Novel 1,3-disubstituted 8-(1-benzyl-1H-pyrazol-4-yl) xanthines: High affinity and selective A2B adenosine receptor antagonists

Article abstract of DOI:10.1021/jm051268+

Adenosine has been suggested to induce bronchial hyperresponsiveness in asthmatics, which is believed to be an A_2B adenosine receptor (AdoR) mediated pathway. We hypothesize that a selective, high-affinity A_2B AdoR antagonist may provide therapeutic benefit in the treatment of asthma. In an attempt to identify a high-affinity, selective antagonist for the A _2B AdoR, we synthesized 8-(C-4-pyrazolyl) xanthines. Compound 22, 8-(1H-pyrazol-4-yl)-1,3-dipropyl xanthine, is a N-1 unsubstituted pyrazole derivative that has favorable binding affinity (K_i = 9 nM) for the A_2B AdoR, but it is only 2-fold selective versus the A₁ AdoR. Introduction of a benzyl group at the N-1-pyrazole position of 22 resulted in 19, which had moderate selectivity. The initial focus of the SAR study was on the preparation of substituted benzyl derivatives of 19 because the corresponding phenyl, phenethyl, and phenpropyl derivatives showed a decrease in A_2B AdoR affinity and selectivity relative to 19. The preferred substitution on the phenyl ring of 19 contains an electron-withdrawing group, specifically F or CF₃ at the m-position, as in 33 and 36 respectively, increases the selectivity while retaining the affinity for the A_2B AdoR. Exploring disubstitutions on the phenyl ring of derivatives 33 and 36 led to the 2-chloro-5-trifluoromethylphenyl derivative 50, which retained the A_2B AdoR affinity but enhanced the selectivity relative to 36. After optimization of the substitution on the 8-pyrazole xanthine, 1,3-disubstitution of the xanthine core was explored with methyl, ethyl, butyl, and isobutyl groups. In comparison to the corresponding dipropyl analogues, the smaller 1,3-dialkyl groups (methyl and ethyl) increased the A_2B AdoR binding selectivity of the xanthine derivatives while retaining the affinity. However, the larger 1,3-dialkyl groups (isobutyl and butyl) resulted in a decrease in both A_2B AdoR affinity and selectivity. This final SAR optimization led to the discovery of 1,3-dimethyl derivative 60, 8-(1-(3-(trifluoromethyl) benzyl)-1H-pyrazol-4-yl)-1,3-dimethyl xanthine, a high-affinity (K_i = 1 nM) A_2B AdoR antagonist with high selectivity (990-, 690-, and 1000-) for the human A₁, A_2A, and A₃ AdoRs.

Full text of DOI:10.1021/jm051268+

3682
J. Med. Chem. 2006, 49, 3682-3692
Novel 1,3-Disubstituted 8-(1-benzyl-1H-pyrazol-4-yl) Xanthines: High Affinity and Selective A2B
Adenosine Receptor Antagonists
Rao V. Kalla,*,† Elfatih Elzein,† Thao Perry,† Xiaofen Li,† Venkata Palle,† Vaibhav Varkhedkar,† Arthur Gimbel,‡ Tennig Maa,‡
Dewan Zeng,‡ and Jeff Zablocki†
Department of Bioorganic Chemistry, Department of Drug Research and Pharmacological Sciences, CV Therapeutics Inc., 3172 Porter DriVe,
Palo Alto, California 94304
ReceiVed December 20, 2005
Adenosine has been suggested to induce bronchial hyperresponsiveness in asthmatics, which is believed to
be an A2B adenosine receptor (AdoR) mediated pathway. We hypothesize that a selective, high-affinity A2B
AdoR antagonist may provide therapeutic benefit in the treatment of asthma. In an attempt to identify a
high-affinity, selective antagonist for the A2B AdoR, we synthesized 8-(C-4-pyrazolyl) xanthines. Compound
22, 8-(1H-pyrazol-4-yl)-1,3-dipropyl xanthine, is a N-1 unsubstituted pyrazole derivative that has favorable
binding affinity (Ki ) 9 nM) for the A2B AdoR, but it is only 2-fold selective versus the A1 AdoR. Introduction
of a benzyl group at the N-1-pyrazole position of 22 resulted in 19, which had moderate selectivity. The
initial focus of the SAR study was on the preparation of substituted benzyl derivatives of 19 because the
corresponding phenyl, phenethyl, and phenpropyl derivatives showed a decrease in A2B AdoR affinity and
selectivity relative to 19. The preferred substitution on the phenyl ring of 19 contains an electron-withdrawing
group, specifically F or CF3 at the m-position, as in 33 and 36 respectively, increases the selectivity while
retaining the affinity for the A2B AdoR. Exploring disubstitutions on the phenyl ring of derivatives 33 and
36 led to the 2-chloro-5-trifluoromethylphenyl derivative 50, which retained the A2B AdoR affinity but
enhanced the selectivity relative to 36. After optimization of the substitution on the 8-pyrazole xanthine,
1,3-disubstitution of the xanthine core was explored with methyl, ethyl, butyl, and isobutyl groups. In
comparison to the corresponding dipropyl analogues, the smaller 1,3-dialkyl groups (methyl and ethyl)
increased the A2B AdoR binding selectivity of the xanthine derivatives while retaining the affinity. However,
the larger 1,3-dialkyl groups (isobutyl and butyl) resulted in a decrease in both A2B AdoR affinity and
selectivity. This final SAR optimization led to the discovery of 1,3-dimethyl derivative 60, 8-(1-(3-
(trifluoromethyl) benzyl)-1H-pyrazol-4-yl)-1,3-dimethyl xanthine, a high-affinity (Ki ) 1 nM) A2B AdoR
antagonist with high selectivity (990-, 690-, and 1000-) for the human A1, A2A, and A3 AdoRs.
Introduction
information on human AdoR antagonists that influenced our
design. Theophylline 1, 1,3-dimethyl xanthine (Figure 1), is a
PDE IV inhibitor and a nonselective AdoR antagonist that has
a Ki of 9 µM for the A2B adenosine receptor (AdoR). Theo-
phylline is currently approved for use in the treatment of asthma
in both iv rescue therapy for acute asthma attacks and chronic
oral treatment.7-9 Theophylline has a low therapeutic index due
to both CNS and cardiac side effects. We hypothesize that a
more selective A2B AdoR antagonist devoid of PDE IV activity
may have an enhanced therapeutic index. Replacing the methyl
groups of 1 with propyl groups as in 7a (Scheme 1) increases
the A2B AdoR affinity (Ki ) 610 nM) without any enhancement
in selectivity.10 Enprofylline 2, a 3-propyl xanthine derivative,
has moderate affinity for the A2B AdoR (Ki ) 4.7 µM) and
also has moderate selectivity against the other AdoR subtypes
(Figure 1).10
Suzuki and co-workers have shown that substitution at the
8-position of xanthine with cycloalkyl groups increases the A1
AdoR affinity. For example, 8-cyclopentyl-1,3-dipropylxanthine,
3 (DPCPX), is a known A1 antagonist that also exhibits
considerable affinity for the A2B AdoR (Ki ) 56 nM, Figure
1).11 Several research groups have synthesized 8-phenyl sub-
stituted xanthines that have high A2B AdoR affinity and
selectivity against the other AdoR subtypes.12-14 Jacobson and
co-workers demonstrated that the introduction of a para-
substituted phenyl derivative at the 8-position of the xanthine
core increases the A2B AdoR affinity and selectivity against the
other AdoRs as illustrated by 4.13,14
Adenosine is an endogenous nonselective agonist that acti-
vates all four subtypes of adenosine receptors (AdoRs): A1,
A
2A, A2B, and A3.1 Adenosine has been implicated to play a
role in inflammatory airway diseases such as asthma.2 High
adenosine levels are observed in the bronchoalveolar lavage
(BAL) fluid and in exhaled breath condensate of asthmatics
compared to those of normal controls.3 It is believed that the
activation of the A2B AdoR on human lung mast cells leads to
mast cell degranulation, releasing inflammatory cytokines (IL-
4, IL-8, and IL-13).4 It has also been shown that the A2B AdoR
subtype is the predominant AdoR expressed in bronchial smooth
muscle cells (BSMC), and its activation increases the expression
and release of interleukin-6 (IL-6) and monocytic chemotactic
peptide-1 (MCP-1).5a The presence and functional coupling of
human A2B AdoRs in different peripheral blood cells that play
a role in immune and inflammatory process in which A2B AdoRs
are thought to be involved have been recently characterized.5b
Therefore, we choose to explore the potential of selective, high-
affinity A2B adenosine receptor (AdoR) antagonists in the
treatment of asthma.6
Prior to a description of our approach to obtain high-affinity
A2B AdoR antagonists, we described relevant background
* To whom correspondence should be addressed. Tel: (650) 384-8568.
Fax: (650) 858-0390. E-mail: rao.kalla@cvt.com.
† Department of Bioorganic Chemistry.
‡ Department of Drug Research and Pharmacological Sciences.
10.1021/jm051268+ CCC: $33.50 © 2006 American Chemical Society
Published on Web 05/20/2006

SelectiVe A2B Adenosine Receptor Antagonists
Journal of Medicinal Chemistry, 2006, Vol. 49, No. 12 3683
Figure 1. Representative structures of xanthine classes that have affinity for the A2B AdoR.
Scheme 1a
as A2B AdoR antagonists, our main focus still remains on the
xanthine class.
Even though several A2B antagonists are known in the
literature with high affinity, there are very few A2B AdoR
antagonists known with good affinity and selectivity.16 Herein,
we report the exploration of the 8-pyrazolyl xanthine derivatives
represented by I (Figure 1) as a new class of adenosine receptor
antagonists with the goal of achieving high affinity for the A2B
AdoR and selectivity over the other AdoRs. To our knowledge,
there were no examples of 8-pyrazolyl xanthines evaluated as
A2B AdoR antagonists in the literature prior to our explora-
tion.22,23 Recently, Baraldi and co-workers have reported the
8-(5-pyrazolyl)-xanthines represented by structure 524 that
typically contain the amide functionality found in the MRS-
1754 (4)14 class of compounds. Our approach to the discovery
of a selective, high-affinity A2B AdoR antagonist through the
preparation of 8-(4-pyrazolyl)-xanthines was guided by a
systematic optimization of the SAR.
Chemistry
a Reagents: (a) CH(OC2H5)3, 70 °C; (b) PhCH2Br, K2CO3, DMF, 80
°C, 90%; (c) NCS, THF, r.t., 75%; (d) NaH, DMF, pyrazole, 75 °C, 70%
(8), 90% (9); (e) Pd(OH)2, cyclohexene, ethanol, 80 °C, 18 h, 20%.
The 8-pyrazolyl xanthine derivatives were synthesized fol-
lowing the synthetic routes illustrated in Schemes 1-4. The
8-(N-1-pyrazolyl) derivatives 8-10 were prepared as shown in
Scheme 1. The 1,3-dipropyl-5,6-diaminouracil (6) was synthe-
sized from 1,3-dipropyl urea following previously described
methods.12 Diamine 6 was treated with triethylorthoformate to
obtain 1,3-dipropyl xanthine (7a).12 The N-7 position of xanthine
derivative 7a was protected by reacting with benzyl bromide
In the search for a nonxanthine class of compounds as A2B
AdoR antagonists, several classes of compounds have been
investigated as AdoR antagonists: adenines, 9-deazaadenines,
8-azadenines, quinoxalines, and pyrazolo[1,5-a]pyridines.15-21
Although many new heterocyclic classes are being discovered

3684 Journal of Medicinal Chemistry, 2006, Vol. 49, No. 12
Kalla et al.
Scheme 2a
1-pyrazolyl)-1,3-dipropyl xanthine 10 was obtained by deben-
zylation of 9 using Pearlman’s catalyst (Pd(OH)2) under transfer
hydrogenation conditions.
The 8-(4-pyrazolyl) xanthine derivatives 11-21 were syn-
thesized as illustrated in Scheme 2. Diamine 6 was selectively
acylated at the 5-position by coupling with the corresponding
pyrazole acids using 1-[3-(dimethylamino)propyl]-3-ethylcar-
bodiimide hydrochloride (EDCI) to furnish the amides followed
by base induced cyclization to yield the corresponding 8-pyra-
zolyl xanthines 11-21 (Table 1).
A second more convergent route was developed for the
synthesis of 8-(4-pyrazolyl)xanthine derivatives that allows for
N-1-pyrazolyl substitution (Scheme 3) through a common late
intermediate. The N-7 position of benzyl derivative 19 was
protected with SEM-Cl using K2CO3 in dimethylformamide to
provide 19a. Debenzylation of derivative 19a using Pearlman’s
catalyst furnished the 8-(N-1H-pyrazol-4-yl) derivative 19b with
the N-7 position SEM protected. The treatment of 19b with 3N
HCl in ethanol furnished the unprotected derivative, 8-(1H-
pyrazol-1-yl)-1,3-dipropyl xanthine (22). The unsubstituted
pyrazole derivative 19b was alkylated with various substituted
benzyl halides using standard alkylation conditions to furnish
a Reagents: (a) R-COOH, EDCI, MeOH, r.t., 16 h, 70-85%; (b) MeOH,
10% NaOH, 80 °C; 5 h, 60-80%.
to yield 7b, followed by treatment with N-chlorosuccinamide
to furnish 8-chloro-7-benzyl-1,3-dipropyl xanthine (7c). The
8-chloro derivative 7c was converted to the 8-(N-1-pyrazolyl)
derivatives 8 and 9 by reacting with the corresponding pyrazole
anions generated by treating with sodium hydride. The 8-(N-
Scheme 3a
a Reagents: (a) SEM-Cl, K2CO3, DMF, 80 °C, 18 h; (b) Pd(OH)2, Cyclohexene, EtOH, 80 °C, 48 h; (c) 3N HCl, EtOH, 70 °C; (d) i)R1-Ph-CH2Br(Cl),
K2CO3, DMF, 80 °C; ii) 3 N HCl, EtOH, 70 °C.
Scheme 4a
a Reagents: (a) ethylcyanoacetate, NaOEt, 70 °C; (b) NaNO2, CH3COOH/H2O (1:1), 70 °C, 1 h; (c) Na2S2O4, 15% NH4OH, 70 °C, 1 h; (d) EDCI,
MeOH, r.t., 16 h; (e) MeOH, 10% NaOH, 80 °C, 5 h.

SelectiVe A2B Adenosine Receptor Antagonists
Journal of Medicinal Chemistry, 2006, Vol. 49, No. 12 3685
Table 1. Exploration of 8-Pyrazolyl Xanthines as Ligands for the A2B Adenosine Receptors
b
compd
R
R1
R2
Ki nMa (A2B
)
8
9
benzyl
benzyl
H
H
H
H
H
H
H
H
I
H
H
H
H
CH3
CH3
H
H
CH3
>5800
>5800
>5800
3700
2100
1100
10
11
12
13
14
15
16
17
18
22
CH3
CH2CH3
Ph
Ph
Ph
Ph
Ph
4-Cl-Ph
H
310
>4100
>6600
>6600
>6600
9
CH2CH2CH3
H
H
H
CF3
CF3
H
a The 95% Confidence intervals are generally within 15% of the mean value. b The binding affinity for the A2B AdoR was determined by the competition
3
for the binding sites labeled by H-ZM241385 (14 nM) in membranes prepared from HEK-A2B cells.
Table 2. Adenosine Receptor Binding Affinities of 8-(N-1-Substituted
Pyrazol-4-yl) Xanthines
the SEM-protected derivatives followed by deprotection with
3N HCl to yield the 8-(1-substituted benzyl-pyrazol-4-yl)-1,3-
dipropyl xanthines 23-52 in good to excellent yields (Scheme
3).
The N-1 and N-3 disubstituted 8-(4-pyrazolyl)xanthine de-
rivatives were synthesized as illustrated in Scheme 4. The
symmetrically substituted ureas, 53a-d were treated with
ethylcyanoacetate in the presence of sodium ethoxide to provide
the corresponding substituted 6-amino uracils 54a-d in good
yields. Nitrosation of the 6-amino uracil derivatives 54a-d was
achieved by the slow addition of solid sodium nitrite to a
solution of the amino uracil in 50% aqueous acetic acid. The
nitroso uracils 55a-d were reduced with Na2S2O4 in 15%
ammonium hydroxide to furnish diamine derivatives 56a-d.
The coupling of the diamine with substituted pyrazole acids
57a-c followed by base-induced cyclization furnished the
N-1and N-3 disubstituted 8-pyrazole xanthines 58-67 following
the conditions described above (Scheme 2).
Ki nMa
A2B selectivity
b
c
d
compd
R
A2B
A1
A2A
A1/A2B
A2A/A2B
14
22
19
20
21
phenyl
310
9
11
74
100
770
20
76
17
77
560
230
290
570
220
2
2
H
2
7
0.2
0.7
23
26
7
benzyl
phenethyl
phenylpropyl
2
a The 95% Confidence intervals are generally within 15% of the mean
value. b The binding affinity for the A2B AdoR was determined by the
competition for the binding sites labeled by 3H-ZM241385 (14 nM) in
membranes prepared from HEK-A2B cells. c The binding affinity for the
A1 AdoR was determined by the competition for the binding sites labeled
by 3H-CPX (0.5 nM) in membranes prepared from CHO-A1 cells. d The
binding affinity for the A2A AdoR was determined by the competition for
the binding sites labeled by 3H-ZM241385 (2 nM) in membranes prepared
from HEK-A2A cells.
Results and Discussion
A series of 8-pyrazolyl xanthines were prepared from
commercially available pyrazole acids with the goal of rapidly
exploring the SAR with respect to A2B AdoR affinity (Table
1). The 8-(N-1-pyrazolyl) xanthine derivatives 8-10 demon-
strated low affinity for the A2B AdoR. This suggests that this
mode of pyrazole attachment is not conducive for binding to
the A2B AdoR receptor. Also, 8-(C-5-pyrazolyl) derivatives 11-
13 demonstrated weak binding affinities (in the range of
1-3 µM) for the A2B AdoR. The incorporation of a C-4-pyrazole
ring at the 8-position of the xanthine as in 22 resulted in high
affinity for the A2B AdoR (9 nM). The effect of the addition
of a N-1 phenyl ring onto 22, as in 14, lowered the A2B AdoR
affinity to 310 nM, but the 4-pyrazolyl moiety had a higher
affinity than that of the corresponding 5-pyrazolyl moiety, as
in 13. The substitution at the 5-position of the pyrazole ring
of 14 either with electron-donating groups (EDG), such as
methyl (15) and propyl (16), or with electron-withdrawing
(EWG) groups, such as trifluoromethyl (17 and 18), results in
a decrease in the A2B AdoR affinity. From our brief survey of
commercially available pyrazole acids, 4-pyrazolyl analogue 22
showed the highest affinity for the A2B AdoR. Therefore, we
chose to further explore the 8-(pyrazol-4-yl) xanthine class in
search of analogues with both high affinity and selectivity for
the A2B AdoR.
An evaluation of the binding selectivity of 4-pyrazolyl
analogue 22 versus that of other AdoR subtypes, such as A1
and A2A, is shown in Table 2. Compound 22 was found to have
modest selectivity over A2A (23-fold) and low selectivity over
A1. Furthermore, N-1-phenyl-substituted derivative 14 demon-
strated low selectivity against both A1 and A2A AdoRs. To
increase the selectivity, we had to explore various substitutions
on 22. The two options for increasing the binding selectivity
are to vary the substitution either at the N-1-position of the
pyrazole or at the N-1 and N-3 positions of the xanthine.
Initially, we explored the substitution at the N-1-position of
pyrazole 22. The introduction of a benzyl group at the N-1
position of the pyrazole ring of 22 resulted in compound 19,
which displayed slightly enhanced selectivity over the A1 AdoR
(Table 2). Increasing the distance between the phenyl group

3686 Journal of Medicinal Chemistry, 2006, Vol. 49, No. 12
Kalla et al.
Table 4. Effect of Symmetric Substitution on the N-1 and N-3
Table 3. Adenosine Receptor Binding Affinities of the Substituted
Phenyls of 8-Pyrazol-4-yl Xanthine Derivatives
Positions of 8-Pyrazolyl Xanthine Derivatives
A2B
selectivity
Ki nMa
A2B
selectivity
Ki nMa
compd
R
R1
A2B
A1
A2A
A1/A2B
A2A/A2B
b
c
d
b
c
d
compd
R1
A2B
A1
A2A
A1/A2B 2A/A2B
A
19
33
36
58
59
60
61
62
63
64
65
66
67
propyl
propyl
propyl
methyl
methyl
H
F
11
14
76
170
170
290
230
400
7
26
17
28
1
7
690
6
58
34
1
1
1
13
12
2
17
990
30
76
44
1
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
2-CH3
3-CH3
4-CH3
33
36
40
37
34
37
20
18
18
28
14
22
37
14
20
240
700
390
35
58
44
640
200
350
22
48
38
59
34
70
59
31
51
66
80
140
70
61
49
79
88
0.6
1
2
2
CF3 14
H
F
2300 >6000 3500
27
1
19
5
1
2
460
990
580
380
570
1400
200
690
120
290
450
890
1100
1300
4300
2-OCH3
3-OCH3
4-OCH3
2-Cl
2
4
methyl CF3
1
2
ethyl
ethyl
ethyl
butyl
ibutyl
ibutyl
ibutyl
H
F
2
1
3
6
CF3 13
3-Cl
4-Cl
2-F
3-F
4-F
2-CF3
3-CF3
4-CF3
2
4
H
H
F
980
1250 620
990
7
19
0.5
0.6
13
1
4
2
18
5
0.5
1
10
60
830
3100
170
23
230
120
230
400
150
nd
nd
nd
280
870
460
nd
CF3 300
14
a The 95% Confidence Intervals are generally within 15% of the mean
value. b The binding affinity for the A2B AdoR was determined by the
competition for the binding sites labeled by 3H-ZM241385 (14 nM) in
membranes prepared from HEK-A2B cells. c The binding affinity for the
A1 AdoR was determined by the competition for the binding sites labeled
by 3H-CPX (0.5 nM) in membranes prepared from CHO-A1 cells. d The
binding affinity for the A2A AdoR was determined by the competition for
the binding sites labeled by 3H-ZM241385 (2 nM) in membranes prepared
from HEK-A2A cells.
44
1
6
170
41
12
2
27
7
2,3-di-F
2,4-di-F
2,6-di-F
3,4-di-F
2-F,3-Cl
2-F-3-CH3
3-CF3-4-F
3-CF3-4-Cl
3,5-di-CF3
3-CF3-4-OCH3 410
2,5-di-Cl
2-Cl-5-F
2-Cl-5-CF3
2-F-3-Cl-5-CF3
2,4,6-tri-F
nde
nd
nd
nd
nd
nd
7
nd
nd
nd
8
170
530
390
nd
9
9
13
10
nd
nd
nd
nd
2
nd
nd
nd
nd
6
nd
nd
nd
nd
nd
nd
70
AdoRs. Similar to 33, the m-CF3 derivative 36 retained the A2B
affinity (Ki ) 14 nM) and showed an increase in selectivity
relative to 19, the unsubstituted benzyl analogue (Table 3). Once
again the o-CF3 and p-CF3 derivatives 35 and 37 did not impart
any binding selectivity just as their corresponding o- and p-F
derivatives 32 and 34.
29
190
4800 nd
nd
nd
54
nd
nd
nd
63
nd
nd
22
360
650
1200 1400
nd
nd
nd
nd
a The 95% Confidence Intervals are generally within 15% of the mean
value. b The binding affinity for the A2B AdoR was determined by the
competition for the binding sites labeled by 3H-ZM241385 (14 nM) in
membranes prepared from HEK-A2B cells. c The binding affinity for the
A1 AdoR was determined by the competition for the binding sites labeled
by 3H-CPX (0.5 nM) in membranes prepared from CHO-A1 cells. d The
binding affinity for the A2A AdoR was determined by the competition for
the binding sites labeled by 3H-ZM241385 (2 nM) in membranes prepared
from HEK-A2A cells. e Not determined.
Encouraged by the enhanced binding selectivity and retained
A2B AdoR affinity of m-F and m-CF3 derivatives 33 and 36,
respectively, we decided to look at the disubstituted benzyl
analogues of 19 with an emphasis on EWGs. In general, various
combinations of difluoro analogues 38-41 showed diminished
affinity for the A2B AdoR with the exception of 3,4-difluoro
analogue 41 (Ki ) 35 nM), which still had lower affinity and
selectivity than the monosubstituted analogue 33. Disubstituted
analogues 42 and 43, which contain a 2-F substituent, were
found to have slightly less favorable A2B AdoR affinity and
selectivity than that of 33 (Table 3). In compounds 44-47, the
m-CF3 group was retained, and the effect of an additional EWG
(44-46) and EDG (47) substituent on the A2B AdoR affinity
was found to be less favorable than 36 (Table 3). We explored
the effect of 2,5-disubstituted EWGs on A2B AdoR affinity (48-
50) with 2-chloro-5-trifluoromethyl derivative 50 resulting in
high A2B AdoR affinity (Ki ) 22 nM) and good selectivity (A1/
A2B ) 50 and A2A/A2B ) 65) (Table 3). Trisubstituted analogues
51 and 52 demonstrated reduced affinity for the A2B AdoR
relative to that of unsubstituted derivative 19.
and the N-1 position of the pyrazole ring from one carbon atom
to two and three carbon atoms, as in compounds 20 and 21,
resulted in a decrease in affinity and selectivity for the A2B AdoR
relative to 19 (Table 2). Therefore, we chose to further
investigate the effect of introducing electron-donating (EDG)
and electron-withdrawing groups (EWG) on the phenyl ring of
moderately selective 19 with the goal of enhancing its A2B
affinity and selectivity (Table 3). Derivatives with electron-
donating groups, such as CH3 (23-25) and OCH3 (26-28),
showed similar affinity at the A1, A2A, and A2B adenosine
receptors, regardless of the position of the substitution (ortho,
meta, or para). Introducing a chloro group, a moderate EWG
in compounds 29-31, also resulted in good binding affinity
for all of the AdoR subtypes. However, m-F derivative 33
retained the A2B AdoR affinity (Ki ) 14 nM) and had higher
binding selectivity for the A2B AdoR compared to that of
compound 19 (Table 3). The corresponding o- and p-F analogues
32 and 34 showed similar affinity for both the A1 and A2B
After optimization of the phenyl substitution of compound
19, which led to 33 and 36, the effect of varying the dialkyl
substitution of the xanthine core N-1 and N-3 on A2B AdoR
affinity and selectivity was evaluated (Table 4). We retained
the m-F benzyl and m-CF3 benzyl N-1-pyrazolyl groups because
these substitutions imparted good affinity and selectivity in the
N-1,3-dipropyl derivatives 33 and 36, respectively, compared

SelectiVe A2B Adenosine Receptor Antagonists
Journal of Medicinal Chemistry, 2006, Vol. 49, No. 12 3687
chromatography. Proton NMR (1H NMR) spectra were recorded
on a Varian Gemini-400 spectrometer (400 MHz). Chemical shifts
are reported in δ units (parts per million) downfield from tetram-
ethylsilane and are assigned as singlets (s), doublets (d), doublet
of doublets (dd), triplets (t), quartet (q), and multiplets (m). Coupling
constants (J) are reported in Hertz (Hz). Mass spectra (MS) were
recorded on Micromass LCZ. Elemental analysis data for final
compounds were obtained from Desert Analytics and were within
(0.4% of the theoretical values for the formulas given.
Table 5. A1, A2A, A2B, and A3 AdoR Affinity and A2B Selectivity of
Selective A2B Antagonists
A2B
selectivity
Ki nMa
b
c
d
e
compd A2B
A1
A2A
A3
23
A1/A2B
A
2A/A2B A3/A2B
22
19
33
36
50
60
9
20
76
230
290
230
400
2
7
13
12
54
990
23
26
17
28
63
690
2
15
4
10
129
1000
11
14
14
22
1
170
56
150
170
170
7-N-Benzyl-1,3-dipropyl-8-(N-1-pyrazolyl)xanthine (8). To a
solution of 7a in DMF, K2CO3 was added followed by benzyl
bromide, and the reaction mixture was heated at 70 °C for 16 h.
K2CO3 was filtered off, concentrated in vacuo, purified by column
chromatography (EtOAc/Hexane 1:3) to furnish the benzylated
derivative 7-N-benzyl-1,3-dipropylxanthine (7b) in 90% yield. A
mixture of 7b (7.0 g, 21.4 mmol) and N-chlorosuccinimide (4.28
g, 32.2 mmol) in 125 mL of THF was stirred under nitrogen at
room temperature for 24 h. The reaction mixture was concentrated
in vacuo, and the residue was partitioned between EtOAc and H2O.
The aqueous layer was extracted with EtOAc. The combined
organics were dried over MgSO4 and concentrated in vacuo to
afford 7c as an oil. Purification by silica gel column chromatography
(EtOAc/Hexane 1:5) gave 5.0 g of 7-N-benzyl-8-chloro-1,3-
dipropylxanthine (7c). A suspension of NaH (60% dispersion in
mineral oil) (467 mg, 11.7 mmol) was washed with hexane (10
mL), and then, DMF (10 mL) was added. To this suspension, the
pyrazole (797 mg, 11.7 mmol) in 10 mL of DMF was slowly added.
The reaction mixture was stirred at room temperature until all
bubbles subsided (10 min). Then, compound 7c (420 mg, 1.17
mmol) was added, and the resulting reaction mixture was heated
at 70 °C for 4 h. The reaction mixture was concentrated in vacuo.
The residue was purified by preparative TLC by eluting with CH2-
Cl2/MeOH (10:1) to provide 130 mg (28%) of the desired product.
1H NMR (CDCl3): δ 8.16 (s, 1H), 7.80 (s, 1H), 7.36-7.15 (m,
5H), 6.50 (s, 1H), 6.15 (s, 2H), 4.15-3.80 (m, 4H), 1.80-1.60
(m, 4H), 1.05-0.85 (m, 6H); MS m/z 392.9 (M + H)+.
7-N-Benzyl-1,3-dipropyl-8-[N-1-(4-iodo)pyrazolyl]xanthine (9).
Following the above procedure, the displacement of the 8-chloro
group with 4-iodo pyrazole furnished the desired product 9 in 92%
yield. 1H NMR (400 MHz, CDCl3): δ 8.22 (s, 1 H), 7.79 (s, 1 H),
7.16-7.27 (m, 6.5 H), 6.07 (s, 2 H), 4.01 (t, J ) 8.0 Hz, 2 H),
3.97 (t, J ) 8.0 Hz, 2 H), 1.74-1.80 (m, 2 H), 1.64-1.72 (m, 2
H),) 0.98 (t, J ) 8.0 Hz, 3 H), 0.96 (t, J ) 8.0 Hz, 3 H); MS m/z
518.8 (M + H)+.
1,3-Dipropyl-8-(N-1-pyrazolyl)xanthine (10). A mixture of 9
(100 mg, 0.255 mmol), palladium hydroxide (20 wt % Pd on
carbon) (137 mg, 0.979 mmol), and cyclohexene (9 mL) in EtOH
(15 mL) was heated at 80 °C for 72 h. The reaction mixture was
filtered through Celite and concentrated in vacuo. The resulting
solid was purified by preparative TLC (EtOAc/Hexane 1:5) to
provide 13 mg of 10 as a white solid, and 55 mg of starting material
9 was recovered (55% yield based on recovered starting material).
1H NMR (CD3OD): δ 8.12 (s, 1H), 7.02 (s, 1H), 6.35 (s, 1H),
4.15-3.80 (m, 4H), 1.80-1.50 (m, 4H), 0.95-0.75 (m, 6H); MS
m/z 302.9 (M + H)+.
1200 1400 2800
990 690 1000
a The 95% Confidence Intervals are generally within 15% of the mean
value. b The binding affinity for the A2B AdoR was determined by the
competition for the binding sites labeled by 3H-ZM241385 (14 nM) in
membranes prepared from HEK-A2B cells. c The binding affinity for the
A1 AdoR was determined by the competition for the binding sites labeled
by 3H-CPX (0.5 nM) in membranes prepared from CHO-A1 cells. d The
binding affinity for the A2A AdoR was determined by the competition for
the binding sites labeled by 3H-ZM241385 (2 nM) in membranes prepared
from HEK-A2A cells. e The binding affinity for A3 AdoR was determined
125
using CHO-A3 cells with
I-AB-MECA as the radioligand.
with those of other substitution patterns. For comparison
purposes, the corresponding unsubstituted benzyl derivatives
were prepared as well. The 1,3-dimethyl xanthine derivative
58 with the unsubstituted benzyl derivative had low affinity for
the AdoRs, whereas the corresponding m-F benzyl derivative
59 had good binding affinity for the A2B receptor and favorable
selectivity (Table 4). The 1,3-dimethyl xanthine m-CF3 deriva-
tive 60 has very favorable A2B AdoR binding affinity (Ki ) 1
nM) and selectivity (A1/A2B ) 990; A2A/A2B ) 690) over the
those of other AdoRs. 1,3-Diethyl xanthine derivatives 61-63
have comparable A2B AdoR affinity similar to that of the
dipropyl derivatives (19, 33, and 36) but displayed higher A2B
AdoR binding selectivity. Increasing the N-1 and N-3 alkyl chain
length to n-butyl or iso-butyl groups, as in 64 and 65-67,
respectively, resulted in lower A2B AdoR receptor affinity
compared to those of the corresponding dipropyl derivative
analogues (Table 4). The A3 AdoR binding affinity of selected
A2B AdoR antagonists was evaluated (Table 5). Compounds 50
and 60, which showed good A2B AdoR binding affinity and
selectivity versus A1 and A2A AdoR, have displayed low affinity
for the A3 AdoR, 2800 and 1000 nM, respectively. Thus, 60
has high affinity (Ki ) 1 nM) and selectivity (A1/A2B ) 990;
A
2A/A2B ) 690 A3/A2B ) 1000) for the A2B AdoR over the
other AdoRs.
Conclusion
A systematic optimization of 8-(pyrazol-4-yl) xanthine de-
rivatives led to the identification of several selective, high-
affinity antagonists for the A2B AdoR subtype. In particular, to
our knowledge, 8-(3-trifluoromethylbenzyl-1H-pyrazol-4-yl)-
1,3-dimethylxanthine 60 has the highest affinity and selectivity
described thus far. This selective A2B antagonist 60 can be useful
in understanding the physiological role of the A2B AdoR and
may serve as a lead toward the discovery of therapeutically
useful agents for asthma.
General Procedure for the Synthesis of Compounds 11-21.
A mixture of diamine 6 (9.42 g, 41.7 mmol), pyrazole acid (8.0 g,
39.6 mmol), and 1-[3-(dimethylamino)propyl]-3-ethylcarbodiimide
hydrochloride (7.6 g, 39.6 mmol) were dissolved in methanol (100
mL) and stirred at room temperature for 16 h. The precipitate was
filtered off and washed with water. The uncyclized product was
dissolved in methanol (20 mL) and 10% aqueous NaOH (20 mL)
and heated at 100 °C for 2 h. Methanol was evaporated, and the
residue was dissolved in water and acidified with 6 N HCl to pH
3-4. The precipitate thus formed was collected and washed with
water and methanol and dried to furnish xanthine derivatives 11-
21 in 60-80% yield.
Experimental Section
Commercial chemicals and solvents were of reagent-grade and
were used without further purification. The following abbreviations
are used for reagents and solvents: DCM, dichloromethane; DMF,
dimethyl formamide; DMSO, dimethyl sulfoxide; EtOAc, ethyl
acetate; Hex, hexane; EtOH, ethanol; and MeOH, methanol.
Whatman silica gel (60 A0, 230-400 mesh) was used for flash
column chromatography. Analtech thin-layer chromatography plates
(20 × 20 cm, 2000 microns) were used for preparative thin-layer
8-(1,3-Dimethyl-1H-pyrazol-5-yl)-1,3-dipropyl-1H-purine-2,6-
(3H,7H)-dione (11). Following the general procedure described
above, the coupling of diamine 6 with 1,3-dimethylpyrazole-5-
carboxylic acid and cyclization furnished xanthine derivative 11
1
in 60% yield. H NMR (DMSO-d6): δ 6.75 (s, 1H), 4.13 (s, 3H),

3688 Journal of Medicinal Chemistry, 2006, Vol. 49, No. 12
Kalla et al.
1
4.01 (t, J ) 8.0 Hz, 2H), 3.86 (t, J ) 8.0 Hz, 2H), 2.18 (s, 3H),
1.80-1.70 (m, 2H), 1.62-1.52 (m, 2H), 0.89 (t, J ) 8.0 Hz, 3H),
0.87 (t, J ) 8.0 Hz, 3H); MS m/z 331.38 (M + H)+.
furnished xanthine derivative 19 in 80% yield. H NMR (DMSO-
d6): δ 13.51 (s, 1 H), 8.45 (s, 1 H), 8.08 (s, 1 H), 7.38-7.27 (m,
5 H), 5.39 (s, 2 H), 3.96 (t, J ) 8.0 Hz, 2 H), 3.83 (t, J ) 8.0 Hz,
2 H), 1.75-1.64 (m, 2 H), 1.60-1.50 (m, 2 H), 0.87 (t, J ) 8.0
Hz, 3 H), 0.85 (t, J ) 8.0 Hz, 3 H); MS m/z 393.37 (M + H)+;
Anal. (C21H24N6O2‚0.75H2O): C,H,N.
8-(1-Ethyl-3-methyl-1H-pyrazol-5-yl)-1,3-dipropyl-1H-purine-
2,6(3H,7H)-dione (12). The coupling of diamine 6 and 1-ethyl-3-
methyl-pyrazole-5-carboxylic acid as described above furnished 12
1
in 70% yield. H NMR (DMSO-d6): δ 6.74 (s, 1H), 4.60 (q, J )
8-(1-Phenethyl-1H-pyrazol-4-yl)-1,3-dipropyl-1H-purine-2,6-
(3H,7H)-dione (20). Following the general procedure described
above, the coupling of diamine 6 with 1-phenethyl-pyrazole-4-
carboxylic acid furnished xanthine derivative 20 in 75% yield. 1H
NMR (DMSO-d6): δ 8.25 (s, 1H), 8.07 (s, 1H), 7.40-7.15 (m,
5H), 4.41 (t, J ) 8.0 Hz, 2H), 3.96 (t, J ) 8.0 Hz, 2H), 3.84 (t, J
) 8.0 Hz, 2H), 3.13 (t, J ) 8.0 Hz, 2H), 1.80-1.50 (m, 4H), 0.88
(t, J ) 8.0 Hz, 3H), 0.86 (t, J ) 8.0 Hz, 3H); MS m/z 405.28 (M
- H)+; Anal. (C22H26N6O2): C,H,N.
8-(1-(3-Phenylpropyl)-1H-pyrazol-4-yl)-1,3-dipropyl-1H-pu-
rine-2,6(3H,7H)-dione (21). Following the general procedure
described above, the coupling of diamine 6 with 1-(3-phenylpropyl)-
pyrazole-4-carboxylic acid furnished xanthine adduct 21 in 75%
yield. 1H NMR (Methanol-d4): δ 8.10 (s, 1 H), 8.04 (s, 1 H), 7.30-
7.11 (m, 5 H), 4.17 (t, J ) 8.0 Hz, 2 H), 4.05 (t, J ) 8.0 Hz, 2 H),
3.93 (t, J ) 8.0 Hz, 2H), 2.61 (t, J ) 8.0 Hz, 2 H), 2.26-2.15 (m,
2 H), 1.84-1.72 (m, 2 H), 1.71-1.59 (m, 2 H), 0.96 (t, J ) 8.0
Hz, 3 H), 0.92 (t, J ) 8.0 Hz, 3 H); MS m/z 418.9 (M - H)+;
Anal. (C23H28N6O2): C,H,N.
8.0 Hz, 2H), 4.00 (t, J ) 8.0 Hz, 2H), 3.86 (t, J ) 8.0 Hz, 2H),
2.18 (s, 3H), 1.73 (q, J ) 8.0 Hz, 2H), 1.57 (q, J ) 8.0 Hz, 2H),
1.32 (t, J ) 8.0 Hz, 3H), 0.89 (t, J ) 8.0 Hz, 3H), 0.87 (t, J ) 8.0
Hz, 3H); MS m/z 345.40 (M + H)+.
8-(1-Phenyl-1H-pyrazol-5-yl)-1,3-dipropyl-1H-purine-2,6-
(3H,7H)-dione (13). The coupling of diamine 6 and 1-phenyl-
pyrazole-5-carboxylic acid as described above furnished xanthine
derivative 13 in 70% yield. 1H NMR (DMSO-d6): δ 7.82 (s, 1H),
7.60-7.40 (m, 5H), 7.12 (s, 1H), 3.82 (t, J ) 8.0 Hz, 2H), 3.64 (t,
J ) 8.0 Hz, 2H), 1.60-1.50 (m, 2H), 1.50-1.30 (m, 2H), 0.85 (t,
J ) 8.0 Hz, 3H), 0.68 (t, J ) 8.0 Hz, 3H); MS m/z 379.42 (M +
H)+.
8-(1-Phenyl-1H-pyrazol-4-yl)-1,3-dipropyl-1H-purine-2,6-
(3H,7H)-dione (14). The coupling of diamine 6 and 1-phenyl-
pyrazole-4-carboxylic acid as described above furnished xanthine
derivative 14 in 80% yield. 1H NMR (DMSO-d6): δ 9.04 (s, 1H),
8.36 (s, 1H), 7.95-7.35 (m, 5H), 4.03 (t, J ) 8.0 Hz, 2H), 3.86 (t,
J ) 8.0 Hz, 2H), 1.85-1.55 (m, 4H), 0.93 (t, J ) 8.0 Hz, 3H),
0.90 (t, J ) 8.0 Hz, 3H); MS m/z 379.43 (M + H)+.
General Procedure for the Synthesis of Compounds 22-54.
A mixture of 19 (2.5 g, 6.38 mmol), potassium carbonate (4.40 g,
31.9 mmol) in DMF (70 mL) was stirred at room temperature under
nitrogen. To this mixture was added dropwise 2-(trimethylsilyl)-
ethoxymethyl chloride (5.33 g, 31.9 mmol). The resulting reaction
mixture was stirred overnight at room temperature, and K2CO3 was
filtered off. The filtrate was concentrated in vacuo, and the resulting
oil was purified by silica gel column chromatography (EtOAc/
Hexane 1:3) to give 3.0 g (90%) of the product, 19a. A mixture of
SEM-protected benzyl derivative 19a (3.0 g, 5.74 mmol), palladium
hydroxide (20 wt % Pd on carbon) (5.0 g), and cyclohexene (50
mL) in EtOH (100 mL) was heated at 80 °C for 2 days. The reaction
mixture was filtered through Celite and concentrated in vacuo. The
residue was purified by silica gel column chromatography (EtOAc/
Hexane 1:3) to provide 1.0 g (40%) of debenzylated derivative 19b
as a white solid. A mixture of SEM-protected debenzylated 19b
(80 mg, 0.185 mmol), alkyl halide (1.85 mmol), and K2CO3 (255
mg, 1.85 mmol) in DMF (4 mL) was stirred overnight at room
temperature, and K2CO3 was filtered off. The filtrate was concen-
trated in vacuo and was purified by preparative TLC. The product
was dissolved in EtOH and treated with 2 N HCl (3 mL) at 100 °C
for 2 h. The reaction mixture was concentrated in vacuo. The residue
was washed with ether to provide the title compounds in 50-82%
yield.
8-(5-Methyl-1-phenyl-1H-pyrazol-4-yl)-1,3-dipropyl-1H-pu-
rine-2,6(3H,7H)-dione (15). Following the general procedure
described above, the coupling of diamine 6 with 5-methyl-1-phenyl-
pyrazole-4-carboxylic acid furnished xanthine derivative 15 in
1
65% yield. H NMR (DMSO-d6): δ 8.34 (s, 1H), 7.70-7.50 (m,
5H), 4.04 (t, J ) 8.0 Hz, 2H), 3.90 (t, J ) 8.0 Hz, 2H), 2.70 (s,
3H), 1.85-1.55 (m, 4H), 1.00-0.80 (m, 6H); MS m/z 393.45
(M + H)+.
8-(1-Phenyl-5-propyl-1H-pyrazol-4-yl)-1,3-dipropyl-1H-purine-
2,6(3H,7H)-dione (16). Following the general procedure described
above, the coupling of diamine 6 with 1-phenyl-5-propyl-pyrazole-
1
4-carboxylic acid furnished 16 in 70% yield. H NMR (DMSO-
d6): δ 8.36 (s, 1H), 7.75-7.45 (m, 5H), 4.03 (t, J ) 8.0 Hz, 3H),
3.89 (t, J ) 8.0 Hz, 3H), 3.07 (t, J ) 8.0 Hz, 2H), 1.90-1.50 (m,
6H), 1.00-0.80 (m, 9H); MS m/z 421.50 (M + H)+.
8-(5-(Trifluoromethyl)-1-phenyl-1H-pyrazol-4-yl)-1,3-dipro-
pyl-1H-purine-2,6(3H,7H)-dione (17). Following the general
procedure described above, the coupling of diamine 6 with
5-trifluoromethyl-1-phenyl-pyrazol-4-carboxylic acid furnished xan-
thine derivative 17 in 75% yield. 1H NMR (DMSO-d6): δ 8.38 (s,
1H), 7.80-7.60 (m, 5H), 4.01 (t, J ) 8.0 Hz, 2H), 3.89 (t, J ) 8.0
Hz, 2H), 1.80-1.50 (m, 5H), 0.94-0.87 (m, 6H); MS m/z 447.42
(M + H)+.
1,3-Dipropyl-8-(1H-pyrazol-4-yl)-1H-purine-2,6(3H,7H)-di-
one (22). A solution of 19b (250 mg) in EtOH (5 mL) was treated
with 2 N HCl (2 mL) at 100 °C for 2 h. The reaction mixture was
concentrated in vacuo, and the residue was treated with ether to
8-(1-(4-Chlorophenyl)-5-(trifluoromethyl)-1H-pyrazol-4-yl)-
1,3-dipropyl-1H-purine-2,6(3H,7H)-dione (18). Following the
general procedure described above, the coupling of diamine 6 with
4-chlorophenyl-5-(trifluoromethyl)-pyrazol-4-carboxylic acid fur-
nished xanthine derivative 18 in 65% yield. 1H NMR (DMSO-d6):
δ 8.28 (s, 1H), 7.30 (d, J ) 8.0 Hz, 2H), 7.20 (d, J ) 8.0 Hz, 2H),
4.01 (t, J ) 8.0 Hz, 2H), 3.88 (t, J ) 8.0 Hz, 2H), 1.80-1.50 (m,
5H), 0.94-0.87 (m, 6H); MS m/z 481.85 (M + H)+.
8-(1-Benzyl-1H-pyrazol-4-yl)-1,3-dipropyl-1H-purine-2,6-
(3H,7H)-dione (19). The 1-benzyl-pyralzol-4-carboxylic acid re-
quired for coupling is synthesized as follows. A mixture of ethyl-
4-pyrazole carboxylate (10 g, 71.4 mmol), potassium carbonate
(49.3 g, 357 mmol), and benzyl bromide (12.2 g, 71.4 mmol) in
acetone (400 mL) was refluxed overnight. K2CO3 was filtered off.
The filtrate was concentrated in vacuo. The residue was taken up
in MeOH (200 mL), and solid potassium hydroxide (20.5 g, 366
mmol) was added, and the resulting reaction mixture was refluxed
overnight. The reaction mixture was concentrated in vacuo. The
residue was partitioned between CH2Cl2 and H2O. The aqueous
layer was acidified to pH 3 with concd HCl. The precipitate was
collected and washed with H2O to give a 97% yield of 1-benzyl-
pyrazol-4-carboxylic acid. The coupling of diamine 6 with 1-benzyl-
pyrazol-4-carboxylic acid as described in the general procedure
1
furnish xanthine derivative 22 in 80% yield as a white solid. H
NMR (DMSO-d6): δ 13.39 (s, 1 H), 13.23 (s, 1 H), 8.28 (s, 1 H),
8.01 (s, 1 H), 3.90 (t, J ) 8.0 Hz, 2 H), 3.77 (t, J ) 8.0 Hz, 2 H),
1.68-1.59 (m, 2 H), 1.53-1.44 (m, 2 H), 0.80 (t, J ) 8.0 Hz, 3
H), 0.78 (t, J ) 8.0 Hz, 3 H); MS m/z 301.24 (M - H)+; Anal.
(C14H18N6O2‚0.7H2O): C,H,N.
8-(1-(2-Methylbenzyl)-1H-pyrazol-4-yl)-1,3-dipropyl-1H-pu-
rine-2,6(3H,7H)-dione (23). Following the general procedure, the
alkylation of 19b with 2-methylbenzyl bromide followed by
1
deprotection furnished 23 in 55% yield. H NMR (DMSO-d6): δ
8.40 (s, 1H), 8.0 (s, 1H), 7.36-7.05 (m, 5H), 5.38 (s, 2H), 4.14 (t,
J ) 8.0 Hz, 2H), 3.46 (t, J ) 8.0 Hz, 2H), 2.38 (s, 3H), 1.86-1.60
(m, 4H), 0.96 (t, J ) 8.0 Hz, 6H); MS m/z 407.2 (M + H)+; Anal.
(C22H26N6O2‚0.5H2O): C,H,N.
8-(1-(3-Methylbenzyl)-1H-pyrazol-4-yl)-1,3-dipropyl-1H-pu-
rine-2,6(3H,7H)-dione (24). Following the general procedure, the
alkylation of 19b with 3-methylbenzyl bromide followed by
1
deprotection furnished 24 in 60% yield. H NMR (DMSO-d6): δ
8.29 (s, 1 H), 7.94 (s, 1 H), 6.92-7.13 (m, 4 H), 5.20 (s, 2H), 3.82

SelectiVe A2B Adenosine Receptor Antagonists
Journal of Medicinal Chemistry, 2006, Vol. 49, No. 12 3689
(t, J ) 8.0 Hz, 2 H), 3.70 (t, J ) 8.0 Hz, 2 H), 2.14 (s, 3 H),
1.51-1.62 (m, 2 H), 1.36-1.48 (m, 2 H), 0.73 (t, J ) 8.0 Hz, 3
H), 0.70 (t, J ) 8.0 Hz, 3 H) MS m/z 407.20 (M + H)+; Anal.
(C22H26N6O2.HCl): C,H,N.
1.52-1.59 (m, 2 H), 0.87 (t, J ) 8.0 Hz, 3 H), 0.85 (t, J ) 8.0 Hz,
3 H); MS m/z 411.08 (M + H)+; Anal. (C21H23FN6O2‚ 0.5H2O):
C,H,N.
8-(1-(3-Fluorobenzyl)-1H-pyrazol-4-yl)-1,3-dipropyl-1H-pu-
rine-2,6(3H,7H)-dione (33). Following the general procedure, the
alkylation of 19b with 3-fluorobenzyl chloride followed by depro-
8-(1-(4-Methylbenzyl)-1H-pyrazol-4-yl)-1,3-dipropyl-1H-pu-
rine-2,6(3H,7H)-dione (25). Following the general procedure, the
alkylation of 19b with 4-methylbenzyl bromide followed by
1
tection furnished 33 in 70% yield. H NMR (DMSO-d6): δ 8.49
1
deprotection furnished 25 in 70% yield. H NMR (DMSO-d6): δ
(s, 1 H), 8.10 (s, 1 H), 7.36-7.45 (m, 1 H), 7.08-7.18 (m, 3 H),
5.42 (s, 2 H), 3.96 (t, J ) 8.0 Hz, 2 H), 3.83 (t, J ) 8.0 Hz, 2 H),
1.67-1.73 (m, 2 H), 1.52-1.59 (m, 2 H), 0.87 (t, J ) 8.0 Hz, 3
H), 0.85 (t, J ) 8.0 Hz, 3 H); MS m/z 411.07 (M + H)+; Anal.
(C21H23FN6O2‚1.25H2O): C,H,N.
8.27 (s, 1 H), 7.92 (s, 1 H), 6.99-7.08 (m, 4 H), 5.19 (s, 2 H),
3.82 (t, J ) 8.0 Hz, 2 H), 3.69 (t, J ) 8.0 Hz, 2 H), 2.13 (s, 3 H),
1.51-1.61 (m, 2 H), 1.36-1.47 (m, 2 H), 0.73 (t, J ) 8.0 Hz, 3
H), 0.72 (t, J ) 8.0 Hz, 3 H); MS m/z 407.18 (M + H)+; Anal.
(C22H26N6O2‚0.5H2O): C,H,N.
8-(1-(4-Fluorobenzyl)-1H-pyrazol-4-yl)-1,3-dipropyl-1H-pu-
rine-2,6(3H,7H)-dione (34). Following the general procedure, the
alkylation of 19b with 4-fluorobenzyl chloride followed by depro-
8-(1-(2-Methoxybenzyl)-1H-pyrazol-4-yl)-1,3-dipropyl-1H-pu-
rine-2,6(3H,7H)-dione (26). Following the general procedure, the
alkylation of 19b with 2-methoxybenzyl bromide followed by
1
tection furnished 34 in 75% yield. H NMR (DMSO-d6): δ 8.38
1
deprotection furnished 26 in 70% yield. H NMR (DMSO-d6): δ
(s, 1H), 7.96 (s, 1H), 5.24 (s, 2H), 3.82 (t, J ) 8.0 Hz, 2H), 3.66
(t, J ) 8.0 Hz, 3H), 1.62-1.30 (m, 4H), 0.84 (t, J ) 8.0 Hz, 3H),
0.82 (t, J ) 8.0 Hz, 3H); MS m/z 411 (M + H)+; Anal. (C21H23-
FN6O2‚H2O): C,H,N.
8.29 (s, 1 H), 8.02 (s, 1 H), 7.26-7.32 (m, 1 H), 6.95-7.05 (m, 2
H), 6.86-6.92 (m, 1 H), 5.30 (s, 2 H), 3.91-3.95 (m, 2 H), 3.78-
3.83 (m, 5 H), 1.64-1.70 (m, 2 H), 1.50-1.56 (m, 2 H), 0.84 (t,
J ) 8.0 Hz, 3H), 0.82 (t, J ) 8.0 Hz, 3 H); MS m/z 423.10 (M +
H)+; Anal. (C22H26N6O3.HCl): C,H,N.
8-(1-(2-(Trifluoromethyl)benzyl)-1H-pyrazol-4-yl)-1,3-dipro-
pyl-1H-purine-2,6(3H,7H)-dione (35). Following the general
procedure, the alkylation of 19b with 2-trifluoromethylbenzyl
8-(1-(3-Methoxybenzyl)-1H-pyrazol-4-yl)-1,3-dipropyl-1H-pu-
rine-2,6(3H,7H)-dione (27). Following the general procedure, the
alkylation of 19b with 3-methoxybenzyl bromide followed by
1
chloride followed by deprotection furnished 35 in 75% yield. H
NMR (DMSO-d6): δ 8.48 (s, 1 H), 8.13 (s, 1 H), 7.79 (d, J ) 8.0
Hz, 1 H), 7.65 (t, J ) 8.0 Hz, 1 H), 7.54 (t, J ) 8.0 Hz, 1 H), 7.08
(d, J ) 8.0 Hz, 1 H), 5.60 (s, 2 H), 3.96 (t, J ) 8.0 Hz, 2 H), 3.84
(t, J ) 8.0 Hz, 2 H), 1.67-1.73 (m, 2 H), 1.52-1.59 (m, 2 H),
0.87 (t, J ) 8.0 Hz, 3 H), 0.85 (t, J ) 8.0 Hz, 3 H); MS m/z 461.17
(M + H)+; Anal. (C22H23F3N6O2‚1.25H2O): C,H,N.
1
deprotection furnished 27 in 75% yield. H NMR (DMSO-d6): δ
8.45 (s, 1 H), 8.08 (s, 1 H), 7.23-7.29 (m, 1 H), 6.82-6.89 (m, 3
H), 5.35 (s, 2 H), 3.96 (t, J ) 8.0 Hz, 2 H), 3.83 (t, J ) 8.0 Hz, 2
H), 3.72 (s, 3 H), 1.65-1.73 (m, 2 H), 1.50-1.59 (m, 2 H), 0.87
(t, J ) 8.0 Hz, 3 H), 0.85 (t, J ) 8.0 Hz, 3 H); MS m/z 423.10 (M
+ H)+; Anal. (C22H26N6O3‚0.5H2O): C,H,N.
8-(1-(3-(Trifluoromethyl)benzyl)-1H-pyrazol-4-yl)-1,3-dipro-
pyl-1H-purine-2,6(3H,7H)-dione (36). Following the general
procedure, the alkylation of 19b with 3-trifluoromethylbenzyl
8-(1-(4-Methoxybenzyl)-1H-pyrazol-4-yl)-1,3-dipropyl-1H-pu-
rine-2,6(3H,7H)-dione (28). Following the general procedure, the
alkylation of 19b with 4-methoxybenzyl bromide followed by
1
chloride followed by deprotection furnished 36 in 80% yield. H
1
deprotection furnished 28 in 75% yield. H NMR (DMSO-d6): δ
NMR (DMSO-d6): δ 8.53 (s, 1 H), 8.11 (s, 1 H), 7.68 (s, 1 H),
7.56-7.62 (m, 3 H), 5.51 (s, 2 H), 3.96 (t, J ) 8.0 Hz, 3 H), 3.83
(t, J ) 8.0 Hz, 3 H), 1.65-1.75 (m, 2 H), 1.52-1.59 (m, 2 H),
0.87 (t, J ) 8.0 Hz, 3 H), 0.85 (t, J ) 8.0 Hz, 3 H); MS m/z 461.06
(M + H)+; Anal. (C22H23F3N6O2‚H2O): C,H,N.
8.25 (s, 1H), 7.94 (s, 1H), 7.13 (d, J ) 8.0 Hz, 2H), 6.77 (d, J )
8.0 Hz, 2H), 5.15 (s, 2H), 3.82 (t, J ) 8.0 Hz, 2H), 3.65 (t, J ) 8.0
Hz, 2H), 3.60 (s, 3H), 1.62-1.36 (m, 4H), 0.74 (t, J ) 8.0 Hz,
3H), 0.70 (t, J ) 8.0 Hz, 3H); MS m/z 423.01 (M + H)+; Anal.
(C22H26N6O3.0.75HCl): C,H,N.
8-(1-(4-(Trifluoromethyl)benzyl)-1H-pyrazol-4-yl)-1,3-dipro-
pyl-1H-purine-2,6(3H,7H)-dione (37). Following the general
procedure, the alkylation of 19b with 4-trifluoromethylbenzyl
8-(1-(2-Chlorobenzyl)-1H-pyrazol-4-yl)-1,3-dipropyl-1H-pu-
rine-2,6(3H,7H)-dione (29). Following the general procedure, the
alkylation of 19b with 2-chlorobenzyl chloride followed by
1
chloride followed by deprotection furnished 37 in 75% yield. H
1
deprotection furnished 29 in 75% yield. H NMR (DMSO-d6): δ
NMR (DMSO-d6): δ 8.52 (s, 1H), 8.10 9s, 1H), 7.76 (d, J ) 8.0
Hz, 2H), 7.46 (d, J ) 8.0 Hz, 2H), 5.50 (s, 2H), 3.96 (t, J ) 8.0
Hz, 2H), 3.82 (t, J ) 8.0 Hz, 2H), 1.78-1.48 (m, 4H), 0.88 (t, J
) 8.0 Hz, 3H), 0.86 (t, J ) 8.0 Hz, 3H); MS m/z 461 (M + H)+;
Anal. (C22H23F3N6O2.0.75CH2Cl2): C,H,N.
8.44 (s, 1 H), 8.10 (s, 1 H), 7.48-7.54 (m, 1 H), 7.32-7.39 (m, 2
H), 7.12-7.18 (m, 1 H), 5.50 (s, 2 H), 3.96 (t, J ) 8.0 Hz, 2 H),
3.83 (t, J ) 8.0 Hz, 2 H), 1.67-1.73 (m, 2 H), 1.52-1.59 (m, 2
H), 0.87 (t, J ) 8.0 Hz, 3 H), 0.85 (t, J ) 8.0 Hz, 3 H); MS m/z
427.04 (M + H)+; Anal. (C21H23ClN6O2): C,H,N.
8-(1-(2,3-Difluorobenzyl)-1H-pyrazol-4-yl)-1,3-dipropyl-1H-
purine-2,6(3H,7H)-dione (38). Following the general procedure,
the alkylation of 19b with 2,3-difluorobenzyl chloride followed by
8-(1-(3-Chlorobenzyl)-1H-pyrazol-4-yl)-1,3-dipropyl-1H-pu-
rine-2,6(3H,7H)-dione (30). Following the general procedure, the
alkylation of 19b with 3-chlorobenzyl chloride followed by
1
deprotection furnished 38 in 70% yield. H NMR (DMSO-d6): δ
1
deprotection furnished 30 in 70% yield. H NMR (DMSO-d6): δ
8.45 (s, 1 H), 8.09 (s, 1 H), 7.07-7.47 (m, 3 H), 5.52 (s, 2 H),
3.96 (t, J ) 8.0 Hz, 3 H), 3.83 (t, J ) 8.0 Hz, 3 H), 1.65-1.74 (m,
2 H), 1.50-1.60 (m, 2 H), 0.87 (t, J ) 8.0 Hz, 3 H), 0.85 (t, J )
8.0 Hz, 3 H) MS m/z 429.33 (M + H)+; Anal. (C21H22F2N6O2.
0.25CH2Cl2): C,H,N.
8.40 (s, 1H), 7.96 (s, 1H), 7.30-7.05 (m, 4H), 5.26 (s, 2H), 3.82-
3.72 (m, 4H), 1.62-1.36 (m, 4H), 0.78 (t, J ) 8.0 Hz, 3H), 0.76
(t, J ) 8.0 Hz, 3H); MS m/z 427 (M + H)+; Anal. (C21H23-
ClN6O2): C,H,N.
8-(1-(4-Chlorobenzyl)-1H-pyrazol-4-yl)-1,3-dipropyl-1H-pu-
rine-2,6(3H,7H)-dione (31). Following the general procedure, the
alkylation of 19b with 4-chlorobenzyl chloride followed by
8-(1-(2,4-Difluorobenzyl)-1H-pyrazol-4-yl)-1,3-dipropyl-1H-
purine-2,6(3H,7H)-dione (39). Following the general procedure,
the alkylation of 19b with 2,4-difluorobenzyl chloride followed by
1
1
deprotection furnished 31 in 75% yield. H NMR (DMSO-d6): δ
deprotection furnished 39 in 75% yield. H NMR (DMSO-d6): δ
8.38 (s, 1H), 7.98 (s, 1H), 7.30 (d, J ) 8.0 Hz, 2H), 7.18 (d, J )
8.0 Hz, 2H), 5.24 (s, 2H), 3.82 (t, J ) 8 Hz, 2H), 3.70 (t, J ) 8.0
Hz, 2H), 1.62-1.36 (m, 4H), 0.78 (t, J ) 8.0 Hz, 3H), 0.74 (t, J
) 8.0 Hz, 3H); MS m/z 426.98 (M + H)+; Anal. (C21H23-
ClN6O2.0.5CH2Cl2): C,H,N.
8.44 (s, 1 H), 8.07 (s, 1 H), 7.07-7.44 (m, 3 H), 5.43 (s, 2 H),
3.96 (t, J ) 8.0 Hz, 3 H), 3.83 (t, J ) 8.0 Hz, 3 H), 1.65-1.74 (m,
2 H), 1.50-1.60 (m, 2 H), 0.87 (t, J ) 8.0 Hz, 3 H), 0.85 (t, J )
8.0 Hz, 3 H); MS m/z 429.30 (M + H)+. Anal. (C21H22F2N6O2.
0.5CH2Cl2): C,H,N.
8-(1-(2-Fluorobenzyl)-1H-pyrazol-4-yl)-1,3-dipropyl-1H-pu-
rine-2,6(3H,7H)-dione (32). Following the general procedure, the
alkylation of 19b with 2-fluorobenzyl chloride followed by depro-
tection furnished 32 in 75% yield. H NMR (DMSO-d6): δ 8.44
(s, 1 H), 8.10 (s, 1 H), 7.17-7.43 (m, 4 H), 5.46 (s, 2 H), 3.96 (t,
J ) 8.0 Hz, 2 H), 3.83 (t, J ) 8.0 Hz, 2 H), 1.67-1.73 (m, 2 H),
8-(1-(2,6-Difluorobenzyl)-1H-pyrazol-4-yl)-1,3-dipropyl-1H-
purine-2,6(3H,7H)-dione (40). Following the general procedure,
the alkylation of 19b with 2,6-difluorobenzyl bromide followed by
deprotection furnished 40 in 65% yield. H NMR (DMSO-d6): δ
8.44 (s, 1 H), 8.02 (s, 1 H), 7.43-7.52 (m, 1 H), 7.13-7.21 (m, 2
H), 5.46 (s, 2 H), 3.96 (t, J ) 8.0 Hz, 3 H), 3.83 (t, J ) 8.0 Hz, 3
1
1

3690 Journal of Medicinal Chemistry, 2006, Vol. 49, No. 12
Kalla et al.
1
H), 1.65-1.74 (m, 2 H), 1.50-1.60 (m, 2 H), 0.87 (t, J ) 8.0 Hz,
3 H), 0.85 (t, J ) 8.0 Hz, 3 H); MS m/z 429.34 (M + H)+; Anal.
(C21H22F2N6O2. 0.5CH2Cl2): C,H,N.
by deprotection furnished 48 in 68% yield. H NMR (DMSO-d6):
δ 8.40 (s, 1H), 8.02 (s, 1H), 7.48 (d, J ) 8.0 Hz, 1H), 7.39 (d, J
) 8.0 Hz, 1H), 7.18 (s, 1H), 5.42 (s, 2H), 3.90 (t, J ) 8.0 Hz, 2H),
3.76 (t, J ) 8.0 Hz, 2H), 1.73-1.40 (m, 4H), 0.82 (t, J ) 8.0 Hz,
3H), 0.80 (t, J ) 8.0 Hz, 3H); MS m/z 461.19 (M + H)+; Anal.
(C21H22Cl2N6O2. 0.5CH2Cl2): C,H,N.
8-(1-(3,4-Difluorobenzyl)-1H-pyrazol-4-yl)-1,3-dipropyl-1H-
purine-2,6(3H,7H)-dione (41). Following the general procedure,
the alkylation of 19b with 3,4-difluorobenzyl chloride followed by
1
deprotection furnished 41 in 75% yield. H NMR (DMSO-d6): δ
8-(1-(2-Chloro-5-fluorobenzyl)-1H-pyrazol-4-yl)-1,3-dipropyl-
1H-purine-2,6(3H,7H)-dione (49). Following the general proce-
dure, the alkylation of 19b with 2-chloro-5-flouorobenzyl chloride
followed by deprotection furnished 49 in 73% yield. 1H NMR
(DMSO-d6): δ 13.49 (s, 1 H), 8.42 (s, 1 H), 8.02 (s, 1 H), 7.29-
7.53 (m, 3 H), 5.52 (s, 2 H), 3.96 (t, J ) 8.0 Hz, 3 H), 3.83 (t, J
) 8.0 Hz, 3 H), 1.65-1.74 (m, 2 H), 1.50-1.60 (m, 2 H), 0.87 (t,
J ) 8.0 Hz, 3 H), 0.85 (t, J ) 8.0 Hz, 3 H); MS m/z 445.26 (M +
H)+; Anal. (C21H22ClFN6O2): C,H,N.
8-(1-(2-Chloro-5-(trifluoromethyl)benzyl)-1H-pyrazol-4-yl)-
1,3-dipropyl-1H-purine-2,6(3H,7H)-dione (50). Following the
general procedure, the alkylation of 19b with 2-chloro-5-trifluo-
romethylbenzyl chloride followed by deprotection furnished 50 in
76% yield. 1H NMR (DMSO-d6): δ 13.55 (s, 1 H), 8.51 (s, 1 H),
8.12 (s, 1 H), 7.76 (br s, 2 H), 7.61 (s, 1 H), 5.60 (s, 2 H), 3.96 (t,
J ) 8.0 Hz, 3 H), 3.83-3.52 (m, 7 H), 1.65-1.74 (m, 2 H), 1.50-
1.60 (m, 2 H), 0.87 (t, J ) 8.0 Hz, 3 H), 0.85 (t, J ) 8.0 Hz, 3 H);
MS m/z 495.18 (M + H)+; Anal. (C22H22ClF3N6O2): C,H,N.
8-(1-(3-Chloro-2-fluoro-5-(trifluoromethyl)benzyl)-1H-pyra-
zol-4-yl)-1,3-dipropyl-1H-purine-2,6(3H,7H)-dione (51). Follow-
ing the general procedure, the alkylation of 19b with 3-chloro-2-
fluoro-5-trifluoromethylbenzyl chloride followed by deprotection
furnished 51 in 66% yield. 1H NMR (DMSO-d6): δ 8.54 (s, 1 H),
8.11-8.15 (m, 1 H), 8.10 (s, 1 H), 7.76-7.80 (m, 1 H), 5.59 (s, 2
H), 3.96 (t, J ) 8.0 Hz, 3 H), 3.83-3.50 (m, 7 H), 1.65-1.74 (m,
2 H), 1.50-1.60 (m, 2 H), 0.87 (t, J ) 8.0 Hz, 3 H) 0.85 (t, J )
8.0 Hz, 3 H); MS m/z 513.36 (M + H)+; Anal. (C22H21ClF4N6O2‚
H2O): C,H,N.
8.49 (s, 1 H), 8.10 (s, 1 H), 7.37-7.48 (m, 2 H), 7.12-7.18 (m, 1
H), 5.39 (s, 2 H), 3.96 (t, J ) 8.0 Hz, 3 H), 3.83 (t, J ) 8.0 Hz, 3
H), 1.65-1.74 (m, 2 H), 1.50-1.60 (m, 2 H), 0.87 (t, J ) 8.0 Hz,
3 H), 0.85 (t, J ) 8.0 Hz, 3 H); MS m/z 429.29 (M + H)+; Anal.
(C21H22F2N6O2. HCl): C,H,N.
8-(1-(3-Chloro-2-fluorobenzyl)-1H-pyrazol-4-yl)-1,3-dipropyl-
1H-purine-2,6(3H,7H)-dione (42). Following the general proce-
dure, the alkylation of 19b with 3-chloro-2-fluorobenzyl chloride
followed by deprotection furnished 42 in 75% yield. 1H NMR
(DMSO-d6): δ 8.49 (s, 1 H), 8.09 (s, 1 H), 7.54-7.61 (m, 1 H),
7.19-7.29 (m, 2 H), 5.51 (s, 2 H), 3.96 (t, J ) 8.0 Hz, 3 H), 3.83
(t, J ) 8.0 Hz, 3 H), 1.65-1.74 (m, 2 H), 1.50-1.60 (m, 2 H),
0.87 (t, J ) 8.0 Hz, 3 H), 0.85 (t, J ) 8.0 Hz, 3 H); MS m/z 443.18
(M - H)+; Anal. (C21H22ClFN6O2): C,H,N.
8-(1-(2-Fluoro-3-methylbenzyl)-1H-pyrazol-4-yl)-1,3-dipropyl-
1H-purine-2,6(3H,7H)-dione (43). Following the general pro-
cedure, the alkylation of 19b with 2-fluoro-3-methylbenzyl bro-
mide followed by deprotection furnished 43 in 75% yield. 1H
NMR (DMSO-d6): δ 8.43 (s, 1 H), 8.07 (s, 1 H), 7.21-7.29
(m, 1 H), 7.04-7.13 (m, 2 H), 5.43 (s, 2 H), 3.96 (t, J ) 8.0 Hz,
3 H), 3.83 (t, J ) 8.0 Hz, 3 H), 2.23 (s, 3 H), 1.65-1.74 (m, 2 H),
1.50-1.60 (m, 2 H), 0.87 (t, J ) 8.0 Hz, 3 H), 0.85 (t, J ) 8.0 Hz,
3 H); MS m/z 425.26 (M + H)+; Anal. (C22H25FN6O2‚1.25H2O):
C,H,N.
8-(1-(4-Fluoro-3-(trifluoromethyl)benzyl)-1H-pyrazol-4-yl)-
1,3-dipropyl-1H-purine-2,6(3H,7H)-dione (44). Following the
general procedure, the alkylation of 19b with 4-fluoro-3-trifluo-
romethylbenzyl bromide followed by deprotection furnished 44 in
75% yield. 1H NMR (DMSO-d6): δ 13.49 (s, 1 H), 8.52 (s, 1 H),
8.11 (s, 1 H), 7.48-7.81 (m, 3 H), 5.48 (s, 2 H), 3.96 (t, J ) 8.0
Hz, 3 H), 3.83 (t, J ) 8.0 Hz, 3 H), 1.65-1.74 (m, 2 H), 1.50-
1.60 (m, 2 H), 0.87 (t, J ) 8.0 Hz, 3 H), 0.85 (t, J ) 8.0 Hz, 3 H);
MS m/z 479.26 (M + H)+; Anal. (C22H22F4N6O2): C,H,N.
8-(1-(4-Chloro-3-(trifluoromethyl)benzyl)-1H-pyrazol-4-yl)-
1,3-dipropyl-1H-purine-2,6(3H,7H)-dione (45). Following the
general procedure, the alkylation of 19b with 4-chloro-3-trifluo-
romethylbenzyl bromide followed by deprotection furnished 45 in
81% yield. 1H NMR (DMSO-d6): δ 13.55 (s, 1 H), 8.53 (s, 1 H),
8.12 (s, 1 H), 7.84 (s, 1 H), 7.73 (d, J ) 8.0 Hz, 1 H), 7.57 (d, J
) 8.0 Hz, 1 H), 5.50 (s, 2 H), 3.96 (t, J ) 8.0 Hz, 3 H), 3.83 (t,
J ) 8.0 Hz, 3 H), 1.65-1.74 (m, 2 H), 1.50-1.60 (m, 2 H), 0.87
(t, J ) 8.0 Hz, 3 H), 0.85 (t, J ) 8.0 Hz, 3 H); MS m/z 495.30 (M
+ H)+; Anal. (C22H22ClF3N6O2): C,H,N.
8-(1-(2,4,6-Trifluorobenzyl)-1H-pyrazol-4-yl)-1,3-dipropyl-
1H-purine-2,6(3H,7H)-dione (52). Following the general proce-
dure, the alkylation of 19b with 2,4,6-trifluorobenzyl chloride
followed by deprotection furnished 52 in 75% yield. 1H NMR
(DMSO-d6): δ 13.51 (s, 1 H), 8.44 (s, 1 H), 8.03 (s, 1 H), 7.24-
7.33 (m, 2 H), 5.42 (s, 2 H), 3.96 (t, J ) 8.0 Hz, 3 H), 3.83-3.50
(m, 7 H), 1.65-1.74 (m, 2 H), 1.50-1.60 (m, 2 H), 0.87 (t, J )
8.0 Hz, 3 H), 0.85 (t, J ) 8.0 Hz, 3 H); MS m/z 447.40 (M + H)+;
Anal. (C21H21F3N6O2.0.25CH2Cl2): C,H,N.
General Procedure for the Preparation of Compounds 56a-
d. Diamines 56a-d were synthesized following the methods
described in the literature.12 To a stirred solution of 1,3-dimethylurea
53a (3.52 g, 40 mmol) in acetic anhydride (30 mL) was added
cyanoacetic acid (3.74 g, 44 mmol), and the resulting mixture was
stirred overnight at 70 °C. The reaction mixture was concentrated,
and the resulting oily residue was diluted with H2O (40 mL) and
treated with 5 N NaOH (15 mL). The precipitate thus formed was
collected by filtration, washed with cold water, and purified by
recrystalization from MeOH/H2O to give 6-amino-1,3-dimethylpy-
rimidine-2,4(1H,3H)-dione (54a) as a light-yellow solid (4.0 g).
Compound 54a (4.00 g, 25.8 mmol) was stirred in 50% aquesous
AcOH solution (160 mL) at 75 °C for 30 min until the reaction
mixture became homogeneous. Once the reaction mixture was
homogeneous, the temperature was reduced to 50 °C, and sodium
nitrite (3.56 g, 51.6 mmol) was added in small portions. After the
completion of the addition, the resulting mixture was cooled to r.t.
and stirred for 1 h. The resulting precipitate was collected by
filtration, washed with water, and dried to afford 6-amino-1,3-
dimethyl-5-nitrosopyrimidine 2,4(1H,3H)-dione (55a) as a purple
solid (4.70 g). A suspension of compound 55a (1.10 g, 6.0 mmol)
was stirred in 14.5% NH4OH (40 mL) at 70 °C for 30 min until
the reaction mixture became homogeneous. The temperature was
reduced to 50 °C, and Na2S2O4 (3.13 g, 18.0 mmol) was added in
small portions. During the addition, the red solution changed to
yellow-green and to light yellow, and the solution was stirred at
room temperature for another 30 min. The volume of the reaction
mixture was reduced to half and cooled in an ice bath for 1 h, and
the precipitate was collected by filtration, followed by washing with
8-(1-(3,5-Bis(trifluoromethyl)benzyl)-1H-pyrazol-4-yl)-1,3-
dipropyl-1H-purine-2,6(3H,7H)-dione (46). Following the general
procedure, the alkylation of 19b with 3,5-bistrifluoromethylbenzyl
1
bromide followed by deprotection furnished 46 in 78% yield. H
NMR (DMSO-d6): δ 8.55 (s, 1 H), 8.14 (s, 1 H), 8.10 (s, 1 H),
8.02 (s, 2 H), 5.61 (s, 2 H), 3.96 (t, J ) 8.0 Hz, 3 H), 3.83 (m, 7
H), 1.65-1.74 (m, 2 H), 1.50-1.60 (m, 2 H), 0.87 (t, J ) 8.0 Hz,
3 H), 0.85 (t, J ) 8.0 Hz, 3 H); MS m/z 529.33 (M + H)+; Anal.
(C23H22F6N6O2‚0.5H2O): C,H,N.
8-(1-(3-(Trifluoromethyl)-4-methoxybenzyl)-1H-pyrazol-4-yl)-
1,3-dipropyl-1H-purine-2,6(3H,7H)-dione (47). Following the
general procedure, the alkylation of 19b with 3-trifluoromethyl-4-
methoxybenzyl bromide followed by deprotection furnished 47 in
1
75% yield. H NMR (DMSO-d6): δ 8.47 (s, 1 H), 8.08 (s, 1 H),
7.63 (s, 1 H), 7.61 (d, J ) 8.0 Hz, 1 H), 7.26 (d, J ) 8.0 Hz, 1 H),
5.38 (s, 2 H), 3.96 (t, J ) 8.0 Hz, 3 H), 3.83 (m, 7 H), 1.65-1.74
(m, 2 H), 1.50-1.60 (m, 2 H), 0.87 (t, J ) 8.0 Hz, 3 H), 0.85 (t,
J ) 8.0 Hz, 3 H); MS m/z 491.35 (M + H)+; Anal. (C23H25F3N6O3‚
0.5H2O): C,H,N.
8-(1-(2,5-Dichlorobenzyl)-1H-pyrazol-4-yl)-1,3-dipropyl-1H-
purine-2,6(3H,7H)-dione (48). Following the general procedure,
the alkylation of 19b with 2,5-dichlorobenzyl chloride followed

SelectiVe A2B Adenosine Receptor Antagonists
Journal of Medicinal Chemistry, 2006, Vol. 49, No. 12 3691
a small amount of water. Compound 56a, 5,6-diamino-1,3-
dimethylpyrimidine-2,4(1H,3H)-dione, was collected as a white
solid (0.56 g, overall yield 60%). Following the general procedure
described above and starting from 1,3-diethyl urea (53b) furnished
diamine derivative 56b in 65% overall yield. Following the general
procedure described above and starting from 1,3-dibutyl urea (53c)
furnished diamine derivative 56c in 68% yield from three steps.
Following the general procedure described above and starting from
1,3-diisobutyl urea (53d) furnished diamine derivative 56d in 66%
overall yield.
General Procedure for the Synthesis of Compounds 57a-c.
To a solution of ethyl-4-pyrazole carboxylate (1 mmol) in acetone,
K2CO3 (5 mmol) and the corresponding benzyl bromide (1.2 mmol)
was added, and the mixture was heated at 50 °C for 16 h. K2CO3
was filtered off, and the filtrate was concentrated and used as such
for the next step. The residue was dissolved in methanol, and solid
KOH (2 mmol) was added, and the mixture was heated at reflux
for 18 h. Mehtanol was distilled off under vacuum, and the residue
was dissolved in water and washed with ethyl acetate, and the
aqueous layer was acidified with 6 N HCl. The resulting precipitate
was filtered, washed with water, and dried.
8-(1-Benzyl-1H-pyrazol-4-yl)-1,3-diethyl-1H-purine-2,6(3H,7H)-
dione (61). Following the general procedure described above, the
coupling of diamine 56b with 57a furnished 61 in 80% yield. H
1
NMR (DMSO-d6): δ 13.55 (s, 1H), 8.47 (s, 1H), 8.10 (s, 1H),
7.29-7.38 (m, 5H), 5.40 (s, 2H), 4.05 (s, 2H), 3.94 (s, 2H), 1.24
(s, 3H), 1.13 (s, 3H); MS m/z 365.40 (M + H)+; Anal. (C19H20N6O2
0.5CH2Cl2): C,H,N.
8-(1-(3-Fluorobenzyl)-1H-pyrazol-4-yl)-1,3-diethyl-1H-purine-
2,6(3H,7H)-dione (62). Following the general procedure described
above, the coupling of diamine 56b with 57b furnished 62 in 76%
yield. 1H NMR (DMSO-d6): δ 13.55 (s, 1H), 8.51 (s, 1H), 8.12 (s,
1H), 7.39-7.45 (m, 1H), 7.11-7.18 (m, 3H), 5.43 (s, 1H), 4.01-
4.08 (s, 2H), 3.90-3.96 (m, 2H), 1.05-1.25 (m, 6H); MS m/z
383.33 (M + H)+, 405.31 (M+ + Na). Anal. (C19H19FN6O2 0.5CH2-
Cl2): C,H,N.
8-(1-(3-(Trifluoromethyl)benzyl)-1H-pyrazol-4-yl)-1,3-diethyl-
1H-purine-2,6(3H,7H)-dione (63). Following the general procedure
described above, the coupling of diamine 56b with 57c furnished
1
63 in 70% yield. H NMR (DMSO-d6): δ 13.59 (s, 1H), 8.56 (s,
1H), 8.13 (s, 1H), 7.69-7.72 (m, 2H), 7.58-7.64 (m, 2H), 5.52
(s, 2H), 4.05 (q, 2H, J ) 6.64 Hz), 3.93 (q, 2H, J ) 7.03 Hz), 1.24
(t, 3H, J ) 7.03 Hz), 1.13 (t, 3H, J ) 6.84 Hz); MS m/z 433.30
(M + H)+, 455.29 (M+ + Na); Anal. (C20H19F3N6O2.0.5CH2Cl2):
C,H,N.
1-Benzyl-1H-pyrazole-4-carboxylic Acid (57a). The coupling
of the ethyl pyrazole carboxylate with benzyl bromide followed
by hydrolysis of the ester as described above furnished pyrazole
1
carboxylic acid 57a in 95% yield. H NMR (CDCl3): δ 12.31 (s,
8-(1-Benzyl-1H-pyrazol-4-yl)-1,3-dibutyl-1H-purine-2,6(3H,7H)-
dione (64). Following the general procedure described above, the
1H), 8.05 (s, 1H), 8.00 (s, 1H), 7.24-7.00 (m, 5H), 5.34 (s, 2H);
MS m/z 203.20 (M + H)+.
1
coupling of diamine 56c with 57a furnished 64 in 75% yield. H
1-(3-Fluorobenzyl)-1H-pyrazole-4-carboxylic Acid (57b). The
coupling of the ethyl pyrazole carboxylate and 3-fluorobenzyl
bromide followed by hydrolysis furnished pyrazole acid 57b in 96%
NMR (DMSO-d6): δ 13.51 (s, 1H), 8.45 (s, 1H), 8.09 (s, 1H),
7.29-7.39 (m, 5H), 5.40 (s, 2H), 4.00 (t, 2H, J ) 7.03 Hz), 3.88
(t, 2H, J ) 8.0 Hz), 1.63-1.70 (m, 2H), 1.49-1.56 (m, 2H), 1.26-
1.33 (m, 4H), 0.88-0.93 (m, 6H); MS m/z 421.37 (M + H)+,
443.37 (M+ + Na); Anal. (C23H28N6O2): C,H,N.
8-(1-Benzyl-1H-pyrazol-4-yl)-1,3-diisobutyl-1H-purine-2,6-
(3H,7H)-dione (65). Following the general procedure described
above, the coupling of diamine 56d with 57a furnished 65 in 80%
yield. 1H NMR (DMSO-d6): δ 13.54 (s, 1H), 8.47 (s, 1H), 8.10 (s,
1H), 7.28-7.38 (m, 5H), 5.40 (s, 2H), 3.84 (d, 2H, J ) 7.03 Hz),
3.74 (d, 2H, J ) 7.42 Hz), 2.20-2.28 (m, 1H), 2.02-2.09 (m,
1H), 0.87 (d, 6H, J ) 6.64 Hz), 0.84 (d, 6H, J ) 6.64 Hz); MS
m/z 421.51 (M + H)+, 443.50 (M+ + Na).
1
yield. H NMR (CDCl3): δ 8.05 (s, 1H), 7.98 (s, 1H), 7.42-7.34
(m, 1H), 7.12-7.04 (m, 2H), 7.00-6.95 (m, 1H), 5.35 (s, 2H);
MS m/z 221.20 (M + H)+.
1-(3-(Trifluoromethyl)benzyl)-1H-pyrazole-4-carboxylic Acid
(57c). The coupling of the ethyl pyrazole carboxylate with
3-trifluoromethyl benzyl bromide followed by hydrolysis furnished
pyrazole acid 57c in 95% yield. 1H NMR (CDCl3): δ 8.04 (s, 1H),
7.98 (s, 1H), 7.40-7.33 (m, 1H), 7.12-7.04 (m, 2H), 7.00-6.94
(m, 1H), 5.36 (s, 2H); MS m/z 271.20 (M + H)+.
General Procedure for the Synthesis of Compounds 58-67.
To a stirred solution of substituted 1H-pyrazol-4-carboxylic acids
(57a-c, 0.7 mmol) and EDCI‚HCl (0.77 mmol) in MeOH (15 mL)
was added 1,3-symmetrically substituted 5,6-diamino uracils (56a-
d, 0.7 mol), and the resulting mixture was stirred overnight at r.t.
The solvent was concentrated in vacuo, and the resulting white solid
was washed with water. The uncyclized product was dissolved in
methanol (10 mL) and 2 N NaOH (10 mL) and stirred at 95 °C for
16 h. The reaction mixture was cooled to r.t. and then acidified
with 6 N HCl to pH 3-4 in an ice bath. The white precipitate was
collected by filtration, washed with water, and dried. Further
washings with methanol furnished compounds 58-67 as white
solids.
8-(1-(3-Fluorobenzyl)-1H-pyrazol-4-yl)-1,3-diisobutyl-1H-pu-
rine-2,6(3H,7H)-dione (66). Following the general procedure
described above, the coupling of diamine 56d with 57b furnished
1
66 in 75% yield. H NMR (DMSO-d6): δ 13.55 (s, 1H), 8.50 (s,
1H), 8.11 (s, 1H), 7.39-7.44 (m, 1H), 7.11-7.18 (m, 3H), 5.43
(s, 2H), 3.85 (d, 2H, J ) 7.42 Hz), 3.74 (d, 2H, J ) 7.81 Hz),
2.21-2.28 (m, 1H), 2.03-2.10 (m, 1H), 0.87 (d, 2H, J ) 6.45
Hz), 0.84 (d, 6H, J ) 6.45 Hz); MS m/z 439.43 (M + H)+, 461.43
(M+ + Na).
8-(1-(3-(Trifluoromethyl)benzyl)-1H-pyrazol-4-yl)-1,3-diisobu-
tyl-1H-purine-2,6(3H,7H)-dione (67). Following the general pro-
cedure described above, the coupling of diamine 56d with 57c
furnished 67 in 80% yield. 1H NMR (DMSO-d6): δ 13.55 (s, 1H),
8.54 (s, 1H), 8.12 (s, 1H), 7.67-7.70 (m, 2H), 7.56-7.63 (m, 2H),
5.52 (s, 2H), 3.84 (d, 2H, J ) 7.81 Hz), 3.73 (d, 2H, J ) 7.42 Hz),
2.20-2.27 (m, 1H), 2.02-2.09 (m, 1H), 0.87 (d, 2H, J ) 6.84
Hz), 0.84 (d, 6H, J ) 6.84 Hz); MS m/z 489.35 (M + H)+, 511.31
(M+ + Na); Anal. (C24H27F3N6O2): C,H,N.
8-(1-Benzyl-1H-pyrazol-4-yl)-1,3-dimethyl-1H-purine-2,6-
(3H,7H)-dione (58). Following the general procedure described
above, the coupling of diamine 56a with 57a furnished 58 in 65%
yield. 1H NMR (DMSO-d6): δ 12.31 (s, 1H), 8.38 (s, 1H), 7.81 (s,
1H), 7.25-7.37 (m, 5H), 5.36 (s, 2H), 3.34 (s, 6H); MS m/z 337.35
(M + H)+; Anal. (C17H16N6O2 0.5CH2Cl2): C,H,N.
Radioligand Binding for A2B Adenosine Receptor.22 Human
A2B adenosine receptor cDNA was stably transfected into HEK-
293 cells (referred to as HEK-A2B cells). The monolayer of the
HEK-A2B cells was washed with PBS once and harvested in a buffer
containing 10 mM HEPES (pH 7.4), 10 mM EDTA, and protease
inhibitors. These cells were homogenized in polytron for 1 min at
setting 4 and centrifuged at 29 000g for 15 min at 4 °C. The cell
pellets were washed once with a buffer containing 10 mM HEPES
(pH7.4), 1 mM EDTA, and protease inhibitors and resuspended in
the same buffer supplemented with 10% sucrose. Frozen aliquots
were kept at -80 °C. Competition assays were started by mixing
10 nM 3H-ZM241385 (Tocris Cookson) with various concentrations
of test compounds and 50 µg of membrane proteins in TE buffer
(50 mM Tris and 1 mM EDTA) supplemented with 1 unit/mL
8-(1-(3-Fluorobenzyl)-1H-pyrazol-4-yl)-1,3-dimethyl-1H-pu-
rine-2,6(3H,7H)-dione (59). Following the general procedure
described above, the coupling of diamine 56a with 57b furnished
1
59 in 60% yield. H NMR (DMSO-d6): δ 13.53 (s, 1H), 8.50 (s,
1H), 8.11 (s, 1H), 7.38-7.44 (m, 1H), 7.11-7.18 (m, 3H), 5.43
(s, 2H), 3.46 (s, 3H), 3.25 (s, 3H); MS m/z 355.34 (M + H)+;
Anal. (C17H15FN6O2. 0.5CH2Cl2): C,H,N.
8-(1-(3-(Trifluoromethyl)benzyl)-1H-pyrazol-4-yl)-1,3-dimeth-
yl-1H-purine-2,6(3H,7H)-dione (60). Following the general pro-
cedure described above, the coupling of diamine 56a with 57c
furnished 60 in 75% yield. 1H NMR (DMSO-d6): δ 13.58 (s, 1H),
8.54 (s, 1H), 8.13 (s, 1H), 7.70-7.72 (m, 2H), 7.57-7.64 (m, 2H),
5.53 (s, 2H), 3.46 (s, 3H), 3.25 (s, 3H); MS m/z 404.34 (M + H)+,
425.30 (M+ + Na). Anal. (C18H15F3N6O2): C,H,N.

3692 Journal of Medicinal Chemistry, 2006, Vol. 49, No. 12
Kalla et al.
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adenosine deaminase. The assays were incubated at 25 °C for 90
min with gentle agitation, stopped by filtration using a Packard
Harvester and washed four times with ice-cold TM buffer (10 mM
Tris, 1 mM MgCl2, pH 7.4). Nonspecific binding was determined
in the presence of 10 µM ZM241385. The affinities of compounds
(i.e., Ki values) were calculated using GraphPad software.
Radioligand Binding for A1, A2A, and A3 Adenosine Recep-
tors.22 Human A1, A2A, and A3 adenosine receptor cDNAs were
stably transfected into either CHO or HEK-293 cells (referred to
as CHO-A1, HEK-A2A, and CHO-A3). The membranes were
prepared from these cells using the same protocol as described
above. Competition assays were started by mixing 0.5 nM 3H-CPX
3
(for CHO-A1), 2 nM H-ZM241385 (HEK-A2A), or 0.1 nM 125I-
AB-MECA (CHO-A3) with various concentrations of test com-
pounds and the perspective membranes in TE buffer (50 mM Tris
and 1 mM EDTA fo CHO-A1 and HEK-A2A) or TEM buffer (50
mM Tris, 1 mM EDTA and 10 mM MgCl2 for CHO-A3)
supplemented with 1 unit/mL adenosine deaminase. The assays were
incubated at 25 °C for 90 min with gentle agitation, stopped by
filtration using a Packard Harvester, and washed four times with
ice-cold TM buffer (10 mM Tris, 1 mM MgCl2, pH 7.4).
Nonspecific binding was determined in the presence of 1 µM CPX
(CHO-A1), 1 µM ZM241385 (HEK-A2A), and 1 µM IB-MECA
(CHO-A3). The affinities of compounds (i.e., Ki values) were
calculated using GraphPad software.
Acknowledgment. We would like to thank Dr. Brent
Blackburn and Dr. Luiz Belardinelli for valuable input and
discussions. We thank Marie Nguyen and Yuzhi Wu for
technical help in the assay group.
Supporting Information Available: Elemental analysis for
selected compounds is included. This material is available free of
charge via the Internet at http://pubs.acs.org.
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