Design and synthesis of 1-(2-alkanamidoethyl)-6-methoxy-7-azaindole derivatives as potent melatonin agonists

Article abstract of DOI:10.1016/j.bmcl.2011.02.097

A series of 7-azaindolic ligands bearing a methoxy group and a N-acetyl chain as melatoninergic pharmacophores were synthesized and their binding affinities towards MT₁ and MT₂ receptors were evaluated. Compounds 7a-c and 12 (cyclohe

Full text of DOI:10.1016/j.bmcl.2011.02.097

Bioorganic & Medicinal Chemistry Letters 21 (2011) 2316–2319
Contents lists available at ScienceDirect
Bioorganic & Medicinal Chemistry Letters
journal homepage: www.elsevier.com/locate/bmcl
Design and synthesis of 1-(2-alkanamidoethyl)-6-methoxy-7-azaindole
derivatives as potent melatonin agonists
Matthieu Jeanty a, Franck Suzenet a, , Philippe Delagrange b, Olivier Nosjean b, Jean A. Boutin b,
⇑
Daniel H. Caignard b, Gérald Guillaumet a
a Institut de Chimie Organique et Analytique (ICOA), Université d0Orléans, UMR-CNRS 6005, BP 6759, rue de Chartres, 45067 Orléans Cedex 2, France
b Département des Sciences Expérimentales, Institut de Recherches Servier, 11 rue des Moulineaux, 92150 Suresnes, France
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of 7-azaindolic ligands bearing a methoxy group and a N-acetyl chain as melatoninergic pharma-
cophores were synthesized and their binding affinities towards MT1 and MT2 receptors were evaluated.
Compounds 7a–c and 12 (cyclohexyl ring connected at C-2 and C-3 position) appears as important mel-
atonin MT2 and MT1 receptors agonists. On the other hand, the presence of basic groups (amines) at posi-
tion C-3 was detrimental to the melatoninergic affinities.
Received 14 December 2010
Revised 22 February 2011
Accepted 23 February 2011
Available online 26 February 2011
Ó 2011 Elsevier Ltd. All rights reserved.
Keywords:
Melatonin
7-Azaindole
MT receptors agonists
Melatonin (N-acetyl-5-methoxytryptamine) is a neurohormone
secreted mainly by the pineal gland.1 In animals, secretion (only
during the dark period) and circulating levels of melatonin vary
in a daily cycle, thereby regulating the circadian rhythms of several
biological functions.2 One of the first applications of melatonin was
for the treatment of diseases associated with the desynchroniza-
tion of biological clock, such as jet-lag. However, melatonin is in-
volved in many others physiological processes within our bodies
and therefore, the possible therapeutic applications for melatonin
ligands are significant. Because of its short half-time, the clinical
use of melatonin is limited and a great attention have been drawn
to the design of novel agonist or antagonist ligands for melatonin
receptors. Only two melatonin agonists are on the market: ramelt-
eon approved for the treatment of sleep disorders, and agomelatine
for the treatment of depression. Agomelatine has the particularity
to be also a 5-HT2C antagonist.3
Melatonin acts on specific high affinity G-protein coupled recep-
tors. Two mammalian melatonin receptor subtypes (MT1 and MT2)
have been identified by molecular cloning studies.4 Both receptors
are expressed mainly in the suprachiasmatic nucleus but they are
also expressed in others parts of the brain and even in some periph-
eral organs.5 More recently, some studies allowed the identification
of a third binding site called MT3.6 This binding site has been
described in hamster as homologue of the cytoplasmic quinone
reductase 2.7
Although several teams have worked on melatonin receptors,
the knowledge of the physiological roles and significance of each
melatonin receptors are still not fully characterized. To gain a dee-
per knowledge about these receptors and to broaden their pharma-
cological interest, a considerable attention has been devoted to the
design of novel selective melatonin ligands,8 discovery of agonist
ligands representing a more challenging milestone.9 Rapidly, it
was proved that the methoxy group as well as the N-acetyl chain
of melatonin were the crucial pharmacophores to retain agonist
activities.10 Therefore, the indole ring of melatonin can be consid-
ered only as a linker and can be replaced easily by a wide range of
others heterocyclic structures without significant loss of binding
affinity to MT1 and MT2 receptors.11 This discovery led, for exam-
ple, to the conception of agomelatine.12
In this Letter, we disclose the design and the synthesis of a new
class of 7-azaindole MT1 and MT2 ligands with agonist activities.
Studies in regards to the possible functionalization at position C2
or C2–C3 without losing affinities over MT1/MT2 receptors lead
to the conception of a highly potent tricyclic ligand.
According to Spadoni et al.13 the indole core of melatonin can be
flipped over without impacting dramatically the binding affinity of
the resulting 1-(2-acetamidoethyl)-6-methoxyindole 1. Indeed, the
affinity of compound 1 for the melatonin receptor isolated from
quail optic tecta was similar to melatonin (Ki of 3.4 nM and 0.6
nM for compound 1 and melatonin, respectively; Fig. 1). In addi-
tion, we have reported that unlike 7-azamelatonin, 4-azamelatonin
was a good melatonin receptors ligand (Table 1).14 We assumed
that the flip of the 7-azaindole core could be the starting point
for the development of a new class of melatoninergic ligands
⇑
Corresponding author.
E-mail address: franck.suzenet@univ-orleans.fr (F. Suzenet).
0960-894X/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2011.02.097

M. Jeanty et al. / Bioorg. Med. Chem. Lett. 21 (2011) 2316–2319
2317
receptors. An agonist stimulates [35S] GTP
cS binding, and this
stimulation is proportional to the efficacy and intrinsic activity of
the molecule. By convention, the natural ligand melatonin has an
efficacy (ECmax) of 100%. Full agonists stimulate [35S]GTP
cS binding
with a maximum efficacy, close to that of melatonin itself. If ECmax
is between 30% and 70%, the compound is considered as partial
agonist. Whereas if ECmax is inferior to 30%, the compound is con-
sidered as antagonist. Compounds 7a–c show very high binding
affinity for MT1 and MT2 receptors closed to those of melatonin
for MT2 receptors. All these compounds exhibit evident agonistic
activity. It is noteworthy that the length of the acyl function (Me,
Et, nPr) has almost no impact on the MT affinities.
These results showed that flipping the 7-azaindole core of
7-azamelatonin allow to keep affinities at nanomolar concentra-
tions for MT receptors. Improvement of the biological properties
of these new ligands was investigated mainly by substituting the
pyrrole ring. Considering the remain biological affinities reported
for the N-1 functionalized endogenous melatonin,16 we started
our investigation by functionalize the C-3 position of 7a.
Taking advantage of the reactivity of the C3 position of azain-
dole towards strong electrophiles, we generated a small library of
C-3 substituted 7-azaindole derivatives (compounds 8a–g, Table 2)
using the Mannich reaction. All these novel compounds have been
evaluated on MT1/MT2 receptors but, unfortunately, a dramatic
negative effect on affinity occurred. None of these new ligands kept
a reasonable activity.
The presence of an extra basic nitrogen atom in compounds
8a–g was though to be responsible of the drop of binding affinities.
The synthesis and evaluation of compounds 8h–i gave this hypoth-
esis credit (Scheme 2, Table 3). Indeed, the introduction of trifluo-
romethyl group (compound 8h) or iodine (compound 8i) at C-3
position did not impact dramatically the binding affinities.
Following these considerations, we decided to introduce an
additional cycloalkyl chain connected to the C-2 and the C-3 posi-
tion. We hypothesized that this bis functionalisation should not be
prejudicial for the melatoninergic affinity by analogy with the re-
main biological affinities reported for the N-1 or C-2 functionalized
endogenous melatonin.8d,17 The synthesis of this original target 12
Figure 1. Structures of some potent melatonin ligands.
considering the similar arrangement of the methoxy, pyridine
nitrogen and ethylacetamido functions compared to the 4-azame-
latonin. In order to validate this pharmacological hypothesis, we
synthesized the compound 7a as depicted in Scheme 1.
The commercially available 7-azaindole was N-oxidized by
treatment with mCPBA. According to the procedure described by
Storz et al.15 the N-oxide function was O-methylated by treatment
with dimethyl sulfate and after addition of sodium methanolate,
the desired 6-methoxy-7-azaindole was isolated in good yield.
Introduction of the N-acetyl chain was achieved in four steps. Com-
pound 4 was first N-alkylated using 1,2-dibromoethane. After reac-
tion of the resulting bromo derivative 5 with sodium azide,
compound 6 was converted into amine by Raney nickel catalyzed
hydrogenation. The desired amide 7a was prepared in situ by add-
ing acetic anhydride at the end of the hydrogenation step. This syn-
thetic approach allowed the easy pharmacomodulation of the
amide group. Compounds 7b and 7c were rapidly prepared by add-
ing the corresponding acyl chloride instead of the acetic anhydride
in the last step.
These three novel 7-azaindole derivatives were evaluated (Ta-
ble 1) for their binding affinity for membranes prepared from hu-
man MT1 and MT2 receptors stably transfected in Human
Embryonic Kidney (HEK 293) cells or Chinese Hamster Ovarian
(CHO) cells, using 2-[125I]iodomelatonin as radioligand. The intrin-
sic activity of the most interesting compounds has been evaluated
only on both melatonin receptor subtypes. The results are shown
in Table 2. The [35S]GTP
cS binding assay used to determine the
functional activity of the compounds was performed using Chinese
Hamster Ovarian (CHO) cell lines stably expressing the human MT2
Table 1
MT1 and MT2 binding affinities (Ki) and functional activities (EC50 and Emax) of compounds 7a–c
Compounds
MT1
MT2
KiMT =KiMT
1
2
Ki SEM (nM)
EC50 SEM (nM)
ECmax SEM (%)
101 18
Ki (nM)
EC50 (nM)
ECmax (%)
Melatonin
7-Azamelatonin
0.25 0.09
40 9.4
0.2 0.004
1.4 0.1
3.3 1.0
1.3 0.1
1.5 0.64
1 100 104
12 1.3
4.0 0.5
5.1 0.6
0.34 0.09
117 15.6
0.3 0.6
0.6 0.05
0.28 0.01
0.3 0.04
0.42 0.17
380 125
1.1 0.2
2.1 0.6
1.4 0.5
0.7 0.1
102
101
99
72
105
2
5
4
4
4
0.7
0.3
92
96
99
87
93
4
3
3
4
3
4-Azamelatonin
0.7
7a
7b
7c
2.3
11.8
4.3
2.1 0.3
77 10
Scheme 1. Reagents and conditions: (a) mCPBA, AcOEt, 0 °C–rt, 18 h; (b) aq K2CO3, 6 h, 4 °C, 57% over two steps; (c) Me2SO4, MeCN, 15 h, 65 °C; (d) MeONa, 18 h, 60 °C, 61%
over two steps; (e) 1,2-dibromoethane, aq NaOH 50%, Bu4NI, 38 h, rt, 90%; (f) NaN3, water/dioxane, 40 h, reflux, 99%; (g) H2, Raney Ni, AcOEt, 2 h, rt; (h) K2CO3, Ac2O or
corresponding acyl chloride, 1–3 h, rt, 76–83% over two steps.

2318
M. Jeanty et al. / Bioorg. Med. Chem. Lett. 21 (2011) 2316–2319
Table 2
Synthesis, MT1, MT2 binding affinities (Ki) and functional activities (EC50 and Emax) of C-3 substituted 7-azaindolic derivatives bearing an extra
basic nitrogen atom
R
N
R2NH, aq. CH2O,
cat. HCl
R
from 8b
8g
N
MeO
N
N
H2, Pd(OH)2
EtOH, 5h, rt
MeO
N
iPrOH, 2-4h,
reflux
7a
8a-f
NHAc
Yield
81%
NHAc
R2N-, compounds no.
Ki MT1 (nM)
Ki MT2 (nM)
KiMT =KiMT
1
2
Me
N
>10,000
>10,000
/
Me
8a
Me
N
N
91%
10,460 613
4007 318
2.6
Ph
8b
8c
81%
70%
510 75
>10,000
742 38
550 28
0.7
N
O
>20
8d
Cl
52%
1210 255
929 60
1.3
N
N
8e
O
59%
9490 211
8295 397
7440 117
1090 24
1.3
7.6
N
F
8f
H
N
30% from 8b
Me
8g
is depicted in Scheme 3. The pyridylhydrazine 10 was synthesized
from the commercially available 2-chloro-6-methoxypyridine 8 by
nucleophilic aromatic substitution of the chlorine with hydrazine.
The Fischer indole methodology was then used as a key step for the
construction of the azaindole core. Although the Fischer cyclization
is known to be tricky with this specific pyridylhydrazine,18 the tri-
cyclic core 10 was obtained with moderate yield under microwave
activation. The N-acetyl chain was then introduced using the syn-
thetic sequence described in Scheme 1.
The biological evaluation of 12 gave good satisfaction (Table 3).
This new derivative appears to be an interesting melatonin ligand
with affinity in the nanomolar range. This result shows that C-2
and C-3 positions can be substituted without losing biological
activities and opens the way to the design of original tricyclic mel-
atonin ligands.
In summary, we have identified a novel series of melatonin ago-
nist for both MT1 and MT2 receptors, built on a 7-azaindole core.
We have shown that extending the terminal alkyl chain of the
amide had a benefic effect on the biological affinities. Basic groups
(amine) at position C-3 were detrimental to the melatonin affini-
Scheme 2. Reagents and conditions: (a) TFAA, CH2Cl2, MW activation, 100 °C,
20 min; (b) MeOH, aq Na2CO3, 86% over two steps; (c) NIS, THF, 2 h, rt, 72%.
Table 3
MT1 and MT2 binding affinities (Ki) and functional activities (EC50 and Emax) of compounds 7a, 8h–i and 12
Compounds
MT1
MT2
KiMT =KiMT
1
2
Ki SEM (nM)
EC50 SEM (nM)
ECmax SEM (%)
101 18
Ki (nM)
EC50 (nM)
ECmax (%)
Melatonin
7-Azamelatonin
0.25 0.09
40 9.4
0.2 0.004
1.4 0.1
91 12
1.5 0.64
1100 104
12 1.3
4.0 0.5
93 9.9
0.34 0.09
117 15.6
0.3 0.6
0.42 0.17
380 125
1.1 0.2
2.1 0.6
10 0.5
102
101
99
2
5
4
4
3
7
1
0.7
0.3
92
96
99
82
37
83
4
3
3
4
5
7
4-Azamelatonin
0.7
7a
8h
8i
0.6 0.05
72
2.3
3.0
3.2
30
5
1
0.5
93
16 1.4
9.0 1.4
32
7
11 1.7
1.3 0.1
37
12
16 4.3
0.68 0.05
89
13.2

M. Jeanty et al. / Bioorg. Med. Chem. Lett. 21 (2011) 2316–2319
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Scheme 3. Reagents and conditions: (a) hydrazine, MW activation, 160 °C, 20 min,
70%; (b) cyclohexanone, aq H2SO4 4%, MW activation, 160 °C, 20 min, 31%; (c) see
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fords good affinities for MT2/MT1 receptors with agonist potency.
Supplementary data
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C.; Renard, P.; Guillaumet, G. J. Enzyme Inhib. Med. Chem. 2003, 18, 187. and
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8339.
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.bmcl.2011.02.097.
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