Synthesis and structure-activity relationships of N-substituted spiropiperidines as nociceptin receptor ligands

Article abstract of DOI:10.1016/j.bmcl.2007.01.069

A series of N-substituted analogs based upon the spiropiperidine core of 1 was synthesized and exhibited high binding affinity to the nociceptin (NOP) receptor. The selectivities against other known opioid receptors were determined.

Full text of DOI:10.1016/j.bmcl.2007.01.069

Bioorganic & Medicinal Chemistry Letters 17 (2007) 2281–2284
Synthesis and structure–activity relationships of N-substituted
spiropiperidines as nociceptin receptor ligands
John P. Caldwell,a,* Julius J. Matasi,a Hongtao Zhang,b
Ahmad Fawzib and Deen B. Tulshiana
aCV & CNS Department of Chemical Research, Schering Plough Research Institute,
2015 Galloping Hill Road, Kenilworth, NJ 07033, USA
bCV & CNS Department of Biological Research, Schering Plough Research Institute,
2015 Galloping Hill Road, Kenilworth, NJ 07033, USA
Received 15 December 2006; revised 15 January 2007; accepted 18 January 2007
Available online 27 January 2007
Abstract—A series of N-substituted analogs based upon the spiropiperidine core of 1 was synthesized and exhibited high binding
affinity to the nociceptin (NOP) receptor. The selectivities against other known opioid receptors were determined.
Ó 2007 Elsevier Ltd. All rights reserved.
Since its discovery in 1994, many advances have been
made regarding the biological significance and functions
of the opioid receptor-like-1 (ORL1/OP4/NOP) receptor
and its endogenous peptide ligand, nociceptin [orphanin
FQ (OFQ) or nociceptin/orphanin FQ (N/OFQ) pep-
tide]. The 17 amino-acid G protein coupled NOP recep-
tor has been found to share a 47% overall homology
(65% homology in the transmembrane domains) with
the classical opioid receptors, l, j, and d; however, the
pharmacology of nociceptin has a low binding affinity
to the classical opioids and opioid antagonists such as
naltrexone do not block the activity of nociceptin.1
of small-molecule NOP agonists and their selectivity
against the classical opioid receptors.
F
N
HN
N
O
F
1
Opioids, such as codeine and butorphanol, are the most
effective drugs available to treat cough associated with
pulmonary diseases. However, these drugs, which acti-
vate l receptors, possess significant side effect liabilities
including respiratory depression, constipation, and
physical dependency. We have reported that OFQ, a
functional agonist, displays the potential to inhibit
cough through a central and peripheral CNS mecha-
nism.2 This antitussive effect is blocked by the small-
molecule selective NOP antagonist, J113397. Therefore,
NOP agonists represent a novel therapeutic approach
for the treatment of cough. Here, we report our SAR
From high-throughput screening of our chemical
library, compound 1 was identified as a lead showing
moderate affinity for NOP with a Ki of 500 nM. Our
SAR development plan was to investigate substitution
on the piperidine nitrogen of the spiropiperidine core
of 1. As summarized in Scheme 1, the commercially
available 1-phenyl-1,3,8-triazaspiro-[4,5]deacan-4-one,
2, was either alkylated in the presence of various benzyl
halides or treated under reductive amination conditions
with benzyl aldehydes to produce 3, where R consists of
primarily benzhydryl, benzyl, and tetralinyl analogs.
The necessary halides were generated via treatment of
the corresponding alcohols with SOCl2 as described in
Scheme 2.
Keywords: Nociceptin; Orphanin FQ; NOP receptor; Agonist.
*
Corresponding author. Tel.: +1 908 740 5199; fax: +1 908 740
7152; e-mail: john.caldwell@spcorp.com
0960-894X/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2007.01.069

2282
J. P. Caldwell et al. / Bioorg. Med. Chem. Lett. 17 (2007) 2281–2284
using [125I]Nociceptin and h-NOP receptor expressing
Chinese hamster ovary (CHO) cell membranes as
described.5 Ki values for human l-, j-, and d-opioid
receptors were determined using [3H]diprenorphine
and CHO cell membranes expressing the opioid recep-
tors as described.5b The functional [35S]GTPc S binding
assays for h-NOP receptor were carried out in CHO cell
membranes as described.5a In cases where the chiral
products were obtained, the compounds were screened
as racemates.
N
N
a
HN
HN
NH
NR1
O
O
2
3
Scheme 1. Reagents and conditions: (a) R1Br, K2CO3, CH3CN, reflux
or R1Cl, K2CO3, KI, CH3CN, reflux or R1CHO, Na(OAc)3BH,
CH2Cl2.
The SAR of the benzyl and benzyhdryl analogs are
shown in Table 1. The benzyl analog 16 exhibited high
affinity for NOP (Ki = 11.0 nM) and was ffi18-fold
selective over KOP. By comparison the cyclohexylmethyl
analog 17 was less potent at NOP and with less
selectivity over KOP (ffi8.5-fold). In the case of the
pyridyl variants (18–20), NOP binding affinity dropped
significantly. Although the thienyl analog 21 displayed
high binding affinity at NOP, selectivity over KOP
remained marginal. Introduction of chloro substituents
on the benzyl ring resulted in single-digit nanomolar
potency at NOP; yet, only slight selectivity at KOP
was observed for 22 and slight selectivity over MOP
was observed for 23. NOP binding was tolerant of a
wide variety of alkyl substitutions (24–29) at the
benzylic position; however, selectivity over the KOP
receptor remained at modest levels. Introduction of
the benzhydryl moiety (30) produced low double-digit
nanomolar potency at NOP and was ffi6-fold selective
over KOP. Constrained analogs of the benzhydryl
varied in NOP binding affinity with the fluorenyl variant
31 being completely inactive while 33 regained similar
potency as 30.
O
OH
Cl
a
b
R
R
R
4
5
6
Scheme 2. Reagents: (a) NaBH4, MeOH; (b) SOCl2, CH2Cl2.
The requisite 8-halo-tetralones were prepared as
described in Scheme 3.3 As outlined in Scheme 4, the
5- and 7-halo-tetralones (12 and 13) were furnished by
subjecting the phenyl halide and butyrolactone 11 under
Friedel–Crafts acylation conditions.4 However, under
similar conditions, the gem-dimethyl butyrolactone 14
gave rise to the 6-halo regioisomer 15.
The compounds described were evaluated in radioligand
binding assays. Ki values against the human NOP recep-
tor were determined from competition binding assays
O
Mono-substitution (34–37) on the benzhydryl substitu-
ent produced compounds with acceptable NOP binding
affinity and excellent selectivity over the opioid recep-
tors. Furthermore, bis-substitution (38–43) enhances
the NOP binding affinity while maintaining this excep-
tional degree of selectivity over the other opioid
receptors.
NH2
NH
O
X
O
a, b
c, d
7
8
9
Scheme 3. Reagents and conditions: (a) Ac2O, py, rt; (b) KMnO4, 9/1/
acetone/15% MgSO4(aq); (c) NaOH(aq), EtOH, reflux; (d) for X = Cl:
NaNO2 0 °C, Cu(I)Cl in 20% HCl(aq), 0 °C 30 min, then rt for 30 min,
then reflux 2 h—for X = F: NOBF4, DCM, 0 °C, add 1,2-dichloro-
benzene, distill DCM, then reflux 2 h.
The SAR of the tetralinyl analogs are shown in Table 2.
In general, these compounds displayed very high affinity
for the NOP receptor with Kis ranging from 15.1 to
0.3 nM as well as high selectivity over DOP. The indanyl
(44) and tetralinyl (45) analogs displayed NOP Kis of 1.2
and 1.4 nM, respectively. The suberonyl derivative, 46,
had a comparatively lower affinity for NOP (Ki =
14.5 nM). Since 45 had a higher degree of selectivity
over the opioids than 44 (46-fold against KOP vs 16-fold
against MOP), we focused our efforts on the tetralinyl
series. The geminal di-methyl analog 47 retained its
NOP potency and increased the selectivity against
KOP but decreased the selectivity against MOP relative
to the parent tetralone. The fluoro derivatives (48–50)
yielded subnanomolar affinity for NOP; yet 48 had sin-
gle-digit nanomolar at MOP, while 49 and 50 displayed
low double-digit binding affinities at KOP. While not
displaying as high NOP affinity as their corresponding
fluoro counterparts, the chloro derivatives (51–53)
exhibited a similar trend in terms of selectivity, with
O
X
O
O
X
a
+
+
O
X
10
11
12
13
X
O
O
a
+
O
X
10
14
15
Scheme 4. Reagents and conditions: (a) AlCl3, heat, 0 °C to 100 °C,
3 h.

J. P. Caldwell et al. / Bioorg. Med. Chem. Lett. 17 (2007) 2281–2284
Table 1. Binding affinities of benzyl and benzhydryl analogs
2283
HN
N
O
N
R
R1
Compound
R
R1
Ki (nM)
NOP
DOP
KOP
MOP
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
Ph
H
11.0
61.2
119.0
133.0
824.0
14.0
3.2
2946
73655
31935
3791
nt
196
517
321
252
nt
500
887
1761
233
nt
Cyclohexyl
2-Pyridyl
3-Pyridyl
4-Pyridyl
3-Thienyl
2-Cl-Ph
2,6-Cl2-Ph
Ph
H
H
H
H
H
8999
724.5
1633
1035
6846
3373
1421
736
26.2
16.4
52.3
50
133
37.4
29.5
74
H
H
2.3
3.5
Me
Et
Ph
Ph
17.0
2.4
60.5
22.5
13
355
244
22
Propyl
Butyl
Isoamyl
Ph
Ph
1.1
1.5
25.5
26
30
Ph
Ph
Cyclopentyl
Ph
4.6
23.0
4680
37890
398
486
137
31
32
na
nt
nt
nt
225.0
9784
1672
2454
33
36.5
11510
776
475
34
35
36
37
38
39
40
41
42
43
2-F-Ph
2-Cl-Ph
2-Me-Ph
4-Cl-Ph
2-Me-Ph
2-Cl-Ph
3-Cl-Ph
2-F-Ph
3-F-Ph
4-F-Ph
Ph
Ph
10.8
9.0
15715
14770
6042
1183
1086
579
818
778
Ph
Ph
11.5
140.0
9.0
1109
5081
346
24805
8600
3529
1602
5887
4628
858
2-Me-Ph
2-Cl-Ph
3-Cl-Ph
2-F-Ph
3-F-Ph
4-F-Ph
6.8
249.0
6.3
150385
26515
16035
10360
5598
595
1616
567
250.0
49.0
1448
2009
3187
787
Values are means of 2–3 experiments (na, not active; nt, not tested).
51 having a high binding affinity at MOP, while 52 and
53 displayed double-digit binding affinities at KOP.
inhibition of the hERG channel at 5 mg/mL; as well
as good oral bioavailabilty in rat at 10 mpk with
an AUC(0–6h) = 2106 nMh, Cmax = 518 nM at 4h, and
C6h = 467 nM.
The mono-halogenated derivatives 54 and 55 in the
geminal di-methyl tetralones produced compounds
which retained single-digit potency at NOP; however,
still demonstrated modest affinity at MOP. The dichlori-
nated tetralone (56) showed acceptable binding at NOP
with excellent selectivity over all of the opioid receptors
with DOP Ki ffi 11.7 lM and near micromolar binding
affinities at KOP and MOP. Compound 56 was
characterized as a full agonist in the functional assay,
as shown in Table 3, since it increased [35S]GTPcS
binding. Furthermore, 56 displayed an acceptable 7%
In summary, we have developed several small-molecule
NOP agonists which display excellent selectivity over
the other opiate receptors. Further work in this area will
be presented in due course.
1
Compound 28: H NMR (CDCl3)d 7.55 (s, 1H), 7.25–
7.45 (m, 7H), 7.02 (d, 2H), 6.96 (t, 1H), 4.77 (br s,
2H), 3.44 (dd, J = 8.8, 5.2 Hz, 1H), 3.07 (m, 1H),
2.60–2.90 (m, 5H), 2.00 (m, 1H), 1.65–1.86 (m, 3H),

2284
J. P. Caldwell et al. / Bioorg. Med. Chem. Lett. 17 (2007) 2281–2284
Table 2. Binding affinities of tetralinyl analogs
HN
N
O
R
N
R1
R2
n
4
R3
R
R4
Compound
R
R1
R2
R3
R4
n
Ki (nM)
NOP
DOP
KOP
MOP
44
45
46
47
48
49
50
51
52
53
54
55
56
H
H
H
H
F
H
H
H
H
H
F
H
H
H
H
H
H
H
H
H
H
Cl
F
H
H
H
H
H
H
F
H
0
1
2
1
1
1
1
1
1
1
1
1
1
1.2
1.4
1854
3511
1906
1790
1231
2650
2400
1375
2511
2266
1291
2461
11655
32
65
19
H
346
673
48
H
14.5
1.3
203
540
145
33.5
13.7
143
66.5
94.5
282
169
938
CH3
H
0.3
0.9
5.1
H
H
Cl
H
H
H
H
H
H
133
72.3
11.1
162
130.5
49.1
59.1
977
H
H
Cl
H
H
H
Cl
H
0.8
2.6
H
H
Cl
H
H
H
H
H
10.8
2.0
H
CH3
CH3
CH3
1.4
8.4
Cl
15.1
Values are means of 2–3 experiments.
Acknowledgments
Table 3. Functional activity of selected compounds
Compound
% Stimulation of [35S]GTPcS at lM
We thank Dr. William Greenlee for his support and
guidance of this work, Dr. Steven Sorota for performing
the hERG activity assay, and Dr. Jesse Wong for
preparation of intermediates.
34
35
39
41
56
87 at 10
87 at 10
90 at 10
104 at 10
104 at 0.1
References and notes
1. See Reviews (a) Bignan, Gilles C.; Connolly, Peter J.;
Middleton, Steven A. Expert Opin. Ther. Patents 2005, 15,
357; (b) Zaveri, Nurulain Life Sci. 2003, 73, 663–678.
2. McLeod, Robbie L.; Parra, Leonard E.; Mutter, Jennifer
C.; Erickson, Christine H.; Carey, Galen J.; Tulshian, Deen
B.; Fawzi, Ahmad B.; Smith-Torhan, April; Egan, Robert
W.; Cuss, Francis M.; Hey, John A. Br. J. Pharmacol. 2001,
132, 1175.
1.25 (m, 1H), 1.0 (m, 1H), 1.07 (m, 1H), 0.93 (d,
J = 6.8 Hz, 6H). Mass Spec ESI (M+1) = 392, 232.
1
Compound 39: H NMR (CDCl3)d 7.65 (d, J = 7.2 Hz,
1H), 6.90–7.40 (m, 12H), 5.49 (s, 1H), 4.74 (s, 2H),
3.00 (t, 2H), 2.60–2.80 (m, 4H), 1.67 (d, J = 13.6 Hz,
2H). Mass Spec ESI (M+1) = 466, 235.
3. Nguyen, Phong.; Corpuz, Evelyn.; Heidelbaugh, Todd M.;
Chow, Ken.; Garst, Michael E. J. Org. Chem 2003, 68, 10195.
4. Kerr, Cheryl A.; Rae, Ian D. Aust. J. Chem. 1978, 31, 341.
5. (a) Fawzi, Ahmad B.; Zhang, Hongtao.; Weig, Blair.;
Hawes, Brian.; Graziano, Michael P. Eur. J. Pharmacol.
1997, 336, 233; (b) Corboz, M. R.; Rivelli, M. A.; Egan, R.
W.; Tulshian, D.; Matasi, J.; Fawzi, A. B.; Benbow, L.;
Smith-Torhan, A.; Zhang, H.; Hey, J. A. Eur. J. Pharmacol.
2000, 402, 171.
1
Compound 56: H NMR (CDCl3)d 8.01 (s, 1H); 7.60 (s,
1H); 7.39 (s, 1H); 7.37 (d, 1H); 7.35 (d, 1H); 6.99 (d,
2H); 6.88 (t, 1H); 4.77 (q, 2H); 3.77 (dd, J = 10.2,
5.1 Hz, 1H); 3.40 (dt, J = 10.2, 2.9 Hz, 1H); 2.97–2.80
(m, 3H); 2.63 (dt, J = 12.8, 5.1 Hz, 1H); 2.44 (d, 1H);
1.97–1.71 (m, 4H); 1.64 (m, 2H); 1.29 (s, 3H); 1.22 (s,
3H); Mass Spec ESI (M+1) = 458, 232.