3H-[1,2,4]-Triazolo[5,1-i]purin-5-amine derivatives as adenosine A2A antagonists

Article abstract of DOI:10.1016/j.bmcl.2006.12.104

A novel series of 3-substituted-8-aryl-[1,2,4]-triazolo[5,1-i]purin-5-amine analogs related to Sch 58261 was synthesized in order to identify potent adenosine A_2A receptor antagonists with improved selectivity over the A₁ receptor, physiochemical properties, and pharmacokinetic profiles as compared to those of Sch 58261. As a result of structural modifications, numerous analogs with excellent in vitro binding affinities and selectivities were identified. Moreover, compound 27 displayed both superior in vitro and highly promising in vivo profiles.

Full text of DOI:10.1016/j.bmcl.2006.12.104

Bioorganic & Medicinal Chemistry Letters 17 (2007) 1659–1662
3H-[1,2,4]-Triazolo[5,1-i]purin-5-amine derivatives
as adenosine A2A antagonists
Lisa S. Silverman,a John P. Caldwell,a,* William J. Greenlee,a Eugenia Kiselgof,a
Julius J. Matasi,a Deen B. Tulshian,a Leyla Arik,b Carolyn Foster,b
Rosalia Bertorelli,c Angela Monopolic and Ennio Onginic
aCV & CNS Departments of Chemical Research, Schering Plough Research Institute, 2015 Galloping Hill Road,
Kenilworth, NJ 07033, USA
bCV & CNS Departments of Biological Research, Schering Plough Research Institute, 2015 Galloping Hill Road,
Kenilworth, NJ 07033, USA
cCV & CNS Research Laboratories, Schering-Plough SPA, Comazzo, Milan, Italy
Received 22 November 2006; accepted 22 December 2006
Available online 8 January 2007
Abstract—A novel series of 3-substituted-8-aryl-[1,2,4]-triazolo[5,1-i]purin-5-amine analogs related to Sch 58261 was synthesized in
order to identify potent adenosine A2A receptor antagonists with improved selectivity over the A1 receptor, physiochemical prop-
erties, and pharmacokinetic profiles as compared to those of Sch 58261. As a result of structural modifications, numerous analogs
with excellent in vitro binding affinities and selectivities were identified. Moreover, compound 27 displayed both superior in vitro
and highly promising in vivo profiles.
Ó 2007 Elsevier Ltd. All rights reserved.
Adenosine, a purine nucleoside, modulates physiological
functions via interaction with specific cell surface recep-
Over the past few years, there has been extensive work
done to synthesize novel xanthine and non-xanthine
potent and selective A2A AR antagonists. KW-6002, a
xanthine based A2A AR antagonist, has been reported
to improve motor disability in the parkinsonian monkey
without provoking dyskinesia.7 In addition, numerous
novel non-xanthine based selective A2A AR antagonists
have been identified as well.8 Several years ago, SCH
58261, a pyrazolo[4,3-e]-1,2,4-triazolo[1,5-c]pyrimidine,
was identified as a very potent but only moderately
selective A2A AR antagonist (A2A Ki = 4.3 nM, A1/
A2A = 35).9 However, SCH 58261 exhibited poor phys-
iochemical and pharmacokinetic (PK) profiles. Conse-
quently, in order to identify novel A2A AR antagonists
with enhanced selectivity over A1 and improved solubil-
ity and PK profiles, we conducted a structure–activity
relationship (SAR) investigation where the pyrazole ring
of SCH 58261 was replaced with an imidazole ring as
shown in general structure 1.
tors. Four adenosine receptor subtypes (A1, A2A, A2B
,
and A3) have been identified and belong to the family
of G protein-coupled receptors (GPCR).1
The adenosine A2A receptor (AR) is abundantly ex-
pressed in striatum, compared to the A1 AR which is
widely distributed throughout the brain.2 The A2A and
A2B receptors stimulate adenylate cyclase activity which
controls intracellular AMP levels and plays an impor-
tant role in regulating motor function.3 A2A AR ago-
nists have been shown to inhibit locomotor activity
and induce catalepsy in rodents.4 In contrast, A2A AR
antagonists prevent the motor disturbances seen in
dopamine D2 receptor null mice.5 It has been discovered
that dopamine deficiency in the brain can initiate the on-
set of Parkinson’s disease (PD), a neurodegenerative
motor disorder.6 Consequently, A2A AR antagonists
may offer potential value as a new class of anti-symp-
tomatic drugs for the treatment of PD.
Using structure 1 as the scaffold, we varied the substitut-
ed side chains at N-3 and then we replaced the 2-furyl at
C-8 with various arene moieties. Herein we report a
summary of the results of our SAR investigation.
Keywords: Adenosine A2A receptor; Antagonist.
*
Corresponding author. Tel.: +1 908 740 5199; e-mail:
john.caldwell@spcorp.com
0960-894X/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2006.12.104

1660
L. S. Silverman et al. / Bioorg. Med. Chem. Lett. 17 (2007) 1659–1662
O
a
O
N
O
O
N
N
HO
N
N
N
N
O
7
6
N
OMe
OMe
O
N
KW-6002
O
N
NH
b
NH2
N
NH2
O
8
5
5
N
N
N
N
8
N
N
2
Aryl
3
N
Scheme 2. Reagents and conditions: (a) 2-bromoethyl methyl ether,
NaH, DMF, 60 °C, 80%; (b) 1—6 N HCl, 80 °C; 2—5 N NaOH, 95%.
7
N
R
O
N
N
1
8
Sch 58261
1
with 2-bromoethyl methyl ether to give intermediate
11. Next, 11 and piperazine were coupled via Buchwald
conditions to give 12. The remainder of piperazines
required to prepare compounds 21, 26, 29, 30, and 33–
38 were either synthesized in a similar fashion or com-
mercially available.
Compounds 21–45 were prepared according to the gen-
eral route described in Scheme 1 or via a slightly modi-
fied route to accommodate the structural modifications
described above. For example, the nucleophilic displace-
ment of bromine on commercially available 2-amino-6-
bromopurine (2) by 2-furoic hydrazide produced HBr
salt 3, which was treated with N,O-bis(trimethylsi-
lyl)acetamide (BSA) to undergo a dehydrative rear-
rangement to produce imidazole tricycle 4. Next, 4
was alkylated with ethylene glycol di-p-tosylate to give
a 1:1 mixture of N-3 (5) and N-1 alkylated products. De-
sired tosylate 5 was isolated by column chromatography
and coupled with piperazine and piperidine derivatives
to give the reported analogs.
Piperidine 17, used for the preparation of 31, was pre-
pared as shown in Scheme 4. Commercially available
4-bromophenol (13) was alkylated with 2-bromoethyl
methyl ether to give intermediate 14 which was lithiated
and added to the C-4 carbonyl of 4-oxo-piperidine-1-
carboxylic acid benzyl ester to yield intermediate 15.
Next, 15 was deoxygenated to yield 16 which was depro-
tected via palladium catalyzed hydrogenolysis to give 17.
The piperidine required to prepare compound 32 was
synthesized in a similar fashion. All compounds report-
ed herein gave satisfactory analytical results.10
The in vitro results of the A2A AR binding assays are
expressed as inhibition constants (Ki, nM) and A1/A2A
describes the selectivity over the A1 AR.11 All assays
were performed in duplicate and reported as mean val-
ues. These compounds were not tested against the other
Piperazine 8, incorporated in 27 and 39–45, was prepared
as shown in Scheme 2. Commercially available 1-acetyl-
4-(4-hydroxyphenyl)-piperazine (6) was alkylated with 2-
bromoethyl methyl ether to give intermediate 7 which
was deacetylated via acid hydrolysis to yield 8.
known adenosine receptor subtypes, A3 and A2B
.
Piperazine 12, used for the preparation of 28, was pre-
pared as shown in Scheme 3. Commercially available
1-bromo-2-fluoro-4-methoxybenzene (9) was demethy-
lated with BBr3 to give phenol 10 which was alkylated
Table 1 shows our initial SAR where the C-8 2-furanyl
moiety is fixed and N-3 is substituted with alkyl and
phenyl groups of varied size, chain length, and electronic
nature. All of the compounds demonstrate acceptable
potency for A2A (Ki = 1–6.3 nM). However compound
21, the phenyl piperazine analog, is the only one that
is selective over A1.
NH2
N
NH2
a
b
N
N
N
H
N
Table 2 shows our follow-up SAR where the C-8 2-fura-
nyl moiety is fixed and the N-3 side chain has been var-
ied with substituted aryl piperazines and piperidines.
With the exception of compound 36 (thiazole), the bal-
ance of analogs exhibit binding affinities (subnanomo-
Br
O
N
.
HN
N
HBr HN
H
N
O
N
2
3
NH2
NH2
N
N
c
d
N
N
N
N
N
F
F
O
F
O
HN
N
TsO
b
N
a
c
N
MeO
Br
HO
Br
O
Br
5
4
10
9
O
11
NH2
N
N
F
N
N
N
O
O
N
NH
R
N
O
12
Scheme 1. Reagents and conditions: (a) 2-furoic hydrazide, n-BuOH,
120 °C, 95%; (b) BSA, 100 °C, 95%; (c) (p-TsOCH2)2, NaH, DMF,
0 °C to rt, 20% (after purification); (d) R = aryl piperazine/aryl
piperidine, DMF, 90 °C, sealed tube, 60%.
Scheme 3. Reagents and conditions: (a) BBr3/CH2Cl2, D, 99%; (b)
K2CO3, 2-bromoethyl methyl ether, CH3CN, D, 98%; (c) piperazine,
Pd2(dba)3, NaOtBu, BINAP, toluene, D, 70%.

L. S. Silverman et al. / Bioorg. Med. Chem. Lett. 17 (2007) 1659–1662
1661
HO
b
Table 2. Receptor affinity of N-3 aryl piperidine/piperazine analogs
a
HO
Br
O
NCO2Bn
O
Br
NH2
13
O
O
14
15
N
N
N
N
d
c
O
O
NCO2Bn
O
NH
N
R
N
O
O
16
17
Compound
R
A2A Ki A1/A2A
(nM)
Scheme 4. Reagents and conditions: (a) K2CO3, 2-bromoethyl methyl
ether, CH3CN, D, 95%; (b) 1—n-BuLi, À78 °C, THF; 2—4-oxo-
piperidine-1-carboxylic acid benzyl ester, À78 °C to rt, 60%; (c)
Et3SiH, TFA, CH2Cl2, À78 °C to rt, 95%; (d) H2, 5% Pd/C, MeOH/
EtOAc, 1 atm, 98%.
21
1.7
182
N
N
Table 1. Receptor affinity of N-3 substituted analogs
26
27
0.1
0.9
1695
669
MeO
MeO
N
N
NH2
N
N
N
O
N
R
N
O
N
N
N
N
Compound
R
A2A Ki (nM) A1/A2A
MeO
MeO
F
28
29
30
0.8
0.4
0.7
517
218
88
18
5.9
50
O
N
N
19
20
21
6.3
4.1
1.7
37
CF3
N
O
O
F
N
33
182
F
N
N
N
N
MeO
22
23
24
25
1.5
4.6
1.7
4.6
17
7
31
0.7
519
O
N
Br
32
33
0.9
0.3
196
403
F
N
Cl
F
9
Cl
Cl
N
N
F3C
30
34
35
0.4
1.4
371
378
N
N
lar) superior to that of analog 21. Moreover, all of the
compounds except 30 (2,4-difluorophenyl) are at least
as selective as compound 21 (A1/A2A P ꢀ170). Com-
pounds 27, 28, 29, and 31 are of particular note since
all of them are extremely selective (A1/A2A > 500) and
they all contain 4-alkoxy phenyl substituents. In addi-
tion, the replacement of piperazine with piperidine
(compounds 27 and 31) does not affect the in vitro
potency and selectivity. Although 26 has the highest
binding affinity and selectivity, after kinetic solubility
and PK evaluations, compound 27 appeared to have
the best overall physiochemical profile. Consequently,
the [4-(2-methoxyethoxy)-phenyl]-1-piperazinyl-4-ethyl
side chain was selected for the final SAR study.
N
N
F
N
N
N
S
36
37
38
1.7
1.0
0.9
138
162
333
N
N
N
Me
Cl
N
N
N
N
N

1662
L. S. Silverman et al. / Bioorg. Med. Chem. Lett. 17 (2007) 1659–1662
Table 3. Receptor affinity of C-8 aryl analogs
Acknowledgments
NH2
We thank Dr. William Greenlee for his support of this
work. We also thank Professor Pier Giovanni Baraldi
et al. for providing us with compounds 18–20 which
were synthesized according to Gatta F. et al. Eur.
J. Med. Chem. 1993, 28, 569.
N
N
N
N
O
Aryl
O
N
N
N
N
Compound
Aryl
A2A Ki (nM)
A1/A2A
669
O
27
0.9
References and notes
39
40
41
42
43
44
45
3.7
2.0
91
83
1. Fredholm, B. B.; Abbracchio, M. P.; Burnstock, G.; Daly,
J. W.; Harden, T. K.; Jacobson, K. A.; Legg, P.; Williams,
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6. Ognini, E.; Fredholm, B. B. Trends Pharmacol. Sci. 1996,
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J. Med. Chem. 2002, 45, 101; (b) Baraldi, P. G.; Cacciari,
B.; Spalluto, G.; Bergonzoni, M.; Dionosotti, S.; Ongini,
E.; Varani, K.; Borea, P. A. J. Med. Chem. 1998, 41, 2126;
(c) Baraldi, P. G.; Cacciari, B.; Spalluto, G.; Borioni, A.;
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1995, 2, 707; (d) Vu, C. B.; Pan, D.; Peng, B.; Sha, L.;
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F
F
9.3
15
CF3
CH3
10.0
20.5
12.6
89.7
181
76
N
108
17
O
N
Table 3 shows the results of the SAR where the N-3 [4-
(2-methoxyethoxy)-phenyl]-1-piperazinyl-4-ethyl moiety
is fixed and the C-8 2-furanyl moiety has been replaced
with various substituted aryl groups. Compared to the
C-8 2-furanyl series, in general the C-8 substituted aryl
derivatives are less potent and selective. Compounds
39 and 40 (phenyl and 3-fluorophenyl) retain good bind-
ing affinity but selectivity against A1 is reduced 7-fold.
The 2-substituted phenyl and pyridinyl ring derivatives
(41–45) are 9- to 90-fold less potent and 4- to 40-fold less
selective than antagonist 27.
9. Zocchi, C.; Ongini, E.; Conti, A.; Monopoli, A.; Negretti,
A.; Baraldi, P. G.; Dionisotti, S. J. Pharmacol. Exp. Ther.
1996, 276, 398.
10. Compound 27: 1H NMR (DMSO-d6,400MHz): d 2.58 (br
s, 4H), 2.75 (t, 2H), 2.95 (br s, 4H), 3.26 (s, 3H), 3.58 (m,
2H), 3.96 (m, 2H), 4.27 (t, 2H), 6.70 (dd, 1H), 6.76–6.84
(m, 4H), 7.22 (m, 1H), 7.78 (br s, 2H), 7.92 (s, 1H), 8.05 (s,
1H).
In summary, we have identified 3-substituted-8-aryl-3H-
[1,2,4]-triazolo[5,1-i]purin-5-amines, a novel series of
adenosine A2A receptor antagonists which display excel-
lent binding affinity and selectivity over the A1 AR.
Most of the N-3 substituted aryl piperazine and piperi-
dine analogs demonstrate in vitro A2A receptor binding
affinity (Ki < 1 nM) and A1 receptor selectivity (A1/
A2A = 300–1700) profiles superior to those of SCH
58261. In addition, compound 27 has demonstrated
superior PK profiles at an oral dose of 3.0 mg/kg
(AUC = 524 ng h/mL) in addition to exhibiting in vivo
activity at 3 mg/kg at 1 and 4 h (80% and 50%) as well
as at 1 mg/kg at 1 and 4 h (55% and 50%) in the rat cat-
alepsy model.13 All studies were carried out in accor-
dance with the National Institute of Health Guide for
the Care and Use of Laboratory Animals.
11. Adenosine A2A and A1 binding assays: [3H]SCH-58261
and [3H]DPCPX binding assays for adenosine A2A and A1
receptors, respectively, were performed as described
previously.12
12. (a) Dionisotti, S.; Ongini, E.; Zocchi, C.; Kull, B.; Arslan,
G.; Fredholm, B. B. Br. J. Pharmacol. 1997, 121, 353; (b)
Klotz, K.-N.; Hessling, J.; Hegler, J.; Owman, C.; Kull,
B.; Fredholm, B. B.; Lohse, M. Naunyn-Schmiedeberg’s
Arch. Pharmacol. 1997, 357, 1.
13. (a) Matasi, J. J.; Caldwell, J. P.; Zhang, H.; Fawzi, A.;
Cohen-Williams, M. E.; Varty, G. B.; Tulshian, D. B.
Bioorg. Med. Chem. Lett. 2005, 15, 3670; (b) Matasi, J. J.;
Caldwell, J. P.; Zhang, H.; Fawzi, A.; Higgins, G. A.;
Cohen-Williams, M. E.; Varty, G. B.; Tulshian, D. B.
Bioorg. Med. Chem. Lett. 2005, 15, 3675.