLEDGEMENTS
This work was supported by the grant from Dong-A
University (2012).
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Synthesis of Benzothiophenones and Naphthothiophenone as Anticholinesterases 5327
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