Synthesis, Pharmacology, and Molecular Docking Studies on 6-Desoxo-N-methylmorphinans as Potent μ-Opioid Receptor Agonists

Article abstract of DOI:10.1021/acs.jmedchem.7b01363

Position 6 of the morphinan skeleton plays a key role in the μ-opioid receptor (MOR) activity in vitro and in vivo. We describe the consequence of the 6-carbonyl group deletion in N-methylmorphinan-6-ones 1-4 on ligand-MOR interaction, signaling, and antinociception. While 6-desoxo compounds 1a, 2a, and 4a show similar profiles to their 6-keto counterparts, the 6-desoxo-14-benzyloxy substituted 3a displays significantly increased MOR binding and agonist potency and a distinct binding mode compared with its analogue 3.

Full text of DOI:10.1021/acs.jmedchem.7b01363

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Brief Article
Synthesis, Pharmacology and Molecular Modeling Studies on 6-
Desoxo-N-Methylmorphinans as Potent µ-Opioid Receptor Agonists
Maria Dumitrascuta, Tanila Ben Haddou, Elena Guerrieri, Stefan M
Noha, Lea Schläfer, Helmut Schmidhammer, and Mariana Spetea
J. Med. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.jmedchem.7b01363 • Publication Date (Web): 20 Oct 2017
Downloaded from http://pubs.acs.org on October 23, 2017
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Synthesis, Pharmacology and Molecular Docking Studies on
6-Desoxo-N-Methylmorphinans as Potent µ-Opioid Receptor Ago-
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Maria Dumitrascuta,‡ Tanila Ben Haddou, ‡ Elena Guerrieri, Stefan M. Noha, Lea Schläfer, Helmut
Schmidhammer, Mariana Spetea*
Department of Pharmaceutical Chemistry, Institute of Pharmacy and Center for Molecular Biosciences Innsbruck (CMBI),
University of Innsbruck, Innrain 80-82, 6020 Innsbruck, Austria
ABSTRACT: Position 6 of the morphinan skeleton plays a key role on the µ-opioid receptor (MOR) activity in vitro and in vivo.
We describe the consequence of the 6-carbonyl group deletion in N-methylmorphinan-6-ones 1-4 on ligand-MOR interaction, sig-
naling and antinociception. While 6-desoxo compounds 1a, 2a and 4a show similar profiles to their 6-keto counterparts, the 6-
desoxo-14-benzyloxy substituted 3a displays significantly increased MOR binding and agonist potency, and a distinct binding
mode than its analogue 3.
It is more than 90 years ago since the structure of morphine,
the naturally-occurring alkaloid in the opium poppy Papaver
somniferum, was clarified.1 Morphine has a long history of
clinical use as an effective analgesic for the treatment of mod-
erate to severe pain. It induces analgesia primarily via activa-
tion of the µ-opioid receptor (MOR).2 The morphine skeleton
and its conversion to analogues has been continuously ex-
plored over the years. The driving force behind the synthetical
efforts has been the search for an alternative to morphine that
would produce powerful analgesia without its adverse effects
(e.g., respiratory depression, sedation, constipation, analgesic
tolerance and addiction).2,3 Subsequently, the morphinan scaf-
fold has been the basis of new drug developments, and ligands
with distinct pharmacological properties are available for
patient use or employed as research probes to explore opioid
pharmacology in vitro and in vivo.2,4
Our laboratory has a longstanding focus in the field of opi-
oid morphinan analgesics. The initial synthetical and pharma-
cological work led to the generation of N-methylmorphinan-6-
ones with different substitution patterns at position 14.2b,4d,5
We established that the introduction of a 14-methoxy group
into the clinically used analgesic oxymorphone (1),2a,b (Figure
1) to yield 2 (14-O-methyloxymorphone)6a (Figure 1) not only
increased MOR affinity by ca. 9-fold, but also produced to a
significant increase (40-fold) in the antinociceptive potency.6a,b
However, 2 induces the typical opioid-like side effects (respir-
atory depression, physical dependence, constipation, and mo-
tor dysfunction).6 Replacement of the 14-methoxy group in 2
cious analgesic in various pain models in animals.5,7 It was
generally described to have an improved side effect profile, by
causing less respiratory depression, bradycardia, constipation,
physical dependence, addiction potential and development of
analgesic tolerance in comparison to conventional MOR anal-
gesics.5,7 Our recent on molecular docking and molecular
dynamics simulations study8 using the active structure of the
MOR (PDB ID: 5C1M)9 in combination with structure-activity
relationships (SAR) on N-methylmorphinan-6-ones revealed
the subtle interplay between substituents at positions 5 and 14
in the morphinan scaffold in the ligand-MOR interaction by
enabling to identify key structural elements that determine
their distinct pharmacological profiles.8
Synthetic approaches have uncovered that functionalizing
position 6 in N-methylmorphinan-6-ones results in a large
diversity of activities.10-14 The 6-carbonyl group can be easily
chemically converted into various functionalities, leading to
hydrazones, oximes, carbazones and semicarbazones,11 which
display antinociceptive efficacies and a favorable side effect
profile regarding respiratory depression and gastrointestinal
motility.11 We and others have reported on the incorporation of
amino acid residues at position 6 of N-methylmorphinan-6-
ones.12 The 6-amino acid zwitterionic conjugates of 2 were
established as MOR agonists inducing potent and long-lasting
peripherally-mediated antinociceptive effects after systemic
administration.4f,5a,13 The 6β-glycine substituted derivative of 2
was equipotent to fentanyl in the tail-flick assay in rats, acting
via activation of peripherally located opioid receptors.13b We
have also described derivatives of 2 with 6-amino and 6-
guanidino substitution that showed high MOR affinity, selec-
tivity and efficacy, and were highly active as antinociceptive
agents.10 By targeting the chemically highly versatile 6-keto
function of N-methylmorphinan-6-ones, we have reported on a
chemically innovative modification giving rise to a novel class
of morphinans with acrylonitrile incorporated substructures.14
The 6-cyano-N-methylmorphinans exhibit high affinity and
selectivity for the MOR, and potent in vitro and in vivo ago-
nism.14
with
a
benzyloxy substituent resulted in
3
(14-O-
benzyloxymorphone)6b (Figure 1), which displayed a 5-fold
increased antinociceptive potency when compared to 2.6b
Moreover, this highly potent MOR agonist showed negligible
inhibition of gastrointestinal motility, while it was much less
constipating than morphine (2.5-fold) and 2 (7-fold).6b Chemi-
cal derivatization in the class of N-methylmorphinan-6-ones
using 2 as the lead compound also targeted position 5, by
introducing a 5-methyl group and giving rise to 4 (14-
methoxymetopon)7 (Figure 1). Compound 4 is a highly effica-
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The present study was undertaken to evaluate the conse- vs 3a, a significant increase up to 9-fold in the MOR affinity
1
2
3
4
5
6
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8
quence of the deletion of the 6-carbonyl group of 14-hydroxy
and 14-alkoxy substituted N-methylmorphinan-6-ones (1-4,
Figure 1) resulting in the respective 6-desoxo-N-
methylmorphinans 1a-4a (Figure 1) on binding and activation
of the MOR, and antinociceptive properties. Towards this aim,
compounds 1a-4a were prepared using Wolff-Kishner condi-
tions similarly to the earlier reported synthesis of 6-
desoxonaltrexone15a and 6-desoxocyprodime15b from their
corresponding 6-keto analogues naltrexone and cyprodime,
respectively. Additionally, molecular modeling and SAR
explorations aided by docking of investigated compounds to
the active conformation of the MOR were performed to gain
insights into their binding mode and interaction mechanisms.
was noticed. Generally, the 6-desoxo derivatives showed a
similar binding affinity to the DOR as their 6-ketomorphinan
counterparts, exception being the pair 1a and 1, where a ca. 3-
fold decrease in affinity to the DOR was observed for 1a (Ta-
ble 1). We also found that KOR affinities of 1a-4a were in the
range of their parent compounds 1-4. The removal of the 6-
keto group in 1 increased selectivity for MOR vs DOR of 1a,
but it did not affect selectivity vs KOR. In the case of the 14-
methoxy substituted 2 and 2a, a slight decrease in MOR vs
KOR selectivity ratios was found, while selectivity for MOR
vs DOR was not changed. The same observation on somewhat
reduced MOR selectivity vs KOR was made when comparing
5-methyl substituted 4 and its 6-deoxo analogue 4a, while a
decrease was also noticed vs DOR (Table 1). Interesting was
the observation on the increase in MOR selectivity upon dele-
tion of the 6-keto function in the 14-benzyloxy substituted 3
leading to 3a. We found that the non-selective 3 was convert-
ed into a MOR selective compound, 3a, which displayed ca. 9-
and 6-fold higher MOR/DOR and MOR/KOR selectivity
ratios, respectively, than 3 (Table 1).
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1
2
3
4
(oxymorphone)
1a
2a
3a
4a
R1 = H, R2 = H
(14-O-methyloxymorphone)
(14-O-benzyloxmorphone)
(14-methoxymetopon)
R1 = CH3, R2 = H
R1 = CH2Ph, R2 = H
R1 = CH3, R2 = CH3
Functional opioid activity of N-methylmorphinan-6-ones 1-
4 and their 6-desoxo counterparts 1a-4a at the human MOR
receptor was next evaluated, where ligand-induced stimulation
of guanosine 5’-O-(3-[35S]thio)-triphosphate ([35S]GTPγS)
binding to membranes from CHO cells expressing the human
MOR receptor was measured (Table 1) as described earlier.14c
Efficacies are presented as percentage stimulation (% stim.)
Figure 1. Structures of N-methylmorphinan-6-ones 1-4 and their
6-desoxo counterparts 1a-4a. Ph, phenyl.
RESULTS AND DISCUSSION
Chemistry The 6-desoxomorphinans 1a-4a reported herein
were prepared from 1, 2,6a 36b and 4,7 respectively by Wolff-
Kishner reduction as depicted in Scheme 1.
relative to the prototypical MOR full agonist DAMGO (EC50
=
27.8 ± 3.7 nM and % stim. = 221 ± 13%). While we have
reported previously on the potent MOR agonism of com-
pounds 2-4 in bioassays using isolated organs (mouse vas
deferens, guinea pig ileum)6b,7 and ligand-stimulated
[35S]GTPγS binding in rat brain membranes,6c herein the first
data on G protein activation upon ligand binding to the human
MOR is presented. Based on in vitro functional activities, all
test compounds produced a concentration-dependent increase
in [35S]GTPγS binding (Figure S2), and showed high efficacy
at the human MOR acting as full agonists (≥ 85% of the re-
sponse to DAMGO), with the most potent agonists being 3
and 3a (Table 1). Our present data indicate that the removal of
the 6-keto function did not largely influence the MOR ago-
nism of 1 vs 1a, 2 vs 2a, and 4 vs 4a. However, an increase of
ca. 2.5-fold in the EC50 value of 3a compared to 3 was ob-
served (Table 1).
We have earlier reported on the replacement of the 6-keto
group in N-methylmorphinans-6-ones with other functionali-
ties (i.e. amino acids, cyano, amino and guanidino) at C-6,
which produced subtle alterations in the in vitro biological
activities, i.e. MOR affinity and agonist potency. Based on
previous and present findings, a 6-keto group is not a require-
ment for high affinity to the MOR of N-methylmorphinans
displaying agonist activity. Earlier observations in the class of
morphinans with opioid antagonism, including naltrexone and
N-cycloproplymethyl-4,14-dimethoxymorphinan (cyprodime),
revealed that deletion of the 6-keto group maintains the MOR
antagonism while MOR selectivity is unchanged or de-
creased.15 The 6-desoxo-N-methylmorphinans 1a-4a were
further assessed for antinociceptive properties after sc admin-
stration in a mouse model of acute thermal nociception, the
hot-plate assay.6b,c Antinociceptive potencies expressed as
ED50 values (and 95% confidence intervals) were calculated at
Scheme 1. Synthesis of Compounds 1a-4aa
1
2
3
4
R1 = H, R2 = H
R1 = CH3, R2 = H
R1 = CH2Ph, R2 = H
R1 = CH3, R2 = CH3
aReagents and conditions: (a) hydrazine hydrate, triethylene
glycol, 180 °C, 1.5 h; then KOH pellets, 180 °C, 2 h.
Pharmacology Binding affinities at the human MOR, δ-
opioid (DOR) and κ-opioid (KOR) receptors of N-
methylmorphinan-6-ones 1-4 and their 6-desoxo counterparts
1a-4a (Figure 1) were first determined in competition binding
assays using membranes from Chinese hamster ovary (CHO)
cells stably transfected with one of the recombinant human
opioid receptors (CHO-hMOR, CHO-hDOR and CHO-hKOR
cells) according to the described procedures.16 Binding affini-
ties expressed as inhibition constant (Ki) values are shown in
Table 1. All compounds displayed high potency to inhibit
[3H][D-Ala2,Me-Phe4,Gly-ol5]enkephalin
([3H]DAMGO)
binding to the human MOR in a concentration-dependent
manner (Figure S1) with Ki values in the subnanomolar range
(Table 1). The high binding affinities to the human MOR
expressed in CHO cells showed by compounds 1-4 corrobo-
rates with our earlier findings at the rat MOR in brain tissue.6b
Evaluation of N-methylmorphinan-6-ones and their 6-desoxo
analogues revealed that the removal of the 6-keto function
does not significantly affect binding affinity at the MOR when
comparing 1 vs 1a, 2 vs 2a, and 4 vs 4a. Only in the case of 3
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Table 1. Opioid receptor binding, functional activities, antinociceptive potencies and physicochemical properties of N-
methylmorphinan-6-ones 1-4 and their 6-desoxo counterparts 1a-4a
1
2
3
4
5
6
7
8
9
Antinociceptive
activityc
Hot-plate assay
ED50 (µg/kg, sc)
(95% C.I.)
Physico-
chemical
propertiesd
clogP
Opioid receptor binding (Ki, nM)a
Functional activityb
[35S]GTPγS MOR
Affinity
Selectivity
DOR/ KOR/
MOR MOR
MOR
DOR
KOR
EC50 (nM)
% stim.
1.79 ± 0.34
1.65 ± 0.5
70.0 ± 4.1
201 ± 80
25.3 ± 8.4
21.6 ± 3.9
10.1 ± 3.1
3.99 ± 0.63
1.27 ± 0.24
0.85 ± 0.05
15.1 ± 0.9
7.92 ± 4.47
39
122
28
36
2.3
21
79
38
14
13
32
17
2.8
18
60
23
7.80 ± 1.61
8.99 ± 2.19
1.45 ± 0.32
1.37 ± 0.26
0.32 ± 0.16
0.13 ± 0.03
3.28 ± 0.81
2.95 ± 0.54
91.5 ± 5.0 382 (186 - 785)
383 (209 - 802)
96.4 ± 5.0 14.2 (7.75 - 26.1)
0.88
1.24
1.45
1.82
3.23
1
1a
2
2a
3
3a
4
104 ± 7
0.32 ± 0.08
0.24 ± 0.08
0.45 ± 0.10
0.048 ± 0.011
0.25 ± 0.04
0.34 ± 0.08
8.80 ± 0.35
8.64 ± 2.25
1.05 ± 0.42
0.89 ± 0.29
19.8 ± 1.3
12.9 ± 1.6
105 ± 6
101 ± 5
103 ± 8
17.3 (8.68 - 34.4)
2.38 (1.29 - 4.38)
2.41 (1.39 - 4.17) 3.60
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99.2 ± 2.8 27.1 (12.7 - 58.1)
1.66
1.90
104 ± 3
25.1 (13.0 - 48.5)
4a
b
aDetermined in competition binding assays using membranes from CHO cells expressing human opioid receptors. Determined in the
[35S]GTPγS binding assay using CHO-hMOR cell membranes. Percentage stimulation (% stim.) relative to the MOR full agonist
DAMGO. a,bValues are means ± SEM of at least three independent experiments. Determined in the hot-plate assay in mice at peak effect
c
(30 min) after sc drug administration (n = 5-6 mice per group). dCalculated logP (clogP) using MarvinSketch 17.10 (ChemAxon,
www.chemaxon.com).
the peak of action, and compared to those of the 6-
ketomorphinans 1-4 (Table 1). All test compounds significant-
ly increased latencies to heat stimulation in a time- and dose-
dependent manner (Figures S3 and S4), with the peak of an-
tinociception generally occurring at 30 min after sc admin-
istration to mice. Compared to morphine (ED50 = 2428 µg/kg,
95% CI of 1382-4268 in the hot-plate assay),14c all compounds
were considerably more efficacious in inducing an antino-
ciceptive response in mice, being up to 1020-fold more potent
than morphine. Within the series, the 14-benzyloxy substituted
3 and 3a were the most active as antinociceptive agent, a find-
ing that correlates well with their in vitro opioid activity pro-
file (Table 1). Overall, as shown in Table 1 and Figure S5, the
6-desoxo compounds 1a-4a exhibit very similar antinocicep-
tive potencies to their analogue 1-4 indicating that the absence
of a 6-carbonyl group does not significantly affect the in vivo
agonism (two-way ANOVA, P > 0.05 when comparing 1-4 to
1a-4a counterparts). In both series of N-methylmorphinan-6-
ones 1-4 and 6-desoxo-N-methylmorphinans 1a-4a, it was
apparent that compounds carrying a 14-methoxy (2, 4, 2a and
4a) and a 14-benzyloxy (3 and 3a) group have enhanced affin-
ity and in vitro and in vivo agonist potency to the MOR than
the 14-hydroxy substituted 1 and 1a (Table 1).
Assessment of physicochemical properties has gained con-
siderable importance over the last years, emerging as a signifi-
cant aspect in drug development, especially for understanding
of the behavior of bioactive molecules and correlation with
pharmacological activities.17 The calculated logP (clogP) of N-
methylmorphinans 1-4 and 1a-4a are indicative for good ca-
pability to enter the CNS (Table 1).
Molecular modeling The X-ray crystal structure of the ac-
tive MOR conformation (PDB ID: 5C1M)9 provides nowadays
novel prospects in the drug discovery process.18 In order to
further understand the effect of a 6-carbonyl group deletion in
N-methylmorphinan-6-ones, we performed molecular docking
using the active conformation of the MOR. Docking studies
were carried out to unravel potential differences in receptor
interactions of 1-4 and their 6-desoxo counterparts 1a-4a using
GOLD19 as outlined in the Supporting Information. Differ-
ences and similarities in the experimentally determined bind-
ing affinities to the MOR (Table 1) are likely to emerge from
the type and number of receptor contacts formed by these
morphinans in those particularly similar docked orientations.
Accounting for pharmacophore models inferred within Lig-
andScout,20 receptor side chains in putative contact with the
bound ligands are depicted in Figure 2 and listed in Table 2. In
line to our recent findings from molecular docking,8 the aro-
matic ring of targeted morphinans was deduced to be embed-
ded by hydrophobic residues (e.g., M151, V236, I296 and
V300), that are responsible for hydrophobic interactions (Ta-
ble 2 and Figures 2). The participation of the phenolic hydrox-
yl group at position 3 in a water-mediated hydrogen-bonding
network with H297 was shown by all ligands.
Figure 2. Docking of N-methylmorphinans-6-ones 1-4 (gray) and
corresponding 6-desoxo counterparts 1a-4a (blue) to the active
structure of the MOR. (A) Overlay of 1 and 1a. (B) Overlay of 2
and 2a. (C) Overlay of 3 and 3a. (D) Overlay of 4 and 4a. The
key residues D147 and H297, involved in a hydrogen-bonding
network, and residues I144 and Y148, involved in hydrophobic
interactions are depicted. Chemical features are color-coded:
red/green arrow: hydrogen bond acceptor/donor; yellow sphere:
hydrophobic interaction; blue asterisk: positively ionizable. Bind-
ing pocket surface is shown, colored according to aggregated
hydrophilicity/hydrophobicity (blue/ochre).
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Table 2. Ligand-MOR interaction pharmacophores inferred from molecular docking solutions of N-methylmorphinan-6-
ones 1-4 and their 6-desoxo counterparts 1a-4a
1
2
Hydrophobic interactions
Hydrogen bonds
3
4
5
6
7
8
9
Inferred from the introduced
Charge-enhanced hydrogen
Interactions mediated by
water molecules
H297
H297
H297
H297
H297
H297
H297
Inferred from the phenol
group
NDa
ND
bond
D147
D147
D147
D147
D147
D147
D147
D147
M151, V236, I296, V300
M151, V236, I296, V300
M151, V236, I296, V300
M151, V236, I296, V300
M151, V236, I296, V300
M151, V236, I296, V300
M151, V236, I296, V300
M151, V236, I296, V300
1
1a
2
2a
3
3a
4
ND
ND
I144b
I144b
ND
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Y148c
H297
4a
aND, not deduced. bFormed by the 14-benzyloxy group. cFormed by the 5-methyl group.
This is recognized as a conserved interaction between
morphinan ligands and the binding pocket.8,9 In addition, the
basic nitrogen forms a charge-enhanced hydrogen bond to the
key residue D147. This observation is supported by earlier
experimental data derived from site-directed mutagenesis
studies demonstrating the crucial role of D147 for binding of
small molecule agonists to the MOR.21 When comparing the
retrieved docking poses of the 6-desoxo compounds 1a-4a to
their corresponding 6-keto analogues 1-4, the observed differ-
ences were more or less subtle regarding critical non-covalent
interactions, which are required for the recognition of these
potent agonists by the MOR (Figure 2). We observed that 1
and 2 and their 6-desoxo analogues 1a and 2a, respectively,
exhibit a similar binding mode regarding the orientation of
these agonists relative to the receptor (Figure 2, A and B).
Differences were noted when evaluating the binding mode of
3 along with its 6-desoxo counterpart 3a, as well as to the
other morphinans (Figure 2C). Docking analysis showed that 3
and 3a adopt an orientation relative to the receptor distinctive
from the other analogues, represented by an additional hydro-
phobic interaction between I144 and the 14-O-benzyl group of
3 and 3a (Figure 2C and Table 2). As a consequence, this
modified relative orientation in the binding pocket may be
responsible for a steric clash, surmised between the 6-keto
group of 3 and residue V300, which is essentially orientated in
the close proximity to the phenol of targeted morphinans (vide
supra). However, in case of 3 the side chain of V300 is getting
in rather close proximity to the 6-keto substituted cyclohexane
ring. This interaction was not observed for the 6-desoxo-14-
benzyloxy substituted 3a, as the docking pose indicated that
the corresponding methylene bridge at position 6 is nicely
accommodated in the binding pocket (Figure 3).
These features may contribute to the improved binding affini-
ties to the MOR of 3a compared to 3 (Ki = 0.048 nM for 3a vs
0.45 nM for 3) experimentally determined in radioligand bind-
ing assays. The projected orientation in the binding pocket was
only slightly different when comparing 5-methyl-14-methoxy-
N-methylmorphinans 4 and 4a. Interestingly, the 6-desoxo
analogue 4a was predicted to form an additional hydrophobic
interaction by the 5-methyl group targeting the nearby Y148
as counterpart (Figure 2D and Table 2). Overall, the docking
analysis pointed out towards a subtle interplay of the substitu-
ents introduced to this series in positions 14 and 5, and in
silico results are in accordance with pharmacological data.
CONCLUSIONS
The present study, combining synthetical, pharmacological
and molecular modeling approaches, established that the dele-
tion of a 6-carbonyl group in targeted N- methylmorphinan-6-
ones 1, 2 and 4 did not fundamentally affect binding, post-
receptor signaling and antinociceptive activities, with the
resulting 6-desoxo analogues evolving as potent MOR ago-
nists. Notable was the observation that the 6-desoxo-14-
benzyloxy 3a displays significantly increased MOR binding
and agonist potency than its 6-keto counterpart 3. At the in
silico level, the absence of the 6-carbonyl function in 3a de-
pleted the steric clash shown by the 6-keto group of 3 with
residue V300, thus explaining the improved binding affinities
to the MOR of 3a compared to 3. These results expand the
understanding on the impact of the 6-carbonyl to 6-CH2 con-
version in N-methylmorphinans on ligand-MOR interaction
and molecular mode of action, and may aid in identifying new
leads for development of opioid analgesics devoid of adverse
effects for treatment of pain.
EXPERIMENTAL SECTION
Chemistry. General methods. All chemicals were of reagent
grade and obtained from standard commercial sources. Melt-
ing points were determined on a Kofler melting point micro-
scope and are uncorrected. 1H NMR (200 MHz) were recorded
on a Varian Gemini 200 spectrometer using tetramethylsilane
(TMS) as internal standard for CDCl3. IR spectra were taken
on a Bruker Alpha FT-IR spectrometer (for detection, an ATR
sensor was used). Mass spectra were recorded on a Varian
MAT 44 S apparatus. Elemental analyses were performed at
the Microanalytic Laboratory of the University of Vienna,
Austria. For column chromatography (MPLC), silica gel 60
(0.040-0.063 mm, Fluka, Switzerland) was used. Compounds
1a-4a were used as bases for testing. The elemental analysis
Figure 3. Overlay of molecular docking solutions for 3 (gray) and
3a (blue) at the active structure of the MOR, illustrating the steric
clash inferred to represent the interaction of the 6-keto group of 3
with V300, orientated in a rather close proximity, and the absence
of this interaction for 3a. Residue V300 is presented, along with
the binding pocket surface shown in wireframe representation.
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Journal of Medicinal Chemistry
0404/1596), the Förderungsbeiträge Aktion D. Swarovski KG
2014, and the University of Innsbruck.
values were found to be within ± 0.4% of the calculated val-
1
2
3
4
5
6
7
8
ues, indicating a purity of the tested compounds of >95%.
General procedure for the synthesis of 6-desoxomorphinans
1a-4a. A mixture of 1, 2, 3 or 4 (0.60 mmol), hydrazine hy-
drate (27 mmol) and triethylene glycol (3 mL) was stirred at
130 °C for 1.5 h. After cooling, KOH pellets (6 mmol) were
added and the mixture stirred at 180 °C for 2 h. After cooling,
the mixture was acidified with 2 N HCl, washed with Et2O (2
x 10 mL), rendered alkaline with NH4OHconc. and extracted
with CH2Cl2 (3 x 10 mL). The organic layer was washed with
H2O (3 x 10 mL), dried over Na2SO4 and evaporated to pro-
vide a solid which was purified by column chromatography
(silica gel, CH2Cl2/MeOH/NH4OH, 98:1:1) to yield the final
compounds.
3,14-Dihydroxy-4,5α-epoxy-17-methylmorphinan (1a). Yield
11% as beige solid. Mp: 90-92 °C. IR (ATR) 3119 cm-1 (OH).
1H NMR (CDCl3): δ 6.71 (d, J = 8.0 Hz, H-C(1)), 6.58 (d, J =
8.0 Hz, H-C(2)), 4.72 (t, J = 8 Hz, H-C(5)), 2.37 (s, CH3N).
MS (ESI) m/z 288.1 [M+1]+. Anal (C17H21NO3∙0.2 MeOH) C,
H, N.
ABBREVIATIONS
CHO, Chinese hamster ovary; DOR, δ-opioid receptor; KOR, ĸ-
opioid receptor; MOR, µ-opioid receptor; PDB, Protein Data
Bank.
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1
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316.2 [M+1]+. Anal (C19H25NO3∙0.1 CH2Cl2 ) C, H, N.
ASSOCIATED CONTENT
Supporting Information.
The supporting Information is available free of charge on the ACS
Publications website.
Additional pharmacological and molecular modeling infor-
mation (PDF)
Molecular formula strings (CSV)
AUTHOR INFORMATION
Corresponding Author
*E-mail: Mariana.Spetea@uibk.ac.at. Phone: +43-512 507 58277.
ORCID
Mariana Spetea: 0000-0002-2379-5358
Author Contributions
‡M.D. and T.B.H. contributed equally to this work.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
We thank Dr. Sonja Sturm for recording the mass spectra. This
research was supported by grants from the Austrian Science Fund
(FWF: TRP19-B18), the Tyrolean Research Fund (TWF: UNI-
8. Noha, S. M.; Schmidhammer, H.; Spetea, M. Molecular
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