Discovery of novel (S)-α-phenyl-γ-amino butanamide containing CCR5 antagonists via functionality inversion approach

Article abstract of DOI:10.1016/j.bmcl.2010.02.023

By using functionality inversion approach, we identified a new scaffold containing (S)-α-phenyl-γ-amino butanamide as CCR5 antagonists derived from the 1,3-propanediamine carboxamide pharmacophore protocol. The (2S)-2-phenyl-4-(8-aza-bicyclo[3.2.1]octan-8

Full text of DOI:10.1016/j.bmcl.2010.02.023

Bioorganic & Medicinal Chemistry Letters 20 (2010) 2219–2223
Contents lists available at ScienceDirect
Bioorganic & Medicinal Chemistry Letters
journal homepage: www.elsevier.com/locate/bmcl
Discovery of novel (S)-a-phenyl-c-amino butanamide containing CCR5
antagonists via functionality inversion approach
Hu-Shan Zhang a, Dong-Zhi Feng a, Li Chen b, Ya-Qiu Long a,
*
a State Key Laboratory of Drug Research, Shanghai Institute of Materia Medica, Shanghai Institutes for Biological Sciences, CAS, 555 Zuchongzhi Road, Shanghai 201203, China
b Shanghai TargetDrug Co. Ltd, 500 Caobao Road, Shanghai 200233, China
a r t i c l e i n f o
a b s t r a c t
Article history:
By using functionality inversion approach, we identified a new scaffold containing (S)-a-phenyl-c-amino
Received 21 December 2009
Revised 25 January 2010
Accepted 5 February 2010
Available online 10 February 2010
butanamide as CCR5 antagonists derived from the 1,3-propanediamine carboxamide pharmacophore
protocol. The (2S)-2-phenyl-4-(8-aza-bicyclo[3.2.1]octan-8-yl)-butanamide derivatives display signifi-
cantly high potency to antagonize CCR5 receptor with nanomolar IC50 values.
Ó 2010 Elsevier Ltd. All rights reserved.
Keywords:
CCR5 antagonist
HIV-1 entry inhibitor
Functionality inversion
Bioisostere
Butanamide
Tropane
2-Methyl-3H-imidazo[4,5-b]pyridine
3H-[1,2,3]Triazolo[4,5-b]pyridine
Human immunodeficiency virus (HIV) is the causative pathogen
of AIDS, a potentially fatal condition resulting from the failure of
the immune system. The AIDS epidemic continues to be a signifi-
cant global threat causing the deaths of 25 million people.1
Although highly active antiretroviral therapy (HAART) has been
successful in reducing the AIDS-related mortality rates dramati-
cally, the rapid emergence of viral strains resistant to current clin-
ical treatments and the dynamic nature of the HIV-1 genome
demand a continued effort to identify alternative points of inter-
vention in the viral life cycle. Targeting the viral entry process
has become a new focus of research for the next generation of
HIV antiretroviral therapies.2
CCR5 is an essential co-receptor for HIV-1 recognition and entry
into CD4+ macrophages and T-cells, but not essential for human
functions.3–5 Many pharmaceutical companies and academic insti-
tutions have been enthusiastically investigating potent antagonists
against CCR5 as viral entry inhibitors,6 and indeed several small-
molecule CCR5 antagonists (Nifeviroc,7 UK-427857,8 GW-873140,9
TAK 22010 as shown in Fig. 1) are now being evaluated in clinical tri-
als. Among them, the UK-427857 (Maraviroc) has become the first
CCR5 antagonist approved by FDA for the treatment of HIV
infection.11
Most of the active structures (represented in Fig. 1) share a
common feature: a basic nitrogen atom which is believed to ‘an-
chor’ the ligand to the key residue E283 within the transmembrane
region of the CCR5 receptor,12 and a proximal lipophilic group,
which likely lies in close contact with a tyrosine residue, Y108.
Combining the structural features of the CCR5 binding pocket,13
we proposed a three-domain pharmacophore model of propane-
1,3-diamine skeleton flanked by two hydrophobic domains for
CCR5 inhibition, from which we identified a new structure CCR5
inhibitor with sub-micromolar IC50 value (designated as TD0444,
Fig. 2).14 A similar six-feature pharmacophore model constructed
via Catalyst was recently reported to accelerate the discovery of
CCR5 antagonists.15
Since our hit compound (TD0444) is structurally related to
Maraviroc, we were intrigued to derive novel scaffold by employing
functionality inversion and bioisostere approach based on the three-
feature pharmacophore model. We were particularly interested in
the Maraviroc-like amide functionality, which has been shown to
impart favorable properties on the Maraviroc series.16 Taking this
amide functionality as a breakthrough point, we designed a distinct
template of (2S)-2-aryl-4-amino-butancarboxamide by inverting
the amide functionality of hit compound TD0444 into carboxamide
as a bioisostere, as depicted inFigure 2. Furthermore, the carboxam-
ide group can be further transformed into carboxylic ester and ether
functionality to derivatize structurally diverse skeletons.
* Corresponding author. Tel./fax: +86 21 50806876.
E-mail address: yqlong@mail.shcnc.ac.cn (Y.-Q. Long).
0960-894X/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2010.02.023

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H.-S. Zhang et al. / Bioorg. Med. Chem. Lett. 20 (2010) 2219–2223
O
O
N
OH
N
O
N
N
NH
N
O
O
HO
HO
O
O
GW-873140(Aplaviroc)
N
Nifeviroc
N
NO2
N
F
F
O
O
H
N
N
N
N
NH2
O
N
O
Cl
UK-427857(Maraviroc)
TAK 220
Figure 1. Representative structures of small-molecule CCR5 antagonists in clinic trials, sharing the common 1,3-propanediamine skeleton flanked by two hydrophobic
domains.
Aromatic Group
with less hydrophobicity
Hydrophobic Group
O
N
N
H
O
R1
O
O
Basic center
R1
O
H
N
H
H
N
O
O
R1
NH
R2
N
N
N
Ar
N
1-6(a-c)
exo-
F3C
a
c
b
TD0444 IC50 = 253 nM
R2:
exo-
N
N
endo-
N
N
N
N
N
N
N
N
Figure 2. The design of (2S)-2-aryl-4-amino-butancarboxyl based CCR5 inhibitors by bioisosteric replacement strategy.
As shown in Figure 2, the substitution around the 4-tropane
dation of the acid by (4-fluorophenyl) methanamine in the pres-
ence of EDCI/HOAt/DIPEA (9a–c?3a–c), and the esterification of
the acid by methanol in the solution of SOCl2/MeOH (9a–c?2a–c).
For the synthesis of the chiral b-phenyl-b-carboxamide aldehyde
(7), the Evans chiral induction protocol was employed as the key
step.17 As shown in Scheme 2, the 2-phenylacetic acid (10) was cou-
pled with (S)-4-benzyloxazolidin-2-one to give the corresponding
amidebearing a chiral auxiliary(11). The subsequentstereoselective
alkylation in the presence of sodium bis(trimethylsilyl)amide affor-
ded the (S)-ethyl 4-((S)-4-benzyl-2-oxooxazolidin-3-yl)-4-oxo-3-
phenylbutanoate (12).18 Because the (S)-4-benzyloxazolidin-2-one
served as the carboxyl protective group as well for further transfor-
mation, we chose DIBAL-H reduction to convert the butanoate (12)
into butanal (7) with the chiral auxiliary intact.
Before the synthesis of the heterocycle substituted tropane, the
exo- and endo-tropanamine (compound 13a,b) were prepared from
3-tropanone according to the literature method. The 3-exo-tropan-
amine (13a) was synthesized from 3-tropinone oxime by reduction
with sodium in 1-propanol,19 whereas 3-endo-tropanamine (13b)
was obtained by a palladium-catalyzed reductive amination of 3-
tropanone with ammonium formate in aqueous methanol.20 Fur-
thermore, the 3-endo-tropanamine (13b) was N-demethylated
with ethyl chloroformate prior to the reductive amination, result-
ing in the N-ethoxycarbonyl protection.
substituted-butancarboxamide skeleton was investigated. The
exo- or endo-2-methyl-3H-imidazo[4,5-b]pyridin-3-yl or exo-3H-
[1,2,3]triazolo[4,5-b]pyridine-3-yl group was incorporated on the
right hand side of the central tropane core. The substituted phenyl
amide or ester or ether group was examined as hydrophobic group
on the left hand side. As a comparison, some intermediate with po-
lar groups on the left hand side was tested too. We hoped that the
novel template with matched functionality substituents could en-
sure CCR5 potency, providing an orthogonal structure to the cur-
rent CCR5 inhibitors.
The synthesis of the
a-aryl-c-aminobutancarboxamide and its
bioisosteric derivatives was achieved by employing convergent
synthesis strategy. As depicted in Scheme 1, starting from the
two building blocks (S)-4-((S)-4-benzyl-2-oxooxazolidin-3-yl)-4-
oxo-3-phenylbutanal (7) and the heterocycle substituted tropane
(8a–c), reductive amination proceeded in the presence of sodium
triacetoxyborohydride and acetic acid to generate the central core
structure (1a–c). Removal of the chiral auxiliary oxazolidinone via
LAH reduction or nucleophilic replacement by (4-fluoro-
phenyl)methanol or hydrolysis by lithium hydroxide/hydrogen
peroxide produced corresponding alcohol (5a), ester (4a) or acid
(9a–c). Further structural derivatization included the etherification
of the alcohol by 4-fluorophenylmethyl bromide (5a?6a), the ami-

H.-S. Zhang et al. / Bioorg. Med. Chem. Lett. 20 (2010) 2219–2223
2221
N
H
F
HO
N
R
O
iii
H
Ph
O
N
HN
O
H
Ph
N
Ph
N
5a
Ph
N
6a
R
H
H
8a-c
H
O
O
HO
Ph
O
ii
R'
O
iv
i
O
v or vi
R
R
R
O
O
Ph
N
9a-c
N
Ph
Ph
N
H
1a-c
7
2a-c: R' = OCH3
3a-c: R' = NHCH2-(4-F-Ph)
vii
N
N
F
a
c
b
exo-
O
O
exo-
R:
endo-
N
N
N
N
N
H
N
N
Ph
N
N
N
4a
N
Scheme 1. Convergent synthesis of the tropane bridged
a-phenyl-c-aminobutanamide and its bioisosteric derivatives by employing reductive amination as the key step.
Reagents and conditions: (i) NaB(OAc)3H/HOAc, 1,2-dichloroethane, rt, 82–90%; (ii) LiAlH4/THF, À78 °C, 1 h, 68–98%; (iii) NaH/DMF, 1-(bromomethyl)-4- fluorobenzene, rt,
85%; (iv) LiOH/H2O2, THF–H2O (3:1/,v/v), rt, 10 h; (v) SOCl2/CH3OH, refluxing to rt, 97% (for 2a–c); (vi) EDCI/HOAT/DIPEA, (4-fluorophenyl)methanamine, CH2Cl2, rt, two steps
overall yield 64–75% (for 3a–c); (vii) n-BuLi/THF, 4-fluorophenylmethanol, À78 to À50 °C, 10 h, 80%.
H
O
O
O
O
O
O
O
O
N
O
N
H
Ph
iv
iii
O
O
O
Ph
N
O
Ph
N
i,ii
OH
O
O
10
11
12
7
Scheme 2. Synthesis of (S)-4-oxo-3-phenylbutanal by using Evans reagent. Reagents and conditions: (i) (COCl)2/CH2Cl2, rt, 3 h; (ii) n-BuLi/THF, À78 °C to 0 °C, 2 h, 91–95%;
(iii) NaN(SiMe3)2/BrCH2COOEt/THF, À78 °C, 3–5 h, 80–85%; (iv) DIBAl-H/CH2Cl2, À78 °C, 3–5 h, 60–80%.
Then, treatment of the exo- or endo-tropanamine (compound
13a,b) with 2-chloro-3-nitropyridine in dry acetonitrile followed
by the Pd-catalyzed hydrogenation afforded the precursor pyri-
dine-2,3-diamine substituted tropane (15a,b), as shown in Scheme
3. The 2-methyl-3H-imidazo[4,5-b]pyridine substituted tropane
(16a,b) was directly formed in refluxing Ac2O, while 3H-
[1,2,3]triazolo[4,5-b]pyridine substituted tropane (16c) was pre-
pared via diazotization in good yield.21 Then, the exo-tropane
derivatives were demethylated by reacting with ethyl chlorofor-
mate at this stage (16a,c). The subsequent deprotection in acidic
conditions afforded the amine component (8a–c) for the final
reductive amination, that is, exo- and endo-2-methyl-3H-imi-
dazo[4,5-b]pyridine substituted tropanes and exo-3H-[1,2,3]triaz-
olo[4,5-b]pyridine substituted tropane.
Based on the new template, a series of tropane substituted
(S)-a-phenyl-c-aminobutanamide and its bioisosteric derivatives
were synthesized, with variation on the hydrophobic domains
(Scheme 1). The heterocycles substituted on the tropane was
R
NO2
Cl
R
R
N
N
N
ii
N
N
N
NH2
NH NO2
NH NH2
i
H
H
H
13a, R = Me,
exo-
14a, R = Me,
exo-
15a, R = Me,
15b, R = EtOCO, endo-
exo-
13b, R = EtOCO, endo-
14b, R = EtOCO, endo-
EtO
H
N
N
O
iii (for 16a,b) or iv (for 16c)
then v (for 16a,c)
N
vi
N
N
N
N
N
X
H
X
H
Y
Y
16a, X=C, Y= CH3, exo-
16b, X=C, Y= CH3, endo-
16c, X= N, Y deleted, exo-
8a, X=C, Y= CH3, exo-
8b, X=C, Y= CH3, endo-
8c, X= N, Y deleted, exo-
Scheme 3. Synthesis of the heterocycle substituted exo- and endo-tropane. Reagents and conditions: (i) K2CO3/CH3CN, reflux, 24 h, 83–99%; (ii) 10% Pd–C/H2 (1 atm), MeOH,
22 h, 90–94%; (iii) Ac2O, 165 °C, 24 h, 85–95%; (iv) NaNO2/HCl(aq), À10 °C 30 min to 10 °C, 1 h, 95%; (v) ethyl chloroformate/CHCl3, K2CO3, reflux, 4 h, 40–80%; (vi) HCl, reflux,
10 h, 52–60%.

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H.-S. Zhang et al. / Bioorg. Med. Chem. Lett. 20 (2010) 2219–2223
Table 1
CCR5 inhibition data of the tropane substituted (S)-
a
-phenyl-
c
-aminobutanamide and its bioisosteric derivatives
H
R1
X
R2
N
1-6(a-c)
Compd
X
O
R1
R2
IC50, nM or inhibition ratea
30% inhibition @ 300 nM
O
O
O
N
exo-
N
N
N
1a
N
N
O
endo-
N
1b
2a
2b
O
O
O
10% inhibition @ 300 nM
22% inhibition @ 300 nM
112
N
N
exo-
N
–OCH3
N
N
endo-
exo-
N
–OCH3
–OCH3
N
N
N
N
2c
3a
O
O
>10 lM
N
F
F
N
H
N
exo-
N
14.4
N
endo-
H
N
N
N
3b
O
30% inhibition @ 300 nM
N
F
F
exo-
N
N
N
H
N
3c
4a
O
O
11% inhibition @ 300 nM
19% inhibition @ 300 nM
N
N
N
N
exo-
exo-
exo-
N
N
N
O
N
N
N
5a
6a
H
H
–OH
NAb
F
O
42% inhibition @ 300 nM
a
Inhibition of RANTES-stimulated [35S]-GTP
cS binding to CCR5-expressing CHO cell membranes.
Inactive @ 10 lM
b
examined with exo- and endo-2-methyl-3H-imidazo[4,5-b]pyri-
dine and exo-3H-[1,2,3]triazolo[4,5-b]pyridine, since the two
heteroaromatic structures have been identified as an optimal
fragment for CCR5 binding. The SAR investigation was focused
on the carboxamide bioisosteres to identify an effective
scaffold.
As shown in Table 1, these compounds were evaluated on the
CCR5 inhibition. For the carboxamide bioisostere examined, com-

H.-S. Zhang et al. / Bioorg. Med. Chem. Lett. 20 (2010) 2219–2223
2223
parison of the exo-2-methyl-3H-imidazo[4,5-b]pyridine substi-
tuted analogs (2a, 3a, 4a, 5a and 6a) revealed that the 4-fluor-
obenzylbutamide motif provided a significant potency advantage
versus butanoate (2a, 4a) and butyl ether (5a), affording the most
potent CCR5 inhibitor in this series (3a, IC50 = 14.4 nM). The polar
butanol counterpart (5a) without hydrophobic group turned out
inactive. This encouraging result proved the rationale of the func-
tionality inversion approach in the design of novel CCR5 antago-
nists. In addition, the potency data of series 2a–c and 3a–c
suggested a preference of 2-methyl-3H-imidazo[4,5-b]pyridine
over 3H-[1,2,3]triazolo[4,5-b]pyridine as an optimal heterocycle
motif, exemplified by the potent inhibitors 2b (IC50 = 112 nM)
and 3a (IC50 = 14.4 nM). And in most cases, the exo-tropane (1a,
3a) was more potent than the endo-tropane counterpart (1b, 3b).
This finding was consistent with the literature-reported activity
difference between the exo and endo-tropane isomers.16 According
to the NMR-based conformational studies, the tropane in the endo
series adopts a pseudo-boat conformation to minimize the 1,3-
diaxial strain between the bulky heterocycle and the carbon bridge
of the tropane, rather than the normally preferred chair conforma-
tion, as adopted by exo series.16 Consequently, the 2-methyl-3H-
imidazo[4,5-b]pyridine lies in approximately the same geometric
position for both the exo (1a, 3a) and endo (1b, 3b) series, but
the greater steric crowding for the endo isomer might result in a
subtle conformational difference that is less well tolerated by the
CCR5 receptor, leading to the deleterious effect.
(07QH14018, 08JC1422200) and National Science & Technology
Major Project (2009ZX09301-001) are greatly appreciated for the
financial supports.
References and notes
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In conclusion, by adopting the functionality inversion and bio-
isostere replacement strategy, a new scaffold of (S)-a-phenyl-c-
aminobutanamide was identified as potent CCR5 inhibitors. An
efficient synthesis was developed to build the tropane substituted
(S)-a-phenyl-c-aminobutanamide and its bioisosteric derivatives,
involving the Evans chiral induction protocol and the reductive
amination reaction. The resulting (2S)-N-(4-fluorobenzyl)-4-(3-
(2-methyl-1H-benzo[d]imidazol-1-yl)-8-aza-bicyclo[3.2.1]octan-
8-yl)-2-phenylbutanamide displayed significantly high potency to
antagonize CCR5 receptor with nanomolar IC50 values, affording a
new scaffold for further development of CCR5 antagonists.
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Acknowledgements
National Natural Science Foundation of China (30672528),
Science and Technology Commission of Shanghai Municipality