Substituted 4-Phenyl-2-(phenylcarboxamido)-1,3-thiazole derivatives as antagonists for the adenosine A1 receptor

Article abstract of DOI:10.1016/S0960-894X(01)00356-0

The synthesis and receptor binding of novel adenosine receptor antagonists is described. We found that non-xanthine 4-phenyl-2-(phenylcarboxamido)-1,3-thiazole derivatives may have high affinity and substantial selectivity for the adenosine A₁

Full text of DOI:10.1016/S0960-894X(01)00356-0

Bioorganic & Medicinal Chemistry Letters 11 (2001) 2017–2019
Substituted 4-Phenyl-2-(phenylcarboxamido)-1,3-thiazole
Derivatives as Antagonists for the Adenosine A1 Receptor
E. W. van Tilburg, P. A. M. van der Klein, M. de Groote,
M. W. Beukers and A. P. IJzerman*
Leiden/Amsterdam Center for Drug Research, Division of Medicinal Chemistry, PO Box 9502, 2300 RA Leiden, The Netherlands
Received 21 March 2001; revised 4 May 2001; accepted 17 May 2001
Abstract—The synthesis and receptor binding of novel adenosine receptor antagonists is described. We found that non-xanthine
4-phenyl-2-(phenylcarboxamido)-1,3-thiazole derivatives may have high affinity and substantial selectivity for the adenosine A1
receptor. # 2001 Elsevier Science Ltd. All rights reserved.
Extracellular adenosine regulates several physiological
functions by activation of specific cell membrane recep-
tors. There are four adenosine receptor subclasses
defined, A1, A2A, A2B and A3. Since the first reports on
the adenosine A1 receptor,1,2 efforts have been made to
identify ligands for this receptor. The first A1 receptor
antagonists were xanthine derivatives, such as theo-
phylline (1, Fig. 1). Since then, a variety of different
classes of heterocyclic compounds has been descri-
bed to possess antagonistic activity at adenosine
receptors, including xanthines, adenines, 7-deazaade-
nines, 7-deaza-8-azapurines, pyrazolo[3,4-c]quinolines,
pyrazolo-[1,5-a]pyridines and 1,8-naphthyridines.3À11
Isoquinoline and quinazoline derivatives from our
laboratory also displayed high affinity for the adenosine
(A1 and A3) receptors, in particular when a spacer-cou-
pled aromatic group was attached to the core ring
system.12À14 With the recent synthesis of a series of
(3-phenyl)-1,2,4-thiadiazoles, another new class of het-
erocyclic compounds as adenosine receptor antagonists
was developed.15 N-(3-Phenyl-1,2,4-thiadiazol-5-yl)-4-
methoxybenzamide (3, LUF 5417) displayed similar
adenosine A3 receptor affinity as N-(3-phenylisoquino-
lin-1-yl)-4-methoxybenzamide (2, Fig. 2). However,
compound 3 displayed a significant increase in affinity for
the adenosine A1 receptor, rendering 3 essentially non-
selective. N-(4-Phenylthiazol-2-yl)-4-methoxybenzamide
(4, LUF 5433, Fig. 2),16 a single compound included in
the study described,15 showed a 9-fold selectivity for the
adenosine A1 receptor.
In the present study, we synthesised a series of 4-phenyl-(2-
phenylcarboxamido)-1,3-thiazole derivatives as potential
antagonists for the adenosine A1 receptor on the basis
of these findings. The synthesis of the substituted 4-
phenyl-2-(phenylcarboxamido)-1,3-thiazoles 17–26 was
achieved by a condensation reaction of 2-amino-4-
phenyl-1,3-thiazole (5) with the appropriate acylchlor-
ide (6–16) yielding the corresponding amide-spaced
compounds (Scheme 1). The HBr salt of 2-amino-4-
phenyl-1,3-thiazole (193 mg, 0.7 mmol) was dissolved in
dioxane (3 mL) and Et3N (195 mL). To this mixture the
appropriate acylchloride (1.07 mmol) in 1 mL dioxane
was added and the solution was refluxed overnight.
After cooling to room temperature the mixure was
Figure 1.
*Corresponding author. Tel.: +31-(0)71-527-4651; fax: +31-(0)71-
527-4565; e-mail:
Figure 2.
0960-894X/01/$ - see front matter # 2001 Elsevier Science Ltd. All rights reserved.
PII: S0960-894X(01)00356-0

2018
E. W. van Tilburg et al. / Bioorg. Med. Chem. Lett. 11 (2001) 2017–2019
filtered, concentrated under vacuo and purified by
column chromatography. The target compounds were
obtained in high yields.17
more favorable at the para-position (19, 20)16 than at the
meta-position (18)21 for high adenosine A1 receptor affi-
nity. The 3,4-dichloro (25) and 2,4-dichloro (26) analo-
gues showed affinities comparable to that of the
unsubstituted compound (17). 2-[(4-Nitrophenyl)carbox-
amido]-4-phenyl-1,3-thiazole (21)16 and 2-[(4-chloro-
phenyl)carboxamido]-4-phenyl-1,3-thiazole (20) had the
highest affinity for the adenosine A1 receptor (Ki values of
22 and 18 nM, respectively). Their selectivity for adeno-
sine A1 versus A3 receptors was approximately 50-fold.
All compounds were tested in radioligand binding
assays to determine their affinities for the adenosine A1
receptor in rat brain cortex, the A2A receptor in rat
striatum and the human A3 receptor as expressed in
HEK 293 cells (Table 1). Displacement experiments
were performed in the absence of GTP. The procedures
used have been described in detail previously.18À20 Table
1 and binding data of the earlier mentioned series of
thiadiazoles (3)15 show that the 1,3-thiazole derivatives
(17–26) have similar adenosine A1 receptor affinities as
their thiadiazole congeners. However, their A1 receptor
selectivity was increased significantly (Table 1). The
unsubstituted 4-phenyl-2-(phenylcarboxamido)-1,3-thia-
zole (17)16 showed affinities in the micromolar range at
the adenosine A2A and A3 receptor, while having high
affinity for the A1 receptor. Introduction of a methyl
group (22)16 did not influence adenosine receptor affi-
nities, whereas a tert-butyl group (23) or a tri-
fluoromethyl group (24) decreased the affinity for the
adenosine A1 receptor. The stronger electron-donating
methoxy analogue (4) increased adenosine A2A and A3
receptor affinity and thus has decreased adenosine A1
receptor selectivity. Introduction of a halogen atom was
Thus, in summary, the 4-phenyl-2-(phenylcarbox-
amido)-1,3-thiazole derivatives constitute a class of
novel high affinity adenosine A1 receptor antagonists.
Furthermore, these compounds display higher A1 selec-
tivity compared to similar series previously described.
Acknowledgements
The authors thank Karl-Norbert Klotz (University of
Wurzburg, Germany) for providing HEK 293 cells
expressing the human adenosine A3 receptor.
References and Notes
1. Londos, C.; Cooper, D. M. F.; Wolff, J. Biochemistry 1980,
77, 2551.
2. Van Calker, D.; Muller, M.; Hamprecht, B. J. Neurochem.
1979, 33, 999.
3. Betti, L.; Biagi, G.; Giannaccini, G.; Giorgi, I.; Livi, O.;
Lucacchini, A.; Manera, C.; Scartoni, V. J. Med. Chem. 1998,
41, 668.
4. Camaioni, E.; Costanzi, S.; Vittori, S.; Volpini, R.; Klotz,
K.-N.; Cristalli, G. Bioorg. Med. Chem. 1998, 6, 523.
5. Collota, V.; Catarzi, D.; Varano, F.; Cecchi, L.; Filac-
chioni, G.; Martini, C.; Trincavelli, L.; Lucacchini, A. J. Med.
Chem. 2000, 43, 3118.
6. Ferrarini, P. L.; Mori, C.; Manera, C.; Martinelli, A.;
Mori, F.; Saccomanni, G.; Barili, P. L.; Betti, L.; Giannaccini,
G.; Trincavelli, L.; Lucacchini, A. J. Med. Chem. 2000, 43,
2814.
7. Katsushima, T.; Nieves, L.; Wells, J. N. J. Med. Chem.
1990, 33, 1906.
8. Kuroda, S.; Akahane, A.; Itani, H.; Nishimura, S.; Durkin,
K.; Kinoshita, T.; Tenda, Y.; Sakane, K. Bioorg. Med. Chem.
Lett. 1999, 9, 1979.
Scheme 1.
Table 1. Affinities of 4-phenyl-2-(phenylcarboxamido)-1,3-thiazole
derivatives (Scheme 1) at adenosine A1, A2A and A3 receptors expres-
sed as Ki values (nMÆSEM, n=3) or percentage displacement
Ki (nM) or % displacement
No.
R1
R2
R3
A1
A2A
A3
a,e
b,e
c,f
9. Muller, C. E. Expert Opin. Ther. Pat. 1997, 5, 419.
10. Pfister, J. R.; Belardinelli, L.; Lee, G.; Lum, R. T.; Milner,
P.; Stanley, W. C.; Linden, J.; Baker, S. P.; Schreiner, G. J.
Med. Chem. 1997, 40, 1773.
11. Poulsen, S.-A.; Quinn, R. J. J. Med. Chem. 1996, 39, 4156.
12. van Muijlwijk-Koezen, J. E.; Timmerman, H.; Link, R.;
van der Goot, H.; IJzerman, A. P. J. Med. Chem. 1998, 41, 3987.
13. van Muijlwijk-Koezen, J. E.; Timmerman, H.; Link, R.;
van der Goot, H.; IJzerman, A. P. J. Med. Chem. 1998, 41,
3994.
14. van Muijlwijk-Koezen, J. E.; Timmerman, H.; van der
Goot, H.; Menge, W. M. P. B.; von Frijtag Drabbe Kunzel, J.;
de Groote, M.; IJzerman, A. P. J. Med. Chem. 2000, 43, 2227.
15. van Muijlwijk-Koezen, J. E.; Timmerman, H.; Vollinga,
R. C.; von Frijtag Drabbe Kunzel, J.; de Groote, M.; Visser,
S.; IJzerman, A. P. J. Med. Chem. 2001, 44, 749.
16. Ramachandraiah, G.; Reddy, K.; Kuroda, S. Indian J.
Chem. Sect. B 1985, 24, 808.
4
H
H
H
H
H
H
H
H
H
H
Cl
H
H
Cl
H
H
H
H
H
H
Cl
H
OCH3
H
H
Br
76Æ8d
39Æ3
1900Æ500d
21%
3%
670Æ100d
42%
42%
58%
63%
35%
66%
27%
18%
17
18
19
20
21
22
23
24
25
26
86Æ1
33Æ4
14%
15%
9%
13%
2%
0%
6%
17%
Cl
18Æ3
NO2
CH3
C(CH3)3
CF3
Cl
22Æ4
36Æ7
1360Æ80
165Æ9
59Æ8
20%
22%
Cl
58Æ7
aDisplacement of [3H]DPCPX from rat cortical membranes.20
bDisplacement of [3H]ZM241385 from rat striatal membranes.19
cDisplacement of [125I]AB MECA from the human A3 receptor
expressed in HEK 293 cells.
dData obtained from van Muijlwijk et al.15
e%Displacement at 10 mM.
f%Displacement at 1 mM.

E. W. van Tilburg et al. / Bioorg. Med. Chem. Lett. 11 (2001) 2017–2019
2019
17. Physical data for the compounds: 2-[(4-Methoxy-
phenyl)carboxamido]-4-phenyl-1,3-thiazole (4): Yield 185 mg
(0.60 mmol, 85%), mp 161–162 ꢀC; 1H NMR(200 MHz,
DMSO-d6) d 12.59 (bs, 1H, NH), 8.14 (d, 2H, J=8.92 Hz,
(C17H14N2OS) C.H.N. 2-{[4-(tert-Butyl)phenyl]carboxamido}-
4-phenyl-1,3-thiazole (23): Yield 167 mg (0.50 mmol, 71%); 1H
NMR(200 Hz, DMSO- d6) d 12.69 (bs, 1H, NH), 8.08 (d, 2H,
J=8.23 Hz, Hortho), 7.95 (d, 2H, J=7.55 Hz, Hortho), 7.66 (s,
1H, SCH), 7.57–7.28 (m, 5H, Harom), 1.30 (s, 9H, CH3) ppm;
m/z 337 (M+); anal. (C20H20N2OS) C.H.N. 4-Phenyl-2-{[4-
H
ortho), 7.94 (d, 2H, J=7.56 Hz, Hmeta), 7.65 (s, 1H, SCH),
7.47–7.32 (m, 3H, Hortho+para), 7.08 (d, 2H, J=8.93 Hz,
meta), 3.84 (s, 3H, CH3) ppm; m/z 311 (M+); anal.
H
(trifluoromethyl)phenyl]-carboxamido}-1,3-thiazole
(24):
(C17H14N2O2S) C.H.N. 4-Phenyl-2-(phenylcarboxamido)-1,3-
thiazole (17): Yield 137 mg (0.49 mmol, 70%), mp 127 ꢀC; H
Yield 146 mg (0.42 mmol, 60%), mp 198–200 ꢀC; 1H NMR
1
(200 MHz, DMSO-d6) d 13.04 (bs, 1H, NH), 8.38–8.22 (m, 2H,
NMR(200 MHz, DMSO- d6) d 12.77 (bs, 1H, NH), 8.13 (d,
2H, J=7.90 Hz, Harom), 7.95 (d, 2H, J=7.21 Hz, Harom), 7.69
(s, 1H, SCH), 7.64–7.32 (m, 6H, Harom) ppm; m/z 281 (M+);
anal. (C16H12N2OS) C.H.N. 2-[(3-Chlorophenyl)carbox-
amido]-4-phenyl-1,3-thiazole (18): Yield 176 mg (0.56 mmol,
80%), mp 136–137 ꢀC; 1H NMR(200 MHz, DMSO- d6) d
12.89 (bs, 1H, NH), 8.19 (s, 1H, COCCH), 8.06 (d, 1H,
J=7.89 Hz, CCHCCl), 7.96–7.92 (m, 2H, Harom), 7.71 (s, 1H,
SCH), 7.71–7.35 (m, 5H, Harom) ppm; m/z 315 (M+); anal.
(C16H11ClN2OS) C.H.N. 2-[(4-Bromophenyl)carboxamido]-4-
phenyl-1,3-thiazole (19): Yield 189 mg (0.53 mmol, 75%), mp
H
H
ortho), 8.05–7.80 (m, 3H, SCH, Hmeta), 7.73–7.64 (m, 1H,
para), 7.52–7.24 (m, 4H, Harom) ppm; m/z 349 (M+); anal.
(C17H11F3N2OS) C.H.N. 2-[(3,4-Dichlorophenyl)carbox-
amido]-4-phenyl-1,3-thiazole (25): Yield 164 mg (0.47 mmol,
67%), mp 192 ꢀC; 1H NMR(200 MHz, DMSO- d6) d 12.89 (bs,
1H, NH), 8.42–8.34 (m, 1H, COCCHCH), 8.12–7.68 (m, 6H,
SCH, Harom), 7.50–7.25 (m, 3H, Harom) ppm; m/z 349 (M+);
anal.
(C16H10Cl2N2OS)
C.H.N.
2-[(2,4-Dichloro-
phenyl)carboxamido]-4-phenyl-1,3-thiazole (26): Yield 198 mg
(0.57 mmol, 81%), mp 180–182 ꢀC; 1H NMR(200 MHz,
DMSO-d6) d 12.85 (bs, 1H, NH), 7.94–7.89 (m, 2H, Harom),
7.79–7.69 (m, 3H, Harom), 7.58–7.32 (m, 4H, Harom) ppm; m/z
349 (M+); anal. (C16H10Cl2N2OS) C.H.N.
198–200 ꢀC; H NMR(200 MHz, DMSO- d6) d 12.81 (bs, 1H,
1
NH), 8.07 (m, 4H, Harom), 7.78–7.69 (m, 3H, Harom), 7.46–7.32
(m, 4H, Harom) ppm; m/z 360 (M+); anal. (C16H11BrN2OS)
C.H.N. 2-[(4-Chlorophenyl)carboxamido]-4-phenyl-1,3-thia-
zole (20): Yield 183 mg (0.58 mmol, 83%), mp 202 ꢀC; 1H
NMR(200 MHz, DMSO- d6) d 12.85 (bs, 1H, NH), 8.13 (d,
2H, J=8.58 Hz, Harom), 7.95 (d, 2H, J=7.21 Hz, Harom), 7.70
(s, 1H, SCH), 7.63 (d, 2H, J=8.59 Hz, Harom), 7.48–7.29 (m,
3H, Harom) ppm; m/z 315 (M+); anal. (C16H11ClN2OS)
C.H.N. 2-[(4-Nitrophenyl)carboxamido]-4-phenyl-1,3-thiazole
(21): Yield 180 mg (0.55 mmol, 79%), mp 213–214 ꢀC; 1H
NMR(200 MHz, DMSO- d6) d 13.02 (bs, 1H, NH), 8.39–8.26
(m, 4H, Harom), 7.94 (d, 2H, J=8.24 Hz, Hortho), 7.72 (s, 1H,
SCH), 7.48–7.29 (m, 3H, Hmeta+para) ppm; m/z 326 (M+);
anal. (C16H11N3O3S) C.H.N. 2-[(4-Methylphenyl)carbox-
amido]-4-phenyl-1,3-thiazole (22): Yield 140 mg (0.48 mmol,
68%), mp 146–149 ꢀC; 1H NMR(200 MHz, DMSO- d6) d
12.78 (bs, 1H, NH), 8.14 (d, 2H, J=7.89 Hz, Hortho), 8.05 (d,
2H, J=7.89 Hz, Hortho), 7.78 (s, 1H, SCH), 7.58–7.55 (m, 5H,
18. Briefly, assays for the human adenosine A3 receptor were
performed in 50/10/1 buffer [50 mM Tris/10 mM MgCl2/1 mM
ethylenediaminetetraacetic acid (EDTA) and 0.01% 3-([3-
cholamidopropyl] - dimethylammonio) - 1 - propanesulfonate
(CHAPS)] in glass tubes and contained 50 mL of a HEK 293
cell membrane suspension (10–30 mg), 25 mL [125I]AB MECA
(final concentration 0.15 nM), and 25 mL of ligand. Incuba-
tions were carried out for 1 h at 37 ꢀC and were terminated by
rapid filtration over Whatman GF/B filters, using a Brandell
cell harvester (Brandell, Gaithersburg, MD, USA). Tubes
were washed three times with 3 mL of buffer. Radioactivity
was determined in a Beckman 5500B g-counter. Nonspecific
binding was determined in the presence of 10À5 M R-PIA.
19. Gao, Z.-G.; IJzerman, A. P. Biochem. Pharmacol. 2000,
60, 669.
20. Pirovano, I. M.; IJzerman, A. P.; Van Galen, P. J. M.;
Soudijn, W. Eur. J. Pharmacol. 1989, 172, 185.
21. Coburn, R. A.; Glennon, R. A. J. Pharm. Sci. 1973, 62, 1785.
H
arom), 2.59 (s, 3H, CH3) ppm; m/z 295 (M+); anal.