Studies on the structure-activity relationship of 1,3,3,4-tetra-substituted pyrrolidine embodied CCR5 receptor antagonists. Part 2: Discovery of highly potent anti-HIV agents

Article abstract of DOI:10.1016/j.bmcl.2010.05.046

Modification of 1,3,3,4-tetra-substituted pyrrolidine embodied CCR5 receptor antagonists revealed that introducing a fluoro group at the 3-position of the 3-phenyl group to reduce metabolism did not adversely affect the high potency against HIV infection, and that replacing the piperidine ring with a tropane ring could deliver the most potent anti-HIV agents. Stereochemistry of the substituted tropane ring is essential for maintaining the potent anti-HIV activity because only exo-isomers displayed subnanomolar whole cell activity.

Full text of DOI:10.1016/j.bmcl.2010.05.046

Bioorganic & Medicinal Chemistry Letters 20 (2010) 5334–5336
Contents lists available at ScienceDirect
Bioorganic & Medicinal Chemistry Letters
journal homepage: www.elsevier.com/locate/bmcl
Studies on the structure–activity relationship of 1,3,3,4-tetra-substituted
pyrrolidine embodied CCR5 receptor antagonists. Part 2: Discovery
of highly potent anti-HIV agents
Ben Li a, , Eric Dale Jones b, , Enkun Zhou c, , Li Chen d, Dean Cameron Baylis b, Shanghai Yu e, Miao Wang e,
c,
Xing He c, Jonathan Alan Victor Coates b, David Ian Rhodes b, Gang Pei a, John Joseph Deadman b, , Xin Xie
,
*
*
Dawei Ma e,
*
a Shanghai Institutes for Biological Sciences, Chinese Academy of Sciences, 319 Yueyang Lu, Shanghai 200031, PR China
b Avexa Ltd, 576 Swan Street, Richmond, Vic. 3121, Australia
c State Key Laboratory of Drug Research, National Center for Drug Screening, Shanghai Institute of Materia Medica, Chinese Academy of Sciences, 189 Guo Shou Jing Road,
Shanghai 201203, PR China
d Shanghai Targetdrug Co., Ltd, 500 Caobao Lu, Shanghai 20023, PR China
e State Key Laboratory of Bioorganic and Natural Products Chemistry, Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences, 354 Fenglin Road,
Shanghai 200032, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Modification of 1,3,3,4-tetra-substituted pyrrolidine embodied CCR5 receptor antagonists revealed that
introducing a fluoro group at the 3-position of the 3-phenyl group to reduce metabolism did not
adversely affect the high potency against HIV infection, and that replacing the piperidine ring with a tro-
pane ring could deliver the most potent anti-HIV agents. Stereochemistry of the substituted tropane ring
is essential for maintaining the potent anti-HIV activity because only exo-isomers displayed subnanom-
olar whole cell activity.
Received 12 April 2010
Revised 11 May 2010
Accepted 12 May 2010
Available online 19 May 2010
Keywords:
CCR5 receptor antagonist
Anti-HIV
Ó 2010 Elsevier Ltd. All rights reserved.
Since the b-chemokine receptor CCR5 was identified as an
essential co-receptor for HIV-1 to dock and gain entry into CD4+
macrophages and T-cells, great efforts have been directed to the dis-
covery of selective CCR5 antagonists in order to develop promising
agents for the treatment and/or prevention of HIV-1 infection.1,2
These efforts have cumulated a variety of CCR5 antagonists that dis-
play significant activity against anti-HIV infection. Among the most
potent compounds ( Fig. 1), Takeda reported that TAK-652 (1)
strongly inhibited the replication of six R5 HIV-1 clinical isolates
(KK, CTV, HKW, HNK, HTN, and HHA) in human peripheral blood
mononuclear cells (PBMCs), with EC50 values ranging from 0.024
to 0.089 nM.3 Schering–Plough researchers described that vicriviroc
(2) potently inhibit all the viral isolates tested, with geometric mean
EC50 values ranging between 0.04 and 2.3 nM.4 Roche’s vicriviroc
analog 3 also showed very potent antiviral activity with an EC50 of
0.5 nM.5 Pfizer’s maraviroc (4) showed potent antiviral activity
against the 43 primary CCR5-tropic HIV-1 isolates tested, as re-
flected by a geometric mean EC90 of 2.0 nM (95% CI, 1.8–2.4 nM).6,7
During the studies on structure–activity relationship of 1,3,3,4-tet-
rasubstituted pyrrolidine embodied CCR5 receptor antagonists,
some of our compounds have been improved to have single digit
picomolar antiviral activity in PBMCs. These molecules may repre-
sent the most potent anti-HIV agents among the known CCR5 recep-
tor antagonists. Herein we wish to disclose our results.
In a previous report, we found that modification of the N-sub-
stituents of nifeviroc (5, Fig. 2) led to the discovery of two urea
analogs (6 and 7) and one amide analog (8) that retained potent
anti-HIV activity. Based on these results, we planned to modify
their benzene ring by introducing different substituents, and their
piperidine ring by freezing its conformation by an ethylene bridge.
Accordingly, the molecules 9 were designed as our new targets.
As depicted in Schemes 1 and 2, our desired compounds were
assembled in a convergent manner. Baylis–Hillman reaction of
methyl acrylate with
provided diesters 11 in with 32–47% yields.8 Michael addition of
(R)- -methyl-benzylamine to these diesters followed by intra-
a-keto esters 10 catalyzed by DABCO
a
molecular condensation in refluxing dioxane afford enantiopure
pyrrolidones 12 in 22–30% yields, upon crystallization from EtOAc
solution. Hydrolysis of 12 accompanied by isomerization provided
acids 13. Reduction of 13 with LAH and subsequent hydrogenolysis
produced free amines, which were reacted with cyclopentanecar-
* Corresponding authors.
E-mail addresses: jdeadman@avexa.com.au (J.J. Deadman), xiexin-2004@163.
com (X. Xie), madw@mail.sioc.ac.cn (D. Ma)
These authors contributed equally to this work.
.
0960-894X/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2010.05.046

B. Li et al. / Bioorg. Med. Chem. Lett. 20 (2010) 5334–5336
5335
i-Bu
Z
O
N
O
HO
methyl acrylate, DABCO
rt, 7-10 d, 32-47%
CO2Et
Z
OEt
H
N
MeO
O
Pr
N
BuO
11
10
O
O
S
O
TAK-652 (1)
Z
O
CF3
1. (R)-α-methylbenzylamine, MeOH
2. TFA (0.3 eq), dioxane, reflux, 3d MeO
2 N NaOH
MeOH
N
HO
85-93%
3. recrystallization from EtOAc
22-30%
O
O
N
N
N
N
N
Ph
O
12
N
Vicriviroc (2)
Z
Z
O
HO
OHC
1. LiAlH4, THF, 0 ºC-reflux
HO
O
HO
2. Pd(OH)2/C, H2 (50 atm), MeOH
N
N
N
O
N
3. cyclopentanecarbonyl chloride,
Et3N, 0 ºC-rt
O
N
N
n-Bu
3
O
N
4. Py·SO3, DMSO, Et3N
40-60%
Ph
OEt
14
13
F
Scheme 1.
F
N
N
H
H
N
N
H
N
O
1. (Boc)2O, NaOH, dioxane, 93%
2. allyl amine, NaBH(OAc)3, AcOH
THF, 0 ºC-r.t., 95%
O
NH
xHCl
HN
NBoc
Maraviroc (4)
15
Figure 1. Structures of some CCR5 antagonists with potent anti-HIV activity.
Z
HO
N
O
NH
N
O
N
O
O
1. 4-nitrobenzyl
chloroformate
HO
14, HOAc
HO
N
N
N
N
O
N
N
NaBH3CN
Et3N, DCM
2. TFA, DCM
86%
O
O
15
MeOH
NH
O
68-83%
N
O
O
NO2
NO2
16
17
NCO
Z
X
NO2
6: X = CF3
nifeviroc (5)
HO
N
N
NH
N
N
1.
7: X = SO2NMe2
O
O
NH
NH
14, HOAc
CF3
NaBH3CN
15
MeOH
2. TFA, DCM
83%
O
R
68-83%
Z
HO
N
N
N
O
HO
N
N
N
CF3
CF3
19
O
18
CO2H
Y
R
Z
O
HO
N
N
N
NH
1.
O
O
O
SO2NHMe
8
9
14, HOAc
SO2NHMe
NaBH3CN
N
15
Figure 2. Structures of some pyrrolidine embodied anti-HIV agents and newly
MeOH
68-83%
EDCI, HOBt
2. TFA, DCM
95%
O
designed molecules.
SO2NHMe
SO2NHMe
20
21
bonyl chloride and then oxidized with PyÁSO3 to deliver aldehydes
14.
Scheme 2.
In a parallel procedure, protection of 4-piperidinone followed
by reductive amination afforded allyl amine 15. Condensation of
15 with 4-nitrobenzyl chloroformate, or an isocyanate or an acid
and subsequent Boc deprotection with TFA produced amines 16,
18, and 20. These amines were subjected to reductive amination
with the aldehydes 14 to give the target molecules 17, 19, and
21 (Scheme 2).
ing CHO cell membranes and their antiviral activity in human
peripheral blood mononuclear cells (R5-tropic HIV-1Ba-L strain).
As summarized in Table 1, it is obvious that introduction of a meth-
oxy or fluoro group at the 4-position of the phenyl ring resulted in
a dramatic loss of antiviral activity and binding inhibition (17a,
17b, 19a, and 21a). However, adding a fluoro group at the 3-posi-
tion still delivered highly potent molecules (17c, 19b, and 21b).
All the compounds synthesized were tested for both their inhi-
bition of RANTES-stimulated [35S]GTP
cS binding to CCR5-express-

5336
B. Li et al. / Bioorg. Med. Chem. Lett. 20 (2010) 5334–5336
Table 1
Table 2
Biological data for compounds 5, 17, 19, and 21
Biological data for compounds 25–30
a
b
a
b
d
Compound
IC50 (nM)
EC50 (nM)
Compound
IC50 (nM)
EC50 (nM)
hERG bindingc
CC50 (lM)
5
2.9
318
30
8.85
38.3
1.38
11.17
2.98
<0.4
>100
>100
<0.4
20
6
>100
4.5
25
26
27
28
29
30
6.77
11.4
30
3.98
14.3
27.1
<0.005
0.0055
1.75
16
>100
>100
89.5
31.3
—
—
88.6
—
12.5
55
42
65
102.7
50
17a: Z = 4-OMe
17b: Z = 4-F
17c: Z = 3-F
19a: Z = 4-F
19b: Z = 3-F
21a: Z = 4-F
21b: Z = 3-F
a
b
c
SPA GTP
Antiviral activity in PBMC.
% inhibition at 1 M.
Cellular toxicity (PBMC).
cS assay.
a
SPA GTP
Antiviral activity in PBMC.
c
S assay.
l
b
d
allyl amine
NBoc
compounds kept good antiviral activity in PBMCs while the endo-
configuration was detrimental to the anti-HIV-1 activity (cf. 25
and 29, 28 and 30). Gratifyingly, compound 25 showed excellent
potency and the mean EC50 value was less than 5 pM. Inspired by
the surprising results, some substituents on the left benzyl ring
were examined in the para-position. The SO2N(CH3)2 substituted
analog 26 had an EC50 of which was 5.5 pM which is similar po-
tency to compound 25.
H
N
NBoc
NBoc
NaBH3CN
+
AcOH, MeOH
86%
NH
O
22
24
23
exo:endo = 3:7
R'
NCO
H
O
N
F
1.
HO
N
N
The compounds were also nontoxic with CC50’s >10 lM, giving a
selectivity index of greater than 2 Â 106 However, substituting of
p-CF3 benzyl by fluoro in 27 decreased the antiviral activity, but
the compound still kept good potency (EC50 = 1.75 nM). The key
finding from this study was that the piperidine ring of our com-
pounds could be substituted by an exo-tropane ring.
R'
2. TFA, DCM
14
N
25: R' = CF3
3. , NaBH3CN
O
26
27
: R' = SO2NMe2
: R' = F
23
O
1. substituted
F
In summary, through further SAR studies on 1,3,3,4-tetrasubsti-
tuted pyrrolidine embodied CCR5 receptor antagonists, we have
demonstrated for the original piperidine based series that intro-
ducing a substituent at the 3-position of the phenyl ring was pos-
sible without loss of activity, while substituents in the 4-position
were not tolerated. Significantly, by replacing the piperidine ring
with a tropane ring, two exo-analogs with single digital picomolar
potency for antiviral activity and nanomolar binding affinity, were
discovered. Their excellent potency was found to be highly depen-
dent on the stereochemistry of the substituted tropane ring,
because endo-isomers showed considerably low potency for anti-
HIV activity, although binding inhibition ability to CCR5 receptor
could not distinguish between them.
2-phenylacetic acid
2. TFA, DCM
HO
N
N
N
MeHNO2S
14
3. , NaBH3CN
28
O
F
HO
N
N
F3C
H
N
N
2. TFA, DCM
O
O
O
3. 14, NaBH3CN
29
24
Acknowledgments
F
1. substituted
HO
N
N
2-phenylacetic acid
2. TFA, DCM
The authors are grateful to the National Natural Science
Foundation of China (Grant 90713047), the Ministry of Science
and Technology (Grants 2008DFB30150, 2009ZX09302-001, and
2009ZX09501-009), and the Chinese Academy of Sciences for their
financial support.
14
3. , NaBH3CN
N
O
MeHNO2S
30
Scheme 3.
References and notes
1. Esté, J. A. Curr. Med. Chem. 2003, 10, 1617.
2. Shaheen, F. Curr. Opin. Infect. Dis. 2004, 17, 7.
For modification of the piperidine part, we planned to replace
this unit with the tropane moiety, and hoped its rigid conformation
would make the resulting molecules more potent. The synthesis of
our target molecules was outlined in Scheme 3, N-Boc protected 8-
aza-bicyclo[3.2.1]octan-3-one 22 was reacted with allylamine un-
der reductive amination conditions to produced exo-isomer 23 and
endo-isomer 24 which were separated in a ratio of 3:7. Following
the same procedures as indicated in Scheme 2, these two amines
were converted into the desired urea analogs 25–27 and 29, and
amide analogs 28 and 30.
3. Baba, M.; Takashima, K.; Miyake, H.; Kanzaki, N.; Teshima, K.; Wang, X.;
Shiraishi, M.; Iizawa, Y. Antimicrob. Agents Chemother. 2005, 49, 4584.
4. Strizki, J. M.; Tremblay, C.; Xu, S.; Wojcik, L.; Wagner, N.; Gonsiorek, W.; Hipkin,
R. W.; Chou, C.-C.; Pugliese-Sivo, C.; Xiao, Y.; Tagat, J. R.; Cox, K.; Priestley, T.;
Sorota, S.; Huang, W.; Hirsch, M.; Reyes, G. R.; Baroudy, B. M. Antimicrob. Agents
Chemother. 2005, 49, 4911.
5. Yang, H.; Lin, X.-F.; Padilla, F.; Gabriel, S. D.; Heilek, G.; Ji, C.; Sankuratri, S.; de
Rosier, A.; Berry, P.; Rotstein, D. M. Bioorg. Med. Chem. Lett. 2009, 19, 209.
6. Dorr, P.; Westby, M.; Dobbs, S.; Griffin, P.; Irvine, B.; Macartney, M.; Mori, J.;
Rickett, G.; Smith-Burchnell, C.; Napier, C.; Webster, R.; Armour, D.; Price, D.;
Stammen, B.; Wood, A.; Perros, M. Antimicrob. Agents Chemother. 2005, 49, 4721.
7. Barber, C. G.; Blakemore, D. C.; Chiva, J.-Y.; Eastwood, R. L.; Middleton, D. S.;
Paradowski, K. A. Bioorg. Med. Chem. Lett. 2009, 19, 1075.
All the tropane-bridged derivatives showed good CCR5 binding
inhibition, whether they are exo- or endo-configuration (Table 2).
However, their antiviral activity was very different: all the exo-
8. Ma, D.; Yu, S.; Li, B.; Chen, L.; Chen, R.; Yu, K.; Zhang, L.; Chen, Z.; Zhong, D.;
Gong, Z.; Wang, R.; Jiang, H.; Pei, G. ChemMedChem 2007, 2, 187.