Methyl 2-(2-(4-formylphenoxy)acetamido)-2-substituted acetate derivatives: A new class of acetylcholinesterase inhibitors

Article abstract of DOI:10.1016/j.bmcl.2007.01.091

A new class of inhibitors of acetylcholinesterase (methyl 2-(2-(4-formylphenoxy)acetamido)-2-substituted acetate derivatives) is described. Compounds 4b and 4i were found to be more potent than galanthamine in inhibiting acetylcholinesterase.

Full text of DOI:10.1016/j.bmcl.2007.01.091

Bioorganic & Medicinal Chemistry Letters 17 (2007) 2123–2125
Methyl 2-(2-(4-formylphenoxy)acetamido)-2-substituted acetate
derivatives: A new class of acetylcholinesterase inhibitors
Huan Wen,a Yayao Zhou,a Chonglan Lin,a Hui Ge,a Lin Ma,a Zihou Wang,c
Wenlie Pengb and Huacan Songa,*
aSchool of Chemistry and Chemical Engineering, Sun Yat-sen University, Guangzhou 510275, China
bSchool of Life Science, Sun Yat-sen University, Guangzhou 510275, China
cCentre for Drug Evaluation, State Drug Administration, Beijing 100050, China
Received 2 November 2006; revised 9 January 2007; accepted 30 January 2007
Available online 2 February 2007
Abstract—A new class of inhibitors of acetylcholinesterase (methyl 2-(2-(4-formylphenoxy)acetamido)-2-substituted acetate deriv-
atives) is described. Compounds 4b and 4i were found to be more potent than galanthamine in inhibiting acetylcholinesterase.
Ó 2007 Elsevier Ltd. All rights reserved.
In the past two decades, treatment for Alzheimer’s dis-
ease (AD) has largely involved replacement of neuro-
transmitters that are known to be lacking in AD,
mostly based on the ‘‘cholinergic hypothesis’’ of AD.1,2
A deficit in central cholinergic transmission caused by
degeneration of the basal forebrain nuclei is an impor-
tant pathological and neurochemical feature of AD.
One possible approach to treating this disease is to re-
store the acetylcholine levels by inhibiting acetylcholines-
terase (AChE) with highly selective inhibitors. AChE
inhibitors such as tacrine, donepezil, rivastigmine and
galanthamine are able to enhance memory in AD pa-
tients (Fig. 1). Unfortunately, the potential effectiveness
offered by most common inhibitors often is limited by
the appearance of central and peripheral side effects.3,4
Recent data focus on the complex nature of AD and dis-
close the involvement of other neurotransmitters such as
serotonin, dopamine, noradrenalin, histamine, excitato-
ry amino acids and neuropeptides among others.5 The
present tendency in the search for new compounds to
treat AD is to look for inhibitors of AChE endowed with
additional therapeutic effects, such as antioxidant6 and
anti-inflammatory agents,7 activation of other neuro-
transmitter systems like GABA or monoaminergic sys-
tems8 and calcium-channel blocking potential.9
4-Hydroxybenzaldehyde (1) is an active constituent of
Gastrodia elata (Chinese name: tianma), which is a very
important Chinese herbal medicine used for the medical
treatment of headaches, migraine, dizziness, epilepsy,
rheumatism, neuralgia, paralysis, and other neuralgic
and nervous disorders.10,11 Gastrodia elata was reported
to contain one or more compounds that dramatically re-
duce amyloid b-peptide induced neuronal cell death
in vitro.12 Recent studies showed that 1 is not only a po-
tent antioxidant agent but also a GABA transaminase
inhibitor (IC50 = 4.1 lg/mL),13 and there was a report
further exactly suggesting that the aldehyde group and
the hydroxyl group at C-4 are necessary for the action.14
Based on these cases, we supposed that 1 might contrib-
ute to the treatment of AD. In our previous high
throughout screening work, 1 was also found to show
weak anti-AChE activity and the value of IC50 was
37.8 mM. Therefore, a series of methyl 2-(2-(4-formyl-
phenoxy)acetamido)-2-substituted acetate derivatives
were designed using 1 and amino-acid methyl esters as
starting materials, in order to explore new AChE inhib-
itors with additional antioxidant and GABA transami-
nase inhibitory effects.
Our strategy is based on the crystallographic structure
of the active-site gorge of AChE from Torpedo californi-
ca.15 The active centre of AChE contains a catalytic tri-
ad (Ser 200, His440, Glu327) which locates at the
bottom of the narrow gorge. Trp84, a residue of the
anionic site, is located near the active center. It is sup-
posed that the aromatic ring of those compounds
Keywords: AChE inhibitors; Methyl 2-(2-(4-formylphenoxy) acetam-
ido)-2-substituted acetate; 4-Hydroxybenzaldehyde; Synthesis.
*
Corresponding author. Tel.: +86 20 84110918; fax: +86 20 8411
2245; e-mail: yjhxhc@mail.sysu.edu.cn
0960-894X/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2007.01.091

2124
H. Wen et al. / Bioorg. Med. Chem. Lett. 17 (2007) 2123–2125
OH
N
NH2
CH3
CH3
N
O
H3C
N
O
O
CH3
H3CO
H3CO
N
H3CO
CH3
O
N
Tacrine
Donepezil
Rivastigmine
CH3
Galanthamine
Figure 1. AChE inhibitors currently used in therapeutics.
H
N
COOCH3
R1
OH
O
O
O
COOH
COOCH3
O
a
b
c
CHO
1
CHO
3
CHO
4
CHO
2
4a R1 = H
4b R1= CH3
4c R1 = CH(CH3)2
4d R1 = CH2CH2SCH3
4e R1 = CH(CH3)C2H5
4f R1 = CH2CH(CH3)2
4g R1 =
CH2Ph 4h R1 = CH2CH2COOCH3
H2
C
N
O
4k R1 =
4i R1 =
4j R1 = p-CH2PhOH
O
C
O
OCH3
N
H
CHO
Scheme 1. Synthesis of 4a–k. Reagents and conditions: (a) ClCH2COOCH3, K2CO3, acetone, 60 °C, 5h, 100%; (b)—(i) NaOH, H2O, (ii) H+, H2O,
95%; (c) amino-acid methyl ester, DCC, DMAP, CHCl3, 0 °C stirred rt, overnight, 26–80%.
synthesized here may interact with Trp84 through p–p
Table 1. AChE inhibition of methyl 2-(2-(4-formylphenoxy)-acetam-
stacking,16 and the amido fragment would be able to
ido)-2-substituted acetate derivatives
interact with the catalytic triad of the active site.
Compound
AChE
a
IC50 (lM)
BuChE
b
IC50 (lM)
Selectivity
for AChEc
Methyl 2-(2-(4-formylphenoxy)acetamido)-2-substituted
acetate derivatives could be prepared by the condensa-
tion of amino-acid methyl esters and 4-formylphenoxy-
acetic acid, and the latter was synthesized by the
reaction of 1 and methyl chloroacetate. From Scheme 1,
when 1 equivalent of 1 and 1.2 equivalents of methyl
chloroacetate reacted in acetone at 60 °C for 5 h in the
presence of anhydrous K2CO3, compound 2 was affor-
ded. Then compound 2 was hydrolysed in 1 M NaOH,
acidified to pH 3–4 with 1 M HCl to produce compound
3.17 Compound 3 condensed with the appropriate ami-
no-acid methyl esters in the presence of N,N0-dic-
yclohexyl-carbodiimide (DCC) and 4-(dimethylamino)
pyridine (DMAP) to provide the desired compounds
(4a–k). All those methyl 2-(2-(4-formylphenoxy)acetam-
ido)-2-substituted acetate derivatives were characterized
by (1H, 13C) NMR, MS and IR spectroscopy. The anti-
AChE activity of compounds 4a–k was evaluated using
a slightly modified colorimetric method of Ellman
et al.18 The obtained IC50 values, together with that of
galanthamine for comparison, are shown in Table 1.
1
4a
>1000
98.86
0.19
Nd
Ndd
Nd
4b
4c
Nd
Nd
4d
4e
Nd
Nd
Nd
Nd
4f
4g
Nd
Nd
Nd
4.96
>200
0.28
Nd
4h
4i
Nd
33.94
Nd
Nd
121.21
27.70
4j
4k
Nd
0.55e
Galanthamine
15.24
a Assay performed using ChE from Electric eel. Values are means of
three different experiments.
b Assay performed using horse serum.
c Selectivity for AChE is defined as IC50 (BuChE)/IC50 (AChE).
d Not determined at 1000 lM concentration.
e Values in the literature16,19 is 0.36–0.61 lM.
three amino-acid moieties, L-alanine, L-tryptophan and
L-phenylalanine into the molecule of 1, respectively,
could make the inhibitory activities of 1 increase obvi-
ously. Second, the introduction of L-glycin into 1 (for
4a) could just slightly increase the AChE inhibitory
activity. Third, for 4c–f, 4h, 4j and 4k, although ami-
no-acid moieties like L-valine, L-methionine, L-leucine,
L-isoleucine, L-aspartic acid, L-tyrosine and L-proline
From Table 1, five results could be found about the
change of AChE inhibitory activity on introduction of
amino-acid moieties into the backbone of 1, as com-
pared with AChE inhibitory activity of 1 itself. First, 1
possessed a very weak AChE inhibitory activity, howev-
er, 4b, 4g and 4i showed a higher AChE inhibitory activ-
ity than 1, therefore, it is sure that the introduction of

H. Wen et al. / Bioorg. Med. Chem. Lett. 17 (2007) 2123–2125
2125
were attached to the structure of 1, respectively, the
References and notes
AChE inhibitory values were almost same as that of 1.
Fourth, both of 4b (IC50 = 0.19 lM) and 4i
(IC50 = 0.28 lM) showed higher AChE inhibitory values
than galanthamine (IC50 = 0.55 lM). Lastly, all of the
tested compounds showed poor inhibition to butyrylch-
olinesterase (BuChE). Especially for compounds 4b and
4i, they all demonstrated a high selectivity, respectively,
for AChE inhibitory activity over BuChE, 4b (selectivi-
ty, >1000) and 4i (selectivity, 121.21), whereas the selec-
tivity value of galanthamine is only 27.70. Therefore, the
remarkable selectivity feature of compounds 4b and 4i
might shed light on the future design and preparation
of AChE inhibitors. However, the reason why 4b and
4i showed a better inhibitory activity and selectivity than
the other synthesized compounds is not clear, which will
be further studied in the future in our laboratory.
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In conclusion, methyl 2-(2-(4-formylphenoxy)acetami-
do)-2-substituted acetate derivatives represented a new
class of highly active and selective AChE inhibitors,
which indicated that the modification based on the
structure of 1 is a practical approach to increase its
AChE inhibitory activities. Among all the tested com-
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Acknowledgment
This work was supported by the Natural Science
Guangdong
Foundation
(2004B30101007).
of
Province,
China
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Supplementary data
Supplementary data associated with this article can be
found, in the online version, at doi:10.1016/
j.bmcl.2007.01.091.