1-Amido-1-phenyl-3-piperidinylbutanes - CCR5 antagonists for the treatment of HIV: Part 2

Article abstract of DOI:10.1016/j.bmcl.2009.01.008

Optimisation of a series of 4-piperidinyltriazoles led to the identification of compound 28a which showed good whole cell antiviral activity, excellent selectivity over the hERG ion channel and complete oral absorption.

Full text of DOI:10.1016/j.bmcl.2009.01.008

Bioorganic & Medicinal Chemistry Letters 19 (2009) 1499–1503
Contents lists available at ScienceDirect
Bioorganic & Medicinal Chemistry Letters
journal homepage: www.elsevier.com/locate/bmcl
1-Amido-1-phenyl-3-piperidinylbutanes – CCR5 antagonists for the
treatment of HIV: Part 2
*
Christopher G. Barber , David C. Blakemore, Jean-Yves Chiva, Rachel L. Eastwood, Donald S. Middleton,
Kerry A. Paradowski
Pfizer Global Research and Development, Sandwich Laboratories, Ramsgate Road, Sandwich, Kent CT13 9NJ, UK
a r t i c l e i n f o
a b s t r a c t
Article history:
Optimisation of a series of 4-piperidinyltriazoles led to the identification of compound 28a which showed
good whole cell antiviral activity, excellent selectivity over the hERG ion channel and complete oral
absorption.
Received 12 December 2008
Revised 5 January 2009
Accepted 6 January 2009
Available online 10 January 2009
Ó 2009 Elsevier Ltd. All rights reserved.
Keywords:
HIV
CCR5
Antagonist
Triazole
Piperidine
HIV is a global health problem which is estimated to have
caused the deaths of 25 million people.1 Decades of research have
not removed the need for new treatments but have identified
many potentia points of intervention that could be targeted. One
essential step in the viral life-cycle is the infection of host cells.
During the early phases of infection, this has been shown to occur
mainly through the CCR5 receptor.2 Further, CCR5 antagonists have
been shown to inhibit cellular infection by R5-tropic HIV highlight-
ing their potential to form part of the treatment of HIV.3 Following
the identification of maraviroc, we wished to identify additional
CCR5 antagonists with the potential to maintain activity against
resistant strains that may develop following long-term clinical
use.4
was N-protected, converted to Weinreb amide 7, and then treated
with methyl Grignard reagent to afford ketone 8. Reductive amina-
tion of 8 with 5 in the presence of titanium tetra isopropoxide gave
9 without control of the nascent stereocentre. Subsequent depro-
tection and acylation furnished 10 which were tested as ꢀ1:1 mix-
tures of isomers (Table 1).
While ureas 10n and 10o and carbamate 10m were not toler-
ated, potency could be enhanced through modification of the ami-
dic group (10a–l). In particular, a significant advantage was
observed when the substituent was cyclic with cyclopentyl and
cyclohexyl analogues 10g and 10h showing sub-nanomolar activ-
ity in our cell fusion assay; however the attendant increase in lipo-
philicity had
a detrimental effect upon metabolic stability.
We have previously described the identification of 3-piper-
idinylbutane 1 (Fig. 1).5 The eutomer showed good activity in our
cell fusion assay and excellent metabolic stability to human liver
microsomes (HLM). Our knowledge of the SAR around maraviroc
(2) led us to conclude that we should be able to increase potency
by 10-fold whilst maintaining the metabolic advantage we had
over the isolipophilic maraviroc.
Synthesis of 4-piperidinyl triazole 5 was achieved in four steps
from 1-benzylpiperidin-4-amine (3) via reaction of acetohydrazide
with the imidoyl chloride formed by treatment of 4 with phospho-
rus pentachloride to give 5 after removal of the N-benzyl-protect-
ing group (Scheme 1). Ethyl 3-amino-3-phenyl-proprionate (6)
Attempts to reduce this metabolic vulnerability through inclusion
of polarity at the 4-position of the cyclohexyl moiety resulted in
significant potency loss (10i–l). This is in contrast to the SAR seen
within the tropane series where polarity at the 4-position had been
well-tolerated.9 Geminal difluoro groups can occasionally be toler-
ated in lipophilic-favouring environments while also blocking
CYP450-mediated metabolism.10 Comparison of 1 and 10q, and
of 10h and 10p showed in both cases improved metabolic stability
without potency loss.
Chromatographic separation of the diastereoisomers of 10p and
10q gave examples 11a–d (Table 2).
We determined the absolute stereochemistry of 11c through
synthesis using the route shown in Scheme 2. Ketone 8 was re-
duced with sodium borohydride to diastereomers 12 and 13 which
were readily separable by column chromatography. Absolute ste-
* Corresponding author. Tel.: +44 1304 641182.
E-mail address: christopher.barber@pfizer.com (C.G. Barber).
0960-894X/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2009.01.008

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C. G. Barber et al. / Bioorg. Med. Chem. Lett. 19 (2009) 1499–1503
O
F
NH
O
NH
F
N
N
N
N
N
N
N
N
1
2 : Maraviroc
IC50 (fusion) = 0.2nM
IC50 (fusion) = 4.3nM
logD = 1.8
HLM < 7 uL/min/mg protein***
logD = 1.9
HLM = 22uL/min/mg protein.
Figure 1. Comparison of maraviroc with a-methyl piperidine 1.
reochemistries were determined by treatment of 12 and 13 with
sodium hydride which resulted in cyclic carbamates 14 and 15,
respectively. nOe determinations of these cyclic carbamates en-
abled stereochemical assignment with reference to published val-
ues.11 Direct conversion of 13 to the mesylate was low yielding
which we believe resulted from competing intramolecular attack
at the mesylate by the b-carbamate oxygen. This forced a change
in protecting group to imine 16 which was successfully converted
to the crude mesylate. This was undertaken at ꢁ40 °C with trieth-
ylamine added last to prevent formation of the corresponding chlo-
ride which would result in the scrambling of stereochemistry.
Displacement of the mesylate by 5 followed by deprotection gave
amine 17 which was converted to 11c as before.
Compounds 11a and 11c showed good levels of metabolic stabil-
ity and potency but the latter showed a superior window over activ-
ity at the hERG ion channel. Compound 11c also showed excellent
activity in a whole cell antiviral assay8 (IC90 = 1.8 nM) comparable
to maraviroc (IC90 = 2.8 nM). Good metabolic stability in rat and
dog liver microsomalpreparations was also demonstrated. However,
a rat PK study gave a relatively high clearance of 92 ml/min/kg which
in conjunction with a modest volume of distribution (Vd) of 10 L/kg
resulted in a half-life of just 1.3 h. Oral bioavailability was limited
to 6% which was ascribed to PGP-mediated efflux on the basis of an
efflux ratio of 39 in MDCK cells. This showed little advantage over
maraviroc which had shown a similar level of bioavailability in rat
pharmacokinetic studies and a slightly shorter t1/2 (0.9 h) as a result
of a modest clearance (74 ml/min/kg) and Vd (6.5 L/kg).
SAR from the tropane series suggested a potential solution to
this poor oral absorption. We had noticed that isomeric triazoles
2 and 18 had shown significantly different biophysical properties
(Table 3). In particular, the efflux ratio in caco-2 cells was im-
proved with 1,2,4-triazole 18 over that seen with the 1,3,4-triazole
2 – a difference that remained when comparing isolipophilic ana-
logues between the two series (data not shown).9 Further, the dis-
connect between logD and clogP led us to determine pKas which in
turn demonstrated that 18 was considerably more basic than 2 –
which has been ascribed to the interaction between the nitrogen
lone pair and the triazole’s dipole.9 This analysis led us to conclude
that the 1,2,4-triazoles could show an improved PK profile as a
consequence of higher caco-2 flux and reduced metabolic clear-
ance despite the increased lipophilicity.
Ph
HN
Ph
N
N
N
NH
N
i
ii, iii
N
NH2
O
3
4
5
Boc
Boc
NH2
O
NH
O
NH
O
O
O
N
iv, v
vi
6
7
8
vii
O
Boc
NH
R2
NH
N
N
viii, ix
N
N
N
N
N
10
9
N
Scheme 1. Preparation of 1-amido-1-phenyl-3-piperidinylbutanes. Reagents and conditions: (i) iBuCOCl, NaOH, Et2O, rt, 96%; (ii) PCl5, CH2Cl2, rt then AcNHNH2 t-amyl
alcohol then
D
toluene, 60%; (iii) NH4CO2H, Pd(OH)2, EtOH, 60 °C, quant.; (iv) Boc2O, Et3N, THF, rt, quant.; (v) MeNHOMe.HCl, iPrMgCl, THF, 4 °C, 54%, (vi) MeMgBr, THF, 90%;
(vii) Ti(OiPr)4, 5, EtOH, rt, 18 h then NaBH3CN, rt, 21%; (viii) HCl, EtOH, rt, quant.; (ix) 3a–k, 3o and 3p—R2O2H, O-(7-azabenzotriazol-1-yl)-N,N,N0,N’-tetramethyluronium
hexafluorophosphate, Et3N, CH2Cl2, rt; 3l–n—RCOCl, Et3N, DMF, rt.

C. G. Barber et al. / Bioorg. Med. Chem. Lett. 19 (2009) 1499–1503
1501
Table 1
1,3,4-Triazoles
Table 2
O
Enantiopure difluorocycloalkyl-1,3,4-triazoles
R1
O
NH
R1
N
N
N
N
N
N
N
N
N
R1
a
Compound
R1
Fusion IC50 (nM)
LogD7.4
HLM Clb
F
F
F
10a
10b
10c
10d
10e
10f
Me
CF3CH2–
CF3CH2OCH2–
cycloPrCH2–
iPr
cycloPr
cyclopentyl
cyclohexyl
89.2
85.3
52.8
3.16
3.11
3.93
0.137
0.513
<8
11.4
61.6
F
>
1.9
—
Compound
11a
—
11b
—
62 (n = 4)
2.0
16
11c
R
11d
S
49.9 (n = 4)
2.2
<7
<8
<8
a-Me Stereo
fusion IC50 (nM)
LogD
1.3 (n = 2)
0.48 (n = 3)
10g
10h
1.9
<7
2.3
2.2
12
>10
2.5
139
HLM
hERGa IC50
2.1
>10
10i
10j
164
10.8
(lM)
O
a
The concentration required to inhibit [3H]dofetilide binding to hERG stably
expressed on HEK-293 cells.
4.87
O
O
S
A small set of 1,2,4-triazoles were prepared with the dif-
luorocyclohexyl amide moiety (Scheme 3).
10k
10l
6.12
28.7
0.5
S
O
O
N-Benzylpiperidin-4-one 19 was reductively coupled to N-Boc-
hydrazine and deprotected to yield 4-piperidinyl hydrazine 21
which was heated with aminoenamines 24 in the presence of ace-
tic acid to yield 1,2,4-triazoles 26 following debenzylation. Reduc-
tive amination with N-Boc ketone 8 followed by deprotection and
amide coupling gave targets 27–33 as a mixture of isomers at the
<8
N
10m
10n
MeO
315
176
1.5
—
<7
<8
O
N
a-methyl centre which were separated by column chromatogra-
phy (Table 4).
10o
187
0.84
0.38
N
N
Potency was reduced by 6-fold for 1,2,4-triazole 28a over the
isomeric 1,3,4-triazole 11c. As expected, lipophilicity was in-
creased but this did not impact upon the rate of microsomal clear-
ance. In fact, 28a shows remarkably little metabolic vulnerability
illustrating the inherent stability of this chemotype despite a mea-
sured logD of 3 and a significant free fraction in human and rat
plasma (fraction unbound = 0.15 and 0.24, respectively). Potency
F
10p6
2.0
2.0
19.0
<7
F
F
10q7
F
a
For details of cell fusion assay, see Ref. 8.
b
Units
l
l/min/mg microsomal protein.
O
Boc
NH OH
N
N
O
O
Boc
NH
ii
ii
O
12
14
+
i
O
Boc
NH
OH
OH
8
13
15
Ph
Boc
NH
OH
Ph
N
NH2
v
N
vi, vii
iii, iv
N
N
13
16
17
N
Scheme 2. Determination of absolute stereochemistry of 11c. Reagents and conditions: (i) NaBH4, EtOH, 70% anti:syn 2:1, chromatographic separation; (ii) NaH, THF, reflux;
(iii) HCl, Et2O; (iv) benzhydrylideneamine, CH2Cl2; (v) MsCl, CH2Cl2, ꢁ40 °C, then Et3N; (vi) 5, K2CO3, MeCN, reflux; (vii) HCl, CH2Cl2.

1502
C. G. Barber et al. / Bioorg. Med. Chem. Lett. 19 (2009) 1499–1503
Table 3
A comparison of 1,2,4 and 1,3,4 triazoles
O
N
F
N
N
NH
N
N
N
N
N
F
R
Compound
clogP
2
18
3.6
2.7
11/26
6
3.3
1.9
1/10
20
logD
Caco-2a
HLM Clint
b
pKa
7.7
0.22
8.4
1.4
Fusion IC50 (nM)
a
Apical-to-basal/basal-to-apical.
b
Units
ll/min/mg microsomal protein.
Ph
Ph
Ph
H
N
N
N
N
N
O
i
ii, iii
N
Boc
NH2
H
19
20
MeO OMe
Me2N
R2
21
O
O
NMe2
R1
N
H
+
R1
NH2
v
iv
R2
22
23
24
O
R1
N
F
F
NH
N
vi
X
N
N
vii, viii, ix
R2
N
N
X = Bn 25
X = H 26
27-33
N
N
Scheme 3. Preparation of 1,2,4-triazoles. Reagents and conditions: (i) H2NNHBoc, AcOH, CH2Cl2, rt, 18 h, 95%; (ii) NaBH4, CH2Cl2, AcOH, rt, 24 h, quant.; (iii) HCl, MeOH, rt,
12 h, quant.; (iv) toluene, 120 °C, 6 h, 80%; (v) AcOH, 90 °C, 3 h, 80%; (vi) Pd(OH)2, NH4CO2H, EtOH, 60 °C, 3 h, 91%; (vii) Ti(OiPr)4, EtOH, 8, rt, 16 h then NaBH3CN, rt; (viii) HCl,
EtOAc, rt, quant.; (ix) acid, O-(7-azabenzotriazol-1-yl)-N,N,N’,N’-tetramethyluronium hexafluorophosphate, Et3N, CH2Cl2, rt.
was reduced for all other difluorocyclohexylamide analogues
tested (27a and 29a–33a). Compound 28a showed the best balance
Table 4
1,2,4-triazoles
of potency and metabolic stability together with an improved
O
MDCK efflux ratio (A ꢁ B/B ꢁ A = 2) together and high flux (api-
cal-basal Papps = 23 ꢂ 10ꢁ6 cm/s). Whole cell antiviral activity was
F
N
also good (IC90 = 2.6 nM) and little activity was seen against the
F
hERG channel (IC50 > 10
inhibition against human CYP isoforms, modest activity against
CYP3A4 was observed for 28a (IC50 = 3.4 M). No activity was ob-
served for other isoforms (1A2, 2C9 or 2D6) up to 30 M.
An oral rat PK study showed 28a to be fully absorbed. This was a
significant improvement over both 2 and 11c. However, the half-
life had not been extended as despite a reduction in clearance to
30 ml/min/kg, the volume of distribution dropped to 2.5 L/kg giv-
ing a t1/2 of 1 h. This was unexpected given the rise in logD mea-
sured from 11c to 28a.
lM). While 11c showed no measureable
N
N
l
R2
N
l
N
R1
Compound
R1
R2
Fusion IC50 (nM)
LogD7.4
HLM Cla
27a (27b)
28a (28b)
29a (29b)
30a (30b)
31a (31b)
32a (32b)
33a (33b)
Et
Me
Me
Et
12 (252)
3.08 (385)
28 (549)
15.7 (292)
15 (1000)
92.3 (4200)
20.9 (302)
2.5 (2.7)
3.0 (2.7)
2.9 (2.8)
3.3 (3.1)
2.9 (2.9)
2.6 (2.6)
3.1 (3.0)
7 (25)
iPr
15 (23)
39 (166)
40 (91)
35 (99)
13 (65)
117 (>440)
Me
Et
cycloPr
CH2OMe
cycloBu
We have demonstrated with 28a that within the
a-methyl
Et
piperidine chemotype, it is possible to combine both maraviroc-like
levels of antiviral activity with complete oral absorption. Replace-
ment of the 1,2,4-triazole of 11c with regioisomeric 1,3,4-triazole
exhibited by 28a led to an increase in lipophilicity that was not
matched by a reduction in metabolic stability. While this change
Me
Me
H
Key: More active isomer (less active isomer).
A
Units ll/min/mg microsomal protein.

C. G. Barber et al. / Bioorg. Med. Chem. Lett. 19 (2009) 1499–1503
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These results have prompted further investigation into alternative
heterocyclic systems which will be disclosed shortly.
Acknowledgements
We thank Manuel Perez for NMR analyses.
References and notes
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