1,3,4-Trisubstituted pyrrolidine CCR5 receptor antagonists: Modifications of the arylpropylpiperidine side chains

Article abstract of DOI:10.1016/S0960-894X(02)00829-6

The 4-(3-phenylprop-1-yl)piperidine moiety of the 1,3,4-trisubstituted pyrrolidine CCR5 antagonist 1 was modified with electron deficient aromatics as well as replacement of the benzylic methylene with sulfones, gem-difluoromethylenes and alcohols in an effort to balance the antiviral potency with reasonable pharmacokinetics.

Full text of DOI:10.1016/S0960-894X(02)00829-6

Bioorganic & Medicinal Chemistry Letters 13 (2003) 119–123
1,3,4-Trisubstituted Pyrrolidine CCR5 Receptor Antagonists:
Modifications of the Arylpropylpiperidine Side Chains
Christopher L. Lynch,a,* Christopher A. Willoughby,a Jeffrey J. Hale,a
Edward J. Holson,a Richard J. Budhu,a Amy L. Gentry,a Keith G. Rosauer,a
a
Charles G. Caldwell,a Ping Chen,a Sander G. Mills,a MalcolmMacCoss, Scott Berk,a
Liya Chen,a Kevin T. Chapman,a Lorraine Malkowitz,b Martin S. Springer,b
Sandra L. Gould,b Julie A. DeMartino,b Salvatore J. Siciliano,b Margaret A. Cascieri,b
c
Anthony Carella,c Gwen Carver,c Karen Holmes,c WilliamA. Schleif, Renee Danzeisen,c
Daria Hazuda,c Joseph Kessler,c Janet Lineberger,c Michael Millerc and Emilio A. Eminic
aDepartment of Medicinal Chemistry, Merck Research Laboratories, PO Box 2000, Rahway, NJ 07065, USA
bDepartment of Immunology Research, Merck Research Laboratories, PO Box 2000, Rahway, NJ 07065, USA
cDepartment of Antiviral Research, Merck Research Laboratories, PO Box 4, West Point, PA 19486, USA
Received 5 July 2002; accepted 18 September 2002
Abstract—The 4-(3-phenylprop-1-yl)piperidine moiety of the 1,3,4-trisubstituted pyrrolidine CCR5 antagonist 1 was modified with
electron deficient aromatics as well as replacement of the benzylic methylene with sulfones, gem-difluoromethylenes and alcohols in
an effort to balance the antiviral potency with reasonable pharmacokinetics.
# 2002 Elsevier Science Ltd. All rights reserved.
Since the discovery of the chemokine receptor CCR5 as
a co-receptor for HIV infection, numerous research
groups have pursued CCR5 antagonists as novel agents
for the treatment and/or prevention of HIV infection.1ꢀ3
Previous reports fromthese laboratories described a
series of 1,3,4-trisubstituted pyrrolidine4 and bicyclic
isoxazolidine5 CCR5 antagonists as potent antivirals
against HIV. Recently, we disclosed zwitterionic pyrro-
lidines as bioavailable and selective CCR5 antago-
nists.6,7 For example, pyrrolidine 1, which contains the
4-(3-phenylprop-1-yl)piperidine
moiety
and
the
3-(cyclobutyl)propionic acid N-1 substituent, possessed
potent antiviral activity and a good pharmacokinetic
profile in rodents. To further explore the balance of
pharmacokinetics with antiviral activity, this paper will
present analogues containing electron deficient
4-(3-phenylprop-1-yl)piperidine subunits as well as
compounds incorporating oxygen, sulfur, fluorine, and
phosphorous functionality at the benzylic position of
the propyl linker within the piperidine side chain.
The synthetic approach for assembling these com-
pounds through a reductive amination with the desired
piperidine has previously been described.7 The required
piperidines were synthesized using chemistry presented
in Schemes 1–7. Since it had previously been demon-
strated that electron withdrawing substituents could
enhance the CCR5 receptor affinity and antiviral activ-
ity of the pyrrolidine series,4 para-substituted electron
deficient 4-(3-phenylprop-1-yl)piperidine analogues
were initially pursued. Two complimentary synthetic
routes for the preparation of the 4-(3-phenylprop-1-yl)-
*Corresponding author. Fax: +1-732-594-5966; e-mail: chris_lynch@
merck.com
0960-894X/03/$ - see front matter # 2002 Elsevier Science Ltd. All rights reserved.
PII: S0960-894X(02)00829-6

120
C. L. Lynch et al. / Bioorg. Med. Chem. Lett. 13 (2003) 119–123
piperidine derivatives were developed (Scheme 1).
Commercially available 4-hydroxyethylpiperidine 2 was
protected as the Boc derivative, followed by conversion
to the iodide and subsequently to the phosphoniumsalt
3. Wittig olefination, followed by catalytic hydrogen-
ation and deprotection, afforded the desired piperidine
derivative 5. The second route involved the oxidation of
the 4-hydroxyethylpiperidine to an intermediate alde-
hyde, followed by a Wittig olefination with methyl-
oxide enantiomers could be separated with preparative
chiral HPLC (Chiralpak-AS). Lastly, the Boc-group
was cleaved with TFA to give 9.
Oxygen-containing functionality was obtained along
several synthetic pathways (Schemes 3–5). Initially, the
triphenyl phosphoniumbroidme to provide
4.
Hydroboration of olefin 4 with 9-BBN, followed by a
Suzuki coupling with the requisite aryl halide or triflate8
and finally treatment with HCl in MeOH, gave the
desired piperidine derivative 5.
Replacement of the benzylic methylene with sulfur ana-
logues was accomplished through an alkylation of 4-
fluorobenzenethiol 6 with iodide 7 to provide thioether
8 (Scheme 2). Oxidation of the thioether linkage could
be controlled by the stoichiometry of Oxone1 yielding
either the sulfoxide or the sulfone. In addition, the sulf-
Scheme 4. (a) MeMgBr, Et2O, 0 ꢁC; (b) H2, Pd/C, MeOH; (c) ethylene
glycol, TsOH(cat), toluene.
Scheme 1. Reagents: (a) Boc2O, CH2Cl2; (b) I2, PPh3, imidazole,
Et2O, CH3CN; (c) PPh3, CH3CN, reflux; (d) KHMDS, THF then
ArCHO; (e) H2, Pd/C, MeOH; (f) HCl, MeOH; (g) (COCl)2, DMSO,
DIEA, CH2Cl2; (h) KHMDS, CH3PPh3Br, THF; (i) 9-BBN, THF,
then aryl halide, Pd(dppf)Cl2, K2CO3, DMF, 50 ꢁC.
Scheme 5. (a) TMSCHN2, MeOH, THF; (b) NaH, iodide 7, DMF;
(c) LiBH4, THF; (d) HCl, MeOH; (e) 1 N NaOH, MeOH.
Scheme 2. (a) NaH, THF; (b) Oxone1, MeOH(aq); (c) TFA, CH2Cl2.
.
Scheme 6. (a) 1,2-Ethanedithiol, BF3 2HOAc, CH2Cl2; (b) 1,3-
ꢁ
.
dibromo-5,5-dimethylhydantoin, HF pyr, CH2Cl2, ꢀ78 C; (c) H2,
Pd(OH)2/C, 95% EtOH; (d) Boc2O, CH2Cl2; (e) TFA, CH2Cl2.
Scheme 3. (a) NMM, iBuOCOCl, CH2Cl2, 0ꢁꢁC; NMM, MeONH-
(d) Cbz–Cl, NaOH, THF; (e) NaBH4, THF; (f) H2, Pd/C, MeOH.
.
Me HCl; (b) 4-F-phenyl-MgBr, THF, Et2O, 0 C; (c) TFA, CH2Cl2;
Scheme 7. (a) PhPOH(OMe), NaHMDS, THF; (b) HCl, MeOH; (c)
TMS-I, CHCl3.

C. L. Lynch et al. / Bioorg. Med. Chem. Lett. 13 (2003) 119–123
121
propionic acid 10 was converted into the Weinreb amide
and reacted with 4-fluorophenylmagnesium bromide,
followed by a protecting group switch to Cbz providing
ketone 11 (Scheme 3). Reduction with NaBH4, followed
by hydrogenation, yielded piperidine 12. In addition,
further elaboration of ketone 11 gave either piperidine
13 or 14 (Scheme 4).
sed on CHO cell membranes10 and for their antiviral
properties in a HeLa cell anti-infectivity single cycle
assay versus the BAL strain of HIV.11 The most potent
analogues were further evaluated in a 7-day PBMC viral
assay (see Table 6).11
Since previous work demonstrated that compound 1
possessed
pharmacokinetics,
a
reasonable balance of activity and
the cyclobutylmethylene or
Assembly of an intermediate ester is presented in
Scheme 5. Esterification of 4-fluorophenylacetic acid 15,
followed by alkylation with iodide 7, gave 16. Inter-
mediate ester 16 was reduced with LiBH4 and depro-
tected with HCl in MeOH to yield 17. Alternatively, 16
was deprotected to provide the piperidinyl-ester 18a,
which was subsequently saponified to yield piperidinyl-
acid 18b.
cyclopropylmethylene side chain and the 4-(3-arylpro-
pyl)-piperidine architecture was generally maintained
throughout our SAR studies. In addition, we had
observed the incorporation of the 3-fluorophenyl at the
4-position of the pyrrolidine led to improved pharmaco-
kinetics and activity for some analogues (unpublished). A
brief look at electron deficient aryl rings produced the
4-fluorophenyl analogue 24 which maintained the activity
of 1 and possessed a longer half life and a lower clearance
in the rat (Table 1). Although nitrile 25 exhibited potent
antiviral properties, the pharmacokinetics failed to
improve. The trifluoromethyl derivative 26 demon-
The desired benzylic gem-difluoro compounds were
obtained through the fluorination method of Katzen-
ellenbogen (Scheme 6).9 Ketone 11 was initially con-
verted to the 1,3-dithiolane. The resulting thioketal was
activated by 1,3-dibromo-5,5-dimethylhydantoin in the
presence of HF-pyridine to give the gem-difluoride 19.
However, separation of the desired product froman
impurity could only be accomplished through a Cbz to
Boc protecting group switch. The piperidine was finally
deprotected with TFA in CH2Cl2 yielding 20.
Table 2. CCR5 receptor affinity and antiviral activity of the sulfur
analogues
Lastly, the phosphinates were pursued as polar sub-
stituents at the benzylic position. Alkylation of methyl
phenylphosphinate with iodide 7, followed by treatment
with HCl in MeOH, provided phosphinic ester 21. Sub-
jecting ester 21 to TMS-I gave phosphinic acid 22.
Compd
R
Y
MIP-1aa (HeLa)b
27
28
29
30
31
32
33
i-Pr
i-Pr
S
SO2
S
SOc
SOd
SO2
SO2
0.8 (100)
0.9 (4)
0.1 (4)
0.5 (33)
0.4 (0.8)
0.2 (0.4)
1.1 (0.4)
The CCR5 receptor affinity and antiviral data for the
compounds are presented in Tables 1–5. Initially, the
analogues were screened for their ability to displace
(c-Bu)CH2
(c-Bu)CH2
(c-Bu)CH2
(c-Bu)CH2
c-Hex
[
125I]-labeled MIP-1a fromthe CCR5 receptor expres-
aSee Table 1, footnote (a).
bSee Table 1, footnote (b).
cMore polar entantiomer (Chiralpak-AS; 85 hexane/15 iso-propanol).
dLess polar entantiomer (Chiralpak-AS; 85 hexane/15 iso-propanol).
Table 1. CCR5 receptor affinity, antiviral activity and the rat
pharmacokinetics of the electron deficient 4-(3-arylpropyl)piperidine
analogues
Table 3. CCR5 receptor affinity and antiviral activity of the gem-
difluoro analogues
Compd
X
Y
MIP-1aa
Clp
(mL/min/kg)
t1/2
(h)
%F
(HeLa)b
1
H
F
F
F
F
H
H
F
CN
CF3
0.1 (1.2)
0.5 (NT)
0.8 (2.0)
0.3 (0.4)
0.2 (11)
26.5
32.0
21.0
47.8
NTc
3.0
3.4
3.4
1.0
NTc
29
24
16
7
NTc
Compd
R
X
MIP-1aa (HeLa)b
23
24
25
26
34
35
36
37
38
i-Pr
F
F
H
F
0.5 (33)
0.06 (0.4)
0.2 (NT)c
0.3 (1.6)
0.3 (4)
(c-Pr)CH2
(c-Bu)CH2
(c-Bu)CH2
c-Hex
aDisplacement of [125I]-labeled MIP-1a fromthe CCR5 receptor
expressed on CHO cell membranes (IC50, nM). Data are reported as a
mean of three determinations. See ref 10 for assay protocol.
bIC90 values obtained in the HeLa cell anti-infectivity assay versus
BAL. See ref 11 for assay protocol.
H
aSee Table 1, footnote (a).
bSee Table 1, footnote (b).
cNot tested.
cNot tested.

122
C. L. Lynch et al. / Bioorg. Med. Chem. Lett. 13 (2003) 119–123
Table 4. CCR5 receptor affinity and antiviral activity of the oxygen-
ated analogues
Table 6. PBMC antiviral activities and pharmacokinetic profiles of
32, 35, 37, 39 and 42
32
35
37
39
42
PBMC (BAL)
IC95 nM (n)11
1.6 (2)
12 (11)
18 (19)
15 (2)
NTa
Rat
Clp (mL/min/kg)
Vdss (L/kg)
t1/2 (h)
78
1.9
0.8
16
14
6.1
7.5
39
18
2.2
3.1
32
76
3.3
0.9
2
54
3.9
1.6
10
Compd
R
X
Y
MIP-1aa
(HeLa)b
%F
39
40
41
42
43
44
45
(c-Pr)CH2
(c-Pr)CH2
(c-Pr)CH2
(c-Pr)CH2
(c-Pr)CH2
(c-Pr)CH2
c-Hex
F
F
F
F
F
F
H
CHOH
CH(OCH2CH2O)
CHCH2OH
C(OH)CH3
CHCO2H
0.4 (1.2)
3.7 (100)
4.8 (33)
1.6 (1.2)
51 (NT)c
1.5 (33)
Dog
Clp (mL/min/kg)
Vdss (L/kg)
t1/2 (h)
NTa
24
3.7
2.4
37
NTa
NTa
NTa
%F
CHCO2Me
C¼O
1.6 (100)
Rhesus
Clp (mL/min/kg)
Vdss (L/kg)
t1/2 (h)
NTa
17.6
2.8
5.6
8
11
1.8
2.4
12
NTa
NTa
aSee Table 1, footnote (a).
bSee Table 1, footnote (b).
cNot tested.
%F
aNot tested.
Table 5. CCR5 receptor affinity and antiviral activity of the phos-
phinate analogues
The benzylic gem-difluoro analogues in Table 3 were
initially prepared to mask the benzylic position against
metabolism. This modification led not only to high
receptor affinity for CCR5, but provided potent anti-
virals in the HeLa assay (35 and 37).
Since cyclopropylmethylene analogue 35 was one of the
most potent antiviral analogues with excellent CCR5
affinity, the oxygenated analogues in Table 4 were pre-
pared containing the cyclopropyl side chain. Within this
series of analogues, benzylic alcohols 39 and 42 were the
most potent analogues against HIV, analogous to sulf-
oxides 30 and 31. Although other functionality pos-
sessed reasonable binding for CCR5, such as ester 44
and ketone 45, the HeLa activity dropped off with these
substituents. With the carboxylic acid 43, the receptor
affinity decreased by 30-fold as compared to ester 44
and ketone 45. Phosphinate analogues 46 and 47 (Table
5) were considerably less active.
Compd
R
MIP-1aa (HeLa)b
46
47
Me
H
19 (NT)c
ꢃ1000 (NT)c
aSee Table 1, footnote (a).
bSee Table 1, footnote (b).
cNot tested.
strated inferior activity. Since the 4-fluorophenyl moiety
maintained the best balance of antiviral activity and
pharmacokinetics within this limited set of analogues, it
was maintained as the SAR of the benzyl methylene was
explored.
The best compounds from Tables 2–5 were tested in the
PBMC assay and screened for their pharmacokinetic
(PK) properties (Table 6). Sulfone 32 was one of the
most potent antivirals prepared (IC95=1.6 nM), but its
poor rat PK with a high clearance and short half-life
precluded it fromhigher species PK experiments. This
trend of high clearance and short half-life was observed
with benzylic alcohols 39 and 42 as well. The gem-
difluoro analogues 35 and 37 possessed better PK pro-
files with lower rates of clearance and longer half-lives
than the other series of analogues. These properties
along with the data fromthe PBMC assay ( 35 and 37,
IC95<20 nM) demonstrated the possibility of balancing
antiviral activity with pharmacokinetics better than 1.
In Table 2 it can be seen that substitution of sulfur
functionality for the benzylic methylene was well toler-
ated by the receptor. However, the antiviral activity of
this series was highly dependent on the oxidation state
of the sulfur. The sulfones exhibited the most potent
activity (e.g., 32 and 33), while lower oxidation states of
sulfur dropped in activity (e.g., 27 and 29). Specifically,
comparing 27 to 28 and 29 to 32 aꢂ10-fold shift in the
HeLa assay was observed. Although the sulfoxide
enantiomers 30 and 31 possessed similar affinity for
CCR5, interestingly the antiviral activity primarily resi-
ded with one of the enantiomers. As previously
observed, receptor binding allowed a wide variation in
the acetic acid side chain.7 The most potent antivirals
possessed the cyclobutylmethylene or the cyclohexyl
side chain (32 and 33).
In our efforts to explore the SAR of the arylpropyl
piperidine side chain, potent antiviral agents were pre-
pared by introducing an electron deficient aryl moiety
and replacing the benzyl methylene of 1 with sulfones,
gem-difluoromethylenes and benzylic alcohols. In con-

C. L. Lynch et al. / Bioorg. Med. Chem. Lett. 13 (2003) 119–123
123
clusion, we have been able to maintain the antiviral
properties of 1 while improving the pharmacokinetic
profile within the trisubstituted pyrrolidine CCR5 class
of antagonists.
K.; Lineberger, J.; Carella, A.; Carver, G.; Emini, E. A.
Bioorg. Med. Chem. Lett. 2001, 11, 2741.
5. Lynch, C. L.; Gentry, A. L.; Hale, J. J.; Mills, S. G.; Mac-
Coss, M.; Malkowitz, L.; Springer, M. S.; Gould, S. L.;
DeMartino, J. A.; Siciliano, S. J.; Cascieri, M. A.; Carella, A.;
Carver, G.; Holmes, K.; Schleif, W. A.; Danzeisen, R.;
Hazuda, D.; Kessler, J.; Lineberger, J.; Miller, M.; Emini,
E. A. Bioorg. Med. Chem. Lett. 2002, 12, 677.
Acknowledgements
6. Hale, J. J.; Budhu, R. J.; Mills, S. G.; MacCoss, M.; Gould,
S. L.; DeMartino, J. A.; Springer, M. S.; Siciliano, S. J.; Mal-
kowitz, L.; Schleif, W. A.; Hazuda, D.; Miller, M.; Kessler, J.;
Danzeisen, R.; Holmes, K.; Lineberger, J.; Carella, A.; Carver,
G.; Emini, E. A. Bioorg. Med. Chem. Lett. 2002, 12, 2997.
7. Lynch, C. L.; Hale, J. J.; Budhu, R. J.; Gentry, A. L.; Hale,
J. J.; Mills, S. G.; Chapman, K. T.; MacCoss, M.; Malkowitz,
L.; Springer, M. S.; Gould, S. L.; DeMartino, J. A.; Siciliano,
S. J.; Cascieri, M. A.; Carella, A.; Carver, G.; Holmes, K.;
Schleif, W. A.; Danzeisen, R.; Hazuda, D.; Kessler, J.; Line-
berger, J.; Miller, M.; Emini, E. A. Bioorg. Med. Chem. Lett.
2002, 12, 3001.
8. Miyaura, N.; Ishiyama, T.; Sasaki, H.; Ishikawa, M.;
Satoh, M.; Suzuki, A. J. Am. Chem. Soc. 1989, 111, 314.
9. Sondej, S. C.; Katzenellenbogen, J. A. J. Org. Chem. 1986,
51, 3508.
10. Siciliano, S. J.; Kuhmann, S. E.; Weng, Y.; Madani, N.;
Springer, M. S.; Lineberger, J. E.; Danzeisen, R.; Miller,
M. D.; Kavanaugh, M. P.; DeMartino, J. A.; Kabat, D. J.
Biol. Chem. 1999, 274, 1905.
The authors would like to thank Dr. G. Kieczykowski,
Mr. J. Leone and Mr. F. Wong for synthetic assistance.
References and Notes
1. Finke, P. E.; Oates, B.; Mills, S. G.; MacCoss, M.; Mal-
kowitz, L.; Springer, M. S.; Gould, S. L.; DeMartino, J. A.;
Carella, A.; Carver, G.; Holmes, K.; Danzeisen, R.; Hazuda,
D.; Kessler, J.; Lineberger, J.; Miller, M.; Schlief, W. A.;
Emini, E. A. Bioorg. Med. Chem. Lett. 2001, 11, 2475.
2. Palani, A.; Shapiro, S.; Clader, J. W.; Greenlee, W. J.; Cox,
K.; Strizki, J.; Endres, M.; Baroudy, B. M. J. Med. Chem.
2001, 44, 3339.
3. Baba, M.; Nishimura, O.; Kanzaki, N.; Okamoto, M.;
Sawada, H.; Iizawa, Y.; Shiraishi, M.; Aramaki, Y.; Okonogi,
K.; Ogawa, Y.; Meguro, K.; Fujino, M. Proc. Nat. Acad. Sci.
U.S.A. 1999, 96, 5698.
4. Hale, J. J.; Budhu, R. J.; Holson, E. B.; Finke, P. E.; Oates,
B.; Mills, S. G.; MacCoss, M.; Gould, S. L.; DeMartino, J. A.;
Springer, M. S.; Siciliano, S.; Malkowitz, L.; Schlief, W. A.;
Hazuda, D.; Miller, M.; Kessler, J.; Danzeisen, R.; Holmes,
11. Hazuda, D. J.; Felock, P.; Witmer, M.; Wolfe, A.; Still-
mock, K.; Grobler, J. A.; Espeseth, A.; Gabryelski, L.; Schleif,
W.; Blau, C.; Miller, M. D. Science 2000, 287, 646.