Synthesis and SAR studies of 3-phenoxypropyl piperidine analogues as ORL1 (NOP) receptor agonists

Article abstract of DOI:10.1016/j.bmcl.2004.11.049

A series of 3-phenoxypropyl piperidine analogues have been discovered as novel ORL1 receptor agonists. Structure-activity relationships have been explored around the 3-phenoxypropyl region with several potent and selective analogues identified.

Full text of DOI:10.1016/j.bmcl.2004.11.049

Bioorganic & Medicinal Chemistry Letters 15 (2005) 589–593
Synthesis and SAR studies of 3-phenoxypropyl piperidine
analogues as ORL1 (NOP) receptor agonists
Ronald Palin,a,* David R. Barn,a John K. Clark,a Jean E. Cottney,b Phillip M. Cowley,a
Marc Crockatt,a, Louise Evans,b Helen Feilden,a Richard R. Goodwin,a
Frank Griekspoor,a, Simon J. A. Grove,a Andrea K. Houghton,b Philip S. Jones,a
Richard J. Morphy,a Alasdair R. C. Smith,a Hardy Sundaram,b David Vrolijk,a,
Mark A. Weston,a Grant Wisharta and Paul Wrenb,à
aDepartment of Medicinal Chemistry, Organon Laboratories Ltd, Newhouse, Lanarkshire ML1 5SH, UK
bDepartment of Pharmacology, Organon Laboratories Ltd, Newhouse, Lanarkshire ML1 5SH, UK
Received 6 August 2004; revised 16 November 2004; accepted 18 November 2004
Available online 13 December 2004
Abstract—A series of 3-phenoxypropyl piperidine analogues have been discovered as novel ORL1 receptor agonists. Structure–
activity relationships have been explored around the 3-phenoxypropyl region with several potent and selective analogues identified.
Ó 2004 Elsevier Ltd. All rights reserved.
The G-protein-coupled receptor opioid receptor-like-1
(ORL1, known as NOP)1 is the fourth member of the
opioid receptor family. It shares a high degree of se-
quence homology with classical opioid receptors mu,
delta and kappa (MOP, DOP, KOP, respectively). How-
ever, typical opioid ligands (such as dynorphin A) do
not bind to the ORL1 receptor with appreciable affin-
ity.2 The endogenous agonist for the ORL1 receptor is
nociceptin (NC, also known as orphanin FQ), a 17-amino
acid neuropeptide. The ORL1 receptor and NC have
been implicated in several physiological pathways
including cognition, pain, locomotion, anxiety, neuroen-
docrine control and modulation of cardiovascular and
respiratory function.3 Both analgesia and hyperalgesia
have been observed after administration of NC in
rodents; the effect appears to be dependent on the dose
of NC and route of administration. In addition to these
complex biological data there are limitations in using
NC because of the inherent poor metabolic stability.
Therefore, the development of highly selective and po-
tent ORL1 ligands could help elucidate the role of the
ORL1 receptor in pain. Recently, several research
groups have disclosed their efforts in the search for small
molecule ORL1 agonists and antagonists.4 In this
communication we would like to report our discovery
and preliminary SAR investigations of a novel series
of selective ORL1 agonists for treatment of peri-opera-
tive pain.
The affinities (Ki values) of compounds at the opioid
receptor family of receptors were determined by radio-
ligand binding experiments performed in triplicate. Com-
petition binding was performed in membranes prepared
from Chinese hamster ovary (CHO) cells expressing
hORL1, hMOP and hDOP receptors using leucyl-
[3H]nociceptin, [3H]diprenorphine and [3H]naltrindole,
respectively.5 Affinity at KOP was determined by
[3H]U69593 binding in a native guinea pig brain prepa-
ration. The functionality of compounds at ORL1 were
TM
determined using Flashplate (Perkin Elmer, UK) tech-
nology by measuring cellular decreases in forskolin-
stimulated cAMP in CHO cells stably transfected with
hORL1 receptor. The potency and efficacy of com-
pounds were determined in duplicate and compared to
the reference ORL1 agonist nociceptin. Additionally a
mouse vas deferens (MVD) preparation was used to
determine whether compounds have ORL1 and/or
hMOP agonist activity in a native system.
Keywords: ORL1; Nociception; Opioid; Analgesia.
*
Corresponding author. Tel.: +44 1698 736128; fax: +44 1698
736187; e-mail: r.palin@organon.co.uk
Present address: Organon NV, PO Box 20, 5340 BH Oss, The
Netherlands.
à Present address: GlaxoSmithKline S.p.A., Neuropharmacology
Research 1, Via A. Fleming 4, 37135 Verona, Italy.
0960-894X/$ - see front matter Ó 2004 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2004.11.049

590
R. Palin et al. / Bioorg. Med. Chem. Lett. 15 (2005) 589–593
H
N
(R = Ph) the synthesis proceeded via epoxidation of
the alkene, using mCPBA, followed by Mitsunobu cou-
pling (8 ! 11 ! 10).6 This procedure avoided the com-
plications of the SN20 type side reactions seen with the
aryl vinyl alcohols.7 In the case of 1, the key intermedi-
ate epoxide (10) is prepared from a-bromination of
cyclohexylaldehyde (12).8 Disappointingly this reaction
could not be achieved for the 4-, 5- and 7-membered
carbocycles. Nucleophilic displacement with 2-chloro-
5-methyl phenol gave the spirocyclohexylphenoxy alde-
hyde (14), which was easily transformed to the epoxide
using trimethylsulfoxonium iodide in the presence of
sodium hydride and DMSO. The epoxides (10) were then
reacted with 4-(2-keto-1-benzimidazolinyl) piperidine to
give the desired aminoalcohols (15–23). Transformation
of the hydroxyl to amine was executed via mesylation
using methylsulfonyl chloride; subsequent displacement
with the amine in DMF resulted in the desired diamines
(24 and 25). For those compounds (15–23) which con-
tain a mixture of diastereomers the ratios were deter-
mined using 1H NMR spectroscopy. The single
diastereomers (22 and 23) were isolated using a combi-
nation of crystallisation from methanol followed by sil-
ica gel chromatography. All compounds were isolated in
racemic form as the hydrochloride salts.
O
N
Cl
OH
N
O
1
Figure 1. Lead compound generated from screening.
Our screening hit 1 (Fig. 1) showed high affinity for
ORL1 with a Ki of 12 nM, and modest selectivity over
MOP (Ki = 58 nM). The compound behaved as an ago-
nist with modest potency (IC50 = 957 nM) and good effi-
cacy (82% of NC response) in the cAMP flashplate
assay. The focus of this study was on the development
of a water soluble ORL1 ligand for use in an intra-
venous formulation. The hydrochloride salt of racemic
1 had a satisfactory solubility in water, which combined
with the ORL1 activity made this an ideal starting point
for optimisation.
During initial hit validation, deletion studies of 1 (Table
1), showed that removal of the phenol ether moiety re-
sulted in a 50-fold reduction in affinity for the ORL1
receptor (2). A further reduction in affinity for ORL1
was seen with the removal of the hydroxy moiety (3).
The need for both the hydroxy and spirocyclohexyl were
emphasised by analogues 4, 5 and 6, as the absence of
either group dramatically reduced affinity for the
ORL1 receptor.
It was found that separation of the diastereoisomeric
mixture was nontrivial and simplification of the lead
structure, by deletion of the hydroxy moiety, was ex-
plored to obtain SAR on the optimal alkyl group
(Scheme 2). Utilising the Friedel–Crafts procedure
developed by Donnelly and co-workers9 acid chlorides
(28) were transformed to the corresponding 3-chloro-
1-alkyl-1-propanones (29) (R = Ph, commercially avail-
able) with aluminium chloride and excess ethene. Subse-
quent reduction to the secondary alcohol with LiAlH4
and coupling using Mitsunobu conditions10 accessed
the phenoxyalkylpropyl chloride (31). Displacement
reaction using 4-(2-keto-1-benzimidazolinyl) piperidine
and potassium carbonate in DMF resulted in the desired
phenoxypropylamines (26–46).11
In order to further explore the preliminary structure–
activity relationships (SAR) developed through deletion
studies we decided to maintain the phenol ether unit and
change the lipophilic spirocyclohexyl region and hydr-
oxy H-bond acceptor/donor region.
The synthesis of novel racemic 3-phenoxypropyl piper-
idine analogues with various 3-alkyl groups is outlined
in Scheme 1. Thus Grignard addition to commercially
available aldehydes (7) with vinylmagnesium bromide
afforded the corresponding alkyl vinyl alcohols (8).
Mitsunobu reaction using 2-chloro-5-methylphenol gave
the desired aryl ethers (9). Subsequent epoxidation with
mCPBA afforded the key intermediate epoxides (10).
When the alkyl vinyl alcohol features an aryl group
With the SAR established for the alkyl linker we then
rapidly explored the SAR in the phenolic region retain-
ing the propyl group for optimisation. Compounds
i
were prepared as described earlier (Scheme 2).
Table 1. Receptor binding of deletion analogues of screening hit 1
H
N
O
N
R4
R1
N
R2 R3
Compd
R1
R2
R3
R4
ORL1 Ki (nM)
MOP Ki (nM)
1
2-Chloro-5-methylphenoxy
H
–(CH2)5–
–(CH2)5–
–(CH2)5–
OH
OH
12
620
58
—
2
3
2-Chloro-5-methylphenoxy
2-Chloro-5-methylphenoxy
2-Chloro-5-methylphenoxy
2-Chloro-5-methylphenoxy
H
1115
282
—
4
H
H
H
H
H
H
H
410
—
5a
6a
OH(S)
OH(R)
905
1920
—
— = Not tested.
a Compounds prepared from homochiral starting materials.

R. Palin et al. / Bioorg. Med. Chem. Lett. 15 (2005) 589–593
591
Cl
O
O
OH
a
b
Cl
O
R
H
R
Cl
R
c
7
8
9
c
O
Cl
h, i
OH
g
R
N
b
O
R
N
O
NR2
O
R
OH
N
NH
O
N
NH
24-25
11
10
15-23
R
O
Cl
O
f
H
H
H
Br
e
d
O
O
O
12
14
13
Scheme 1. Reagents and conditions: (a) 1.0 M vinyl magnesium bromide in THF; (b) 2-chloro-5-methyl phenol, DIAD, PPh3, THF; (c) mCPBA,
DCM; (d) bromine, DCM; (e) 2-chloro-5-methyl phenol, NaH, toluene; (f) trimethylsulfoxonium iodide, NaH, DMSO; (g) 4-(2-keto-1-
benzimidazolinyl) piperidine, MeOH; (h) methanesulfonyl chloride, DiPEA, DCM; (i) imidazole or dimethylamine, DMF.
R2
R1
O
R2
R1
R
N
O
O
d
O
b
a
c
OH
O
N
NH
R
Cl
R
Cl
R
Cl
R
Cl
28
26-46
29
30
31
Scheme 2. Reagents and conditions: (a) ethene, AlCl3, DCM; (b) LiAlH4, THF; (c) phenol, DIAD, PPh3, THF; (d) 4-(2-keto-1-benzimidazolinyl)
piperidine, K2CO3, NaI (cat.), DMF.
The SAR of the 3-phenoxy region is shown in Tables 2
hydroxy unit with either an imidazole moiety (24) or
dimethylamine group (25) was detrimental to ORL1
affinity.
n
i
and 3. While propyl (17), butyl (20) and phenyl (22)
groups show comparable affinity to the lead (1) there
was no improvement in functional activity over the ori-
ginal hit (Table 2). Other alkyl groups (15–16) show a
decrease in affinity for ORL1 and tend to be more selec-
tive for the MOP. Furthermore replacement of the
Surprisingly, compounds 26 and 27 (Table 2) showed
comparable affinity for ORL1 as the lead 1 with similar
selectivity over MOP. More encouraging was the
Table 2. SAR of the propyl linker
H
N
O
N
Cl
R3
N
O
R1 R2
Compd
R1
R2
R3
Diast. ratio
ORL1 Ki (nM)
MOP Ki (nM)
cAMP
% NC response
82
IC50 (nM)
1
–(CH2)5–
OH
OH
OH
OH
OH
OH
OH
OH
OH
OH
Imidazole
NMe2
H
—
53:47
3:1
12
838
60
58
118
45
957
—
15
16
17
18
19
20
21
22
23
24
25
26
27
Me
Et
nPr
iPr
cHex
iBu
H
H
H
H
H
H
H
H
H
—
—
—
84:16
2:1
20
27
88
1058
—
97
108
462
28
—
—
55:45
1:1.8
1:1.6
96:4
6:94
—
103
166
355
114
144
477
102
16.7
65.2
—
1081
—
88
Benzyl
Ph
Ph
768
38.2
125
509
815
8.3
—
797
—
93
—
—
—
–(CH2)5–
–(CH2)5–
—
—
—
—
Ph
iPr
H
H
240
140
81
101
H
—
10.8
— = Not tested.

592
R. Palin et al. / Bioorg. Med. Chem. Lett. 15 (2005) 589–593
Table 3. SAR of the phenol region
R2
H
N
R1
O
N
N
O
Compd
R1
R2
ORL1 Ki (nM)
MOP Ki (nM)
cAMP
% NC response
IC50 (nM)
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
2-Me
2-Et
H
42
37
24
113
25
39
218
14
11
4
21
23
24
20
67
62
196
47
65
59
65
58
56
38
103
—
—
—
—
H
2-CF3
2-iPr
2-F
H
1730
—
79
H
—
5-Me
5-Me
5-NO2
5-NH2
5-Me
5-OMe
5-Me
5-OMe
5-F
244
165
—
70
58
2-NO2
2-Me
2-Me
2-Me
2-Me
2-Cl
—
128
131
38
104
89
100
101
101
51
11
3
141
80
2-Cl
2-Cl
126
32
52
5179
9837
1532
2-Me
3-Me
4-F
4-F
67
78
— = Not tested.
dramatic improvement (7-fold) in functional activity for
compound 27. This suggests the hypothesis that the
H-bond properties of the hydroxy group is required is
misguided and conformational constraints may play a
bigger role in this region.
has affinity for the MOP receptor it is not a functional
MOP agonist. Solubility is compatible with an intrave-
nous formulation and preliminary in vivo experiments
have shown 41, to have an ED50 = 1.07 lmol/kg (intra-
venous administration) in the second phase of the mouse
formalin paw test,12 thus suggesting this compound has
antinociceptive properties. Further work with 41 should
aid in the elucidation of the role of the ORL1 receptor in
the processing of nociceptive information and determine
the therapeutic potential of ORL1 agonists as analge-
sics. Further in vivo studies will be the subject of future
publications for this series of compounds.
Earlier deletion studies confirmed the need for an aryl-
ether group (compound 2, Table 1). It appears from the
results shown in Table 3 that 2,5-disubstitution of the
phenol is preferred. Bulky alkyl groups in the 2-position
show a lower affinity for ORL1 (35) but retain MOP
affinity. However, small alkyl or halogen groups are well
tolerated, suggesting that the 2-position is involved in
orientating the molecule into a particular conformation.
It appears that electronic effects in the 2-position have
little bearing on affinity with the electron withdrawing
nitro group (37) showing only a slight decrease in affin-
ity compared to the corresponding methyl group in this
position (40). Electron withdrawing groups in the 5-po-
sition reduce affinity for ORL1, whereas 5-amino (39),
5-methyl (40) and 5-methoxy (41) are all well tolerated.
The highest potency compound is the 2-methyl-5-meth-
oxyphenol analogue (41) with improved affinity for
ORL1 and a ꢀ20-fold improvement in functional activ-
ity compared to the original hit 1.
Acknowledgements
We would like to thank our colleagues in the Depart-
ment of Analytical Chemistry for structure and purity
determination.
References and notes
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In summary we have developed a simplified synthetic
route to 3-phenoxypropyl piperidine analogues to ex-
plore ORL1 SAR around the original hit compound 1.
These compounds all show agonist activity with high
to moderate affinity towards ORL1. In particular 41 dis-
played high affinity with good functional potency and
efficacy and modest selectivity (MOP/ORL1 = 15,
KOP/ORL1 = 25 and DOP/ORL1 = 191). Furthermore,
the activity of 41 in the MVD assay (IC50 = 977nM) was
attenuated by a selective ORL1 antagonist, but not by
naloxone, demonstrating that although the compound

R. Palin et al. / Bioorg. Med. Chem. Lett. 15 (2005) 589–593
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(CDCl3) d 8.98 (1H, s, NH), 7.85 (1H, d), 7.17 (1H, m),
7.05 (2H, d), 7.03 (1H, s), 6.43 (2H, m), 4.70 (1H, m), 4.25
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