Design and synthesis of cyclic acylguanidines as BACE1 inhibitors

Article abstract of DOI:10.1016/j.cclet.2015.07.020

Based on the lead compound 1 reported in literature, a series of novel BACE1 inhibitors were designed and synthesized, among which compound 11 exhibited a 14-fold improvement in potency over the lead compound 1. This represents a good lead for the discovery of more promising BACE1 inhibitors for the potential treatment of AD.

Full text of DOI:10.1016/j.cclet.2015.07.020

G Model
CCLET 3397 1–4
Chinese Chemical Letters xxx (2015) xxx–xxx
Contents lists available at ScienceDirect
Chinese Chemical Letters
journal homepage: www.elsevier.com/locate/cclet
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Original article
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Design and synthesis of cyclic acylguanidines as BACE1 inhibitors
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Q1 Jia-Kuo Liu, Wei Gu, Xiao-Rui Cheng, Jun-Ping Cheng, Wen-Xia Zhou , Ai-Hua Nie
Beijing Institute of Pharmacology and Toxicology, Beijing 100850, China
A R T I C L E I N F O
A B S T R A C T
Article history:
Based on the lead compound 1 reported in literature, a series of novel BACE1 inhibitors were designed
and synthesized, among which compound 11 exhibited a 14-fold improvement in potency over the lead
compound 1. This represents a good lead for the discovery of more promising BACE1 inhibitors for the
potential treatment of AD.
Received 27 March 2015
Received in revised form 27 April 2015
Accepted 22 May 2015
Available online xxx
ß 2015 Chinese Chemical Society and Institute of Materia Medica, Chinese Academy of Medical Sciences.
Published by Elsevier B.V. All rights reserved.
Keywords:
Alzheimer’s disease
BACE1 inhibitor
Cyclic acylguanidine
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1. Introduction
discovered [8], among which the most promising MK-8931 [9] has 32
entered a phase 3 clinical trial.
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Alzheimer’s disease (AD), a neurodegenerative disorder, is the
most common form of dementia and no disease modification
therapies are currently available. It is affecting more and more
people worldwide and the gradual loss of mental and physical
functions of the patients poses great emotional and economic
threat to the families and is a financial burden on the society for
long-term care. This disease is characterized pathologically by the
presence of intracellular neurofibrillary tangles (NFTs) and
extracellular amyloid plaques. According to the amyloid cascade
Taking compound 1 as a lead, we aimed at discovering new 34
compounds with improved BACE1 inhibitory potency while 35
maintaining its ability to cross the BBB. Analyzing the X-ray 36
crystal structure of compound 1 binding to the BACE1 active site 37
(PDB: 4DJU [10]) revealed that the H-bond interactions between 38
the guanidine and Asp228 and Asp32 are crucial for its inhibitory 39
activity while the two phenyl rings extended to the S1 and S20 40
pockets, respectively, leaving the S3 pocket unoccupied. Therefore, 41
it might be possible to increase the inhibitory activity against 42
BACE1 if a hydrophobic group is attached at the meta-position of 43
one phenyl ring to extend to the S3 pocket. Based on this 44
hypothesis, compounds described in Fig. 2a were designed. The 45
phenolic ester group was introduced to bind to the S3 pocket. 46
Another reason for this design was that it is the pharmacophore of 47
a brain penetrable AChE inhibitor Rivastigmine [11,12], which is 48
now the main therapeutic drug used for AD. We hope that the 49
integration of two brain penetrable pharmacophores would retain 50
the brain accessibility while increasing the BACE1 inhibitory 51
activity. We also want to replace the guanidine with urea or 52
thiourea, as they are also capable of forming the essential H-bond 53
interactions but the polarity of the molecule was greatly reduced. 54
The reduction of the molecule polarity might help the brain 55
penetration. One target compound (X = N atom) was docked into 56
the BACE1 active binding site to validate this design and the 57
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11
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hypothesis, accumulation of amyloid peptides (A
which is the main component of senile plaques, results in neuronal
toxicity and cell death [1]. A is produced from -amyloid
precursor protein ( -APP) by the sequential cleavage of the - and
-secretase [2]. -secretase (also known as BACE1, memapsin-2 or
b) in the brain,
b
b
b
b
g
b
Asp-2) has been considered an attractive therapeutic target for the
treatment of AD [3].
In this research we focused on a lead compound, 2-imino-3-
methyl-5,5-diphenylimidazolidin-4-one (1, Fig. 1). It is identified
by several unique studies as a weak BACE1 inhibitor with an IC50 of
7.1
mmol/L [4,5]. What makes this compound attractive is its
ability to cross the blood–brain barrier (BBB), which precluded
many excellent BACE1 inhibitors (like the HEA type, the first
generation BACE1 inhibitor [6,7] from further development. Based
on this, a series of brain penetrable BACE1 inhibitors have been
docking results confirmed our hypothesis above (2b).
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2. Experimental
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*
Corresponding authors.
The synthesis of designed compounds 9, 10 and 11 were shown 60
E-mail addresses: zhouwx@nic.bmi.ac.cn (W.-X. Zhou), ahnie512@163.com
(A.-H. Nie).
in Scheme 1. 3-hydroxybenzoic acid was used as the starting 61
http://dx.doi.org/10.1016/j.cclet.2015.07.020
1001-8417/ß 2015 Chinese Chemical Society and Institute of Materia Medica, Chinese Academy of Medical Sciences. Published by Elsevier B.V. All rights reserved.
Please cite this article in press as: J.-K. Liu, et al., Design and synthesis of cyclic acylguanidines as BACE1 inhibitors, Chin. Chem. Lett.
(2015), http://dx.doi.org/10.1016/j.cclet.2015.07.020

G Model
CCLET 3397 1–4
2
J.-K. Liu et al. / Chinese Chemical Letters xxx (2015) xxx–xxx
70%. A mixture of 7, NaCN and (NH4)2CO3 in ethanol/H2O was
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heated to reflux to give 8 (yield 54%). Target compound 9 was
obtained by methylation of 8 with CH3I, acetone and K2CO3 in a
yield of 48%. Lawesson’s reagent was used to transform the C55O
group of 9 to C55S to afford 10 (yield 63%). The C55S group of 10 was
finally converted to C55N bond with tert-butyl hydroperoxide and
ammonium hydroxide to give 11 in a yield of 21%.
The synthesis of designed compounds 16a–16k were shown in
Scheme 2. 3-Iodophenol was converted to 12 by reacting with
ethynyltrimethylsilane under the catalysis of CuI/PdCl2(PPh3)2/
TEA (yield 100%). PdCl2(PPh3)2/TBAF catalyzed cross coupling of 12
with 1-bromo-3-fluorobenzene (take R = ꢀF as an example) at
80 8C resulted in 13 in a yield of 100%. 14 was obtained by the
oxidation of the ethynyl group with KMnO4 in the presence of
MgSO4 and Na2CO3 (yield 65%). Cyclization and rearrangement
happened between 14 and 1-methylguanidine hydrochloride at
90 8C giving 15 in a yield of 58%. 15 reacted with ethyl (methyl)
carbamic chloride to give the final product 16 (yield 24%).
The structure of the new compounds was characterized by 1H
NMR and MS.
Fig. 1. Compound 1 (1a) and its interaction model with BACE1 active site (PDB:
4DJU) (1b).
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material. The phenol group was protected by reacting with CH3I in
the presence of K2CO3 to give 3. Intermediate 4 was obtained by the
hydrolysis of
3 with 4 mol/L NaOH (100% yield). The 3-
methoxybenzoyl chloride prepared from 4 by reacting with SOCl2
under reflux underwent an AlCl3 catalyzed Friedel–Crafts reaction
with benzene to afford 5 in a total yield of 77%. Deprotection of the
methyl group using AcOH/HBr under reflux gave 6 (yield 100%).
The phenol group was then transformed to a phenol ester group by
reacting with ethyl(methyl)carbamic chloride to give 7 in a yield of
3. Results and discussion
91
BACE1 inhibitor research has long been stagnating for not being
able to cross the BBB until the discovery of compound 1. As an
analogue of phenytion, it is brain penetrable and showed weak
BACE1 inhibitory activity. This brings great hope to the BACE1
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Fig. 2. Designed compounds (2a, type I) and docking result of compound 11 with BACE1 active binding site (2b).
Scheme 1. Synthetic route of the designed compounds (type I). Reagents and conditions: (a) K2CO3, CH3I, r.t.; (b) NaOH, r.t.; (c) i. SOCl2, reflux ii. AlCl3, benzene, r.t.; (d) AcOH,
48%HBr, reflux; (e) ethyl(methyl)carbamic chloride, acetone, K2CO3, r.t.; (f) NaCN, (NH4)2CO3, 120 8C; (g) acetone, K2CO3, CH3I, r.t.; (h) Lawesson’s reagent, reflux; (i) t-BuOOH,
NH3ꢁH2O, r.t.
Please cite this article in press as: J.-K. Liu, et al., Design and synthesis of cyclic acylguanidines as BACE1 inhibitors, Chin. Chem. Lett.
(2015), http://dx.doi.org/10.1016/j.cclet.2015.07.020

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J.-K. Liu et al. / Chinese Chemical Letters xxx (2015) xxx–xxx
3
Scheme 2. Synthetic route of designed compounds (type II). Reagents and conditions: (a) PdCl2(PPh3)2, TEA, CuI, r.t.; (b) TBAF, PdCl2(PPh3)2, 80 8C; (c) KMnO4, MgSO4,
Na2CO3, r.t.; (d) 1-methylguanidine hydrochloride, Na2CO3, EtOH/H2O, 90 8C; (e) DIPEA, THF, reflux.
Table 1
Inhibitory activity of target compounds against BACE1.
Compounds
X
R–
Inhibition rate (%) at 10.0
m
mol/L
IC50 (mmol/L)
9
O
S
–H
N/A
N/A
N/A
0.5
10
–H
N/A
11
N
N
N
N
N
N
N
N
N
N
N
N
–H
92.65
27.1
29.8
13.4
18.5
32.8
16.8
57.3
16.6
62.6
7.1
16a
16b
16c
16d
16e
16f
16g
16h
16i
16j
16k
–CH3
N/A
N/A
N/A
N/A
N/A
N/A
N/A
N/A
N/A
N/A
N/A
–OCF3
–Cl
–F
–CF3
–Br
–3–OCH3–Ph
–N(CH3)2
–3–Cl–Ph
–3,4,5-(–OCH3)3
–OH
12.0
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inhibitor research. In this report, 14 analogues of compound 1 were
designed and successfully synthesized. Compounds 9, 10, 11 were
tested for their BACE1 inhibitory activity using the time-resolved
fluorescence (TRF) method (Table 1). It was found that at a
Compounds 16a–16k were also tested for their BACE1 115
inhibitory activity using the time-resolved fluorescence (TRF) 116
method (Table 1). The results showed that inhibitory activity of 117
these compounds (type II) against BACE1 was lower than that of 118
compound 11. It suggests that when a hydrophobic group was 119
attached to one phenyl of the lead compound 1 the other phenyl 120
could not tolerate any substitutions. This is maybe due to the 121
conformational changes induced by the meta- substitutions, 122
making it difficult for the molecule to reach to the BACE1 binding 123
site to form the interactions essential for its enzymatic activity. 124
Efforts to optimize the structure of 11 to further improve potency 125
100
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103
104
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107
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110
111
112
113
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concentration of 10
inhibitory activity against BACE1 while compound 11 showed
an IC50 of 0.5 mol/L, 14-fold more potent than the lead compound
mmol/L, 9 and 10 exhibited almost no
m
1, which is in accordance with our expectations. As the
replacement of guanidine with urea and thiourea leads to a large
bioactivity loss, it is suggested that the guanidine was essential for
the BACE1 inhibitory activity. Unable to change the phenol ester
group or the guanidine, we tried to introduce a group on the other
phenyl ring to design another kind of compounds (type II,
compounds 16a–16k) and investigate whether this modification
further improves the potency (Fig. 3). Different type (electron-
withdrawing or electron-donating) and size of groups were
introduced. We hope that this modification could help enhancing
the hydrophobic interactions between the small molecule and the
protein active binding site thus improves the enzymatic potency.
are ongoing.
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4. Conclusion
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In conclusion, based on the lead compound 1, a series of cyclic 128
acylguanidine BACE1 inhibitors were designed and synthesized. 129
Enzymatic inhibition tests showed that compound 11 exhibited a 130
14-fold improvement in potency, which implies that the designing 131
strategy of extending the hydrophobic phenyl ester group from the 132
S1 pocket to occupy the S3 pocket is of great value. This compound 133
represents a good lead for the discovery of new BACE1 inhibitors. 134
Acknowledgments
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This work was supported by grants from The National Natural Q2136
Science Foundation of China (No. 81172924) and Beijing Municipal 137
Fig. 3. Designed compounds (type II).
Natural Science Foundation (No. 7112106).
138
Please cite this article in press as: J.-K. Liu, et al., Design and synthesis of cyclic acylguanidines as BACE1 inhibitors, Chin. Chem. Lett.
(2015), http://dx.doi.org/10.1016/j.cclet.2015.07.020

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Appendix A. Supplementary data
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Please cite this article in press as: J.-K. Liu, et al., Design and synthesis of cyclic acylguanidines as BACE1 inhibitors, Chin. Chem. Lett.
(2015), http://dx.doi.org/10.1016/j.cclet.2015.07.020