CCR5 antagonists: 3-(pyrrolidin-1-yl)propionic acid analogues with potent anti-HIV activity

Article abstract of DOI:10.1021/ol034707c

(Matrix presented) A novel approach to α,α-disubstituted-β -amino acids (β^2,2-amino acids) was employed in the synthesis of a series of 3-(pyrrolidin-1-yl)-propionic acids possessing high affinity for the CCR5 receptor and potent anti-HIV activity. The rat pharmacokinetics for these new analogues featured higher bioavailabilities and lower rates of clearance as compared to cyclopentane 1.

Full text of DOI:10.1021/ol034707c

ORGANIC
LETTERS
2003
Vol. 5, No. 14
2473-2475
CCR5 Antagonists:
3-(Pyrrolidin-1-yl)propionic Acid
Analogues with Potent Anti-HIV Activity
,†
Christopher L. Lynch,* Jeffrey J. Hale,† Richard J. Budhu,† Amy L. Gentry,†
Paul E. Finke,† Charles G. Caldwell,† Sander G. Mills,† Malcolm MacCoss,†
Dong-Ming Shen,† Kevin T. Chapman,† Lorraine Malkowitz,‡ Martin S. Springer,‡
Sandra L. Gould,‡ Julie A. DeMartino,‡ Salvatore J. Siciliano,‡
Margaret A. Cascieri,‡ Anthony Carella,§ Gwen Carver,§ Karen Holmes,§
William A. Schleif,§ Renee Danzeisen,§ Daria Hazuda,§ Joseph Kessler,§
Janet Lineberger,§ Michael Miller,§ and Emilio Emini§
Department of Medicinal Chemistry, Merck Research Laboratories, P.O. Box 2000,
Rahway, New Jersey 07065, Department of Immunology Research, Merck Research
Laboratories, P.O. Box 2000, Rahway, New Jersey 07065, and Department of AntiViral
Research, Merck Research Laboratories, P.O. Box 4, West Point, PennsylVania 19486
christopher.lynch@abbott.com
Received April 28, 2003
ABSTRACT
A novel approach to r,r-disubstituted-â-amino acids (â2,2-amino acids) was employed in the synthesis of a series of 3-(pyrrolidin-1-yl)-
propionic acids possessing high affinity for the CCR5 receptor and potent anti-HIV activity. The rat pharmacokinetics for these new analogues
featured higher bioavailabilities and lower rates of clearance as compared to cyclopentane 1.
Recently a series of zwitterionic cyclopentane CCR5 an-
tagonists were disclosed bearing a 4-(3-benzyl-pyrazol-5-
yl)piperidine side chain and possessing both potent antiviral
activity and acceptable pharmacokinetics (e.g. 1).1 Since
these structures were inspired by the previously reported
1,3,4-trisubstituted pyrrolidines,2 we envisioned a new series
of antagonists by transposing the R-amino functionality of
1 into the cyclopentyl scaffold, thus producing a 3-(pyrro-
lidin-1-yl)propionic acid (e.g. 2). This approach simplified
the structure of 1 by replacing two stereocenters with
symmetrical substituents. This paper presents the novel
synthesis and the initial structure-activity relationships
(SAR) of these new â-pyrrolidinyl acids as CCR5 antagonists
with potent anti-HIV activity.
Previous approaches to â2,2-amino acids have included
dialkylation of cyanoacetates and â-alanine esters, Mannich
reactions with silyl ketene acetates, aminations of â-halo-
propionates, and Reformatsky reactions with benzotriazole
derivatives.3 Our general route utilizes a reductive amination
(2) Lynch, C. L.; Hale, J. J.; Budhu, R. J.; Gentry, A. L.; 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.;
Lineberger, J.; Miller, M.; Emini, E. A. Bioorg. Med. Chem. Lett. 2002,
12, 3001.
† Department of Medicinal Chemistry, Merck Research Laboratories.
‡ Department of Immunology Research, Merck Research Laboratories.
§ Department of Antiviral Research, Merck Research Laboratories.
(1) Finke, P. E. Inflammation Research Association, 11th National
Convention, Lake George, NY; October 5-9, 2002.
10.1021/ol034707c CCC: $25.00 © 2003 American Chemical Society
Published on Web 06/11/2003

Scheme 2. Syntheses of the R-Formyl Esters Required for the
Reductive Amination in Scheme 1a
Figure 1. CCR5 antagonist 1 and 3-(pyrrolidin-1-yl)propionic acid
analogue 2.
with R-formyl esters derived from disubstituted malonates
(Schemes 1 and 2). Although a related reductive amination
a Reagents and conditions: (a) TEA, PMB-Cl, DMF (56%);
(b) (COCl)2, DMSO, DIEA, CH2Cl2, -78 to 0 °C (99%); (c)
Cs2CO3, Et-I, DMF (99%) or K2CO3, dihalide, DMSO (67-99%);
(d) DIBAL, CH2Cl2, -78 °C (54-98%); (e) TEA, Bn-Br, DMF
(31%); (f) DMAP, Bn-OH, K2CO3, CH2Cl2, (R ) H; 64%) or
TEA, Bn-Br, DMF (R ) Me; 44%); (g) LDA, HMPA, THF, -78
°C; iPr-I, THF, -78 to -25 °C (45-60%); (h) O3, CH2Cl2, -78
°C; Me2S (58-66%).
Scheme 1. General Synthesis of the
3-(Pyrrolidin-1-yl)propionic Acid Analoguesa
rolidine 35 and an R-formyl ester from Scheme 2, followed
by treatment with TBAF and a Swern oxidation, to provide
aldehydes 4 (Scheme 1). A reductive amination with 4-(3-
benzyl-1-ethyl-pyrazol-5-yl)piperidine, followed by unmask-
ing of the carboxylic acids, gave the final targets.
The required aldehydes were obtained through a variety
of routes (Scheme 2). The gem-dimethyl aldehyde 6 was
synthesized in a two-step sequence of esterification and
Swern oxidation of the commercially available hydroxy
propionic acid 5. The gem-diethyl 8 and cyclic side chains
9 (ring sizes greater than or equal to 5 atoms) were
synthesized from dibenzyl malonate 7 and the appropriate
dihalide in the presence of cesium carbonate or potassium
carbonate. The intermediate diester was reduced to the
R-formyl ester by DIBAL, using the method of Burton.6 The
cyclobutane 11 was obtained from the commercially available
diacid 10, using chemistry analogous to that used to prepare
8 and 9. The isopropyl side chain in 14 and 15 was
introduced by an alkylation of a cinnamate or tiglate ester
with isopropyl iodide,7 followed by ozonolysis of the
resulting â,γ-unsaturated ester.
a Reagents and conditons: (a) NaB(OAc)3H, aldehyde from
Scheme 2, CH2Cl2 (31-85%); (b) TBAF, THF (86-99%); (c)
(COCl) , DMSO, CH2Cl2, -78 °C; DIEA (70-88%); (d) Na-
2
B(OAc) H, 4-(3-benzyl-1-ethyl-pyrazol-5-yl)piperidine, CH2Cl2
3
(52-84%); (e) H2, Pd-C, MeOH (benzyl esters; 37-99%) or
HCO2H, ∆, (PMB-esters; 66%).
has been reported for the preparation of an R,R-dimethyl-
â-amino peptide mimetic,4 the current method allows a novel
and efficient manner to vary the side chains of the amino
acids. In this paper a series of 3-(pyrrolidin-1-yl)propionic
acids is synthesized that illustrate the utility of this chemistry
in studying the SAR of â2,2-amino acids.
(5) Hale, J. J.; Budhu, R. J.; Mills, S. G.; MacCoss, M.; Malkowitz, L.;
Siciliano, S.; Gould, S. L.; DeMartino, J. A.; Springer, M. S. Bioorg. Med.
Chem. Lett. 2001, 11, 1437.
In the execution of the chemistry, the desired analogues
were obtained through a reductive amination between pyr-
(6) Davis, C. R.; Swenson, D. C.; Burton, D. J. J. Org. Chem. 1993, 58,
6843.
(7) Kajikawa, A.; Morisaki, M.; Ikekawa, N. Tetrahedron Lett. 1975,
16, 4135.
(3) For a review see: Abele, S.; Seebach, D. Eur. J. Org. Chem. 2000,
1.
(4) Kim, H.; Carroll, B.; Lee, M. S. Synth. Commun. 1997, 27, 2505.
2474
Org. Lett., Vol. 5, No. 14, 2003

Table 1. CCR5 Receptor Affinity, Antiviral Activity, and Rat
Pharmacokinetics
Table 2. CCR5 Receptor Affinity and Antiviral Activity for
the gem-Difluoro and Sulfone Analogues
MIP-1Ra
(HeLa)b (mL/min/kg) (h) % F
Clp
t1/2
compd
no.
MIP-1Ra
(HeLa)
compd
no.
R
R′
X
R
R′
24
25
26
27
28
29
30
Me
Me
Et
Me
Me
Et
CF2
SO2
SO2
CF2
SO2
CF2
SO2
0.6 (100)
1.4 (33)
0.2 (1.2)
0.2 (33)
0.5 (33)
0.1 (NT)b
0.2 (NT)b
16
17
18c
19c
20
21
22
23
Me
Et
i-Pr
i-Pr
(CH2)3
(CH2)4
Me 0.7 (33)
Et 0.8 (0.4)
1.4
7.9
3.4
1.4
1.8
1.0
NTd
NTd
0.9 27
1.1 22
1.5 48
1.6 16
0.8 36
0.9 35
NTd NTd
NTd NTd
H
0.6 (11)
(CH2) 3
(CH2) 3
(CH2) 4
(CH2) 4
Me 0.8 (0.13)
0.6 (33)
0.4 (0.13)
0.2 (0.13)
1.5 (11)
(CH2)5
(CH2CH2-
a See Table 1, footnotes a and b. b Not tested.
OCH2CH2)
a Displacement of [125I]-labeled MIP-1R from the CCR5 receptor
expressed on CHO cell membranes (IC50, nM). Data are reported as a mean
of three determinations. See ref 8 for assay protocol. b IC90 values obtained
in the HeLa cell anti-infectivity assay vs BAL (nM). See ref 9 for assay
protocol. c Racemic. d Not tested.
judged by their IC50’s in the MIP-1R assay. Unfortunately,
this potency did not translate into better antiviral activity as
compared to the corresponding benzyl pyrazole analogues
of Table 1. The best compound from Table 2 (26) was 10-
fold less active in the HeLa assay as compared to the most
active compounds of Table 1 (19, 21, and 22). As a result
of this decline in antiviral activity, pharmacokinetic studies
were not carried out with the hybrid analogues.
In conclusion, a novel approach to studying the side chain
SAR of â2,2-amino acids was developed in the synthesis of
the 3-(pyrrolidin-1-yl)propionic acids. These analogues pos-
sessed high affinity for the CCR5 receptor and potent anti-
HIV activity. In addition, the rat pharmacokinetics for this
class of antivirals featured enhanced bioavailabilities and
lower rates of clearance as compared to 1. This new series
of antagonists departed from the previously reported R-amino
acids,2,10 which never rivaled the potency of the cyclopen-
tane-based analogues, and allowed access to pyrrolidine-
derived CCR5 antagonists comparable to 1.
The CCR5 receptor affinity and antiviral data for the
compounds are presented in Table 1. Initially, the analogues
were screened for their ability to displace [125I]-labeled MIP-
1R from the CCR5 receptor expressed on CHO cell
membranes8 and for their antiviral properties in a HeLa cell
anti-infectivity single cycle assay versus the BAL strain of
HIV.9 All of the compounds in this series possessed
subnanomolar affinity for the CCR5 receptor (16-22), with
the exception of pyran 23. Analogues with side chains greater
than or equal to 4 carbon atoms (17, 19, 21, and 22) also
possessed subnanomolar antiviral activity in the HeLa cell
antiviral assay. These 3-(pyrrolidin-1-yl)propionic acids were
further evaluated to determine their rat pharmacokinetics.
In general, these â2,2-amino acids had lower clearance rates
and comparable half-lives to 1. Oral bioavailability was
acceptable across this series of antivirals.
Acknowledgment. The authors thank Dr. G. Kieczykow-
ski, Mr. J. Leone, and Mr. F. Wong for synthetic assistance
and the Basic Chemistry Analytical Services for the phar-
macokinetic studies.
To further examine the 3-(pyrrolidin-1-yl)propionic acids,
some other previously described piperidine subunits10 were
incorporated into this new scaffold (Table 2). These hybrid
molecules possessed high affinity for the CCR5 receptor as
OL034707C
(10) Lynch, C. L.; Willoughby, C. A.; Hale, J. J.; Holson, E. J.; Budhu,
R. J.; Gentry, A. L.; Rosauer, K. G.; Caldwell, C. G.; Chen, P.; Mills, S.
G.; MacCoss, M.; Berk, S.; Chen, L.; Chapman, K. T.; 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. 2003, 13, 119.
(8) 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.
(9) Hazuda, D. J.; Felock, P.; Witmer, M.; Wolfe, A.; Stillmock, K.;
Grobler, J. A.; Espeseth, A.; Gabryelski, L.; Schleif, W.; Blau, C.; Miller,
M. D. Science 2000, 287, 646.
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