Synthesis and structure-activity relationships of aminoalkylazetidines as ORL1 receptor ligands

Article abstract of DOI:10.1016/S0960-894X(02)00652-2

A series of aminoalkylazetidines has been discovered as novel ORL1 receptor ligands. Structure-activity relationships have been investigated at the azetidine N and the alkyl side chain sites. Several potent and selective analogues have been identified.

Full text of DOI:10.1016/S0960-894X(02)00652-2

Bioorganic & Medicinal Chemistry Letters 12 (2002) 3157–3160
Synthesis and Structure–Activity Relationships of
Aminoalkylazetidines as ORL1 Receptor Ligands
Wen-Lian Wu,* Mary Ann Caplen, Martin S. Domalski, Hongtao Zhang, Ahmad Fawzi
and Duane A. Burnett*
Schering Plough Research Institute, 2015 Galloping Hill Road, MS 2800, Kenilworth, NJ07033-0539, USA
Received 23 May 2002; accepted 24 July 2002
Abstract—A series of aminoalkylazetidines has been discovered as novel ORL1 receptor ligands. Structure–activity relationships
have been investigated at the azetidine N and the alkyl side chain sites. Several potent and selective analogues have been identified.
# 2002 Elsevier Science Ltd. All rights reserved.
The ORL1 receptor is a G-protein coupled receptor
with high homology to the opiate receptors.1 Its endogen-
ous ligand is called Nociceptin (or Orphanin FQ) and is a
17-aminoacid neuropeptide that is structurally related to
the opiate peptide dynorphin A. Nociceptin, however,
shows little affinity for the other opiate receptors.2
Nociceptin has been shown to have many different
behavioral effects, including interfering with motor
performance, stimulating feeding, producing hyperalgesia,
reversing stress-induced analgesia, producing analgesia
and anxiolytic-like effects.3 Recently, the discovery of
non-peptidic small molecule ORL1 ligands has been
disclosed.4 In this communication, we would like to
report our discovery and SAR investigations of a novel
series of ORL1 ligands.
condensation of the commercially available ethyl dibro-
mobutyrate and benzhydrylamine followed by DIBAL
reduction of the resulting azetidine ester gave the
N-benzhydrylazetidine-2-carboxaldehyde. Grignard addi-
tion gave the aminoalcohols as a mixture of erythro and
threo isomers (erythro-4b/threo-4a 3:1) in >50% yield.
Because initial screening results demonstrated that the
threo aminoalkylazetidines are consistently more potent
than their erythro counterparts, we focused our efforts
on the threo diamine analogues. Taking advantage of
this natural selectivity, we subsequently inverted the
stereochemistry of the major erythro aminoalcohol via a
Mitsunobu reaction giving the desired threo aminoazide
product.7 Reduction of the azide generated the desired
diamine analogues as racemates.8,9
Our aminoalkylazetidine lead compound 1 (Fig. 1) was
identified as an agonist from high-throughput screening
of our chemical library, as it showed high affinity for
ORL1 with a Ki of 31 nM and modest selectivity over m-,
k, and d receptors. Optical resolution was achieved by
using a chiral column (Chiracel OJ, eluting with 95:5:0.2
hexane, EtOH, Et2NH). The absolute stereochemistry
of the less active enantiomer 1a was determined by X-ray
crystallography of its l-tartaric acid salt.5 In a GTPgS
assay both the racemate and its two enantiomers behave
as agonists at the ORL1 receptor.
The compounds described were evaluated in radioligand
binding assays (Table 1). Ki values against the human
ORL1 receptor were determined from competition
binding assays using [125I]Nociceptin and h-ORL1
receptor expressing Chinese hamster ovary (CHO) cell
membranes as described.10 Ki values for human m-, k-,
and d-opioid receptors were determined using
[3H]diprenorphine and CHO cell membranes expressing
the opioid receptors as described.10b Functional
[35S]GTPgS binding assays for h-ORL1 receptor were
carried out in CHO cell membranes expressing the
receptor as described, using a 1 h incubation prior to
initiation of assays.10 Compounds that induce an
increase in [35S]GTPgS binding were identified as ago-
nists, while compounds that have no effect on basal
[35S]GTPgS binding, but inhibit Nociceptin induced
increase in the binding were identified as antagonists.
The synthesis of racemic aminoalkylazetidine analogues
with various side chains is outlined in Scheme 1.6 Thus
*Corresponding authors. Fax:+1-908-740-7152; e-mail: duane.burnett@
spcorp.com
0960-894X/02/$ - see front matter # 2002 Elsevier Science Ltd. All rights reserved.
PII: S0960-894X(02)00652-2

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W.-L. Wu et al. / Bioorg. Med. Chem. Lett. 12 (2002) 3157–3160
are reasonably well tolerated, but p-phenoxy substitution
dramatically reduces affinity.
Derivatizations at the azetidine N were carried out
using a parallel synthesis approach as shown in Scheme
2. The N-allylazetidine carboxaldehyde was prepared as
above starting with allylamine. Addition of commer-
cially available 3-chlorophenylmagnesium bromide to
the aldehyde gave a mixture of diastereomers (erythro/
threo 3:1). After the threo isomer was transformed to
the desired erythro one by a two-step procedure,11 the
erythro amino alcohol 9b was converted to the threo
aminoazide 10. Reduction and protection gave the pri-
mary amine as an advanced intermediate for parallel
synthesis. The azetidine nitrogen was revealed by
removal of the allyl group12 and then alkylated with
various substituents. In cases where two diastereomers
were generated, chromatographic separation gave pure
Figure 1. Aminoalkylazetidine lead compounds.
Nociceptin shows an EC50 of 1.11ꢀ0.08 nM (meanꢀSEM,
n=10) and a 100% increase in [35S]GTPgS binding over
basal level in this assay. In the meta substituted series,
the methoxy and methyl moieties are poorly tolerated,
while simply substituting the m-trifluoromethyl sub-
stituent with m-chloro or m-fluoro yields compounds
with significant affinity and agonist functional properties.
In the para position, chloro, methyl and methoxy groups
Scheme 1. (a) NaHCO3, CH3CN, 55%; (b) DIBAL-H, 50%; (c) ArMgBr, THF, >50%; (d) Zn(N3)2(Pyr)2, Ph3P, DIAD, toluene, ꢁ75%; (e)
NiCl2(H2O)6, NaBH4, MeOH, ꢁ70%.
Table 1. Receptor binding of azetidine substituted phenyl analoguesa
Compd
R
ORL1 Ki (nM)
m Ki (nM)
k Ki (nM)
d Ki (nM)
Functional assay
(ꢀ)-1
(+)-1a
(ꢂ)-1b
6a
6b
6c
6d
6e
6f
6g
CF3
CF3
CF3
3-F
31
236
14
191
1967
6088
516
1766
1654
Nd
Nd
586
488
865
1634
3565
641
997
1232
Nd
Nd
607
660
1341
2772
150
6088
10960
3312
2923
8344
Nd
Agonist
Agonist
Agonist
Agonist
Nd
Nd
Nd
Agonist
Nd
Nd
3-Cl
37
3% @ 10 mM
45% @ 10 mM
152
94
3-Me
3-MeO
4-Cl
4-Me
4-MeO
4-PhO
3-F-4-Me
3,5-F2
Nd
1439
1856
2919
7722
634
198
7222
58
6h
6i
6j
5401
152
4245
Nd
Agonist
Agonist
105
4015
9533
aResults shown are mean values of 2–3 independent determinations. All assays were performed in duplicates. Kd values used for radiolabeled ligands
were: [125I]Nociceptin 23 pM for hORL1 receptor; [3H]Diprenorphine 0.8 nM for hd-, 0.31 nM for hk-, and 0.12 nM for hm-opioid receptors.

W.-L. Wu et al. / Bioorg. Med. Chem. Lett. 12 (2002) 3157–3160
3159
Scheme 2. Reagents and conditions: (a) allylamine, CH3CN, 50%; (b) DIBAL, ether; (c) m-chlorophenylmagnesium bromide, THF, 68% two steps;
(d) Ph3P, DIAD, 4-nitrobenzoic acid; K2CO3, MeOH, 72%; (e) Zn(N3)2(Pyr)2, Ph3P, DIAD, toluene, 70%; (f) Ph3P, THF–H2O, 87%; (g) (Boc)2O,
TEA, 71%; (h) Pd2(dba)3, dppb, thiosalicylic acid, 73%; (i) RX, K2CO3; (j) TFA.
racemic isomers. Finally, the desired azetidine diamine
analogues were obtained by removal of the Boc groups.
with the receptor. Pyrrolidine analogues of this series
had remarkably reduced affinity for ORL1.
The SAR of the azetidine N-substituted analogues is
shown in Tables 2 and 3. While the 3,30-difluoro benz-
hydryl analogue 13a gave comparable potency as the
lead compound, other substituents on the benzhydryl
moiety generally decreased the Ki dramatically (Table
2). Replacement of one phenyl ring of the benzhydryl
group with acyclic alkyl groups generated compounds
with high affinity and improved selectivity versus other
opioid receptors. In each example the ‘B’ diastereomer
was more potent than the ‘A’ diastereomer. These design-
ations were assigned by their chromatographic mobility
upon SiO2 chromatography and no effort has been
made to determine the relative stereochemistry. The
potency peaked at incorporation of a butyl group (13k,
n=3). Other analogues with a butyl chain demonstrated
similar binding profiles. Replacement of the benzhydryl
with a 5-nonanyl group resulted in an inactive com-
pound. These results may suggest that the two phenyl
groups of the benzhydryl probably interact differently
In summary, we have developed a practical, stereo-
selective synthetic route to threo aminoalkylazetidines
to explore the ORL1 structure–activity relationships
around the lead compound 1. These compounds show
stereoselective binding to the ORL1 receptor and have
modest to good potency. Selectivity over other opiate
receptors is generally good. In particular compound
13k–b shows high affinity (Ki=30 nM) and good selec-
tivity (m/ORL1=61, k/ORL1=114, and d/ORL1=268).
Compound 13k–b is an agonist at the ORL1 receptor.
Although both agonist and antagonists of this receptor
have been observed, the series typically yields agonists
when high affinity is present. Further work in this area
will be presented in due course.
Table 3. Receptor binding of azetidine N-alkylsubstituted analoguesa
Table 2. Receptor binding of azetidine N-substituted benzhydryl
analoguesa
Compd
R
n
ORL1 Ki m Ki
(nM)
k Ki
(nM) (nM) (nM)
d Ki Functional
assay
13h
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
0
1
1
2
2
3
3
4
4
5
5
3
3
3
3
3
1062
570
94
367
79
745
30
772
34
1016
185
680
26
347
49
10620 4099 39120
8067 13130 22410
7217 7718 22370 Agonist
4130 2938 10090 Nd
6015 7172 16460 Agonist
2248 3684 8819
1815 3432 8048
2148 3911 7579
1939 2993 5958
1591 1945 4978
Nd
Nd
13i–a
13i–b
13j–a
13j–b
13k–a
13k–b
13l–a
13l–b
13m–a
13m–b
13n–a
13n–b
13o–a
130–b
13p
Nd
Agonist
Nd
Nd
Nd
Compd
R
ORL1 Ki m Ki
(nM)
k Ki
d Ki
(nM) (nM) (nM)
Functional
assay
Ph
Ph
Ph
13a
13b
13c
13d
13e
13f
13g
3,30-F2
3,30-Cl2
3,30-(CF3)2
4,40-F2
4,40-Cl2
4,40-Br2
4,40-Me2
64
420
2593
684
3221
2532
1135
4317 3533 4787
7159 6037 2709
Agonist
Agonist
Nd
Nd
Nd
1494 2855 3913 Antagonist
Nd
Nd
Nd
3-F-Ph
3-F-Ph
3,5-F2-Ph
3,5-F2-Ph
n-Bu
4705 4125 12270
1599 2134 8710
5551 6488 9372
3975 4219 11240 Agonist
9666 4262 31780 Nd
Nd
Agonist
Nd
2176 2240 7430
3452 3251 5013
2502 1484 4987
Nd
Nd
801
1841 5011
1345
aSee Table 1 notes.
aSee Table 1 notes.

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W.-L. Wu et al. / Bioorg. Med. Chem. Lett. 12 (2002) 3157–3160
Jenck, F.; Monsma, F. J., Jr.; Wichmann, J.; Dautzenberg,
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