Design, synthesis, and evaluation of indanone derivatives as acetylcholinesterase inhibitors and metal-chelating agents

Article abstract of DOI:10.1016/j.bmcl.2012.04.029

A series of novel indanone derivatives was designed, synthesised and evaluated as potential agents for Alzheimer's disease. Among them, compound 6a, with a piperidine group linked to indone by a two-carbon spacer, exhibited the most potent inhibitor activity, with an IC₅₀ of 0.0018 μM for AChE; the inhibitory activity of this compound was 14-fold more potent than that of donepezil. Furthermore, these compounds also exhibited good metal-chelating ability.

Full text of DOI:10.1016/j.bmcl.2012.04.029

Bioorganic & Medicinal Chemistry Letters 22 (2012) 4462–4466
Contents lists available at SciVerse ScienceDirect
Bioorganic & Medicinal Chemistry Letters
journal homepage: www.elsevier.com/locate/bmcl
Design, synthesis, and evaluation of indanone derivatives
as acetylcholinesterase inhibitors and metal-chelating agents
⇑
⇑
Fan-Chao Meng, Fei Mao, Wen-Jun Shan, Fangfei Qin, Ling Huang , Xing-Shu Li
Institute of Drug Synthesis and Pharmaceutical Process, School of Pharmaceutical Sciences, Sun Yat-sen University, Guangzhou 510006, 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 novel indanone derivatives was designed, synthesised and evaluated as potential agents for
Received 28 January 2012
Revised 27 March 2012
Accepted 6 April 2012
Available online 13 April 2012
Alzheimer’s disease. Among them, compound 6a, with a piperidine group linked to indone by a two-car-
bon spacer, exhibited the most potent inhibitor activity, with an IC50 of 0.0018 lM for AChE; the inhib-
itory activity of this compound was 14-fold more potent than that of donepezil. Furthermore, these
compounds also exhibited good metal-chelating ability.
Ó 2012 Elsevier Ltd. All rights reserved.
Keywords:
Alzheimer’s disease
Indanone derivatives
Acetylcholinesterase inhibitors
Metal-chelating agents
Alzheimer’s disease (AD) is a neurodegenerative disorder asso-
ciated with a selective loss of cholinergic neurons in the brain and
decreasing levels of acetylcholine (ACh).1 AD leads to a progressive
decline in cognitive, executive and memory functions and eventu-
ally to incapacitating dementia before death.2 The prevalence of AD
increases dramatically with age and doubles for every five-year
interval after the age of 65.3 Although several diverse hallmarks
to the high affinity and selectivity of donepezil for AChE.12 Inspired
by this fact, we combined the indanone moiety with an aromatic
ring via an aldol condensation to enhance the affinity for the
PAS. In addition, the introduction of the double bonds may endow
this type of compound with other activities, such as metal-chelat-
ing activity. Based on this strategy, we designed a series of
indanone derivatives with various amine groups linked to the
6-position of indanone by carbon spacers of different lengths.
Herein, we report the design and synthesis of these molecules
and the biological evaluation of the abilities of these molecules
to serve a highly effective inhibitor of acetylcholinesterase and
metal-chelating agents.
such as b-amyloid (Ab) deposits, s-protein aggregation, oxidative
stress, and low levels of acetylcholine (ACh) have been thought
to play significant roles in the pathophysiology of AD, acetylcholin-
esterase inhibitors (AChEIs) are the only drugs used clinically for
the treatment of this disease (Fig. 1).4,5 According to the ‘choliner-
gic hypothesis’, AChEIs could increase the levels of ACh in AD pa-
tients through the inhibition of acetylcholinesterase (AChE) and,
therefore, relieve some symptoms experienced by AD patients.6
Among the several AChEIs drugs approved by the US Food and
Drug Administration (FDA) for the treatment of AD, donepezil
(Fig. 1) has been demonstrated to improve the cognitive function
of patients with mild to severe moderate AD.7 This drug shows
excellent tolerability without hepatotoxicity and is thought to be
the most effective AChE inhibitor in the world at present.8,9
Recently, a number of donepezil analogues have been synthesised
and evaluated as AChE inhibitors.10,11 However, from academic and
practical standpoints, it is still desirable to develop more effective
agents for the treatment of Alzheimer’s disease. It is well known
that the indanone donepezil moiety can interact with the PAS of
AChE via aromatic stacking interactions, which contributes greatly
⇑
Corresponding authors. Tel./fax: +86 20 3994 3050.
E-mail addresses: lixs@mail.sysu.edu.cn, lixsh@mail.sysu.edu.cn (X.-S. Li).
Figure 1. Stucture of reported AChE inhibitors.
0960-894X/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.bmcl.2012.04.029

F.-C. Meng et al. / Bioorg. Med. Chem. Lett. 22 (2012) 4462–4466
4463
Scheme 1. Reagents and conditions: (i) Pd/C, H2, MeOH, 40 °C; (ii) methanesulfonic acid, 120 °C; (iii) aldehyde, PTSA, toluene, 120 °C; (iv) Br(CH2)nBr, K2CO3, acetonitrile,
80 °C; (v) amines, acetonitrile, 80 °C; (vi) 10% Pd/C, H2, CH3OH.
The synthetic routes for 6-hydroxy-5-methoxy indanone 3,
pyridinylmethylene-indanone 5 and their derivatives 6 and 7 are
shown in Scheme 1. First, ferulic acid 1 was hydrogenated in the
presence of Pd/C and then cyclised at 120 °C, catalysed by
methanesulfonic acid, to yield the indone 3 with 70% yield. The
which the hydrogen at double bond and methylene display strong
coupling in NOESY spectra (See Supplementary data).
The AChE-inhibitory effects of these pyridinylmethylene-inda-
none derivatives were determined by the spectroscopic method
of Ellman et al.16 using tacrine donepezil and galantamine as
the reference standards. The BuChE inhibitory activity against
equine serum BuChE was also determined using the same method.
The IC50 values for AChE and BuChE inhibition are summarised
in Table 1.
alkylation of 3 with different
a,x-dibromoalkanes compounds in
acetonitrile provided 4a–4f with 50–80% yields, and these com-
pounds were then treated with different aldehydes in the presence
of p-toluenesulphonic acid to afford compounds 5a–5f in approxi-
mately 80% yields. Finally, products 6a–6g were obtained by the
reaction of 5a–5f with commercially available secondary amines
(e.g., diethylamine and piperidine) in 55–80% yields.13 Compound
5g was prepared by the condensation of 3 with pyridylaldehyde.14
Compounds 7a–7c, were prepared via a displacement reaction of
4a–4c with piperidine.15 Compounds 6k and 6l, which have the
piperidine group linked to pyridin-2-ylmethylene-indanone and
to pyridin-2-ylmethylene-indanone and benzylidene by two- to
nine-carbon spacers, respectively, were obtained by the reaction
of 7a with corresponding aldehydes under the same reaction con-
ditions as those used to prepare 5. The reduction product (6m)
from 6l was prepared by reducing the double bond using 10% Pd/
C as catalyst. All of the compounds were characterised using ana-
lytical and NMR spectroscopic data. The Z configuration at the dou-
ble bond was justified by NMR experiments of compound 6a,
The lead compound 6-hydroxy-5-methoxy indanone 3 exhib-
ited poor inhibitory activity for AChE and BuChE (Table 1, entry
1, IC50 value = 91.96
pound 5g, the condensation product of 3 with pyridylaldehyde,
provided better results (Table 1, entry 2, IC50 value = 49.55
lM for AChE and 258.03 lM for BuChE). Com-
lM
for AChE). It is exciting that all of the indanone 6 derivatives exhib-
ited much better inhibitory activities against AChE than the lead
compounds 3 and 5. The most potent inhibitor was 6a, in which
the piperidine group was linked to indanone by two-carbon spac-
ers; this compound had an IC50 value of 0.0018 lM (i.e., it was 47-
fold more potent than tacrine and 14-fold more potent than
donepezil). The carbon spacers seem to be very important for the
inhibitory activities of these compounds. For example, compounds
6a–f, which have the piperidine group linked with pyridinylmeth-
ylene-indanone by two to nine-carbon spacers, exhibited very

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F.-C. Meng et al. / Bioorg. Med. Chem. Lett. 22 (2012) 4462–4466
Table 1
In vitro inhibition IC50
(l
M) and selectivity of indanone derivatives, tacrine and donepezil for AChE and BuChE
Entry
Compounds
n
IC50
(
lM)
Selectivity for AChEc
AChE SEMa
BuChE SEMb
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
18
19
20
3
5g
7a
7b
7c
6a
6b
6c
6d
6e
6f
6g
6h
6i
6j
6k
0
0
2
3
5
2
3
4
5
6
9
4
4
6
6
2
2
2
91.96 0.867
49.55 0.430
3.11 0.362
258.03 1.334
246.8 1.506
136.5 1.372
32.95 0.148
114.97 0.969
9.5 0.241
42.72 0.347
13.84 0.467
9.15 0.169
4.47 0.005
1.24 0.142
17.48 0.236
28.92 0.672
18.50 0.832
13.36 0.569
7.48 0.432
2.24 0.332
9.20 0.31
2.8
4.9
43.9
0.916 0.061
0.361 0.084
0.0018 0.00007
0.062 0.0001
0.081 0.002
0.288 0.004
0.232 0.005
0.734 0.053
0.156 0.006
0.205 0.009
2.87 0.351
0.271 0.002
0.0044 0.00081
0.0091 0.00039
1.27 0.12
0.0858 0.0049
0.026 0.0005
0.623 0.099
35.9
319.4
5248.6
684.6
170.2
31.8
19.3
1.7
112.2
141.1
6.4
49.3
1703.9
246.4
7.24
0.2
179.5
25.3
6l
6m
Tacrine
Donepezil
Galantamine
0.013 0.001
4.66 0.503
15.7 0.787
SEM: standard error of the mean.
a
50% inhibitory concentration (means SEM of three experiments) of AChE from electric eel.
50% Inhibitory concentration (means SEM of three experiments) of BuChE from equine serum.
Selectivity for AChE = IC50 (BuChE)/IC50 (AChE).
b
c
different inhibitory activities against AChE (6a, two-carbon spacer,
IC50 value = 0.0018 M; 6b, n = 3, 0.062 M; 6c, n = 4, 0.081 M;
6d, n = 5, 0.288 M; 6f, n = 9, 0.734 M). These results indicated
l
l
l
l
l
that long carbon spacers generally had an unfavourable effect on
the inhibitory activity of AchE, and it seemed that the shorter car-
bon spacer is, the better the inhibitory activity that it exhibits. In
addition to the length of the carbon spacer, the substituted amino
groups had important effects on the inhibitory activity.
Compounds 6c, 6g, and 6h, in which pyridinylmethylene-indanone
is linked piperidinyl, pyrrolidinyl and N,N-diethyl amino groups by
a four-carbon spacer, exhibited IC50 values of 0.081, 0.156 and
0.205 lM, respectively. Similar phenomenon were also found for
compounds 6e, 6i, and 6j, in which pyridinylmethylene-indanone
is linked by a six-carbon spacer to pyrrolidinyl, imidazolyl and
N,N-diethyl amino groups (6e: 0.232
0.271 M). Compounds 7a (3.11 M), 7b (0.916
(0.3599 M) did not exhibit high levels of activity, a result that
lM, 6i: 2.87
lM, 6j:
l
l
lM) and 7c
l
indicated that the pyridinylmethylene group in these compounds
was very important for the inhibitory activity. In addition, com-
pounds 6k (0.0044
inhibitory activities than donepezil (0.026
reduction product 6m showed poor inhibitory activity (1.27
l
M) and 6l (0.0091
l
M) both exhibited better
M). However, the
M),
Figure 2. Steady state inhibition by compound 6a of AChE hydrolysis of ACh; the
plots show mixed type inhibition for compound 6a on AChE.
l
l
which indicated that the double bond is crucial for the activity.
The level of in vitro BuChE inhibition was also determined using
the same method. Most of these compounds exhibited moderate
inhibitory potency against BuChE, but there were no apparent pat-
terns in the levels of inhibition.
To study the inhibitory mechanism for this class of AChEIs,
compound 6a was chosen for kinetic studies based on the
in vitro inhibition experiments, and the results are shown in Figure
2. The intersection point of the Lineweaver–Burk reciprocal plot
was in the third quadrant, which is indicative of mixed-type inhi-
bition, which consists of both uncompetitive inhibition and non-
competitive inhibition,17 the latter of which involves binding to
the allosteric site(s) of the enzyme.
Figure 3 showed that the electronic spectra of 6a exhibited a blue
shift (the peak at 360 nm shift to 352 nm) after the addition of
Cu2+, whereas 6k and 6l exhibited a red shift (from 315 nm to
318 nm and from 320 to 325 nm, respectively). These results indi-
cated that all of the compounds could interact with the Cu2+ ion.
Similar results were also obtained when Fe3+ was added; the peak
in the electronic spectra of 6a at 360 nm shifted to 346 nm, the
peaks of 6k and 6l changed from 315 nm to 318 nm and from
320 to 326 nm, respectively. The electronic spectra of 6k and 6d
exhibited no obvious change when Zn2+ was added, whereas that
of 6a exhibited a shift from 360 to 357 nm and that of 6l exhibited
a shift from 320 to 324 nm. To determine the key functional motifs
responsible for the observed chelator properties, compounds 6d,
which has five carbon linkers between the nitrogen of the
The chelation abilities of compounds 6a, 6d, 6k 6l and
Clioquinol (CQ) towards biometals such as Cu2+, Fe3+ and Zn2+ in
ethanol were studied by UV–vis spectrometry.18,19 The results in

F.-C. Meng et al. / Bioorg. Med. Chem. Lett. 22 (2012) 4462–4466
4465
Figure 3. UV spectrum of compound 6a, 6d, 6k, 6l and CQ (50 lM) alone or at the presence of 50 l .
M Fe3+, Cu2+ and Zn2+
piperidine ring and the oxygen atom at the phenyl ring, was also
evaluated as a chelator with iron (Fig. 3). There is no significant
shift was observed in the UV spectrum of 6d after adding Fe3+
and Cu2+. The relatively poor chelating ability of 6d indicated that
the nitrogen of the piperidine ring and oxygen of phenolic group in
an appropriate distance was the main role of chelating metal.
In conclusion, a series of indanone derivatives was designed,
synthesised, and biologically evaluated as inhibitors of AChE and
BuChE. Most of these compounds were potent inhibitors of AChE,
with IC50 values ranging from micromolar to sub-micromolar. In
addition, all of these compounds showed moderate activity
towards BuChE. Among them, compound 6a, which contained a

4466
F.-C. Meng et al. / Bioorg. Med. Chem. Lett. 22 (2012) 4462–4466
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as a multifunctional drug candidate for the treatment of AD.
Acknowledgement
We thank the Natural Science Foundation of China (No.
20972198) for financial support of this study.
Supplementary data
Supplementary data associated with this article can be found, in the
online version, at http://dx.doi.org/10.1016/j.bmcl.2012.04.029.
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