Essential structure of opioid κ receptor agonist nalfurafine for binding to the κ receptor 2: Synthesis of decahydro(iminoethano) phenanthrene derivatives and their pharmacologies

Article abstract of DOI:10.1016/j.bmcl.2012.05.122

To clarify the essential structures of an opioid κ receptor selective agonist, nalfurafine, for binding to the κ receptor, we designed and synthesized some nalfurafine derivatives and the decahydro(iminoethano) phenanthrene derivatives with a cyclohexene moiety as a surrogate for the phenol ring. In addition to the 6-amide side chain and the 17-nitrogen substituted by a cyclopropylmethyl group, the 4,5-epoxy ring, phenolic hydroxy group, and angular hydroxy group played important roles in eliciting the binding properties of nalfurafine but these three moieties were not indispensable for binding to the κ receptor. Moreover, the phenol ring was also not essential for the binding to the κ receptor, and the cyclohexene moiety would play an important role in fixing the conformation of decahydro(iminoethano)phenanthrene derivatives to effectively raise the amide side chain, rendering a conformation that resembled the active one of nalfurafine.

Full text of DOI:10.1016/j.bmcl.2012.05.122

Bioorganic & Medicinal Chemistry Letters 22 (2012) 5071–5074
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Bioorganic & Medicinal Chemistry Letters
journal homepage: www.elsevier.com/locate/bmcl
Essential structure of opioid
j
receptor agonist nalfurafine for binding
to the
j
receptor 2: Synthesis of decahydro(iminoethano)phenanthrene
derivatives and their pharmacologies
⇑
Hiroshi Nagase , Satomi Imaide, Shigeto Hirayama, Toru Nemoto, Hideaki Fujii
Department of Medicinal Chemistry, School of Pharmacy, Kitasato University, 5-9-1, Shirokane, Minato-ku, Tokyo 108-8641, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
To clarify the essential structures of an opioid
j receptor selective agonist, nalfurafine, for binding to the
Received 9 May 2012
Revised 30 May 2012
Accepted 31 May 2012
Available online 7 June 2012
j
receptor, we designed and synthesized some nalfurafine derivatives and the decahydro(iminoethan-
o)phenanthrene derivatives with a cyclohexene moiety as a surrogate for the phenol ring. In addition
to the 6-amide side chain and the 17-nitrogen substituted by a cyclopropylmethyl group, the 4,5-epoxy
ring, phenolic hydroxy group, and angular hydroxy group played important roles in eliciting the binding
properties of nalfurafine but these three moieties were not indispensable for binding to the
j receptor.
Keywords:
Opioid
Moreover, the phenol ring was also not essential for the binding to the receptor, and the cyclohexene
j
moiety would play an important role in fixing the conformation of decahydro(iminoethano)phenan-
threne derivatives to effectively raise the amide side chain, rendering a conformation that resembled
the active one of nalfurafine.
j
Receptor
Decahydro(iminoethano)phenanthrene
Nalfurafine
Ó 2012 Elsevier Ltd. All rights reserved.
OH
Opioid receptors are now classified into three types (l, d, and j)
17
N
O
based on not only pharmacological studies, but also by molecular
6
5
biological studies.1 Narcotic addiction is believed to be derived
N
O
Me
from the
drug targets for analgesics without addiction. Recently, one of our
designed selective agonists, nalfurafine hydrochloride (TRK-820,
Fig. 1),2–6 was launched in Japan as an antipruritic for patients
l receptor type, and therefore d and j types are promising
O
4
.
HCl
OH
j
nalfurafine hydrochloride (TRK-820 )
Cl
undergoing dialysis.4,5 Although prototypical
j agonists with an
O
O
O
arylacetamide structure such as U-50,488H7,8 and U-69,593
(Fig. 1)9 had serious side effects like psychotomimetic and aversive
reactions,10,11 nalfurafine showed neither aversive nor addictive
effects.12 This disparity of properties between nalfurafine and
N
Cl
N
Me
Me
N
N
U-50,488H
U-69,593
arylacetamide
ences in
receptor subtypes13–16 that each compound interacted
with: arylacetamide
subtype,14,15 whereas nalfurafine may interact with the
j agonists was proposed to stem from the differ-
j
Figure 1. Structures of nalfurafine hydrochloride, U-50,488H, and U-69,593.
j
agonists would interact with the
j
j
1 receptor
3 receptor
subtype.17–21 We were interested in the differences in the
pharmacological effects between nalfurafine and the arylaceta-
mide derivatives. We therefore designed and synthesized decahy-
droisoquinoline derivatives lacking the phenol ring moiety to
clarify the essential structural features of nalfurafine that mediate
exhibited binding affinities and selectivities for the
indicating that these two functional groups should be essential
for the binding to the
receptor.24 Moreover, the subnanomolar
Ki values of nalfurafine as opposed to the micromolar order binding
affinity of 1 for the receptor suggested that the phenol moiety in
j receptor,
j
j
nalfurafine could force the C-ring into the boat form and effectively
binding to the
sional pharmacophore model of
decahydroisoquinoline derivatives 1 with a 6-amide side chain
j
receptor on the basis of the new three-dimen-
j
agonists.22,23 The trans-fused
increase the population of nalfurafine molecules in the most favor-
able conformation for binding to the
j
receptor (Fig. 3).24 There-
fore, we attempted to elucidate the role of the phenol ring in
nalfurafine in binding. In general, the phenol ring moiety in the
morphinan derivatives is thought to be the most important phar-
and a cyclopropylmethyl substituent at the nitrogen (Fig. 2)
⇑
Corresponding author.
E-mail address: nagaseh@pharm.kitasato-u.ac.jp (H. Nagase).
macophoric unit to provide a p–p interaction and a hydrogen bond
0960-894X/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.bmcl.2012.05.122

5072
H. Nagase et al. / Bioorg. Med. Chem. Lett. 22 (2012) 5071–5074
H
X
N
O
N
N
i), ii)
6
O
5
N
H
Me
X = H, OH
O
O
1
9
Figure 2. Structures of decahydroisoquinoline derivatives.
H
O
iii)-v)
6
N
Me
O
O
H
O
O
10b (6β)
N
Me
N
Scheme 2. Reagents and conditions: (i) SOCl2, pyridine, rt, 46%; (ii) H2, Pd/C, MeOH,
rt, 40%; (iii) 2 M HCl, MeOH, 80 °C, 82%; (iv) MeNH2ÁHCl, NaBH3CN, MeOH, 70 °C,
C-ring
O
then separation of 6a- and 6b-amines; (v) (2E)-3-(furan-3-yl)prop-2-enoyl chloride,
OH
Et3N, CH2Cl2, rt, 20% from 9 via 6b-amine.
Figure 3. Proposed active conformation of nalfurafine binding to the
j receptor.
olefin moiety in 5 was resistant to hydrogenation under various
conditions. Ketone 6, obtained by hydrolysis of 5, was submitted
to reductive amination to provide amines 7,30 of which 6b-isomer
7b was converted into 8b by acylation. The other designed com-
pound 10b30 (X = H, Fig. 4) was synthesized by a similar manner
as shown in Scheme 1. Preparation of compound 9 (Scheme 2)
was accomplished by dehydration of 5 using thionyl chloride and
subsequent catalytic hydrogenation.31 The synthesis of nalfurafine
derivative 14 without a 4,5-epoxy ring commenced with morphi-
nan 11 (Scheme 3). The reductive amination of 1132 with benzylm-
ethylamine stereoselectively gave 6b-amine 12 in the same
manner as for 4,5-epoxymorphinans.33,34 Nalfurafine derivative
14 was obtained from 6b-amine 12 by debenzylation, acylation,
and subsequent demethylation. The key reaction in the synthesis
of nalfurafine derivative 18, which lacked the phenolic hydroxy
group, was deoxygenation via 1-phenyltetrazol-5-yl ether 16
(Scheme 4).35,36 Ether 16 prepared from naltrexone (15) was trea-
ted under hydrogenolysis conditions to afford 3-dehydroxynaltrex-
one (17).36 Compound 17 was converted to nalfurafine derivative
18 by similar methods shown in Scheme 3. Nalfurafine derivative
20, without an angular hydroxy group, was prepared from 14-
dehydroxynaltrexone (19)31 by methods similar to those shown
in Scheme 3 (Fig. 5).
X
N
O
6
N
Me
O
Designed compound 2 (X = H, OH)
4. Structures of designed decahydro(iminoethano)phenanthrene
Figure
derivatives.
with the opioid receptor.25–27 So, to examine the role of the phenol
ring, we designed decahydro(iminoethano)phenanthrene deriva-
tives 2 (Fig. 4) whose cyclohexene moiety would mimic the phenol
part in nalfurafine, but could not provide either a p–p interaction
or a hydrogen bond with the receptor. The olefin moiety in 2 would
fix the cyclohexene moiety in a position resembling that of the
phenol ring in nalfurafine. Herein, we report the synthesis of some
nalfurafine derivatives and the designed decahydro(iminoethan-
o)phenanthrene derivatives 2. We also evaluate their binding
affinities for the opioid receptors.
We attempted to synthesize the designed compound 8b
(X = OH, Fig. 4) (Scheme 1) starting from morphinan 3 prepared
by a reported method.28,29 Morphinan 3 was reduced under the
Birch conditions to give cyclohexadiene 4 and cyclohexene 5. The
The binding affinities of the prepared compounds for the opioid
receptors were evaluated with competitive binding assays. The re-
sults were shown in Table 1 with the affinities of nalfurafine and
OH
OH
OH
N
N
N
N
ii)
i)
O
O
O
+
O
O
O
OMe
OMe
3
4
5
OH
OH
OH
N
N
O
iii)
iv)
6
6
NHMe
N
O
Me
O
6
7a (6α)
7b (6β)
8b (6β)
Scheme 1. Reagents and conditions: (i) Li, NH3 (liq.), EtOH, THF, À33 °C, 4: 53%, 5: 46%; (ii) 2 M HCl, MeOH, 80 °C, 92% from 5; (iii) MeNH2ÁHCl, NaBH3CN, MeOH, 60 °C, 7a:
64%, 7b: 22%; (iv) (2E)-3-(furan-3-yl)prop-2-enoyl chloride, Et3N, CH2Cl2, rt, 99% from 7b.

H. Nagase et al. / Bioorg. Med. Chem. Lett. 22 (2012) 5071–5074
5073
Table 1
OH
OH
Binding affinities of 6–8b, 10b, 14, 18, 20, nalfurafine, and decahydroisoquinoline
N
N
derivative 1 (X = OH, 6b) for opioid receptorsa
i)
Bn
Me
O
N
compound
Ki (nM)
Selectivity
lb
dc
jd
l
/
j
d/
j
OMe
OMe
Nalfurafine
0.582
>100,000
391
>1000
83.2
24.6
0.43
169
0.51
96.5
>100,000
939
>1000
308
204
2.09
>1000
8.82
0.195
19,130
787
2.6
—
0.50
—
14.7
1.94
2.42
59.8
1.47
456
—
11
12
f
f
1e
OH
6
1.19
g
g
7b
8b
10b
14
18
20
352
—
N
O
5.64
12.7
0.18
2.82
0.35
54.5
16.1
11.8
—
25.3
ii), iii)
N
Me
O
g
OR
13 (R = Me)
14 (R = H)
iv)
a
Binding assays were carried out in duplicate (j: cerebellum of guinea pig, l and
d: whole brain without cerebellum of mouse).
[3H] DAMGO was used.
b
Scheme 3. Reagents and conditions: (i) p-TsOHÁH2O, MeNHBn, PhCO2H, PhH,
reflux, then NaBH3CN, EtOH, MS4A, 0 °C to rt, 36%; (ii) H2, Pd/C, MeOH, rt, 60%; (iii)
(2E)-3-(furan-3-yl)prop-2-enoyl chloride, Et3N, CH2Cl2, rt, 74%; (iv) BBr3, CH2Cl2,
0 °C to rt, 83%.
[3H] DPDPE was used.
c
[3H] U-69,593 was used.
d
e
Ref. 24.
f
Selectivity was not calculated due to the Ki value for the
l
l
or d receptor was
or d receptor was
over 100,000 nM.
g
Selectivity was not calculated due to the Ki value for the
over 1000 nM.
OH
OH
N
N
i)
O
N
O
O
O
O
N
group were lower for all three types of the opioid receptors (espe-
cially for the receptor, with 290-fold lower affinity than that of
nalfurafine), but 18 had acceptable affinity for the receptor. As
a result, the selectivity of 18 over the receptor increased mark-
edly. This observation indicates that the phenolic hydroxy group,
which is believed to be one of the most important pharmacophores
to bind with the opioid receptor, would not be an essential phar-
N
l
N
Ph
OH
j
15
16
OH
j
l
N
ii)
iii) - v)
O
O
macophore for binding to the
20 without an angular hydroxy group maintained its affinities to
the receptor compared to nalfurafine, whereas the affinity of
20 for the d receptor increased. While the decrease in the affinity
of 20 for the receptor would result from lacking the interaction
between 6-amide side chain and 14-hydrogen. These outcomes
indicated that the 4,5-epoxy ring, phenolic hydroxy group, and
angular hydroxy group played important roles in eliciting the bind-
ing properties of nalfurafine but these structural features were not
j receptor. Nalfurafine derivative
17
l
OH
N
O
j
N
O
Me
O
18
indispensable for binding to the
signed compounds 8b and 10b, which have a cyclohexene moiety
as a surrogate for the phenol ring, showed high affinity for the
receptor and low affinities to the and d receptors. These out-
j receptor. Surprisingly, the de-
Scheme 4. Reagents and conditions: (i) 5-chloro-1-phenyl-1H-tetrazole, K2CO3,
DMF, rt, 89%; (ii) H2, Pd/C, AcOH, 50 °C, 89%; (iii) p-TsOHÁH2O, MeNHBn, PhCO2H,
PhH, reflux, then NaBH3CN, EtOH, MS4A, 0 °C to rt, 74%; (iv) H2, Pd/C, MeOH, rt, 71%;
(v) (2E)-3-(furan-3-yl)prop-2-enoyl chloride, Et3N, CH2Cl2, rt, 99%.
j
l
comes indicated that the benzene ring, which is believed to be
one of the most important pharmacophores for binding with the
opioid receptor, was not essential for binding to the
j receptor.
H
H
The affinities of 8b and 10b for the receptor were about 3400-
j
N
N
O
and 1500-fold higher than that of decahydroisoquinoline deriva-
tives 1 (X = OH, 6b), respectively.24 This result would stem from
the ability of the cyclohexene moiety to force the conformation
of 8b and 10b to effectively raise the amide side chain, a conforma-
tion in which the amide side chain at the upper side would resem-
ble the active conformation of nalfurafine (Fig. 3). On the other
hand, decahydro(iminoethano)phenanthrene derivatives 6 and 7b
O
N
O
O
Me
O
OH
OH
19
20
Figure 5. Structures of 14-dehydroxynaltrexone (19) and nalfurafine derivative 20.
lacking the 6-amide side chain showed low affinities for the
receptor, supporting the idea that the amide side chain would play
an important role in binding to the receptor.
In conclusion, to clarify the essential structures of nalfurafine
for binding to the receptor, we designed and synthesized some
j
decahydroisoquinoline 1 (X = OH, 6b) for comparison. The assays
were performed by previously reported procedures.37
j
Nalfurafine derivative 14 without the 4,5-epoxy ring showed
j
affinities for the
l
and
j
receptors comparable to nalfurafine itself,
nalfurafine derivatives and the decahydro(iminoethano)phenan-
threne derivatives with the cyclohexene moiety serving as a surro-
gate for the phenol ring. In addition to the 6-amide side chain and
the 17-nitrogen bearing the cyclopropylmethyl substituent, the
4,5-epoxy ring, phenolic hydroxy group, and angular hydroxy
group played important roles in eliciting the binding properties
but higher affinity to the d receptor than nalfurafine. The 4,5-epoxy
ring may hardly facilitate nalfurafine to acquire an active confor-
mation as shown in Figure 3, which would result in the affinity
of 14 for the
j receptor comparable to that of nalfurafine. The affin-
ities of nalfurafine derivative 18 without the phenolic hydroxy

5074
H. Nagase et al. / Bioorg. Med. Chem. Lett. 22 (2012) 5071–5074
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of nalfurafine, but these three moieties were not indispensable for
binding to the receptor. Moreover, the decahydro(iminoethan-
o)phenanthrene derivatives permitted us to determine that the
phenol ring of nalfurafine was not essential for binding to the
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j
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Acknowledgments
19. Tsuji, M.; Yamazaki, M.; Takeda, H.; Matsumiya, T.; Nagase, H.; Tseng, L. F.;
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24. Nagase, H.; Imaide, S.; Yamada, T.; Hirayama, S.; Nemoto, T.; Yamaotsu, N.;
Hirono, S.; Fujii, H. Chem. Pharm. Bull., in press.
We acknowledge the financial supports from Shorai Foundation
for Science, Uehara Memorial Foundation, and JSPS KAKENHI
(23590133) [Grant-in-Aid for Scientific Research (C)]. We also
acknowledge the Institute of Instrumental Analysis of Kitasato
University, School of Pharmacy for its facilities.
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