Synthesis of 2-arylfuro[3,2-b]pyridines: Effect of the C2-aryl group on melatoninergic activity

Article abstract of DOI:10.1016/j.ejmech.2016.01.008

We report herein an efficient synthesis of 2-substituted furo[3,2-b]pyridines and their biological evaluation as melatonin receptors ligands. The proposed eight-step sequence ending with a Suzuki coupling allowed a rapid access to various analogues. The steric hindrance and the conformation of the aryl group in C2-position were evaluated regarding the selectivity of the molecule for one of the two high affinity melatonin receptors as well as the activity profile of the compound. Introduction of 1-naphthyl substituent gave the best result in terms of selectivity with a MT₁/MT₂ ratio of about 150 (MT₁ K_i = 198 nM, MT₂ K_i = 1.3 nM).

Full text of DOI:10.1016/j.ejmech.2016.01.008

European Journal of Medicinal Chemistry 109 (2016) 268e275
Contents lists available at ScienceDirect
European Journal of Medicinal Chemistry
journal homepage: http://www.elsevier.com/locate/ejmech
Research paper
Synthesis of 2-arylfuro[3,2-b]pyridines: Effect of the C2-aryl group on
melatoninergic activity
a
b
b
a
Audrey Couhert , Philippe Delagrange , Daniel-Henri Caignard , Agn eꢀ s Chartier ,
a,
*
a, **
Franck Suzenet , G eꢁ rald Guillaumet
a
Institut de Chimie Organique et Analytique, Universit ꢁe d’Orl ꢁe ans, UMR CNRS 7311, B.P. 6759, 45067 Orl ꢁe ans Cedex 2, France
Institut de Recherche Servier, Sciences Exp ꢁe rimentales, 125 Chemin de Ronde, 78290 Croissy, France
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 20 October 2015
Received in revised form
We report herein an efficient synthesis of 2-substituted furo[3,2-b]pyridines and their biological eval-
uation as melatonin receptors ligands. The proposed eight-step sequence ending with a Suzuki coupling
allowed a rapid access to various analogues. The steric hindrance and the conformation of the aryl group
in C2-position were evaluated regarding the selectivity of the molecule for one of the two high affinity
melatonin receptors as well as the activity profile of the compound. Introduction of 1-naphthyl sub-
7
January 2016
Accepted 9 January 2016
Available online 12 January 2016
stituent gave the best result in terms of selectivity with a MT
1
/MT
2
ratio of about 150 (MT
1
K
i
¼ 198 nM,
MT
2
K
i
¼ 1.3 nM).
Keywords:
©
2016 Elsevier Masson SAS. All rights reserved.
Melatonin receptors
Furopyridine
SAR
Melatoninergic activity
1
. Introduction
[15e17]. The mechanism of action of melatonin is getting an
increasing interest and one of the challenges is to develop MT -or
MT -selective ligands as pharmacological tools to understand the
2
function of each receptor subtypes. Recently, many series of MT -
selective ligands have been reported, such as ligand II [18] (Fig. 1),
with selectivity ratios from good (124-fold) to excellent (until
1200-fold) [19e23]. However, full agonist selective ligands are still
1
Melatonin (N-acetyl-5-methoxytryptamine, I, Fig. 1) is
a
2
neurohormone which is synthesized and secreted in the pineal
gland during the period of darkness. This hormone acts as a regu-
lator of the biological clock and is involved in several physiological
and metabolic processes including central nervous system disor-
ders [1e4]. Moreover, melatonin has been described as anti-
inflammatory [5], pain modulator [6], antioxidant [7,8], retinal
2
required to explain the intrinsic biological properties of MT . Sub-
type selectivity seems to be correlated with the presence in the C2-
position of a hydrophobic aryl substituent out-of-the-plane of the
core skeleton [24,25]. C2-arylation might also be responsible for the
[9], vascular [10] and antitumoral agent [11]. However, melatonin is
not often used in clinical treatments due to its short half-life [12]
but it is used as a dietary complement. Research on melatonin
analogues lead to the marketing of Ramelteon (Rozerem ), a
antagonist behavior observed for a lot of MT
[18,26].
2
selective ligands
®
melatonin agonist, employed to treat insomnia and nocturia [13].
Since 2009, a melatonin receptor agonist and a 5-HT2C antagonist,
Agomelatine (Valdoxan ), is indicated for the treatment of
Among all melatonin analogues (biological affinity are reported
in Table 1), the benzofuran scaffold (Fig. 1, ligand III) appeared to be
as efficient as the original indole skeleton [27]. The furo[2,3-c]
pyridine ligand (Fig. 1, ligand Vb) has been previously synthesized
in our group [28] and biologically evaluated on MT receptors. It
®
depression [14].
Biological functions of melatonin are mediated through activa-
tion of two high affinity G-protein coupled receptors MT
1
and MT
2
1 2
showed a good activity on both MT and MT receptors [29],
however, it was not the most potent melatoninergic ligand we
could expect. Indeed, in the pyrrolopyridine series, the 4-azaindole
isomer (Fig. 1, ligand IVa) gives a best biological profile than the 6-
and 7-azaindoles (Fig. 1, ligands IVb and IVc respectively) [30], and
*
Corresponding author.
* Corresponding author.
E-mail addresses: franck.suzenet@univ-orleans.fr (F. Suzenet), gerald.
*
confirmed by the MT
2
-selective radioligand recently published
guillaumet@univ-orleans.fr (G. Guillaumet).
[31]. As a result, we chose to work on the furo[3,2-b]pyridine series
http://dx.doi.org/10.1016/j.ejmech.2016.01.008
0223-5234/© 2016 Elsevier Masson SAS. All rights reserved.

A. Couhert et al. / European Journal of Medicinal Chemistry 109 (2016) 268e275
269
2
Fig. 1. Natural neurohormone I, MT -selective ligand II, benzofuran analogue III, azaindole and furopyridine derivatives IVaec and Vaeb.
Table 1
Biological bindings.
with sodium hydride in acetonitrile and was subjected to an O-
alkylation with 4-bromobut-2-enenitrile [33] to yield a 65/35 Z/E
Compound
K
i
± SEM (nM)
Selectivity fold MT
1
/MT
2
mixture
enenitrile 4 in 88% yield. Radical cyclization of the Z/E mixture of
and formation of the dihydrofurane ring was achieved with
tributyltin hydride (3 equivalents) in the presence of AIBN (30 mol
) to yield dihydrofuropyridine 5 as a single regioisomer: the 5-exo-
of
4-((2-bromo-6-methoxypyridin-3-yl)oxy)but-2-
MT
1
MT
2
4
I
II
III
IVa
IVb
IVc
Vb
0.25 ± 0.09
1350
0.15 ± 0.02
0.24 ± 0.004
9.5 ± 2.2
40 ± 5.1
0.34 ± 0.09
1.7
0.7
790
0.5
0.7
0.3
%
0.34 ± 0.02
0.36 ± 0.6
35 ± 2.6
117 ± 12
59 ± 15
trig adduct was isolated in 78% yield. The oxidation of compound 5
was carried out with o-chloranil to afford the furopyridine skeleton
0.3
1.2
6
in 55% yield [34]. Despite our efforts to optimize this aromati-
73 ± 11
zation step, full oxidation of dihydrofuropyridine 5 into furopyr-
idine 6 was unachievable and 30% of starting material (which were
engaged in a new aromatization process) were recovered. Direct
(
Fig. 1, ligand Va) as an hybrid pattern between benzofuran and 4-
conversion of the nitrile-bearing furopyridine
6
into 2-
azaindole scaffolds. The methodology we previously developed
using a palladium-copper catalyzed synthesis of the furopyridine
ring described was not convenient for an easy modulation of sub-
stituents at position C2. For each new substituent, the reaction
conditions had to be adjusted. To avoid the weakness of this pre-
vious strategy, we herein presented the development of a new
synthetic pathway to access structures derived from furo[3,2-b]
pyridines (Fig. 1, Va). Our new methodology ends with a Suzuki-
Miyaura cross-coupling at C2 position, allowing a wide range of
substituents to be coupled.
ethylacetamide 7 was achieved in a one-pot reaction: Raney
nickel-catalyzed hydrogenation (5 bar) in acetic anhydride with
sodium acetate gave 2-ethylacetamide 7 in 50% overall yield. Access
to the targeted products by Suzuki-Miyaura cross-coupling reaction
first required the halogenation of the 3-substituted furopyridine
7
at position C2. Bromination reaction was performed at low tem-
perature by metalation and electrophilic trapping with carbon
tetrabromide to afford 8 [35], while iodination was achieved using
iodine as electrophile to afford the iodo derivative 9. The last step of
the synthesis consists in a Suzuki-Miyaura cross coupling reaction
between compound 8 and different boronic acids to afford the 2-
substituted furopyridines 10e16. This step can also be performed
from 9 as shown with the phenyl boronic acid and afford the 2-
phenylfuro[3,2-b] analogue 10 in 80% yield. Yields and final struc-
tures are described in Table 2.
The main advantage of this new synthetic pathway to 2-
substituted furo[3,2-b]pyridines is the large range of substituent
that could be introduced in the last step in very good yields after a
straightforward sequence.
2
. Chemistry
C2-substituted furopyridines were prepared as shown in
Scheme 1. First, the 5-methoxy-3-pyridinol 2 was successfully
synthesized from commercially available 5-bromo-2-
methoxypyridine 1 in a one-pot three-steps sequence in a good
7% overall yield: 5-bromo-2-methoxypyridine 1 initially under-
5
went an halogen-metal exchange at low temperature followed by
the lithiated species trapping with trimethylborate and subsequent
ꢀ
oxidation step of the aryldimethylborate with peracetic acid at 0 C
3. Biological assays
[32]. Regioselective bromination of 2 was carried out with N-bro-
mosuccinimide in acetonitrile to afford 3 in good 73% yield. In order
to build the furanic cycle and to introduce the 2-ethylacetamide
side chain precursor in one step, pyridinol 3 was deprotonated
The binding affinities of the final compounds 10e16 and also of
compounds 7 and 9 were determined in competition radioligand
binding assays using 2-[ I]-iodomelatonin on human MT and
1
125

270
A. Couhert et al. / European Journal of Medicinal Chemistry 109 (2016) 268e275
ꢀ
ꢀ
Scheme 1. Synthetic pathway to 2-substituted furo[3,2-b]pyridines. Reagents and conditions: a) i) n-BuLi (1.5 eq.), THF, ꢁ78 C, 20 min; ii) B(OMe)
3
(1.5 eq.), ꢁ78 C, 2 h; iii)
ꢀ
ꢀ
CH CO H (1.5 eq.), 0 C, 1 h; iv) NaHCO (1.5 eq.), r.t., 1 h, 57%; b) NBS (1.1 eq.), MeCN, r.t., 2 h, 73%; c) NaH (1.5 eq.), MeCN, 0 C, 30 min then 4-bromobut-2-enenitrile (2 eq.), r.t.,
3
3
3
overnight, 88%; d) Bu
3
SnH (3 eq.), AIBN (0.3 eq.), benzene, reflux, 24 h-48 h, 78%; e) o-chloranil (2 eq.), toluene, reflux, 24 h, 55%; f) Ac
2
O, NaOAc (1.5 eq.), Raney Ni (cat.), H
2
(5 bar),
CO
2 3
ꢀ
ꢀ
(2 eq.) in THF, ꢁ78 ꢀC, 1 h, 67%; h) n-BuLi (10 eq.), THF, ꢁ78 ꢀC, 1 h then I
(2 eq.) in THF, ꢁ78 ꢀC, 1 h, 52%; i) Cs
60
C, 24 h, 60%; g) n-BuLi (10 eq.), THF, ꢁ78 C, 1 h then CBr
4
2
(
3 4
6 eq.), boronic acid (1.5 eq.), Pd(PPh ) (5 mol%), water/dioxane (1/2).
MT
The activity behaviors are evaluated by the [35S]-GTP
assay performed on membranes from CHO cells.
Results in binding affinities as well as activity behaviors and
MT /MT selectivity ratios are reported in Table 3.
2
receptors expressed in HEK-293 cells.
Compound 14 with a 2-naphthyl group in C2, exhibits a sharp loss
of selectivity due to good affinities for each receptor subtype. Both
gS binding
ligands are MT
2
1
antagonists but 13 behaves as an MT antagonist
while 14 behaves as an MT
1
agonist. Once again, as for 12, the MT
1
1
2
antagonism of 13 might be explained by the perpendicular orien-
tation of the 1-naphthyl group from the indole.
Increasing once more the size of the substituent leads to com-
pounds 15 and 16, bearing at the C2 position a 9-anthracenyl and a
4
. Results and discussion
9
-phenanthrenyl group respectively. The increase steric hindrance
resulted in low binding affinities and even decreased selectivity
ratios but confirmed the MT antagonist profile of C-2 steric hin-
Compared to 4-azaindole IVa, compound 7 shows slightly lower
affinities for MT
melatoninergic ligand. The introduction of the bulky iodine atom
improves the affinity for MT and MT by 8 and 15 times respec-
1 2
and MT receptors but remains a very good
2
dered ligands [37]. Although the C2-aryl groups might be posi-
tioned in an orthogonal way from the plane core nucleus, 15 and 16
show partial agonist profiles at MT , which can be attributed to the
1
1
2
tively. This tendency is in agreement with the improve affinity of 2-
iodomelatonin vs melatonin [36]. The introduction of a phenyl
group or a 4-fluorophenyl group at the C2-position leads to 10 and
bigger steric factors.
11, presenting similar binding profiles compared to the iodinated
derivative with subnanomolar affinities. These four ligands exhibit
2
no selectivity toward MT and behave as full agonists for both
melatonin receptors.
5. Conclusion
Replacement of the phenyl group by the 2,6-dimethylphenyl
group causes a loss of affinity for melatoninergic binding sites.
This might be due to the orthogonal positioning of the aryl group
towards the plan core. Compared to 10, the activity of the resulting
We described herein a new synthetic pathway towards C2-
substituted furo[3,2-b]pyridines in an eight-steps sequence. A
large range of aryl substituents can be easily and efficiently intro-
duced thanks to this methodology.
In the furo[3,2-b]pyridine series we have developed three
compounds (9, 10, 11) with subnanomolar binding affinities and
ligand 12 is also modified from an agonist profile on MT
to an antagonist profile towards MT subtype and partial agonist
profile on MT subtype.
Introduction of 1-naphthyl in C2-position maintains a good af-
finity for the MT receptor but leads to a sharp decrease in MT
affinity (124-fold compared to 7, 4900-fold compared to 10). This
loss of affinity for MT subtype results in an enhancement of the
selectivity ratio, up to 152, and gives the best ligand in the series.
1 2
and MT
1
2
MT
ratio for MT
1
/MT
2
agonist activities. Ligand 13 exhibits a 152-fold selectivity
receptors and behaves as an antagonist towards both
2
1
2
melatoninergic receptors. Other analogues bearing bulky groups a
perpendicular conformation show only moderate selectivity ratios
for MT receptor but seem to behave as MT antagonists
2 1
1

A. Couhert et al. / European Journal of Medicinal Chemistry 109 (2016) 268e275
271
Table 2
warm to r.t. The media is partially concentrated, diluted in DCM and
washed with water. The organic layer was dried over MgSO and
concentrated. The residue was purified by column chromatography
(SiO , 7/3 petroleum ether/EtOAc) to afford 2 as a yellow solid.
Yield: 57%; m.p.: 77e79 C; IR (ATR Diamond, cm
Structures and yields for compounds 10e16 from the Suzuki-Miyaura cross-coupling
reaction.
4
Entry Starting material Compound Structure
Yield (%)a
90
2
ꢀ
1
ꢁ1
n
1
2
3
8
9
8
10
10
11
)
3050, 2660,
1
601, 1497, 1262, 1047; H NMR (400 MHz, CDCl
OCH
), 6.7 (d, 1H, J ¼ 8.9 Hz, H
), 7.8 (d, 1H, J ¼ 3.0 Hz, H
3
) d 3.9 (s, 3H,
3
5
), 7.2 (dd, 1H, J ¼ 8.9 Hz, J ¼ 3.0 Hz,
13
H
4
2 3 3
); C NMR (100 MHz, CDCl ) d 54 (CH ),
80
78
111 (CH), 129 (CH), 133 (CH), 148 (Cq), 159 (Cq); HRMS (EI-MS):
calculated for C
6
H
8
NO
2
126.05495 [MþH]þ found 126.05530
þ
[MþH] .
6.1.2. 2-Bromo-6-methoxypyridin-3-ol (3)
N-bromosuccinimide (1.4 g, 7.9 mmol) was added to a solution
of 2 (900 mg, 7.2 mmol) in acetonitrile (30 mL) and the mixture was
stirred during 2 h at r.t. The media was quenched with water and
extracted 3 times with EtOAc. The organic layer was dried over
4
5
8
8
12
13
87
83
MgSO
chromatography (SiO
yellow solid. Yield: 73%; m.p.: 86e88 C; IR (ATR Diamond, cm
4
and concentrated. The residue was purified by column
2
, 9/1 petroleum ether/EtOAc) to afford 3 as a
ꢀ
ꢁ1
) n
1
2
941, 2845, 1571, 1547, 1476, 1274, 1041, 810; H NMR (400 MHz,
CDCl 3.9 (s, 3H, OCH ),
), 5.0 (bs, 1H, OH), 6.7 (d, 1H, J ¼ 8.6 Hz, H
.3 (d, 1H, J ¼ 8.6 Hz, H ), 111 (CH),
3
)
d
3
5
13
7
4 3 3
); C (100 MHz, CDCl ) d 54 (CH
1
C
25 (Cq), 127 (CH), 144 (Cq), 157 (Cq); HRMS (EI-MS): calculated for
203.96546 [MþH]þ found 203.96574 [MþH] .
þ
6
7
8
8
14
15
60
80
6
H
7
BrNO
2
6.1.3. 4-((2-Bromo-6-methoxypyridin-3-yl)oxy)but-2-enenitrile (4)
Under argon, sodium hydride (150 mg, 3.7 mmol) was added
portionwise to a solution of 3 (500 mg, 2.5 mmol) in freshly
ꢀ
distilled acetonitrile (15 mL) cooled to 0 C. The mixture was stirred
ꢀ
at 0 C for 30 min and 4-bromobut-2-enenitrile (540 mg, 3.7 mmol)
was added. The solution was allowed to warm to r.t. and was stirred
at r.t. overnight. The media was quenched with water and extracted
8
8
16
95
3
times with EtOAc. The organic layer was dried over MgSO
concentrated. The residue was purified by column chromatography
SiO , 9/1 petroleum ether/EtOAc) to afford compound 4 as a brown
4
and
(
2
oil (cis isomer) and a white solid (trans isomer). Global yield: 88%
ꢀ
(
cis/trans ratio ¼ 65/35); m.p. trans isomer: 100e102 C; IR (ATR
a
Isolated yield after column chromatography.
ꢁ1
1
Diamond, cm
)
n
3056, 2220, 1471, 1254, 1096,1054, 816; H NMR
cis isomer 3.9 (s, 3H, OCH ), 4.9 (dd, 2H,
J ¼ 6.1 Hz, J ¼ 1.7 Hz, H ), 5.6 (dt, 1H, J ¼ 11.2 Hz, J ¼ 1.7 Hz, H10), 6.7
d, 1H, J ¼ 8.7 Hz, H ), 6.8 (dt, 1H, J ¼ 11.2 Hz, J ¼ 6.1 Hz, H ), 7.3 (d,
H, J ¼ 8.7 Hz, H ) trans isomer 3.9 (s, 3H, OCH ), 4.7 (dd, 2H,
J ¼ 3.5 Hz, J ¼ 2.4 Hz, H
(400 MHz, CDCl
3
)
d
3
6
. Experimental section
8
(
1
5
9
6
.1. Chemistry
4
3
8
), 6.0 (dt, 1H, J ¼ 16.3 Hz, J ¼ 2.4 Hz, H10), 6.7
1H NMR and 13C NMR spectra were recorded on a Bruker DPX
50 MHz or 400 MHz instrument using CDCl . The chemical shifts
scale in ppm) and all coupling
(d, 1H, J ¼ 8.8 Hz, H ), 6.8 (dt, 1H, J ¼ 16.3 Hz, J ¼ 3.5 Hz, H ), 7.2 (d,
5
9
13
1
6
1
(
H, J ¼ 8.8 Hz, H
4
); C NMR (100 MHz, CDCl
3
)
d
3
cis isomer 54 (CH ),
2
3
9 (CH
2
), 102 (CH), 110 (CH), 115 (CN), 128 (CH), 130 (Cq), 146 (Cq),
are reported in part per million (
d
48 (CH), 159 (Cq) trans isomer 55 (CH
3
), 69 (CH
2
), 102 (CH), 110
constant (J) values are in Hertz (Hz). The following abbreviations
were used to explain the multiplicities: s (singlet), d (doublet), t
triplet), m (multiplet), dd (doublet doublet) and bs (broad singlet).
Melting points are uncorrected. IR absorption spectra were ob-
tained on a Perkin Elmer PARAGON 1000 PC and values are re-
CH), 117 (CN), 127 (CH), 130 (Cq), 146 (Cq), 148 (CH), 159 (Cq);
þ
HRMS (EI-MS): calculated for C10
H
10BrNO
2
268.99202 [MþH]
(
þ
found cis isomer 268.99186 [MþH] and found trans isomer
þ
2
68.99193 [MþH] .
ꢁ
1
ported in cm
. HRMS were recorded on a Maxis Bruker
6
.1.4. 2-(5-Methoxy-2,3-dihydrofuro[3,2-b]pyridin-3-yl)
instrument. Column chromatographies were performed using silica
gel 60 (0.063e0.200 mm, Merck).
acetonitrile (5)
Under argon, tributyltin hydride (2 mL, 11.1 mmol) and AIBN
(180 mg, 1.1 mmol) were added to a solution of 4 (1 g, 3.7 mmol) in
6
.1.1. 6-Methoxypyridin-3-ol (2)
Under argon, n-butyllitium (20 mL, 31.8 mmol) was added
degassed benzene (100 mL). The mixture was stirred under reflux
for 24e48 h. The media was partially concentrated and acetone
(50 mL) and a saturated aqueous solution of potassium fluoride
(50 mL) were added. After precipitation of tin salts, the media was
filtered through a silica pad. The filtrate was partially concentrated
and the resulting aqueous phase was extracted 3 times with EtOAc.
dropwise to a solution of 1 (4 g, 21.3 mmol) in THF (140 mL) cooled
to e 78 C. After 20 min, trimethylborate (3.7 mL, 31.8 mmol) was
ꢀ
ꢀ
added slowly and the media is stirred at ꢁ78 C for 2 h. A solution of
peracetic acid (6.7 mL, 31.8 mmol) was added and the media was
ꢀ
ꢀ
allowed to warm to 0 C. After 1 h at 0 C, the reaction was
quenched with an aqueous solution of NaHCO and allowed to
The combined organic layers were washed with NH
4
Cl, dried over
3
MgSO and concentrated. The residue was dissolved in acetonitrile
4

272
A. Couhert et al. / European Journal of Medicinal Chemistry 109 (2016) 268e275
Table 3
Biological assay results.
K
i
± SEM (nM)
Selectivity fold MT
1
/MT
2
Activity
Compound
R
MT
1
MT
2
7
H
1.6 ± 0.2
1.2 ± 0.3
1.5
2.5
0.3
Agonist MT
Agonist MT
Agonist MT
Agonist MT
Agonist MT
Agonist MT
1
2
1
2
1
2
9
I
0.2 ± 0.5
0.08 ± 0.01
0.14 ± 0.04
1
1
1
0
1
2
0.04 ± 0.003
0.2 ± 0.04
120 ± 14
0.03 ± 0.009
9 ± 0.7
6.7
Agonist MT
Agonist MT
1
2
13.3
Antagonist MT
Partial agonist MT
1
2
13
198 ± 18
1.3 ± 0.2
152.3
Antagonist MT
Antagonist MT
1
2
1
1
4
5
2.2 ± 0.2
31 ± 0.5
0.09 ± 0.003
5 ± 0.4
22.2
6.2
Agonist MT
Antagonist MT
1
2
Partial agonist MT
Antagonist MT
1
1
2
1
6
68 ± 7
5 ± 0.3
13.6
Partial agonist MT
Antagonist MT
2
and washed with hexane to afford compound 5 as a yellow oil.
with EtOAc. Filtrate was washed with an aqueous solution of
NaHCO . The aqueous layer was extracted twice with EtOAc. The
combined organic layers were washed twice with distilled water,
dried over MgSO and concentrated. The residue was purified by
column chromatography (SiO
ꢁ1
Yield: 78%; IR (ATR Diamond, cm
)
n
2953, 2248, 1472, 1418, 1232,
2.7 (dd, 1H, J ¼ 17.0 Hz, J ¼ 8.5 Hz,
10a), 2.9 (dd, 1H, J ¼ 17.0 Hz, J ¼ 4.5 Hz, H10b), 3.7e3.8 (m, 1H, H ),
.9 (s, 3H, OCH
m, 1H, H8b), 6.6 (d, 1H, J ¼ 8.8 Hz, H
NMR (100 MHz, CDCl 21 (CH ), 40 (CH), 54 (CH
3
1
3
1024; H NMR (250 MHz, CDCl ) d
H
9
4
3
(
3
), 4.4 (dd, 1H, J ¼ 10.0 Hz, J ¼ 6.5 Hz, H8a), 4.8e4.9
2
, 8/2 petroleum ether/EtOAc) to
13
ꢀ
5
), 7.1 (d, 1H, J ¼ 8.8 Hz, H
4
);
C
afford 7 as a white solid. Yield: 60%; m.p.: 163e165 C; IR (ATR
ꢁ
1
1
3
)
d
2
3
), 75 (CH
2
), 110
Diamond, cm
(400 MHz, CDCl
(m, 2H, CH ), 4.0 (s, 3H, OCH
NH), 7.6 (s, 1H, H
), 7.7 (d, 1H, J ¼ 8.9 Hz, H
CDCl 23 (CH ), 24 (CH ), 39 (CH ), 54 (CH
3
)
n
)
3271, 3101, 2943,1633, 1406,1236,1033; H NMR
2.0 (s, 3H, CH ), 2.9e3.0 (m, 2H, CH ), 3.6e3.7
), 6.7 (d, 1H, J ¼ 8.9 Hz, H ), 6.9 (bs, 1H,
); C NMR (100 MHz,
), 107 (CH), 118 (Cq),
(
CH), 118 (CN),121 (CH), 144 (Cq),148 (Cq),160 (Cq); HRMS (EI-MS):
3
d
3
2
calculated for C10
11
H N
2
O
2
191.08150 [MþH]þ found 191.08165
2
3
5
þ
13
[MþH] .
2
4
3
)
d
3
2
2
1
21 (CH), 131 (Cq), 144 (Cq), 145.0 (Cq), 162 (Cq), 170 (Cq); HRMS
6
.1.5. 2-(5-Methoxyfuro[3,2-b]pyridin-3-yl)acetonitrile (6)
o-Chloranil (242 mg, 1.0 mmol) was added to a solution of 5
þ
(EI-MS): calculated for C12
14
H N
2
O
3
235.10772 [MþH] found
þ
2
35.10744 [MþH] . After analysis by high performance liquid
(
100 mg, 0.5 mmol) in toluene (7 mL). The mixture was stirred
chromatography (HPLC) the purity was found to be 82% and 87% at
under reflux for 24 h. The media was allowed to warm to r.t. and
was filtered through a silica pad rinsed with DCM. The filtrate was
concentrated and the residue was purified by column chromatog-
2
54 nm and 323 nm, respectively.
2
raphy (SiO , 95/5 petroleum ether/EtOAc) to afford 6 as a white
solid. 30% of the starting material was recovered and engaged in a
6
.1.7. N-(2-(2-Bromo-5-methoxyfuro[3,2-b]pyridin-3-yl)ethyl)
acetamide (8)
ꢀ
new aromatization reaction Yield: 55%; m.p.: 71e73 C; IR (ATR
Under argon, n-butyllithium (5 mL, 8.0 mmol) was added
dropwise to a solution of 7 (190 mg, 0.8 mmol) in dry THF (10 mL)
ꢁ1
1
Diamond, cm
NMR (400 MHz, CDCl
H, J ¼ 9.0 Hz, H ), 7.7 (d, 1H, J ¼ 9.0 Hz, H
100 MHz, CDCl 12 (CH ), 54 (CH ), 108 (CH), 112 (CN), 117 (Cq),
)
n
2950, 2920, 2257, 1589, 1407, 1253, 1026;
H
3
)
d
3.8 (s, 2H, H10), 4.0 (s, 3H, OCH ), 6.7 (d,
3
ꢀ
ꢀ
cooled to e 78 C. The mixture was stirred at ꢁ78 C for 1 h and a
1
3
1
5
4 8
), 7.8 (s, 1H, H ); C NMR
solution of carbon tetrabromide (400 mg, 1.6 mmol) in dry THF
(
3
)
d
2
3
ꢀ
(
5 mL) was added dropwise. The mixture was stirred at ꢁ78 C for
1
22 (CH), 142 (Cq), 145 (Cq), 146 (CH), 165 (Cq); HRMS (EI-MS):
1
h and was then quenched with water. This aqueous layer was
extracted 3 times with EtOAc and the combined organic layers were
dried over MgSO and concentrated. The residue was purified by
column chromatography (SiO
calculated for C10
9
H N
2
O
2
189.06585 [MþH]þ found 189.06567
þ
[MþH] .
4
2
, 5/5 petroleum ether/EtOAc) to
ꢀ
6.1.6. N-(2-(5-Methoxyfuro[3,2-b]pyridin-3-yl)ethyl)acetamide (7)
afford 8 as a white solid. Yield: 67%; m.p.: 139e141 C; IR (ATR
Diamond, cm
1020; H NMR (400 MHz, CDCl
2H, CH ), 3.6e3.7 (m, 2H, CH
J ¼ 8.9 Hz, H ), 7.6 (d, 1H, J ¼ 8.9 Hz, H
(CH ), 24 (CH ), 39 (CH ), 54 (CH ), 107 (CH), 118 (Cq), 121 (CH), 131
ꢁ1
In an autoclave, 6 (790 mg, 4.2 mmol) was dissolved in anhy-
) n 3282, 2919, 1635, 1569, 1402, 1258, 1230, 1122,
1
dride acetic (50 mL) and sodium acetate (520 mg, 6.3 mmol) and
Raney nickel (cat.) were added. The mixture was stirred under
3
)
d
2.0 (s, 3H, COCH
3
), 2.9e3.0 (m,
2
2
), 4.0 (s, 3H, OCH
3
), 6.7 (d, 1H,
ꢀ
13
hydrogen atmosphere (5 bars) at 60 C for 24 h. The media was
5
4 3
); C (100 MHz,CDCl ) d 23
allowed to cool to r.t. and was filtered through a silica pad rinsed
3
2
2
3

A. Couhert et al. / European Journal of Medicinal Chemistry 109 (2016) 268e275
273
(
Cq), 144 (Cq), 145 (Cq), 162 (Cq), 170 (Cq); HRMS (EI-MS): calcu-
6.1.9.3. N-(2-(2-(2,6-Dimethylphenyl)-5-methoxyfuro[3,2-b]pyridin-
3-yl)ethyl)acetamide (12). Synthesized according to the general
procedure using 2,6-dimethylphenylboronic acid and purified by
lated for
C
12
H14BrN
2
O
3
313.01823 [MþH]þ found 313.01828
[
MþH]þ.
column chromatography (SiO
afford 12 as a pale yellow gum. Yield: 87%; IR (ATR Diamond, cm
2
, 5/5 petroleum ether/EtOAc) to
ꢁ1
6
.1.8. N-(2-(2-Iodo-5-methoxyfuro[3,2-b]pyridin-3-yl)ethyl)
)
)
1
acetamide (9)
n
3291, 2924, 1651, 1585, 1407, 1240,; H NMR (400 MHz, CDCl
1.9 (s, 3H, CH ), 2.1 (s, 6H, 2xCH ), 2.7e2.8 (m, 2H, CH
), 4.1 (s, 3H, CH ), 7.1 (d, 2H,
), 6.7 (d, 1H, J ¼ 8.8 Hz, H
J ¼ 8.8 Hz, HAr), 7.2 (bs, 1H, NH), 7.2e7.3 (m, 1H, HAr), 7.7 (d, 1H,
3
Under argon, n-butyllithium (5 mL, 8.0 mmol) was added
dropwise to a solution of 7 (190 mg, 0.8 mmol) in dry THF (10 mL)
d
3
3
2
), 3.5e3.6
(m, 2H, CH
2
3
5
ꢀ
ꢀ
cooled to e 78 C. The mixture was stirred at ꢁ78 C for 1 h and a
13
solution of iodine (395 mg, 1.6 mmol) in dry THF (5 mL) was added
J ¼ 8.8 Hz, H
23 (CH ), 40 (CH
(2xCH), 129 (Cq), 130 (CH), 139 (2xCq), 144 (Cq), 144 (Cq), 156 (Cq),
4
); C NMR (100 MHz, CDCl
3 3 3
) d 20 (2xCH ), 23 (CH ),
ꢀ
dropwise. The mixture was stirred at ꢁ78 C for 1 h and was then
2
2
), 54 (CH ), 107 (CH), 116 (Cq), 122 (CH), 128
3
quenched with water. This aqueous layer was extracted 3 times
with EtOAc and the combined organic layers were dried over
162 (Cq), 170 (Cq); HRMS (EI-MS): calculated for C20
H
22
N
2
O
3
339.17032 [MþH]þ found 339.17020 [MþH] .
þ
MgSO
chromatography (SiO
white cottony solid. Yield: 52%; m.p.: 170e172 C; IR (ATR Diamond,
4
and concentrated. The residue was purified by column
2
, 5/5 petroleum ether/EtOAc) to afford 9 as a
ꢀ
6.1.9.4. N-(2-(5-Methoxy-2-(naphthalen-1-yl)furo[3,2-b]pyridin-3-
yl)ethyl)acetamide (13). Synthesized according to the general pro-
cedure using 1-naphthylboronic acid and purified by column
ꢁ
1
1
cm
(
(
)
n
3276, 2945, 1627, 1583, 1434, 1231, 1100, 1028; H NMR
400 MHz, CDCl 1.9 (s, 3H, COCH ), 2.8e2.9 (m, 2H, CH ), 3.6e3.7
m, 2H, CH ), 4.0 (s, 3H, OCH ), 6.7 (bs, 1H,
), 6.6 (d, 1H, J ¼ 8.9 Hz, H
24 (CH ), 25
), 102 (Cq), 107 (CH), 122 (CH), 126 (Cq), 143
Cq), 148 (Cq), 162 (Cq), 170 (Cq); HRMS (EI-MS): calculated for
3
)
d
3
2
2
3
5
d
chromatography (SiO
ꢁ1
white solid. Yield: 83%; m.p.: 117e119 C; IR (ATR Diamond, cm ) n
2
, 5/5 petroleum ether/EtOAc) to afford 13 as a
13
ꢀ
NH), 7.6 (d, 1H, J ¼ 8.9 Hz, H
4
); C (100 MHz,CDCl
3
)
3
1
(
CH
2
), 39 (CH
2
), 54 (CH
3
3
3241, 2932, 1631, 1585, 1418, 1239, 1053; H NMR (400 MHz, CDCl )
(
d
1.8 (s, 3H, CH
3
), 2.9e3.0 (m, 2H, CH
2
2
), 3.5e3.6 (m, 2H, CH ), 4.1 (s,
þ
þ
C
12
H13IN
2
O
3
361.00437 [MþH] found 361.00424 [MþH] . After
3H, CH
3
), 6.8 (d, 1H, J ¼ 8.8 Hz, H
5
), 7.0 (bs, 1H, NH), 7.5e7.6 (m, 4H,
analysis by HPLC, the purity was found to be 98.5% and 99% at
2
H
Ar), 7.8 (d, 1H, J ¼ 8.8 Hz, H
4
), 7.7e7.8 (m, 1H, HAr), 7.9e8.0 (m, 2H,
23 (CH ), 23 (CH ), 40 (CH ), 54
), 107 (CH), 117 (Cq), 122 (CH), 125 (CH), 126 (CH), 127 (CH), 127
13
54 nm and 306 nm, respectively.
HAr); C NMR (100 MHz, CDCl
3
)
d
3
2
2
(CH
3
6
.1.9. General procedure for the synthesis of 2-arylfuro[3,2-b]
(CH), 128 (CH), 129 (CH), 129 (CH), 131 (CH), 132 (Cq), 134 (Cq), 144
(Cq), 145 (Cq), 156 (Cq), 162 (Cq), 170 (Cq); HRMS (EI-MS): calcu-
pyridines
Cesium carbonate (170 mg, 1.2 mmol) and arylboronic acid
0.3 mmol) were added to solution of 8 (70 mg, 0.2 mmol) in water
2 mL) and dioxane (5 mL). The mixture was degassed and tetra-
þ
þ
lated for C22H
20
N
2
O
3
361.15467 [MþH] found 361.15484 [MþH] .
(
(
After analysis by HPLC, the purity was found to be 99% and 99.1% at
254 nm and 315 nm, respectively.
kis(triphenylphosphine)palladium (13 mg, 5 mol%) was added. The
mixture was stirred under reflux for 4 h. The media was filtered
6.1.9.5. N-(2-(5-Methoxy-2-(naphthalen-2-yl)furo[3,2-b]pyridin-3-
yl)ethyl)acetamide (14). Synthesized according to the general pro-
cedure using 2-naphthylboronic acid and purified by column
over
a silica pad rinsed with EtOAc and the filtrate was
concentrated.
chromatography (SiO
white solid. Yield: 60%; m.p.: 112e114 C; IR (ATR Diamond, cm
2
, 5/5 petroleum ether/EtOAc) to afford 14 as a
ꢀ
ꢁ1
6
.1.9.1. N-(2-(5-Methoxy-2-phenylfuro [3,2b]pyridin-3-yl)ethyl)acet-
amide (10). Synthesized according to the general procedure using
phenylboronic acid and purified by column chromatography (SiO
/5 petroleum ether/EtOAc) to afford 10 as a white solid. Yield: 90%;
) n
3
) d 1.9
1
3281, 2926, 1633, 1412, 1245, 1026; H NMR (400 MHz, CDCl
(s, 3H, CH ), 3.2e3.3 (m, 2H, CH ), 3.7e3.8 (m, 2H, CH ), 3.8 (s, 3H,
CH ), 7.0 (bs,1H, NH), 7.5e7.6 (m, 2H, HAr),
), 6.7 (d, 1H, J ¼ 8.8 Hz, H
7.7 (d, 1H, J ¼ 8.8 Hz, H ), 7.9e8.0 (m, 4H, HAr), 8.3 (s, 1H, HAr);
NMR (100 MHz, CDCl 23 (CH ), 24 (CH ), 40 (CH ), 54 (CH ), 107
2
,
3
2
2
5
3
5
ꢀ
ꢁ1
13
m.p.: 134e136 C; IR (ATR Diamond, cm
)
n
3296, 2945, 1633,
1.9 (s, 3H, CH ),
), (s, 3H, CH ), 6.7 (d, 1H,
4
), 7.0 (bs, 1H, NH), 7.4e7.5 (m, 3H, HAr), 7.7 (d, 1H,
4
C
1
1566, 1412, 1242, 1026; H NMR (400 MHz, CDCl
3
)
d
3
3
)
d
3
2
2
3
3
.1e3.2 (m, 2H, CH
2
), 3.7e3.8 (m, 2H, CH
2
3
(CH), 115 (Cq), 122 (CH), 124 (CH), 127 (CH), 127 (CH), 127 (CH), 128
(CH), 128 (Cq), 129 (CH), 129 (CH), 133 (Cq), 133 (Cq), 144 (Cq), 146
(Cq), 155 (Cq), 162 (Cq), 170 (Cq); HRMS (EI-MS): calculated for
J ¼ 8.8 Hz, H
13
J ¼ 8.8 Hz, H
5
), 7.7e7.8 (m, 2H, HAr); C NMR (100 MHz, CDCl
), 40 (CH ), 54 (CH ), 77 (CH), 107 (CH), 114 (Cq),
21(CH), 122 (CH), 126 (Cq), 127 (CH), 129 (CH), 129 (Cq), 131 (Cq),
3
) d 23
þ
þ
(
1
CH
3
), 23 (CH
2
2
3
C
22
H
20
N
2
O
3
361.15467 [MþH] found 361.15487 [MþH] . After
analysis by HPLC, the purity was found to be 98.6% at 336 nm.
1
C
44 (Cq), 146 (Cq), 162 (Cq), 170 (Cq); HRMS (EI-MS): calculated for
þ
þ
H
18 19
N
2
O
3
311.13902 [MþH] found 311.13945 [MþH] . After
6.1.9.6. N-(2-(2-(Anthracen-9-yl)-5-methoxyfuro[3,2-b]pyridin-3-yl)
ethyl)acetamide (15). Synthesized according to the general proce-
dure using 9-anthraceneboronic acid and purified by column
analysis by HPLC, the purity was found to be 90.4% and 99% at
2
54 nm and 304 nm, respectively.
chromatography (SiO
white solid. Yield: 80%; m.p.: 117e119 C; IR (ATR Diamond, cm
2
, 5/5 petroleum ether/EtOAc) to afford 15 as a
ꢀ
ꢁ1
6.1.9.2. N-(2-(2-(4-Fluorophenyl)5-methoxyfuro[3,2-b]pyridine-3-yl)
) n
3
) d 1.8
1
ethyl)acetamide (11). Synthesized according to the general proce-
dure using 4-fluorophenylboronic acid and purified by column
3338, 2923, 1672, 1533, 1242, 1022; H NMR (400 MHz, CDCl
(s, 3H, CH ), 2.9e3.0 (m, 2H, CH ), 3.6e3.7 (m, 2H, CH ), 4.1 (s, 3H,
CH ), 7.0 (bs,1H, NH), 7.6e7.8 (m, 5H, HAr),
), 6.8 (d,1H, J ¼ 8.9 Hz, H
7.8 (d, 1H, J ¼ 8.9 Hz, H ), 7.9 (d, 1H, J ¼ Hz, HAr), 7.9 (s, 1H, HAr), 8.0
(d, 1H, J ¼ Hz, HAr), 8.7e8.8 (m, 2H, HAr); C NMR (100 MHz, CDCl
23 (CH ), 24 (CH ), 40 (CH ), 54 (CH ), 108 (CH), 117 (Cq), 122 (CH),
3
2
2
chromatography (SiO
white solid. Yield: 78%; m.p.: 160e162 C; IR (ATR Diamond, cm
2
, 3/7 petroleum ether/EtOAc) to afford 11 as a
3
5
ꢀ
ꢁ1
)
4
1
13
n
3279, 2947, 1610, 1414, 1248, 1230, 1023; H NMR (400 MHz,
CDCl 1.9 (s, 3H, CH ), 3.1e3.2 (m, 2H, CH ), 3.7e3.8 (m, 2H, CH ),
.0 (s, 3H, CH ), 6.9 (bs, 1H, NH), 7.1e7.2
), 6.7 (d, 1H, J ¼ 8.9 Hz, H
m, 2H, HAr), 7.7 (d,1H, J ¼ 8.9 Hz, H ), 7.7e7.8 (m, 2H, HAr); C NMR
100 MHz, CDCl 23 (CH ), 27 (CH ), 40 (CH ), 54 (CH ), 107 (CH),
14 (Cq), 116 (2xCH), 122 (CH), 127 (Cq), 129 (2xCH), 144 (Cq), 145
Cq), 162 (Cq), 170 (Cq); HRMS (EI-MS): calculated for C18
3
)
3
)
d
3
2
2
d
3
2
2
3
4
3
4
123 (CH), 123 (CH), 126 (CH), 127 (Cq), 127 (CH), 127 (CH), 127 (CH),
128 (CH), 129 (CH), 131 (CH), 131 (Cq), 131 (Cq), 131(Cq), 131(Cq),
13
(
(
4
3
)
d
3
2
2
3
144 (Cq), 145 (Cq), 156 (Cq), 162 (Cq), 170 (Cq); HRMS (EI-MS):
þ
1
calculated for C26
H
23
N
2
O
3
411.17032 [MþH] found 411.017049
þ
(
3
H18FN
2
O
3
[MþH] . After analysis by HPLC, the purity was found to be 95.4%
þ
þ
29.12960 [MþH] found 329.12934 [MþH] .
and 96.7% at 254 nm and 302 nm, respectively.

274
A. Couhert et al. / European Journal of Medicinal Chemistry 109 (2016) 268e275
6
3
.1.9.7. N-(2-(5-Methoxy-2-(phenanthren-9-yl)furo[3,2-b]pyridine-
-yl)ethyl)acetamide (16). Synthesized according to the general
were respectively for CHO-MT
and 2000 for basal activity, 4800 and 8000 in the presence of
melatonin 1 M and 160 and 180 in the presence of GTP S 10 mM
1 2
and CHO-MT membranes 1000
procedure using 9-phenanthreneboronic acid and purified by col-
m
g
umn chromatography (SiO
2
, 5/5 petroleum ether/EtOAc) to afford
which defined the non specific binding. Data from the dose
response curves (seven concentrations in duplicate) were analyzed
by using the program PRISM (Graph Pad Software Inc., San Diego,
CA, USA) to yield EC50 (Effective concentration 50%) and Emax
(maximal effect) for agonists. Antagonist potencies are expressed as
ꢀ
1
cm
6 as a white solid. Yield: 95%; m.p.:167e169 C; IR (ATR Diamond,
ꢁ
1
1
)
n
3248, 3078, 2939, 1636, 1440, 1404, 1241, 1027; H NMR
1.7 (s, 3H, CH ), 2.7e2.8 (m, 2H, CH ), 3.4e3.5
), 6.7 (bs, 1H, NH), 6.8 (d, 1H, J ¼ 8.9 Hz,
23 (CH ), 24 (CH ), 40
), 108 (CH), 119 (Cq), 122 (CH), 123 (Cq), 125 (2xCH),
(
(
H
400 MHz, CDCl
3
)
d
3
2
m, 2H, CH
2
), 4.1 (s, 3H, CH
3
13
5
), 7.4e7.5; C NMR (100 MHz, CDCl
3
)
d
3
2
K
B
¼ IC50/1 þ ([Ago]/EC50 ago), where IC50 is the inhibitory con-
35
(
CH
2
), 54 (CH
3
centration of antagonist that gives 50% inhibition of [ S] GTPgS
126 (2xCH), 127 (2xCH), 129 (2xCH), 130 (CH), 131 (2xCq), 132
binding in the presence of a fixed concentration of melatonin
(
2xCq), 144 (Cq), 145 (Cq), 154 (Cq), 162 (Cq), 170 (Cq); HRMS (EI-
([Ago]) and EC50 agi is the EC50 of the molecule when tested alone.
þ
MS): calculated for C26
23
H N
2
O
3
411.17032 [MþH] found 411.17065
þ
[
MþH] . After analysis by HPLC, the purity was found to be 99.6%
Acknowledgment
and 98.7% at 254 nm and 314 nm, respectively.
The authors thank Nathalie Percina (ICOA, Univ Orleans) for her
help and HPLC skills.
6
.1.10. Control of purity by high performance liquid
chromatography (HPLC) followed by UV-DAD detection
Purifications of prepared compounds were performed on a
liquid chromatography system equipped with a Diode Array De-
tector (DAD) (Agilent). Samples were injected on a Kinetex C18
Appendix A. Supplementary data
Supplementary data related to this article can be found at http://
dx.doi.org/10.1016/j.ejmech.2016.01.008.
(
Phenomenex) column, 2.1 mm diameter, 150 mm length and
.7 m particle size. . The mobile phase consisted of 70%/30%
1
m
methanol/water (v/v) and with 0.1% formic acid. The determination
of the purity was performed in isocratic mode for compounds 10,
References
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5, and 16. A gradient with a 70%/30% concentration of methanol/
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0
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1
2
1
2
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1
m
M melatonin. After a 120-min incubation at 37 C, reaction was
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v/v) polyethylenimine. Filters were washed three times with 1 mL
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(
[
[
[
[
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response curves (seven concentrations in duplicate) were analyzed
using the program PRISM (Graph Pad Software Inc., San Diego, CA,
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