Novel racemic tetrahydrocurcuminoid dihydropyrimidinone analogues as potent acetylcholinesterase inhibitors

Article abstract of DOI:10.1016/j.bmcl.2013.03.069

The synthesis of racemic tetrahydrocurcumin- (THC-), tetrahydrodemethoxycurcumin- (THDC-) and tetrahydrobisdemethoxycurcumin- (THBDC-) dihydropyrimidinone (DHPM) analogues was achieved by utilizing the multi-component Biginelli reaction in the presence of copper sulphate as a catalyst. The evaluation of acetylcholinesterase inhibitors for Alzheimer's disease of these compounds showed that they exhibited higher inhibitory activity than their parent analogues. THBDC-DHPM demonstrated the most potent inhibitory activity with an IC₅₀ value of 1.34 ± 0.03 μM which was more active than the approved drug galanthamine (IC₅₀ = 1.45 ± 0.04 μM).

Full text of DOI:10.1016/j.bmcl.2013.03.069

Bioorganic & Medicinal Chemistry Letters 23 (2013) 2880–2882
Contents lists available at SciVerse ScienceDirect
Bioorganic & Medicinal Chemistry Letters
journal homepage: www.elsevier.com/locate/bmcl
Novel racemic tetrahydrocurcuminoid dihydropyrimidinone
analogues as potent acetylcholinesterase inhibitors
Sarawalee Arunkhamkaew, Anan Athipornchai, Nuttapon Apiratikul, Apichart Suksamrarn,
⇑
Vachiraporn Ajavakom
Department of Chemistry and Center of Excellence for Innovation in Chemistry, Faculty of Science, Ramkhamhaeng University, Bangkok 10240, Thailand
a r t i c l e i n f o
a b s t r a c t
Article history:
The synthesis of racemic tetrahydrocurcumin- (THC-), tetrahydrodemethoxycurcumin- (THDC-) and tet-
rahydrobisdemethoxycurcumin- (THBDC-) dihydropyrimidinone (DHPM) analogues was achieved by uti-
lizing the multi-component Biginelli reaction in the presence of copper sulphate as a catalyst. The
evaluation of acetylcholinesterase inhibitors for Alzheimer’s disease of these compounds showed that
they exhibited higher inhibitory activity than their parent analogues. THBDC–DHPM demonstrated the
Received 31 October 2012
Revised 11 March 2013
Accepted 21 March 2013
Available online 1 April 2013
most potent inhibitory activity with an IC50 value of 1.34 0.03
l
M which was more active than the
Keywords:
approved drug galanthamine (IC50 = 1.45 0.04 M).
l
Alzheimer’s disease
Acetylcholinesterase inhibitor
Curcumin
Ó 2013 Elsevier Ltd. All rights reserved.
Dihydropyrimidinone
Alzheimer’s disease (AD) is a progressive neurogenerative disor-
der of the central nervous system, affecting approximately 18 mil-
lion people worldwide and this number is estimated to grow up to
34 million people in 13 years time.1 This disease is one of the most
common forms of dementia, which is determined clinically by
multiple cognitive deficits including memory loss, emotional dis-
turbance, personality changes and finally the total eradication of
the nervous system.1 This dreadful disease is particularly charac-
terized by the loss of cholinergic neurons which make an essential
acetylcholine (ACh) neurotransmitter in the neocortex and hippo-
campus. The deficiency of a cholinergic neurotransmitter, espe-
cially the decline of the level of ACh neurotransmitter in the
brain, is intimately involved in memory loss in AD. The deactiva-
tion or termination of ACh by acetylcholinesterase (AChE) within
the synapse itself together with the formation of extracellular
amyloid plaque (amyloid-b) and intracellular neurofibrillary tan-
gles are generally thought to be associated with the reduction in
the release of neurotransmitters.2 To the present date many re-
search approaches to the discovery of the effective AChE inhibitor
(AChEI) for the treatment of AD have been reported.3 Among them,
there are some important examples such as the first generation of
AChE inhibitor (AChEI), tacrine (Cognix™),4 which is rarely used
nowadays because of its potential liver toxicity, donepezil (Air-
cept™),5 rivastigmine (Exelon™)6 and galanthamine (Reminyl™),7
which have been proved as effective drugs in treatment of AD. De-
spite these great efforts, no drug that can provide the complete
protection of neurons has ever been found. However, effective
AD treatments are realized by blocking ACh breakdown by use of
AChEI and thus prolonging ACh action. We are therefore interested
to develop a novel AChEI by modifying curcuminoid natural prod-
ucts, which are an abundant resource in Asian countries.
Curcuminoids can be obtained from rhizomes of turmeric, Cur-
cuma longa L. (Zingiberaceae).8 By possessing attractive biological
activities including antioxidant, anti-inflammatory, anticancer,
anti-HIV and recently discovered anti-Alzheimer’s, curcuminoids,
particularly the three main components; curcumin (1), demethoxy-
curcumin (2) and bisdemethoxycurcumin (3), have been drawing
an enormous amount of attention from researchers.9 However, its
hydrogenated curcuminoids, tetrahydrocurcumin (THC, 4), tetra-
hydrodemethoxycurcumin (THDC, 5) and tetrahydrobisdemthoxy-
curcumin (THBDC, 6), have been evaluated as antioxidants and a
biomarker in a clinical study.10,11 The evaluation of these curcumi-
noid congeners (4À6) as the AChEI is therefore very interesting.
Also, the comparison between AChE inhibitory activities of curc-
uminoids (1À3) and THCs (4À6) on AChE may provide an important
discovery for the cure of AD (Table 1). The Ellman method was uti-
lized for the test of the AChE inhibitory activities of both curcumi-
noids (1–3) and THCs (4À6).12 According to the results, each THC
exhibited approximately twofold better IC50 compared to that of
its curcuminoid pair. This preliminary evaluation demonstrated
that the absence of double bond and methoxy group within the
curcuminoid structure tends to increase in the inhibitory activity.
The results demonstrated that curcuminoids (1À3) and THCs
(4À6) displayed potential inhibitory activity against AChE,
however, the activity was not high when compared to that of the
⇑
Corresponding author. Tel.: +66 2 319 5112; fax: +66 2 310 8401.
E-mail address: a_vachiraporn@ru.ac.th (V. Ajavakom).
0960-894X/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.bmcl.2013.03.069

S. Arunkhamkaew et al. / Bioorg. Med. Chem. Lett. 23 (2013) 2880–2882
2881
Table 1
Table 2
Inhibitory activity IC50
(l
M) of curcuminoids and THCs on AChE
Synthesis and AChE inhibitory activity of racemic THC–DHPM analogues
Compound Structure
IC50
(lM)
Compound R1
R2
R3
Yield % IC50 (lM)
8
9
H
OCH3
OCH3
OCH3
OCH3
OCH3
OCH3
OCH3
OCH3
OCH3
OCH3
OCH3
OCH3
OCH3
OCH3
OCH3
OCH3
74
59
67
37
44
27
48
44
36.33 0.50
NDc
O
O
3-NO2
4-NO2
4-OH
4-OCH3
2,4-OCH3
2,5-OCH3
3-OCH3-4-OH
H
H3CO
OCH3
OH
NDc
1
81.05 2.51
37.74 0.16
18.27 0.09
41.16 0.17
20.55 0.12
10
11
12
13
14
15
16
16a
16b
17
17a
17b
18
19
7
17.31 0.17
14.28 0.09
NDc
HO
O
O
O
O
NDc
OCH3
2
56.42 0.58
5.36 0.20
10.65 0.58
20.94 0.54
2.89 0.10
5.78 0.64
9.10 0.16
2.04 0.11
1.34 0.03
1.45 0.04
OCH3 or H H or OCH3 39b
HO
OH
H
H
OCH3
H
H
OCH3
OCH3 or H H or OCH3 21b
3
4-OCH3
4-OCH3
4-OCH3
H
OCH3
H
H
H
OCH3
H
HO
OH
O
O
55
25
H3CO
OCH3
4-OCH3
H
H
4
Galanthaminea
HO
OH
OCH3
OH
a
b
c
Reference compound.
Yield given for the mixture (1:1) of two isomers.
Not determined.
O
O
O
O
5
HO
CuSO4Á5H2O gave acceptably promising results (Scheme 1). The
divalent copper metal may have the coordinating property with
the 1,3-dicarbonyl group and stabilizes the enol form generated
in situ which prompts to react with other two components to pro-
vide the DHPM product.
6
7.89 0.03
1.45 0.04
HO
OH
7
Galanthaminea
a
Reference compound.
In our initial investigations, the THC readily available through
the catalytic hydrogenation reaction of curcumin was subjected
to Biginelli reaction incorporating with urea and various aromatic
aldehydes to create a number of THC–DHPM analogues (Table 2,
compounds 8À15). It is worth noticing that all cyclized products
are racemic mixtures. The reaction of THC (4), urea and benzalde-
hyde in the presence of CuSO4Á5H2O under refluxing temperature
in EtOH gave the smooth cyclocondensation to the corresponding
THC–DHPM derivative 8 in good yield. Various substituents such
as nitro, hydroxy, and methoxy groups were introduced onto the
aromatic aldehydes to provide a series of THC–DHPM derivatives
9À15, in order to evaluate the substituent effect on their biological
activities. THC–DHPMs (11 and 12) bearing a hydroxy or methoxy
group at the para-position of the starting aromatic aldehyde satis-
factorily revealed high inhibitory activities of AChE with IC50 val-
ues of 17.31 0.17 and 14.28 0.09, respectively. Encouraged by
these results, the synthesis of THDC–DHPMs (16 and 17) from
THDC (5), benzaldehyde and 4-methoxybenzaldehyde were carried
out in a similar manner to that of the preparation of THC–DHPMs
(8À15). The mixtures of structural isomers from both THDC–
DHPMs (16 and 17) were obtained. The regioisomers of each of
compounds 16 and 17 were separated using normal phase HPLC
to give 16a and 16b, 17a and 17b, respectively, and biologically
reference AChEI, galanthamine. According to the molecular model-
ing studies reported by Tomassoli et al. some dihydropyrimidinone
(DHPM) derivatives have been reported as the potent compound
against AChE.13 These six-membered heterocyclic analogues also
have wide range of biological properties such as antibacterial, anti-
fungal, anti-inflamatory, calcium channel blocker, and anti-cancer
activity.14 It is therefore of interest to combine the THCs (4À6)
with the DHPM units, which may lead to the novel drug candidate
with much improved AChE inhibitory activity. In order to do so, a
synthetic strategy was designed to construct the DHPM building
block out of the THC unit. Observation of the curcumin structure
led us to consider the 1,3-dicarbonyl moiety of THC as one compo-
nent for a multi-component cyclocondensation reaction.15 The rep-
resentative Biginelli reaction, the acid-catalyzed three-component
condensation of an aldehyde, a urea and a 1,3-dicarbonyl com-
pound, is capable of creating the DHPM derivatives.16 Though sul-
furic acid can be traditionally used as the acid catalyst in this
reaction, various Brønsted and Lewis acids, for example p-TsOH,
Sr(OTf)2, Mn(OAc)2Á2H2O, CuCl2Á2H2O and CuSO4Á5H2O, have been
proven to be very effective.17 In this work, after screening several
Lewis acids as catalysts for the Biginelli reaction of THC,
Scheme 1. Synthesis of racemic THC–DHPM analogues by a multi-component Biginelli reaction.

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S. Arunkhamkaew et al. / Bioorg. Med. Chem. Lett. 23 (2013) 2880–2882
evaluated together with their mixtures as AChEI. The THBDC (6)
was also introduced to the Biginelli reaction with benzaldehyde
and urea to furnish the THBDC–DHPM analogue (18). As a methoxy
group in the aromatic ring of the aldehyde precursor is considered
to be essential for inhibitory activity, we therefore synthesized the
desired THBDC–DHPM analogue (19) by treatment of THBDC (6)
with 4-methoxybenzaldehyde.
Supplementary data
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.bmcl.2013.03.
069.
References and notes
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in the same range IC50 value of 36.33 0.50
the corresponding THC (IC50 = 41.16 0.17
l
M when compared to
lM) (Table 2). From this
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duction of one more electron donating group in the aromatic moi-
ety did not increase the activities against AChE, as 2,4-OCH3-, 2,5-
OCH3- and 3-OCH3-4-OH-phenyl THC–DHPMs (13À15) exhibited
poor inhibitory activities. A mixture isomers of THDC–DHPM ana-
logue (17) bearing a 4-OCH3 phenyl group demonstrated much bet-
ter activity with an IC50 value of 2.89 0.10
DHPM (16) without the 4-OCH3 phenyl group (5.36 0.20
well as in the case of both THBDC–DHPMs (18 and 19), IC50 value
of 2.04 0.11 M and 1.34 0.03 M were obtained from the com-
lM than that of THDC-
lM), as
l
l
pound without 4-OCH3 phenyl group and with 4-OCH3 phenyl
group, respectively. However, the individual regioisomers 16a and
16b, 17a and 17b were less active than their respective mixture
16 and 17. This was probably due to the synergistic effect of the
regioisomers in the mixture. From the screening test of AChEIs,
the methoxy substituent on the phenyl group originated from alde-
hyde and an absence of methoxy group on curcuminoid component
seem to be crucially important for the inhibitory activity of these
compounds.
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In conclusion, THC-, THDC- and THBDC–DHPM analogues were
synthesized and evaluated as inhibitors of AChE. The inhibitory
potencies of the synthesized compounds against the AChE unveiled
that THBDC–DHPM (19) bearing a 4-OCH3 phenyl group has a
highly effective inhibitory activity of AChE18 with an excellent
IC50 value of 1.34 0.03
that of galanthamine.
lM, which is slightly more potent than
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This work was supported by the Center of Excellence for Inno-
vation in Chemistry (PERCH-CIC), The Thailand Research Fund
and the Department of Chemistry, Faculty of Science, Ramkhamha-
eng University. We would also like to acknowledge the Depart-
ment of Chemistry, Faculty of Science, Chulalongkorn University
for HPLC separation.