N-{2-[2-(4-Phenylbutyl)benzofuran-4-yl]cyclopropylmethyl}acetamide: An orally bioavailable melatonin receptor agonist

Article abstract of DOI:10.1016/j.bmcl.2004.07.055

N-{2-[2-(4-Phenylbutyl)benzofuran-4-yl]cyclopropylmethyl}acetamide 3a was synthesized as an orally bioavailable agonist at MT₁ and MT ₂ melatonin receptors with significantly low vasoconstrictive activity. Elsevier Ltd. All rights re

Full text of DOI:10.1016/j.bmcl.2004.07.055

Bioorganic & Medicinal Chemistry Letters 14 (2004) 5157–5160
N-{2-[2-(4-Phenylbutyl)benzofuran-4-yl]cyclopropylmethyl}-
acetamide: an orally bioavailable melatonin receptor agonist
Li-Qiang Sun,* Katherine Takaki, Jie Chen, Lawrence Iben, Jay O. Knipe, Lori Pajor,
Cathy D. Mahle,ꢀ Elaine Ryan and Cen Xu
Bristol–Myers Squibb Pharmaceutical Research Institute, 5 Research Parkway, Wallingford, CT 06492, USA
Received 28 June 2004; revised 26 July 2004; accepted 27 July 2004
Available online 13 August 2004
Abstract—N-{2-[2-(4-Phenylbutyl)benzofuran-4-yl]cyclopropylmethyl}acetamide 3a was synthesized as an orally bioavailable
agonist at MT1 and MT2 melatonin receptors with significantly low vasoconstrictive activity.
Ó 2004 Elsevier Ltd. All rights reserved.
Sleep disorders have an estimated prevalence of 15–27%
in the adult population.1 Insomnia, the most common
sleep disorder, affects 20–40% of American adults2 with
the incidence increasing significantly with age. While
benzodiazepines have been the most widely prescribed
pharmacologic agents for the short-term treatment of
sleep disorders, these compounds have profound effects
on the CNS with the common side effects of residual
daytime sedation, rebound insomnia, amnesia, irritabil-
ity, and dependence, alterations in sleep architecture and
synergistic interaction with alcohol.3 Insomnia has a
myriad of causes, one of which is disruption of the nor-
mal circadian sleep–wake cycle. Dysynchrony in the
sleep–wake cycle can result from physiological changes.
A therapeutic potential for the treatment of such disor-
ders is resynchronization of the sleep–wake cycle via
modulation of the melatoninergic system. The hormone
melatonin (N-acetyl-5-methoxytryptamine) (Fig. 1) is
synthesized and released primarily by the pineal gland
in a circadian manner that closely follows the daily
light/dark cycle.4,5 It plays a major role in the regulation
of circadian rhythms and the control of seasonal cy-
cles.6,7 Melatonin alleviates jet lag, regulates delayed
sleep phase syndrome,8 and induces sleep.9 It has been
demonstrated that many of the effects of melatonin are
mediated through G-protein-coupled receptors ex-
pressed primarily in the brain, retina, pituitary, and
O
HN
O
N
H
Melatonin
Ph
Ph
O
O
O
H
N
N
N
R
N
H
R
O
1
2
Ph
Ph
O
H
O
O
N
R
N
R
H
O
3
4
Ph
N
O
R
NH
O
5
Figure 1.
Keywords: Melatonin; Melatonin receptors; Benzofuran; Agonist.
*
Corresponding author. Tel.: +1 203 677 7460; fax: +1 203 677
7702; e-mail: sunl@bms.com
ꢀ Present address: Bayer Pharmaceutical Research Center, 400 Morgan
Lane, West Haven, CT 06516, USA.
blood vessels.10 The recent cloning of several G-pro-
tein-coupled melatonin receptor genes has revealed at
0960-894X/$ - see front matter Ó 2004 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2004.07.055

5158
L.-Q. Sun et al. / Bioorg. Med. Chem. Lett. 14 (2004) 5157–5160
least three melatonin receptor subtypes, two of which
are defined as MT1 and MT2 and are found in
mammals.11
OH
O
O
b,c
O
a
HO
OEt
OEt
OEt
OEt
As a pharmacological tool and therapeutic entity, mela-
tonin is less than ideal due to several major handicaps:
the short biological half-life (about 19min in rat),12 con-
tractile effects on vascular smooth muscle,13 low aque-
ous solubility, and poor oral bioavailability. Therefore,
a need exists for the development of its surrogates to
overcome these liabilities of the endogenous ligand.
6
7
9
O
Ph
O
O
O
d,e
f,g
O
OEt
8
Recently, we reported the discovery of the benzoxazole
nucleus as a melatoninergic pharmacophore, which
served as an isosteric replacement for the previously
established alkoxyaryl core.14,15 It is of particular inter-
est to note that compounds from this series bearing a 4-
phenylbutyl chain at the 2-position of the benzoxazole
moiety, 1 and 2 (Fig. 1), exhibited the highest binding
affinity at both human MT1 and MT2 receptor subtypes,
and the highest selectivity (up to 35-fold) for MT1 with
subnanomolar activity against MT2, respectively. Given
the increased activity and selectivity of compounds
incorporating a 4-phenylbutyl moiety at the 2-position
of the benzoxazole moiety, this substituent was retained
in the context of an examination to identify substitutes
for the benzoxazole scaffold. An initial study focused
on the benzofuranyl moiety with the 4-phenylbutyl
and amide groups disposed as shown in compounds 3
and 4. Moreover, we sought to extend studies evaluating
the replacement of the amide side chains in 3 and 4. To
this end, we examined application of the aminopyrroli-
dine heterocycle that we have previously demonstrated
to confer substantial activity to melatoninergic lig-
ands.16 In the present paper, we report the identification
of a novel series of potent, orally active agonists of mela-
tonin receptors based on the benzofuran nucleus and
exemplified by structures 3–5.
Ph
O
Ph
O
h
i
O
O
10
Ph
11
Ph
Ph
O
O
O
j,k,l
m
OEt
NH2
NH2
O
O
12
Ph
13
H
N
n
O
R
O
3a-f
14
Scheme 1. Reagents and conditions: (a) MCPBA, NaHCO3, CH2Cl2,
rt, 80%; (b) Ac2O, Py, rt, 98%; (c) DDQ, dioxane, reflux, 85%; (d)
Ph3P(CH2)3PhBr, n-BuLi, THF, 0–25°C, 85%; (e) 10% Pd/C, H2,
EtOAc, 99%; (f) LiAlH4, THF, rt, 96%; (g) (COCl)2, DMSO, CH2Cl2,
Et3N, 90%; (h) Ph3PCHCO2Et, THF, reflux, 99%; (i) CH2N2,
Pd(OAc)2, THF, 80%; (j) NaOH, MeOH, reflux, 99%; (k) SOCl2,
CH2Cl2, reflux; (l) NH3, THF, À78–25°C, 89% (two steps); (m) Red-
Al, toluene, 0–25°C, 88%; (n) R1COCl, Et3N, CH2Cl2 or R2NCO,
benzene.
The compounds discussed in this report contain a 2-(4-
phenylbutyl)benzofuran nucleus and a general route to
the first series of analogues, compounds 3a–f, is de-
scribed in Scheme 1. Epoxidation of 2-allylphenol 617
with MCPBA was accompanied by a spontaneous intra-
molecular epoxide opening by the phenolic hydroxyl to
afford dihydrobenzofuran 7 directly in 80% yield. An at-
tempt to directly oxidize 7 to benzofuran 8 with DDQ
failed, necessitating conversion of 7 to aldehyde 8 via
a two step process that proceeded with 83% overall
yield. This comprised acetylation of alcohol 7 with acetic
anhydride followed by oxidation with excess DDQ in
dioxane at reflux. Wittig olefination of 8 followed by
hydrogenation in the presence of 10% Pd on charcoal
as a catalyst gave compound 9. The ester moiety of 9
was reduced with LiAlH4 and the resultant alcohol oxi-
dized under Swern conditions to produce aldehyde 10. A
second Wittig homologation with Ph3PCHCO2Et pro-
vided the trans-cinnamic ester 11 in 99% yield. Cyclo-
propanation of 11 with diazomethane using Pd(OAc)2
as a catalyst gave rise to ( )-trans-cyclopropane 12 in
80% yield. The next series of reactions involved convert-
ing the ester functionality of 12 to the corresponding re-
verse amide or urea derivatives. In the event, ester 12
was treated with NaOH to afford the corresponding
acid, which, in turn, was converted into the amide 13
using SOCl2 and NH3. Red-Al-mediated reduction of
13 provided the primary amine 14, which was acylated
with a series of acid chlorides or reacted with ethyl iso-
cyanate to produce the amide and urea products 3a–f,
respectively. It should be noted that these compounds
were prepared in racemic form and with the aforemen-
tioned trans-stereochemistry about the cyclopropane
ring system.
The benzofuran derivatives 4a–f incorporating a simple
alkyl side chain were synthesized as shown in Scheme 2.
The aldehyde 10 was subjected to a Wittig reaction using
(triphenylphosphoranylidene)acetonitrile to afford the
unsaturated nitrile 11, which was directly converted to
the final amides 4a–d by hydrogenation over Raney-
nickel in the presence of the appropriate carboxylic acid
anhydride. On the other hand, compounds 4e–f were
prepared through a two step sequence in which nitrile
11 was first reduced to the primary amine 12 and then
acylated using cyclopropanecarbonyl chloride or reacted
with ethyl isocyanate.

L.-Q. Sun et al. / Bioorg. Med. Chem. Lett. 14 (2004) 5157–5160
5159
Table 1. Ki of compounds 3a–f, 4a–f, and 5a–f competing for the
binding of 2-[125I]-iodomelatonin to membrane preparations of
NIH3T3 cells stably expressing human MT1 or MT2 melatonin
receptor
Ph
O
Ph
a
c
O
O
N
Ph
Ph
Ph
N
O
10
Ph
11
b
O
R
H
N
NH
O
O
O
R
N
H
R
Ph
O
O
d
3a-f
4a-f
5a-f
O
O
NH2
N
H
R
Compd
R
MT1 Ki (nM)
MT2 Ki (nM)
12
4a-f
Mel
3a
3b
3c
3d
3e
3f
––
0.3
2.6
2.1
10
0.7
10
Me
Et
Scheme 2. Reagents and conditions: (a) Ph3PCHCN, THF, reflux,
95%; (b) (RCO)2O, H2, Ra-Ni, THF; (c) NaBH4, CoCl2ÆH2O, MeOH,
90%; (d) R1COCl, Et3N, CH2Cl2 or R2NCO, benzene.
2.8
10
n-Pr
i-Pr
23
16
54
33
c-Pr
NHEt
Me
33
19
90
The synthetic route to the aminopyrrolidine derivatives
5a–f is outlined in Scheme 3. The Baeyer–Villiger oxida-
tion of aromatic aldehyde 10 followed by hydrolysis of
the resulting aryl formate under acidic conditions pro-
vided phenol 13. Exposure of 13 to triflic anhydride in
the presence of pyridine provided the triflate 14 in 69%
yield. Coupling of 14 with (S)-pyrrolidin-3-yl-carbamic
acid tert-butyl ester in the presence of Pd(OAc)2 and BI-
NAP using Cs2CO3 as a base in toluene under reflux fur-
nished carbamate 15 in 90% yield. Deprotection of 15
with HCl in ethyl acetate gave amine 16 in 63% yield.
The amide and urea products 5a–f were prepared from
16 by treatment with a series of acid chlorides or reac-
tion with ethyl isocyanate.
4a
4b
4c
4d
4e
4f
127
60
Et
7.5
16
n-Pr
i-Pr
82
83
3.7
6
c-Pr
NHEt
Me
95
52
21
22
5a
5b
5c
5d
5e
5f
14
6
Et
2.4
11
n-Pr
i-Pr
7.2
162
152
98
105
42
c-Pr
NHEt
39
propionamide 3b from the cyclopropyl series demon-
strated high affinity for both MT1 and MT2 receptors.
The binding affinity of these compounds is sensitive to
the identity of the N-acyl group. Replacement of the
methyl or ethyl group of the amide side chain by propyl
(3c), iso-propyl (3d), or cyclopropyl (3e) resulted in a
slight decrease in both MT1 and MT2 affinity. Replacing
the terminal amide of 3a with a simple ethyl urea pro-
vided compound 3f, which showed a more marked
reduction in binding affinity at both receptors. Replace-
ment of the constrained cyclopropane moiety of right
hand side chain of the compounds 3a–f by a conforma-
tionally flexible alkyl side-chain provided compounds
4a–f, which demonstrated high affinity for the MT1
receptor and weaker MT2 affinity. Further structural
evolution of the cyclopropyl-bearing side chain in 3a–f
to the previously reported heterocyclic aminopyrrolidine
gave compounds 5a–f. From this set of derivatives, the
propionamide 5b proved to be the most active ligand
for both MT1 and MT2 receptors, while acetamide 5a
and butyramide 5c demonstrated good affinity for both
MT1 and MT2 receptor subtypes. However, iso-propion-
amide 5d, cyclopropylcarboxamide 5e, and urea 5f
showed weak binding at both receptors.
The Ki values of compounds 3–5 for human MT1 and
MT2 melatonin receptor subtypes were determined in
assays using 2-[125I]-iodomelatonin according to the pre-
viously described assay method.16,18 The chemical struc-
tures and Ki values of these compounds are reported in
Table 1. As can be seen from Table 1, acetamide 3a and
Ph
OH
Ph
O
a, b
c
O
O
13
10
Ph
OTf
Ph
N
O
O
O
d
e
NH
O
14
15
Ph
N
Ph
O
R
f
NH2
NH
O
O
N
These studies led to the identification of compounds 3a,
3b, and 5b as the three most potent ligands for MT1 and
MT2 receptors. While the propionamides 3b and 5b were
more potent than acetamide 3a, these two compounds
were less attractive than 3a since previous studies in this
program had indicated that acetamides generally show
better oral bioavailability than propionamides. Conse-
quently, acetamide 3a was chosen for further evaluation.
16
5a-f
Scheme 3. Reagents and conditions: (a) MCPBA, CHCl3; (b) HCl,
MeOH, 22% (two steps); (c) Tf2O, pyridine, 0–25°C, 63%; (d) (S)-
pyrrolidin-3-yl-carbamic acid tert-butyl ester, Pd(OAc)2, BINAP,
Cs2CO3, toluene, reflux, 90%; (e) HCl/EtOAc, 69%; (f) R1COCl,
Et3N, CH2Cl2 or R2NCO, benzene.

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L.-Q. Sun et al. / Bioorg. Med. Chem. Lett. 14 (2004) 5157–5160
Table 2. Pharmacokinetic parameters of 3a
and Dr. Nicholas A. Meanwell for valuable discussion
and assistance with the preparation of the manuscript.
PK parameters
Rata
1
IV
Dose (mg/kg)
t1/2 (h)
3.8 1
27.5 2.8
557 76
References and notes
Cl (mL/min/kg)
Vd (mL/kg)
1. (a) Wake up America: a national sleep alert, Report of the
National Commission on Sleep Disorders Research, 1993;
(b) US Bureau of the Census, 1993.
2. Dement, W. C.; Mitler, M. M. J. Am. Med. Assoc. 1993,
269, 1548.
PO
Dose (mg/kg)
F (%)
1
90
3. Takaki, K. S.; Epperson, J. R. Ann. Rep. Med. Chem.
1999, 34, 41.
a Compound dosed in rats as a solution in PEG-400.
4. Yu, H.-S.; Reiter, R. J. Melatonin: Biosynthesis, Physio-
logical Effects, and Clinic Applications; CRC Press: Boca
Raton, 1993.
5. Arendt, J. Melatonin and the Mammalian Pineal Gland;
Chapman and Hall: London, 1995.
6. Cassone, V. M. Trends in Neuroscience 1990, 13, 457.
7. Bartness, T. J.; Goldman, B. D. Experientia 1989, 45, 939.
8. Oldani, A.; Ferini-Strambi, L.; Zucconi, M.; Stankov, B.;
Fraschini, F.; Smirne, S. NeuroReport 1994, 6, 132.
9. Anton-Tay, F.; Diaz, J. L.; Fernandez-Guardiola, A. Life
Sci. 1971, 10, 841.
It has been reported that melatonin has a marked ability
to enhance a-adrenoceptor-mediated vasoconstriction
of the rat tail artery.13 Thus, the effect of compound
3a on vascular smooth muscle was evaluated using the
method previously described.19 Compared to melatonin,
compound 3a showed reduced vasoconstrictive activity
in assays conducted with rat caudal arteries (0.09 rela-
tive to melatonin).20
10. Morgan, P. J.; Barrett, P.; Howell, H. E.; Helliwell, R.
Neurochem. Int. 1994, 24, 101.
11. Reppert, S. M.; Weaver, D. R.; Godson, C. Trends
Pharmacol. Sci. 1996, 17, 100.
12. Yeleswaram, K.; McLaughlin, L. G.; Knipe, J. O.;
Schabdach, D. J. Pineal Res. 1997, 22, 45.
Compound 3a was further tested for functional activity
in NIH3T3 cells expressing melatonin MT1 or MT2
receptor using methodology previously described.19
The agonist activity of the compound was assessed by
comparing its ability to inhibit forskolin-stimulated
cAMP accumulation with that of melatonin. Full agon-
ist activity was confirmed for compound 3a at both the
human melatonin MT1 and MT2 receptor subtypes with
EC50 s of 0.6 and 21nM at the MT1 and MT2 receptors,
respectively. The intrinsic activity relative to melatonin
of 3a was 0.97 and 0.79 at the MT1 and MT2 receptors,
respectively.
13. (a) Viswanathan, M.; Laitinen, J. T.; Saavedra, J. M.
Proc. Natl. Acad. Sci. U.S.A. 1990, 87, 6200; (b) Krause,
D. N.; Barrios, V. E.; Duckles, S. P. Eur. J. Pharmacol.
1995, 276, 207; (c) Ting, K. N.; Dunn, W. R.; Davies, D.
J.; Sugden, D.; Delagrange, P.; Guardiola-Lemaitre, B.;
Scalbert, E.; Wilson, V. G. Br. J. Pharmacol. 1997, 122,
1299.
14. Sun, L.-Q.; Chen, J.; Takaki, K.; Johnson, G.; Iben, L.;
Mahle, C. D.; Ryan, E.; Xu, C. Bioorg. Med. Chem. Lett.
2004, 14, 1197.
15. Sun, L.-Q.; Chen, J.; Bruce, M.; Deskus, J. A.; Epperson,
J. R.; Takaki, K.; Johnson, G.; Iben, L.; Mahle, C. D.;
Ryan, E.; Xu, C. Bioorg. Med. Chem. Lett. 2004, 14,
3799.
16. Sun, L.-Q.; Chen, J.; Mattson, R. J.; Epperson, J. R.;
Deskus, J. A.; Li, W.-S.; Takaki, K.; Hodges, D. B.; Iben,
L.; Mahle, C. D.; Ortiz, A.; Molstad, D.; Ryan, E.;
Yeleswaram, K.; Xu, C.; Luo, G. Bioorg. Med. Chem.
Lett. 2003, 13, 4381.
Compound 3a was further characterized in pharmacoki-
netic studies that are summarized in Table 2. The oral
bioavailability in rats was 90%, significantly superior
to the oral bioavailability of melatonin at the same dose
(24%). The oral bioavailability of 3a is presumably due
to a combination of good absorption, predicted by the
measured Caco-2 permeability (Pc 75nm/s), and moder-
ate clearance.
In conclusion, the benzoxazole scaffold that projects the
4-phenylbutyl and alkylamide groups was successfully
replaced by an isosteric benzofuranyl moiety. This struc-
tural modification led to the discovery of a series of ben-
zofuran derivatives as novel melatoninergic ligands and
the subsequent identification of compound 3a as an or-
ally bioavailable agonist at MT1 and MT2 melatonin
receptors with significantly lower vasoconstrictive activ-
ity in vitro in the rat tail artery and a longer biological
half-life in rats than the natural ligand, melatonin.
17. Catt, J. D.; Johnson, G.; Keavy, D. J.; Mattson, R. J.;
Parker, M. F.; Takaki, K. S.; Yevich, J. P. U.S. Patent
5,856,529, 1999.
18. Ki Values represent mean from experiments performed in
duplicate. Data reported in the text are means of 1–2
experiments run at five different concentrations in dupli-
cates. Standard errors were typically within 10% of mean
value. Melatonin was run as standard reference in every
assay with reproducible Ki.
19. Mattson, R. J.; Catt, J. D.; Keavy, D.; Sloan, C. P.;
Epperson, J.; Gao, Q.; Hodges, D. B.; Iben, L.; Mahle, C.
D.; Ryan, E.; Yocca, F. D. Bioorg. Med. Chem. Lett. 2003,
13, 1199.
20. The rat tail artery tension response is measured in grams.19
The force values (N = 3) at the concentration tested
(100nM) were 0.506 and 0.045g for melatonin and 3a,
respectively, 0.09 relative to melatonin.
Acknowledgements
We thank and acknowledge Ms. Yi-Xin Li for NMR
data support, Mr. Robert Kane for MS data support,