Modifications and structure-activity relationships at the 2-position of 4-sulfonamidopyrimidine derivatives as potent endothelin antagonists

Article abstract of DOI:10.1016/S0960-894X(01)00682-5

To improve water solubility and to study structure-activity relationships, we modified the structure of the pyrimidine nucleus of each of a series of potent ET_A antagonists, 3a and 4a, at the 2-position. In a previous study, each of these antag

Full text of DOI:10.1016/S0960-894X(01)00682-5

Bioorganic & Medicinal Chemistry Letters 12 (2002) 81–84
Modifications and Structure–Activity Relationships at the
2-Position of 4-Sulfonamidopyrimidine Derivatives as Potent
Endothelin Antagonists
Hiroshi Morimoto, Hideshi Shimadzu, Toshihiro Hosaka, Yasushi Kawase,
Kosuke Yasuda, Kohei Kikkawa, Rikako Yamauchi-Kohno and Koichiro Yamada*
Discovery Research Laboratory, Tanabe Seiyaku Co. Ltd., 2-2-50 Kawagishi, Toda, Saitama 335-8505, Japan
Received 21 August 2001; accepted 10 October 2001
Abstract—To improve water solubility and to study structure–activity relationships, we modified the structure of the pyrimidine
nucleus of each of a series of potent ETA antagonists, 3a and 4a, at the 2-position. In a previous study, each of these antagonists
showed an extremely high affinity for the ETA receptor in porcine aortic membrane (IC50 3a; < 0.001 nM, 4a; 0.0039 nM). Two
modification methods, one being the addition of organolithium followed by DDQ oxidation and the other being the nucleophilic
substitution of 2-(methylsulfonyl)pyrimidine, were applied individually to synthesize 2-substituted-4-sulfonamidopyrimidine deri-
vatives. The introduction of aryl, heteroaryl, alkyl, amino, alkoxy, or alkylthio groups into the 2-position varied the affinity. Deri-
vatives with hydrophilic groups at the 2-position showed higher water solubility but tended to reduce the affinity for the ETA
receptor. # 2001 Elsevier Science Ltd. All rights reserved.
Endothelin (ET)-1 is a 21-amino acid peptide first iso-
lated from cultured porcine vascular endothelial cells in
1988. It belongs to a family of three isopeptides ET-1,
ET-2, and ET-3 each of which has a distinct specificity
for receptor subtypes ETA (binding affinity: ET-1=ET-
2>ET-3) and ETB (binding affinity: ET-1=ET-2=ET-
3).1À3 ETs have been implicated as pathogenic factors
for a variety of disease states, including essential hyper-
tension, congestive heart failure, pulmonary hyperten-
sion, subarachnoid hemorrhage, cerebral ischemia,
vasospasm, cyclosporin-induced renal failure, athero-
sclerosis, and asthma.4À7 The ET antagonists have been
suggested to offer potential utility in the treatment of
these disorders.8À10
antagonists, 3a and the related compounds 4a and 5
(shown in Fig. 1), and we have reported the structure–
activity relationships (SAR) at the 4- and 5-positions of
the pyrimidine nucleus.13 The IC50s of 3a, 4a, and 5 for
the ETA receptor in porcine aortic membrane were <
0.001, 0.0039, and 0.051 nM, respectively. At the 4-
position, the benzenesulfonamide moiety with a bulky
hydrophobic group at para-position showed the high
affinity. With respect to the 5-position, the phenyl or
phenoxy group with small substituents allowed the high
affinity to be maintained. However, these derivatives
showed low water solubility (3a, 0.0016 mg/mL at pH
7.5). Therefore, we intended to improve the water solu-
bility by introducing hydrophilic groups into the 2-
position of the pyrimidine nucleus, which had not been
modified. Here we describe the efficient modifications
and SAR at the respective 2-positions of the compounds
3a and 4a.
The first orally active non-peptidic sulfonamide deriva-
tives, Ro.46-2005 (1)11 and bosentan (2),12 were ETA/B
mixed type antagonists disclosed in 1993 (Fig. 1). The
selective ETA antagonist is expected to be a useful ther-
apeutic agent for these cardiovascular diseases because
the effect of ET-1 on the cardiovascular system is medi-
ated predominantly by the ETA receptor.3 We have
already developed a series of potent and ETA-selective
The general method of synthesizing the substituted
pyrimidines is shown in Scheme 1 (Method A).14À17
First, we chlorinated 2,5-disubstituted-4,6-dihydroxypy-
rimidine 8, prepared from the substituted malonate 6
and the amidine derivative 7. This was followed by
substitution at the 4- and 6-positions to afford the 2,4,5,6-
tetrasubstituted pyrimidines 3b–c and 4b–d (see Table 1).
With this method, the modification at the 2-position
*Corresponding author. Tel.:+81-48-433-2602; fax: 81-48-433-2610;
e-mail: koichiro@tanabe.co.jp
0960-894X/02/$ - see front matter # 2001 Elsevier Science Ltd. All rights reserved.
PII: S0960-894X(01)00682-5

82
H. Morimoto et al. / Bioorg. Med. Chem. Lett. 12 (2002) 81–84
would not be efficient since it requires a variety of
starting material 7. Therefore, we developed the follow-
ing two routes of synthesis for convenient modification
at the 2-position of the pyrimidine nucleus.
By method C shown in Scheme 3, a variety of heteroa-
toms were introduced at the 2-position at the last syn-
thetic
step.
2-Methylthio-4,6-dihydroxypyrimidine
derivative 14, prepared from 6 and thiourea, was con-
verted to the 4,6-dichloro-2-methylthiopyrimidine deri-
vative 15, which led to the compound 17. After
Scheme 2 summarizes the method to introduce the aryl
group into the 2-position of the pyrimidine nucleus
(Method B). Nucleophilic addition of an aryl lithium to
11, followed by DDQ oxidation, provided 12.18 Sub-
stitution by 4-tert-butylbenzenesulfonamide and ethyl-
ene glycol at the 4- and 6-positions, followed by the
introduction of 5-bromopyrimidine into the terminal
hydroxyl group, afforded the targeted 2-aryl derivatives
3d–f. By this method, however, heteroatoms, such as
amine, alcoholate, and thiolate, are introduced not into
the 2-position but into the 4-position of 11.
Scheme 2. Nucleophilic addition at the 2-position and following oxi-
dation of pyrimidine (Method B): (a) RLi, THF, À60 ꢀC; (b) DDQ,
aqueous AcOH, 0 ꢀC; (c) 4-tert-butylbenzenesulfonamide, K2CO3,
DMSO, 80 ꢀC; (d) ethylene glycol, NaH, 100ꢀC; (e) 5-bromo-2-chloro-
pyrimidine, NaH, THF–DMAc, room temperature.
Figure 1. Structure of Ro.46-2005, bosentan and our N-(6-(2-((5-
bromo-2-pyrimidinyl)oxy)ethoxy)-4-pyrimidinyl)sulfonamide deriva-
tives.
Scheme 3. Nucleophilic substitution into (2-methylsulfonyl)py-
rimidine with a variety of heteroatoms (Method C). (a) thiourea,
NaOMe, MeOH, room temperature; (b) MeI, K2CO3, H2O, room
temperature; (c) POCl3, PhNMe2, reflux; (d) 4-tert-butylbenzenesulf-
onamide, K2CO3, DMSO, 80 ꢀC; (e) ethylene glycol, NaH, 100 ꢀC; (f)
5-bromo-2-chloropyrimidine, NaH, THF–DMAc, room temperature;
(g) mCPBA, CHCl3, room temperature; (h) amine, alcoholate, or
thiolate, room temperature À140 ꢀC.
Scheme 1. General method of synthesizing the substituted pyrimidine
(Method A): (a) NaOMe, MeOH, room temperature; (b) POCl3,
reflux; (c) 4-tert-butylbenzenesulfonamide, K2CO3, DMSO, 80 ꢀC; (d)
ethylene glycol, NaH, 100 ꢀC; (e) 5-bromo-2-chloropyrimidine, NaH,
THF–DMAc, room temperature.

H. Morimoto et al. / Bioorg. Med. Chem. Lett. 12 (2002) 81–84
83
oxidative conversion of the methylthio group of 17 to
methylsulfone as an activated leaving group (18), a
variety of heteroatoms, such as N-alkyl, O-alkyl, and S-
alkyl groups, were introduced into the 2-position by
nucleophilic substitutions (4e–o).
groups at this position, such as the 2-thienyl (3e), phenyl
(3f), and alkyl groups (3b and c), reduced the affinity
drastically. With respect to the 5-(2-methoxy)phenoxy
derivatives (4), the 2-hydroxyethylamino (4g) and 4-
hydroxypiperidino (4n) derivatives maintained the high
affinity for the ETA receptor and showed high ETA-
selectivity (IC50s for ETB receptors in rat cerebellum
membrane were larger than 1 nM). Minor changes to
the substituents in 4g and 4n, such as replacement of the
NH or OH group with another (4i, j, or o) and chain
elongation (4h), decreased the affinity for the ETA
receptor. Furthermore, carboxylic acid (4m) and other
smaller substituents, such as Me, CF3, OMe, and NH2
(4b, d, e, and f), also reduced the affinity. These results
suggest that the ETA receptor could have specific inter-
actions with the 2-hydroxyethylamino and 4-hydroxy-
piperidino groups at the 2-position of the pyrimidine
nucleus. In addition, the introduction of these groups
into the 2-position improved the water solubility. The
water solubility of 4a, 4g, and 4n at pH 7.4 were 0.16,
7.32, and 2.57 mg/mL, respectively.
Tables 1 and 2 show the SAR of the modified com-
pounds at the 2-position. With respect to the 5-(4-
methyl)phenyl derivatives (3), the compound with 2-
pyridyl (3d) group showed a relatively high affinity for
the ETA receptor (IC50=0.043 nM). However, other
Table 1. SAR of the 2-substituted derivatives 3
In summary, to study SAR for ETA receptor antagonist
activity, modifications at the 2-position of the sulfon-
amidopyrimidine derivatives 3a and 4a with a variety of
substituents were carried out. The 2-hydroxyethylamino
(4g) and 4-hydroxypiperidino (4n) derivatives each
showed an affinity nearly equal to or a little higher than
that for the ETA receptor compared with unsubstituted
ones (3a and 4a).13 The potency of each of these com-
pounds was about 1000 times higher than that of
bosentan. Further investigation is now under way.
Compd
R
IC50 (nM)a
Method
Mp (ꢀC)
3a
3b
3c
3d
3e
3f
H
i-Pr
n-Pr
2-Pyridyl
2-Thienyl
Phenyl
<0.001
1.1
1.6
0.043
3.9
>10B
A
A
B
166–168
184–186
169–171.5
209–210
B
173.5–174.5
aInhibition of [125I]ET-1 binding in vitro to ETA receptors in porcine
aortic membrane. Values are from a single experiment.
Table 2. SAR of the 2-substituted derivatives 4
Compd
R
IC50 (nM)a
Method
Yield (%) from 18
Mp (ꢀC)
4a
4b
4c
4d
4e
4f
4g
4h
4i
4j
4k
4l
4m
4n
H
Me
n-Pr
CF3
OMe
NH2
0.0039b
2.7
A
A
A
–
148–149.5
143–145
0.026
>10A
>10C
>10C
0.0053b
>10C
>10C
>10C
0.15
0.20
10C
0.0025b
0.45
7.5
Amorphous
78.5–80
42
c
99e
12
197–198.5
Amorphous
NH(CH2)2OH
NH(CH2)3OH
NMe(CH2)2OH
NH(CH2)2NMe
S(CH2)2OH
O(CH2)2OH
S(CH2)2COOH
4-hydroxy-piperidino
4-methyl-piperazin-1-yl
C
14
26
51
C
Amorphous
137–140
Amorphous
84
91
47f
16
47
Amorphous
Amorphous
Amorphous
Amorphous
Amorphous
C
d
C
C
4o
Bosentan
aInhibition of [125I]ET-1 binding in vitro to ETA receptors in porcine aortic membrane. Values are from a single experiment.
bIC50 for ETB receptors in rat cerebellum membrane were larger than 1 nM. Values are from single experiment.
cPrepared by reduction of R=N3 derivative.
dPrepared by hydrolysis of R=S(CH2)2COOMe derivative.
eYield of the R=N3 derivative.
fYield of the R=S(CH2)2COOMe derivative.

84
H. Morimoto et al. / Bioorg. Med. Chem. Lett. 12 (2002) 81–84
Experimental
ET receptor binding assay. These binding experiments
were carried out according to the reported method by
Ihara et al.19
Compound 12. 1.66 M n-Butyllithium in hexane solution
(11.4 mL) was added to a solution of thiophene (1.69 g)
in dry THF (20mL) dropwise at 0 ꢀC. After 30min, a
solution of 11 (4.00 g) in dry THF (20 mL) was added to
the mixture at À60 ꢀC, and the whole was allowed to stir
at 0 ꢀC for 1.5 h. 85% aqueous AcOH then DDQ (5.70
g) were added. After 1 h, the reaction mixture was dilu-
ted with 10% aqueous citric acid and extracted with
EtOAc. The extract was washed with saturated aqueous
NaHCO3, H2O, and brine, dried over anhydrous
Na2SO4, and concentrated. Purification by silica gel
column chromatography followed by crystallization
from hexane afforded 12 (2.64 g, 66%): mp 119.5–
120 ꢀC.
Acknowledgements
The authors wish to thank the staff of the analytical
section of our laboratory for spectral measurements and
elemental analyses.
References and Notes
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piperidine (250mg) in DMSO (3 mL) was stirred at
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brine, dried over anhydrous Na2SO4, and concentrated.
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with i-Pr2O afforded 4n as amorphous powder (58 mg,
16%).