Design and synthesis of benzoxazole derivatives as novel melatoninergic ligands

Article abstract of DOI:10.1016/j.bmcl.2003.12.052

A novel series of benzoxazole derivatives was synthesized and evaluated as melatoninergic ligands. The binding affinity of these compounds for human MT₁ and MT₂ receptors was determined using 2-[ ¹²⁵I]-iodomelatonin as the

Full text of DOI:10.1016/j.bmcl.2003.12.052

Bioorganic & Medicinal Chemistry Letters 14 (2004) 1197–1200
Design and synthesis of benzoxazole derivatives as novel
melatoninergic ligands
Li-Qiang Sun,* Jie Chen, Katherine Takaki, Graham Johnson,y Lawrence Iben,
Cathy D. Mahle,{ Elaine Ryan and Cen Xu
Bristol-Myers Squibb Pharmaceutical Research Institute, 5 Research Parkway, Wallingford, CT 06492, USA
Received 20 October 2003; revised 11 December 2003; accepted 12 December 2003
Abstract—A novel series of benzoxazole derivatives was synthesized and evaluated as melatoninergic ligands. The binding affinity
of these compounds for human MT1 and MT2 receptors was determined using 2-[125I]-iodomelatonin as the radioligand. The results
of the SAR studies in this series led to the identification of compound 28, which exhibited better MT1 and MT2 receptor affinities
than melatonin itself. This work also established the benzoxazole nucleus as a melatoninergic pharmacophore, which served as an
isosteric replacement to the previously established alkoxyaryl core.
# 2003 Elsevier Ltd. All rights reserved.
Melatonin (N-acetyl-5-methoxy-tryptamine) (Fig. 1) is
an indole-derived neurohormone present in all mam-
malian species that is synthesized and secreted primarily
by the pineal gland in a circadian manner that closely
follows the daily light/dark cycle.1,2 It plays a major role
in the regulation of circadian rhythms and the control
of seasonal cycles.3,4 Melatonin alleviates jet lag, reg-
ulates delayed sleep phase syndrome,5 and induces
sleep.6 Moreover, melatonin has been reported to have
antioxidant,7 neuroprotective,8 antiproliferative,9 and
immunomodulatory10 properties. It has been demon-
strated that many of the effects of melatonin are medi-
ated through G-protein-coupled receptors expressed
primarily in the brain, retina, pituitary, and blood
vessels.11 Cloning of several G-protein-coupled melato-
nin receptor genes has revealed at least three melatonin
receptor subtypes, two of which are defined as MT1 and
MT2, and have been found in mammals.12
constrained melatoninergic agents.13 Interestingly, a
consistent structural motif found in almost all these
melatonin receptor ligands is the presence of an amido
group and an alkoxyaryl moiety. Hence, from the SAR
available in the literature, these groups appear to be
critical in determining the binding affinity and biological
activity of these melatoninergic ligands. It should also
be noted that as part of this body of work, melatonin
receptor ligands have been identified which are selective
14ꢀ21
21ꢀ22
for either the MT2
or MT1
receptor. Despite
these findings, at the outset of this study there was sur-
prisingly limited SAR established with respect to isos-
teric replacements of the alkoxyaryl and amido
pharmacophoric elements. Herein we report the synth-
esis and biological activity of benzoxazole derivatives as
novel and potent melatoninergic ligands.
A generic structure of the proposed melatonin pharma-
cophore is shown in Figure 1. The topology of the
oxazole ring is such that the oxazole nitrogen may serve
as a surrogate hydrogen bond acceptor23 in place of the
The development of synthetic compounds capable of
mimicking the effects of melatonin has progressed con-
siderably during the past decade. These compounds are
structurally diverse, and range from simple indole deriv-
atives and its bioisosteres, to phenylalkyl amides and
* Corresponding author. Tel.: +1-230-677-7460; fax: +1-203-677-
7702; e-mail: sunl@bms.com
y
Present address: Rib-X Pharmaceuticals, Inc., 300 George Street,
New Haven, CT 06511, USA.
Present address: Bayer Pharmaceutical Research Center, 400 Morgan
Lane, West Havewn, CT 06516, USA.
{
Figure 1.
0960-894X/$ - see front matter # 2003 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2003.12.052

1198
L.-Q. Sun et al. / Bioorg. Med. Chem. Lett. 14 (2004) 1197–1200
C5 methoxy group of melatonin. Moreover, the cyclo-
propyl bearing side chain in 9–31 was designed as a repla-
cement for the C3 ethylamide side chain of melatonin.
assays using 2-[125I]-iodomelatonin according to the
previously described assay method.24 The chemical
structures, Ki values and MT1 and MT2 selectivity ratios
of these compounds are reported in Table 1. As is evi-
dent from the results, the oxazole nucleus conferred
substantial activity to this series of melatoninergic
ligands. Moreover, the SARs at both ends of the mole-
cule are well defined. As seen in Table 1, compared to
melatonin, 2-ethyl-benzoxazoles 9–14 exhibit excellent
affinity for both human MT1 and MT2 melatonin
receptors. The binding affinity and receptor subtype
selectivity of these compounds are sensitive to the identity
of the N-acyl group. Thus, while acetamide 9 displayed
a similar MT1 affinity to melatonin, this analogue
showed a 3-fold decrease in MT2 affinity. On the other
hand, replacement of the methyl group of the amide side
chain by an ethyl (10), propyl (11), iso-propyl (12), or
cyclopropyl (13) resulted in a greater increase in MT1
affinity than in MT2 binding and therefore increased the
overall selectivity for MT1 receptors. From this set of
compounds the cyclopropane carboxamide 13 proved to
be the most active ligand for both MT1 and MT2
receptors with an 8- and 1.6-fold increased affinity when
compared to melatonin at the respective receptors.
However, replacing the terminal amide with a simple
ethyl urea, as found in compound 14, resulted in a 9-
and 3-fold loss in activity at the MT1 and MT2 receptor
subtypes, respectively.
The general route to the melatonin analogues 9–31 is
described in Scheme 1. Wittig reaction of commercially
available 3-nitrosalicylaldehyde 1 with Ph3PCHCO2Et
provided exclusively the (ꢁ)-trans-cinnamic ester 2 in
90% yield. Cyclopropanation of 2 with diazomethane
using Pd(OAc)2 as a catalyst gave rise to (ꢁ)-trans-
cyclopropane 3 in 70% yield. The nitro moiety of 3 was
reduced and acetylated by catalytic hydrogenation in
the presence of an appropriate carboxylic acid anhy-
dride or acid chloride to produce amide 4. The requisite
benzoxazole functionality, as found in 5, was intro-
duced by subjecting the hydroxy amide 4 to a solution
of catalytic amount PPTS in xylene at reflux. The next
series of reactions involved converting the ester func-
tionality of 5 to the corresponding reverse amide or urea
derivatives. In the event, ester 5 upon treatment with
LiAlH4 afforded the corresponding alcohol 6. Swern
oxidation of 6 to the corresponding aldehyde provided
intermediate 7, which was converted to the parent
oxime and subsequently reduced to provide the primary
amine 8. Intermediate 8 was then acylated using a series
of acid chlorides or reacted with isocyanates to produce
the amide and urea products, respectively, 9–31. It
should be noted that these compounds were prepared in
racemic form and with the aforementioned anti-stereo-
chemistry about the cyclopropane ring system.
Table 1. Ki of compounds 9–31 competing for the binding of 2-[125I]-
iodomelatonin to membrane preparations of NIH3T3 cells stably
expressing human MT1 or MT2 melatonin receptors. Values represent
mean from experiments performed in duplicate
The Ki values of compounds 9–31 for human MT1 and
MT2 melatonin receptor subtypes were determined in
Compd
R
R1
MT1 Ki
(nM)
MT2 Ki
(nM)
Ratio
MT2/MT1
Mel
9
—
Et
Et
Et
Et
Et
Et
nPr
nPr
nPr
nPr
nPr
nPr
iPr
iPr
iPr
—
Me
Et
nPr
iPr
cPr
NHEt
Me
Et
nPr
iPr
cPr
NHEt
Me
Et
nPr
iPr
cPr
NHEt
Me
Et
nPr
iPr
0.4
0.35
0.05
0.05
0.2
0.05
3.1
0.96
0.22
0.94
4.9
0.18
3.5
5.1
4.9
1.8
1.3
0.6
0.3
0.88
0.44
0.33
0.75
0.19
2.2
0.13
0.08
0.04
0.28
0.14
1.0
1.2
0.32
0.8
2.5
2.2
12.6
0.37
0.07
0.24
1.9
0.7
2.5
8.8
6.6
3.8
3.8
0.7
0.1
0.4
0.04
0.06
0.8
0.3
0.2
0.07
0.4
1.9
3.7
1.1
7.4
1.4
4.8
14.6
9.6
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
iPr
iPr
iPr
11.8
0.05
0.05
0.05
0.13
0.05
Ph(CH2)4
Ph(CH2)4
Ph(CH2)4
Ph(CH2)4
Ph(CH2)4
Scheme 1. (a) Ph3PCHCO2Et, THF, reflux, 90%; (b) Pd(OAc)2,
CH2N2, ether, 0–25ꢂC, 86%; (c) (RCO)2O, 10% Pd/C, H2, THF, rt;
(d) PPTS, xylene, reflux; (e) LiAlH4, THF; (f) COCl2, DMSO,
.
CH2Cl2, Et3N; (g) NH2OH HCl, NaOH, THF; (h) NaBH4,
cPr
0.48
.
CoCl2 H2O, MeOH; (i) RCOCl, Et3N, CH2Cl2 or RNCO, benzene.

L.-Q. Sun et al. / Bioorg. Med. Chem. Lett. 14 (2004) 1197–1200
1199
Modifications to the left hand aromatic moiety were
somewhat limited but included the replacement of the
ethyl substituent at the 2-position of the benzoxazole
ring system with the homologous n-propyl group, com-
pounds 15–20. When compared to the aforementioned
2-ethyl-benzoxazoles, the majority of the 2-propyl-
bearing analogues showed a simultaneous decrease in
MT1 affinity along with an increase in MT2 affinity,
albeit the latter was generally slight. As a consequence,
‘reversed’ selectivity was observed in this series, with
analogues showing greater affinity to the MT2 receptor
subtype. For example, compound 11, a 2-ethyl analogue,
showed moderate MT1 selectivity (MT2/MT1=6.6),
while the corresponding 2-propyl compound 17 pre-
sented much better MT2 selectivity (MT1/MT2=24). As
observed with the 2-ethyl series, the amide cyclopropyl
derivative 19 proved to be a highly potent analogue in
the 2-propyl series, but once again at the expense of
receptor selectivity.
based on the fact that the oxazole nitrogen is also a
hydrogen bond acceptor.23
In conclusion, benzoxazole derivatives26 have been
described as novel melatoninergic ligands that demon-
strate high binding affinity to both the MT1 and MT2
receptors. SARs have been defined around both the
terminal amide moiety and the C2 position of the ben-
zoxazole ring. These findings establish the substituted
benzoxazole ring system as a novel isostere for the
alkoxyaryl moiety of the endogenous ligand and its
close analogues.
References and notes
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2-iso-Propyl-benzoxazoles 21–26 were derived by sub-
stitution of the 2-ethyl group in the benzoxazole with an
iso-propyl group. This structural modification has a
greater effect on reducing MT1 affinity compared to
MT2 binding. Thus, substitution of the ethyl group in
compound 9 with an iso-propyl moiety (21) caused a 15-
fold decrease in MT1 affinity with negligible impact on
MT2 receptor binding. Changing the methyl group of
the amide side chain of 21 to an ethyl (22), propyl (23)
or iso-propyl (24) only slightly affected the potency at
both receptor subtypes. Pursuing a similar substitution
pattern with the cyclopropyl derivative 25 resulted in
the achievement of subnanomolar MT1 affinity that was
accompanied by a 2-fold decrease in MT2 binding.
However, as previously observed, the ethylamino sub-
stituent, as in 26, diminished binding to both receptors.
In addition, these compounds showed MT2/MT1 selec-
tivity ratios ranging between 1 to 4 and MT1/MT2
selectivity ratios varying between 2 to 15.
13. Takaki, K. S.; Mahle, C. D.; Watson, A. J. Curr. Pharm.
Des. 1997, 3, 429.
Of particular interest, a marked increase in binding
affinity was achieved by replacing the 2-ethyl group of
the benzoxazole 10 with a 4-phenyl-butyl group as in
compound 28. This modification imparted a 6-fold
increase in MT2 affinity while retaining MT1 affinity
comparable to the prototype 10. Compound 28 was the
most potent of this series with MT1 and MT2 affinities
8- and 4-fold more potent than melatonin, respectively.
Replacement of the propionamide of 28 by acetamide
(27), butyramide (29), or cyclopropylcarboxamide (31)
caused a slight decrease in MT2 activity but maintained
MT1 affinity. Pursuing a similar substitution with the
iso-propyl derivative 30 resulted in a larger decrease in
MT2 potency than in MT1 affinity, which, in turn,
enhanced the MT1 selectivity (MT2/MT1=14.6).
14. Teh, M.-T.; Sugden, D. Naunyn-Schmiedeberg’s Arch.
Pharmacol. 1998, 358, 522.
15. Faust, R.; Garratt, P. J.; Jones, R.; Yeh, L. K.; Tsotinis,
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Barrett, P.; Drew, J. E.; Delagrange, P.; Lesieur, D.;
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J. A.; Audinot, V.; Nicolas, J.-P.; Bennejean, C.; Delagrange,
P.; Renard, P.; Lesieur, D. J. Med. Chem. 2002, 45, 2788.
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. Yous, S.; Durieux-Poissonnier, S.; Lipka-Belloli, E.;
Guelzim, H.; Bochu, C.; Audinot, V.; Boutin, J. A.;
Delagrange, P.; Bennejean, C.; Renard, P.; Lesieur, D.
Bioorg. Med. Chem. 2003, 11, 753.
21. Audinot, V.; Mailliet, F.; Lahaye-Brasseur, C.; Bonnaud,
A.; Le Gall, A.; Amosse, C.; Dromaint, S.; Rodriguez,
It has been reported that the methoxy group of melato-
nin and melatonergic ligands plays an important role in
modulating melatonin receptor affinity. It has generally
been assumed that the 5-OMe group of melatonin
functions as a hydrogen bond acceptor for the atom
- OH or NH of serine 115 of the MT1 receptor.25 The
results of the current study support this hypothesis

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L.-Q. Sun et al. / Bioorg. Med. Chem. Lett. 14 (2004) 1197–1200
M.; Nagel, N.; Galizzi, J.-P.; Malpaux, B.; Guillaumet,
(m, 4H); 13C NMR (75 MHz, CDCl3) d 173.7, 166.8,
149.3, 141.0, 125.7, 124.0, 121.2, 116.7, 43.5, 30.5, 29.6,
21.3, 20.2, 17.1, 13.7, 13.1, 9.8; MS (ESI) 287 (M+H)+.
Compound 19: 1H NMR (300 MHz, CDCl3) d 7.42 (d,
J=7.9 Hz, 1H), 7.14 (t, J=7.8 Hz, 1H), 6.84 (d, J=7.6
Hz, 1H), 6.45 (br, s, 1H), 3.51–3.42 (m, 1H), 3.26–3.17 (m,
1H), 2.86 (t, J=7.4 Hz, 2H), 2.09–2.01 (m, 1H), 1.94–1.82
(m, 2H), 1.59–1.48 (m, 1H), 1.42–1.33 (m, 1H), 1.21–1.14
(m, 1H), 1.05–0.92 (m, 6H), 0.70–0.64 (m, 2H); 13C NM R
(75 MHz, CDCl3) d 173.5, 166.8, 149.3, 141.0, 125.8,
124.0, 121.2, 116.6, 43.7, 30.4, 29.5, 21.3, 20.2, 17.1, 14.6,
13.3, 7.0; MS (ESI) 299 (M+H)+. Compound 28: 1H
NMR (300 MHz, CDCl3) d 7.47 (d, J=7.8 Hz, 1H), 7.30–
7.26 (m, 2H), 7.20–7.16 (m, 4H), 6.89 (d, J=7.6 Hz, 1H),
5.86 (br, s, 1H), 3.52–3.43 (m, 1H), 3.28–3.19 (m, 1H),
2.95 (t, J=7.3 Hz, 2H), 2.70 (t, J=7.6 Hz, 2H), 2.22 (q,
J=7.6 Hz, 2H), 2.10–2.03 (m, 1H), 1.99–1.90 (m, 2H),
1.83–1.73 (m, 2H), 1.59–1.49 (m, 1H), 1.25–1.14 (m, 4H),
1.05–0.98 (m, 1H); 13C NMR (75 MHz, CDCl3) d 173.7,
166.7, 149.4, 141.9, 141.0, 128.3, 125.8, 124.1, 121.3,
116.7, 43.6, 35.4, 30.8, 29.6, 28.5, 26.4, 21.3, 17.1, 13.2,
9.8; MS (ESI) 377 (M+H)+. Compound 29: 1H NM R
(300 MHz, CDCl3) d 7.46 (d, J=7.8 Hz, 1H), 7.30–7.26
(m, 2H), 7.20–7.16 (m, 4H), 6.89 (d, J=7.5 Hz, 1H), 5.85
(br, s, 1H), 3.51–3.42 (m, 1H), 3.28–3.19 (m, 1H), 2.95 (t,
J=7.3 Hz, 2H), 2.70 (t, J=7.6 Hz, 2H), 2.17 (t, J=7.3
Hz, 2H), 2.11–2.05 (m, 1H), 2.00–1.90 (m, 2H), 1.83–1.73
(m, 2H), 1.72–1.62 (m, 2H), 1.59–1.48 (m, 1H), 1.24–1.17
(m, 1H), 1.04–0.98 (m, 1H), 0.94 (t, J=7.3 Hz, 3H); 13C
NMR (75 MHz, CDCl3) d 172.9, 166.7, 149.4, 141.9,
141.0, 128.3, 125.8, 124.1, 121.3, 116.7, 43.5, 38.7, 35.4,
30.8, 28.5, 26.4, 21.3, 19.1, 17.1, 13.7, 13.2; MS (ESI) 391
(M+H)+.
G.; Lesieur, D.; Lefoulon, F.; Renard, P.; Delagrange, P.;
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C.; Renard, P.; Lesieur, D. J. Med. Chem. 2003, 46, 1127.
23. It is established that the N atom rather than the O atom
of oxazoles and benzoxazoles functions as a hydrogen
bond acceptor. See: (a) Meanwell, N. A.; Romine, J. L.;
Rosenfeld, M. J.; Martin, S. W.; Trehan, A. K.; Wright,
J. J. K.; Malley, M. F.; Gougoutas, J. Z.; Brassard, C. L.;
Buchanan, J. O.; Federici, M. E.; Fleming, J. S.; Gam-
berdella, M.; Hartl, K. S.; Zavoico, G. B.; Seiler, S. M. J.
Med. Chem. 1993, 36, 3884. (b) Edwards, P. D.; Meyer,
E. F., Jr.; Vijayalakshmi, J.; Tuthill, P. A.; Andisik, D. A.;
Gomes, B.; Strimpler, A. J. Am. Chem. Soc. 1992, 114,
1854. (c) Abraham, M. H.; Duce, P. P.; Prior, D. V.;
Barratt, D. G.; Morris, J. J.; Taylor, P. J. J. Chem. Soc.,
Perkin Trans. 2 1989, 1355.
24. 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.
25. Sugden, D.; Chong, N. W. S.; Lewis, D. F. V. Br. J.
Pharmacol. 1995, 114, 618.
26. Selected spectral data. Compound 16: 1H NM
(300 MHz, CDCl3) d 7.44 (d, J=7.9 Hz, 1H), 7.16 (t,
J=7.8 Hz, 1H), 6.86 (d, J=7.6 Hz, 1H), 5.99 (br, s, 1H),
3.51–3.42 (m, 1H), 3.26–3.17 (m, 1H), 2.88 (t, J=7.6 Hz,
2H), 2.22 (q, J=7.6 Hz, 2H), 2.11–2.03 (m, 1H), 1.97–1.84
(m, 2H), 1.58–1.47 (m, 1H), 1.25–1.13 (m, 4H), 1.08–0.97
R