Synthesis of a new series of 1H-imidazol-1-yl substituted 8-phenylxanthines as adenosine receptor ligands

Article abstract of DOI:10.1002/cbdv.201000141

A new series of 1H-imidazol-1-yl substituted 8-phenylxanthine analogs has been synthesized to study the effects of the imidazole group on the binding affinity of compounds for adenosine receptors. Competition binding studies of these compounds were carrie

Full text of DOI:10.1002/cbdv.201000141

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CHEMISTRY & BIODIVERSITY – Vol. 8 (2011)
Synthesis of a New Series of 1H-Imidazol-1-yl Substituted 8-Phenylxanthines
as Adenosine Receptor Ligands
by Ranju Bansal*a), Gulshan Kumara), Deepika Gandhia), Louise C. Youngb), and Alan L. Harveyb)
a) University Institute of Pharmaceutical Sciences, Panjab University, Chandigarh – 160014, India
(phone: þ91-1722541142; e-mail: ranju29in@yahoo.co.in)
b) Strathclyde Institute for Drug Research and Strathclyde Institute of Pharmacy and Biomedical
Sciences, University of Strathclyde, GB-27 Taylor Street, Glasgow G4 0NR
A new series of 1H-imidazol-1-yl substituted 8-phenylxanthine analogs has been synthesized to study
the effects of the imidazole group on the binding affinity of compounds for adenosine receptors.
Competition binding studies of these compounds were carried out in vitro with human cloned receptors
using [3H]DPCPX and [3H]ZM 241385 as radioligands at A1 and A2A adenosine receptors, respectively.
The effect of the substitution pattern of the (imidazolyl)alkoxy group on various positions of the phenyl
ring at C(8) was also studied. The xanthine derivatives displayed varying degrees of affinity and
selectivity towards A1 and A2A receptor subtypes despite a common but variedly substituted ArꢀC(8).
1. Introduction. – Adenosine receptors represent important pharmacological
targets in the treatment of a variety of diseases such as anti-inflammatory conditions,
sepsis, heart attack, asthma, diabetes, obesity, and Parkinsonꢀs disease [1][2]. Over the
past years, the search for ligands that show selectivity and potency towards individual
receptor subtypes has intensified, as the role of these receptors in many therapeutic
areas is continuously expanding [3–5]. ZM 241385 and DPCPX (Fig.) have been
reported as potent selective adenosine A2A and A1 receptor ligands, respectively [4].
Substituted xanthines (¼ 3,7-dihydro-1H-purine-2,6-diones) represent the most potent
class of adenosine receptor antagonists reported to date. 8-Phenyltheophylline is the
parent member of a variety of potent adenosine receptor antagonists. It has been
established that appropriate substituents on the phenyl ring at C(8) affects the potency
Figure. Structures of adenosine receptor ligands ZM 241385 (for A2A receptors) and DPCPX (for A1
receptors)
ꢁ 2011 Verlag Helvetica Chimica Acta AG, Zꢂrich

CHEMISTRY & BIODIVERSITY – Vol. 8 (2011)
1291
and selectivity towards adenosine receptors and thus their pharmacological effects
[4][6].
As a part of our ongoing research program directed towards the search for new 8-
phenyl-substituted xanthine-based highly effective ligands for the adenosine receptors
[6][7], here we report a new series of 8-{[(imidazolyl)alkoxy]phenyl}xanthines along
with variable substitutions at 1-, 3-, and 7-positions of the 8-phenylxanthine scaffold.
The substitution pattern of the 8-phenyl substituent was also changed to study its effects
on the affinity for adenosine A1 and A2A receptors. 1H-Imidazole, a pharmacologically
active heteroaromatic ring has been selected for substitution at C(8) of the phenyl ring,
as such a moiety has been shown to induce selectivity for adenosine receptors and may
also result in synergistic effects [8–10]. The substituents at 1-, 3- and 7-positions of the
skeleton in some selected xanthines were also modified to study the structureꢀactivity
relationship, as alterations in substitutions at these positions lead to noticeable changes
on the adenosine receptor affinity [11–13]. The newly synthesized compounds were
evaluated by radioligand-binding studies using [3H]DPCPX and [3H]ZM 241385 as
radioligands at A1 andA2A adenosine receptors, respectively [14][15].
2. Results and Discussion. – 2.1. Synthesis. A variety of synthetic pathways have
been followed by various research groups involved in the development of new
therapeutically useful xanthine derivatives [16–18]. The synthesis of 8-substituted
xanthine derivatives reported in this study has been carried out according to a
previously described procedure from uracil and corresponding aryl aldehydes [16] as
shown in Schemes 1–3. Oxidative cyclization of the resulting imine intermediate using
SOCl2 afforded the xanthines in good yields. Synthesis of 5,6-diamino-1,3-dimethylur-
acil (1), a key intermediate to the synthesis of all the 8-substituted xanthine derivatives,
was performed according to the general method as reported earlier [19][20].
Treatment of substituted aldehydes 2a–2d, which in turn were synthesized by
treating vanillin, isovanillin, and 3-hydroxybenzaldehyde with 1-bromo-3-chloropro-
pane or 1-bromo-2-chloroethane in refluxing ethyl methyl ketone, with 1 in MeOH/
AcOH 4 :1 at room temperature, resulted in the formation of the corresponding
(benzylidene)amino derivatives 3a–3d. A singlet integrating for one H-atom appeared
at ca. d(H) 9.75 for N¼CH in the 1H-NMR spectra of 3a–3d. Subsequent ring closure
of these intermediates by refluxing in SOCl2 for 30–40 min afforded the chloroalkoxy
compounds 4a–4d (Scheme 1). The incorporation of an imidazolyl group was achieved
by fusion of 4a–4d with powdered 1H-imidazole at 1608 for 2 h to afford
(imidazolyl)alkoxy derivatives 5a–5d. Resonances for imidazolyl H-atoms in the
1H-NMR spectra appeared in the range of d(H) 6.90–8.00. The effects of varying the
alkyl substituents at the 1-, 3-, and 7-positions of 8-substituted xanthines on adenosine
receptor affinity are well-known [21–23]. Therefore, we also modified the substitution
pattern at 1-, 3-, and 7-positions in some of the newly synthesized 8-(imidazolyl-
substituted)phenylxanthines to study the structureꢀactivity relationship. Propylation
was carried out by adding PrBr to a heated mixture of 4a and 4d, respectively, in DMF
in the presence of anhydrous K2CO3 to afford the corresponding 7-propyl derivatives 6
and 10 (Scheme 2), which showed NMR signals at d(H) 0.88 (t, MeCH2), 1.86 (m,
MeCH2), and 4.29 (t, CH2N) for H-atoms of the Pr group. Further, reaction of 6 and 10
with 1H-imidazole at 1208 for 2 h yielded the imidazolyl-substituted counterparts 7 and

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CHEMISTRY & BIODIVERSITY – Vol. 8 (2011)
Scheme 1. Synthetic Route to 8-Substituted Xanthines 4a–4d and 5a–5d
11, respectively (Scheme 2). For the synthesis of 8-(imidazolyl-substituted phenyl)-
xanthines with a Me group at 7-position, 4a and MeI were heated at 70–808 in DMF in
the presence of anh. K2CO3 to give 8, which, on treatment with imidazole, gave the
desired tetra-substituted xanthine 9. Three singlets for MeN groups of the purine
nucleus appeared in the NMR spectra of 8 and 9, confirming the formation of 1,3,7-
trimethylxanthine derivatives.
Condensation of 5,6-diamino-1-methyluracil [7][24] and 5,6-diamino-1-methyl-3-
propyluracil [7][24] with 2a in MeOH/AcOH 4 :1 afforded the corresponding
(benzylidene)amino derivatives 12a and 12b, which, on subsequent ring closure in
refluxing SOCl2, gave the corresponding chloroalkoxy derivatives 13a and 13b
(Scheme 3). Further, treatment with imidazole afforded (imidazolyl)propoxy deriva-
tives 14a and 14b, respectively.
2.2. Biological Studies. The newly synthesized compounds were evaluated using
radioligand-binding studies at cloned human A1 and A2A adenosine receptors.
[3H]DPCPX and [3H]ZM241385 were used as radioligands for A1 and A2A adenosine
receptors, respectively [14][15].
The observed affinities of various newly synthesized 8-phenylxanthine derivatives
in radioligand-binding assays at human A1 and A2A receptors are compiled in the Table.
The three series, i.e., vanillin-, isovanillin-, and 3-hydroxybenzaldehyde-based xanthine
derivatives displayed varying degrees of affinity and selectivity towards A1 and A2A
receptor subtypes despite a common substitution of chloro/(imidazolyl)alkoxy group

CHEMISTRY & BIODIVERSITY – Vol. 8 (2011)
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Scheme 2. Synthetic Route to 7-Alkylated Xanthines 6–11
on 8-phenyl ring in accordance with our earlier report [6]. This is attributed to the
varied positioning of the phenyl substituent. In general, the chloro precursors 4a–4d
(except 4c), 6, 8, and 10 were found to be more potent and selective ligands for A2A
over A1 receptors in comparison to their imidazolyl-substituted analogues 5a–5d, 7, 9,
and 11, respectively. The incorporation of an imidazole group in the side chain resulted
in increased affinity for A1 receptors and thereby decreased selectivity for A2A
receptors in case of vanillin-derived xanthine derivatives 5a, 7, 9, and 11. However, in
the case of 3-(1H-imidazol-1-yl)propoxy derivative 5d, which is without an ortho MeO
group, binding was observed only at A1 receptors (Ki of 3.6 mm) with little effect on A2A
receptors at 100 mm. The isovanillin derivatives 4c and 5c exhibited substantial loss of
binding affinity for both adenosine receptor subtypes.
7-Alkylation (propylation and methylation) of vanillin-based products 4a and 5a
resulted in marginal loss of affinity and selectivity for adenosine receptors in both
chloropropoxy (i.e., 6 and 8) and (imidazolyl)propoxy (i.e., 7 and 9) derivatives;
however, the binding pattern remains the same. The effect of propylation of
monosubstituted 8-phenylxanthines 4d and 5d was also tested. Although the effects
were insignificant for chloroalkoxy derivative 10, more than 100-fold increase in

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CHEMISTRY & BIODIVERSITY – Vol. 8 (2011)
Scheme 3. Synthesis of 1-Unsubstituted/Substituted 8-Substituted Xanthines 13a, 13b, 14a, and 14b
Table. Adenosine A1 and A2A Binding Affinities of Various Compounds
Compound
Code
Ki [mm]a)
A1
A2A
A1/A2A
4a
4b
4c
4d
5a
5b
5c
5d
6
7
8
9
10
11
13a
13b
14a
14b
RG- DPJ-28
RG-DPJ-104
RG-DPJ-69
RG-DPJ-95
RG-DPJ-29
RG-DPJ-105
RG-DPJ-70
RG-DPJ-96
RG-DPJ-239
RG-DPJ-241
RG-DPJ-237
RG-DPJ-295
RG-DPJ-252
RG-DPJ-251
RG-DPJ-256
RG-DPJ-264
RG-DPJ-257
RG-DPJ-296
DPCPX
>100
>100
>100
>100
3.1 (0.3–28)
>100
>100
3.6 (0.8–16)
>100
17.7 (0.7–43)
>100
18.9 (4.2–86)
>100
10.8 (2.2–52)
>100
>100
>100
0.32 (0.2–0.6)
0.18 (0.08–0.37)
>100
0.1 (0.06–0.18)
0.5 (0.25–0.98)
0.7 (0.6–0.9)
>100
>100
1 (0.6–1.5)
>312
>555
–
>1000
6.2
>142
–
0.036
>100
21
>138
13.5
>34
0.84 (0.7–1.1)
0.7 (0.4–1.2)
1.4 (0.8–2.5)
2.9
2.0 (1.2–3.4)
5
–
–
–
2.5
0.73
8.43
>100
>100
>100
1.1 (0.6–2.2)
0.44 (0.3–0.7)
0.13b)
0.064 (0.03–0.14)
0.095 (0.06–0.15)
ZM241385
0.54b)
a) Ki Values are given with 95% confidence limits. b) From [25].

CHEMISTRY & BIODIVERSITY – Vol. 8 (2011)
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binding affinity was observed for A2A receptors in case of (imidazolyl)propoxy
derivative 11 in comparison to 5d. 1-Unsubstituted analogs 13a and 14a showed little
variation of binding at A1 and A2A receptors at concentrations up to 100 mm. In contrast,
introduction of 1-Pr group instead of Me led to loss of affinity for A2A receptors in case
of chloropropoxy 13b but almost no change in pattern of activity for (imidazolyl)pro-
poxy derivative 14b in comparison to their corresponding methylated derivatives 4a
and 5a, respectively.
The newly synthesized xanthine derivatives displayed slightly to markedly weaker
binding affinity for adenosine receptors in comparison to the reference adenosine
receptor ligands DPCPX and ZM241385 as shown in the Table. However, compound 5d
displayed 28 times more selectivity for A1 vs. A2A in comparison to DPCPX, which is
only 1.5 times more selective, and compound 4d exhibited 1000 times more selectivity
on A2A vs. A1 as compared to ZM241385 with A2A vs. A1 selectivity of 8.43.
3. Conclusion. – A new series of 8-{[(imidazolyl)alkoxy]phenyl}xanthines has been
synthesized and evaluated for adenosine-binding affinity. Varied substitutions at 1-, 3-,
and 7-positions and the substitution pattern of an 8-phenyl substituent cause marked
changes in the binding affinity of xanthines for adenosine A1 and A2A receptors.
Suitable selection and positioning of substituents in xanthine skeleton may lead to
development of potent and selective adenosine receptor ligands of therapeutic value.
The authors are thankful to the Haryana State Council of Science & Technology, India, and the
Association of Commonwealth Universities, United Kingdom, for providing financial support.
Experimental Part
Chemistry. General. All solvents were freshly distilled and dried prior to use according to standard
procedures. Anh. Na2SO4 was used as drying agent. TLC: silica gel G according to the method of Stahl
(E. Merck) with AcOEt as solvent and activated at 1108 for 30 min; I2 was used to develop the TLC
plates. M.p.: Veego melting-point apparatus; uncorrected. IR Spectra: Perkin-Elmer 882 and RX1 FT-IR
spectrophotometer models in KBr (˜nmax in cmꢀ1). 1H-NMR: Bruker AC-300F, 300 MHz and Varian EM-
360, 60 MHz spectrometers in CDCl3/(D6)DMSO; d in ppm (TMS¼0 ppm). Elemental analyses:
Perkin-Elmer-2400 CHN elemental analyzer.
General Procedure for the Synthesis of the Substituted Aldehydes 2a–2d. 1-Bromo-3-chloropropane
(1.5 ml, 14.15 mmol) was added to a stirred and refluxing suspension of vanillin, isovanillin, and 3-
hydroxybenzaldehyde (1.0 g, 6.57 mmol) in the presence of anh. K2CO3 (2.0 g, 14.47 mmol) and ethyl
methyl ketone (40 ml). In an analogous manner, vanillin was treated with 1-bromo-2-chloroethane. The
mixture was further refluxed for 6 h with continuous stirring, and the reaction was monitored by TLC.
The mixture was cooled, filtered, and the excess of solvent was removed under reduced pressure to afford
4-(3-chloropropoxy)-3-methoxybenzaldehyde (2a), 4-(2-chloroethoxy)-3-methoxybenzaldehyde (2b), 3-
(3-chloropropoxy)-4-methoxybenzaldehyde (2c), and 3-(3-chloropropoxy)benzaldehyde (2d) [26][27],
resp., as oily residues which were used as such for further reaction.
General Procedure for the Synthesis of the (Benzylidene)amino Derivatives 3a–3d and 12a–12b. To a
stirred soln. of 5,6-diamino-1,3-dimethyluracil (1; 1.0 g, 5.87 mmol) in MeOH/AcOH 4 :1 (40 ml) was
slowly added the soln. of the oily 2a–2d in MeOH (24 ml). A precipitate formed after some time, and the
mixture was further stirred overnight at r.t. The precipitate thus obtained was filtered off, washed with
MeOH, and dried to afford the corresponding benzylidene derivatives 3a–3d, resp., which were used as
such for further cyclization. For the synthesis of 12a and 12b, 2a was reacted with 5,6-diamino-1-
methyluracil and 5,6-diamino-1-methyl-3-propyluracil, resp.

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CHEMISTRY & BIODIVERSITY – Vol. 8 (2011)
6-Amino-5-{[4-(3-chloropropoxy)-3-methoxybenzylidene]amino}-1,3-dimethylpyrimidine-2,4(1H,3H)-
dione (3a). 1H-NMR (300 MHz, CDCl3/(D6)DMSO): 2.24 (quint., CH2); 3.25 (s, MeN); 3.46 (s, MeN);
3.77 (t, J¼4.40, ClCH2); 3.89 (s, MeO); 4.15 (t, J¼4.02, CH2O); 6.93 (br. s, NH2; disappeared on D2O
exchange); 7.25 (d, Jo ¼8.19, 1 arom. H); 7.46 (s, 1 arom. H); 9.66 (s, HꢀC¼N).
6-Amino-5-{[4-(2-chloroethoxy)-3-methoxybenzylidene]amino}-1,3-dimethylpyrimidine-2,4(1H,3H)-
dione (3b). 1H-NMR (300 MHz, CDCl3/(D6)DMSO): 3.38 (s, MeN); 3.48 (s, MeN); 3.83–3.87 (m,
ClCH2); 3.91 (s, MeO); 4.33 (t, J¼5.79, CH2O); 5.66 (br. s, NH2; disappeared on D2O exchange); 6.92 (d,
Jo ¼8.50, 1 arom. H); 7.28 (dd, Jo ¼8.26, Jm ¼2.00, 1 arom. H); 7.37 (d, Jm ¼1.99, 1 arom. H); 9.73 (s,
HꢀC¼N).
6-Amino-5-{[3-(3-chloropropoxy)-4-methoxybenzylidene]amino}-1,3-dimethylpyrimidine-2,4(1H,3H)-
dione (3c). 1H-NMR (60 MHz, CDCl3/(D6)DMSO): 2.10–2.60 (m, CH2); 3.40 (s, MeN); 3.50 (s, MeN);
3.7–4.0 (m, ClCH2); 3.95 (s, MeO); 4.28 (t, J¼5.40, CH2O); 5.81 (br. s, NH2; disappeared on D2O
exchange); 6.85–7.15 (m, 1 arom. H); 7.3–7.7 (m, 2 arom. H); 9.85 (s, HꢀC¼N).
6-Amino-5-{[3-(3-chloropropoxy)benzylidene]amino}-1,3-dimethylpyrimidine-2,4(1H,3H)-dione
(3d). 1H-NMR (60 MHz, CDCl3/(D6)DMSO): 1.90–2.50 (m, CH2); 3.35 (s, MeN); 3.45 (m, MeN,
ClCH2); 4.12 (t, J¼4.50, CH2O); 5.97 (br. s, NH2; disappeared on D2O exchange); 6.90 (m, 1 arom. H);
7.32 (m, 3 arom. H); 9.70 (s, HꢀC¼N).
6-Amino-5-{[4-(3-chloropropoxy)-3-methoxybenzylidene]amino}-1-methylpyrimidine-2,4(1H,3H)-
dione (12a). 1H-NMR (300 MHz, CDCl3/(D6)DMSO): 2.22–2.25 (m, CH2); 3.51 (s, MeN); 3.77 (t,
ClCH2); 3.89 (s, MeO); 4.23 (t, J¼6.0, CH2O); 5.60 (br. s, NH2; disappeared on D2O exchange); 6.92 (d,
Jo ¼8.18, 1 arom. H); 7.30–7.33 (m, 2 arom. H); 9.72 (s, HꢀC¼N).
6-Amino-5-{[4-(3-chloropropoxy)-3-methoxybenzylidene]amino}-1-methyl-3-propylpyrimidine-
2,4(1H,3H)-dione (12b). 1H-NMR (300 MHz, CDCl3): 0.96 (t, J¼7.42, MeCH2); 1.65–1.70 (m, MeCH2);
2.28–2.34 (m, ClCH2CH2); 3.51 (s, MeN); 3.77 (t, J ¼ 6.28, ClCH2); 3.90 (s, MeO); 3.92–3.97 (m,
CH2N); 4.21 (t, J¼6.0, CH2O); 5.62 (br. s, NH2; disappeared on D2O exchange); 6.92 (d, Jo ¼8.18, 1
arom. H); 7.29–7.36 (m, 2 arom. H); 9.74 (s, HꢀC¼N).
General Procedure for the Synthesis of 8-[(Chloroalkoxy)phenyl]xanthines 4a–4d and 13a–13b. The
Schiff bases 3a – 3d and 12a–12b (1.0 g, 2.63 mmol) were heated in SOCl2 (20 ml) for 30–40 min to
undergo cyclization. The excess SOCl2 was removed under reduced pressure to obtain a solid product.
Ice-cold H2O was added to it, and the resultant suspension was neutralized with NH4OH soln. The
precipitate obtained was collected by filtration, dried, and recrystallized from a mixture DMF/MeOH to
obtain the desired products 4a–4d and 13a–13b, resp.
8-[4-(3-Chloropropoxy)-3-methoxyphenyl]-3,7-dihydro-1,3-dimethyl-1H-purine-2,6-dione (4a).
Yield: 0.75 g (75.45%). M.p. >2708. IR (KBr): 3200, 2960, 1690, 1640, 1490. 1H-NMR (300 MHz,
CDCl3/(D6)DMSO): 2.22 (quint., CH2); 3.28 (s, MeN); 3.51 (s, MeN); 3.79 (t, J ¼ 6.37, ClCH2); 3.87 (s,
MeO); 4.15 (t, J¼5.98, CH2O); 7.02 (d, Jo ¼8.31, 1 arom. H); 7.68–7.73 (m, 2 arom. H); 13.48 (br. s, NH).
Anal. calc. for C17H19ClN4O4 (378.80): C 53.89, H 5.05, N 14.79; found: C 53.76, H 4.78, N 14.24.
8-[4-(2-Chloroethoxy)-3-methoxyphenyl]-3,7-dihydro-1,3-dimethyl-1H-purine-2,6-dione (4b). Yield:
0.68 g (68.68%). M.p. 296–2988. FT-IR (KBr): 3176, 2950, 1690, 1640, 1490, 1235. 1H-NMR (300 MHz,
CDCl3/(D6)DMSO): 3.33 (s, MeN); 3.58 (s, MeN); 3.64–3.90 (m, J¼5.80, ClCH2); 3.92 (s, MeO); 4.29 (t,
J¼5.79, CH2O); 6.96 (d, Jo ¼8.50, 1 arom. H); 7.70 (dd, Jo ¼8.26, Jm ¼2.00, 1 arom. H); 7.74 (d, Jm ¼1.99, 1
arom. H); 13.2 (s, NH). Anal. calc. for C16H17ClN4O4 (364.5): C 52.68, H 4.69, N 15.36; found: C 52.33, H
4.17, N 14.90.
8-[3-(3-Chloropropoxy)-4-methoxyphenyl]-3,7-dihydro-1,3-dimethyl-1H-purine-2,6-dione (4c).
1
Yield: 0.96 g (96.98%). M.p. 236–2388. IR (KBr): 3300, 1680, 1650, 1480, 1200. H-NMR (300 MHz,
CDCl3/(D6)DMSO): 2.30 (quint., J¼6.07, CH2); 3.43 (s, MeN); 3.66 (s, MeN); 3.76 (t, J¼6.24, ClCH2);
3.90 (s, MeO); 4.23 (t, J¼5.89, CH2O); 6.90 (s, 1 arom. H); 7.50 (s, 1 arom. H); 12.50 (s, NH). Anal. calc.
for C17H19ClN4O4 (378.80): C 53.89, H 5.05, N 14.79; found: C 53.76, H 4.85, N 14.42.
8-[3-(3-Chloropropoxy)phenyl]-1,3-dimethyl-3,7-dihydro-1H-purine-2,6-dione (4d). Yield: 0.43 g
1
(43.46%). M.p. 240–2428. IR (KBr): 3190, 2960, 1700, 1650, 1480, 1210. H-NMR (300 MHz, CDCl3/
(D6)DMSO): 2.23–2.39 (m, CH2); 3.40 (s, MeN); 3.63 (s, MeN); 3.79 (t, J¼5.79, ClCH2); 4.18 (t, J¼5.77,
CH2O); 6.95–7.0 (m, 1 arom. H); 7.35 (t, Jo ¼8.11, 1 arom. H); 7.71–7.77 (m, 2 arom. H); 13.50 (br. s,
NH). Anal. calc. for C16H17ClN4O3 (348.78): C 55.09, H 4.91, N 16.06; found: C 54.88, H 4.44, N 15.98.

CHEMISTRY & BIODIVERSITY – Vol. 8 (2011)
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8-[4-(3-Chloropropoxy)-3-methoxyphenyl]-3,7-dihydro-3-methyl-1H-purine-2,6-dione (13a). Yield:
0.75 g (75.45%). M.p. >3008. FT-IR (KBr): 3161, 3016, 1679, 1592, 1493, 1250, 1031. 1H-NMR
(300 MHz, CDCl3/(D6)DMSO): 2.30–2.34 (m, CH2); 3.69 (s, MeN); 3.78 (t, J¼6.06, ClCH2); 3.96 (s,
MeO); 4.27 (t, J¼5.87, CH2O); 7.06 (d, Jo ¼8.73, 1 arom. H); 7.61 (s, 1 arom. H); 7.69 (d, Jo ¼ 8.37, 1 arom.
H). Anal. calc. for C16H17ClN4O4 (364.78): C 52.6, H 4.70, N 15.30; found: C 52.22, H 4.38, N 15.19.
8-[4-(3-Chloropropoxy)-3-methoxyphenyl]-3,7-dihydro-3-methyl-1-propyl-1H-purine-2,6-dione
(13b). Yield: 0.80 g (80.64%). M.p. 212–2168. FT-IR (KBr): 3148, 3003, 2819, 1687, 1593, 1493, 1427,
1254, 1229, 1023. 1H-NMR (300 MHz, CDCl3): 0.97 (t, J¼7.35, MeCH2); 1.69–1.77 (m, MeCH2); 2.29–
2.37 (m, ClCH2CH2); 3.76 (s, MeN); 3.79 (t, J¼6.13, ClCH2); 3.99 (s, MeO); 4.07 (t, J¼7.48, CH2N); 4.24
(t, J¼5.89, CH2O); 6.99 (d, Jo ¼8.35, 1 arom. H); 7.75 (s, 1 arom. H); 7.79 (d, Jo ¼8.34, 1 arom. H). Anal.
calc. for C19H23ClN4O4 (406.85): C 56.09, H 5.69, N 13.77; found: C 55.95, H 5.57, N 13.58.
General Procedure for the Synthesis of 7-Alkylated Xanthine Derivatives 6, 8, and 10. 1-
Bromopropane (1.2 ml, 13.98 mmol) was added to a stirred slurry of 4a and 4c (0.7 g, 1.85 mmol) in
presence of anh. K2CO3 (1.5 g, 10.85 mmol) in DMF (15 ml). Further, the mixture was stirred at 70–808
for 5 h, and completion of the reaction was monitored by TLC. The resultant suspension was filtered,
cooled, and H2O was added. The precipitate obtained was filtered, washed thoroughly with cold H2O,
dried, and recrystallized from MeOH to afford corresponding 7-propylated chloroalkoxy derivatives 6
and 10. The 7-methylated derivative 8 was synthesized by the same procedure by adding MeI as an
alkylating agent to the stirred slurry of 4a.
8-[4-(3-Chloropropoxy)-3-methoxyphenyl]-3,7-dihydro-1,3-dimethyl-7-propyl-1H-purine-2,6-dione
(6). Yield: 0.4 g (51.9%). M.p. 108–1128. IR (KBr): 2965, 1695, 1537, 1438, 1275, 1036. 1H-NMR
(300 MHz, CDCl3): 0.88 (t, J¼7.40, MeCH2); 1.83–1.88 (m, MeCH2); 2.09–2.15 (m, ClCH2CH2); 3.44 (s,
MeN); 3.64 (s, MeN); 3.90 (t, J ¼ 5.94, ClCH2); 3.94 (s, MeO); 4.25–4.33 (m, CH2N, CH2O); 7.00 (d, Jo ¼
8.79, 1 arom. H); 7.13–7.16 (m, 2 arom. H). Anal. calc. for C20H25ClN4O4 (420.88): C 57.07, H 5.99, N
13.31; found: C 56.91, H 5.97, N 13.05.
8-[4-(3-Chloropropoxy)-3-methoxyphenyl]-3,7-dihydro-1,3,7-trimethyl-1H-purine-2,6-dione (8).
Yield: 0.65 g (75%). M.p. 182–1848. FT-IR (KBr): 2945, 1694, 1656, 1538, 1486, 1427, 1235, 1031.
1H-NMR (300 MHz, CDCl3): 2.31–2.39 (m, CH2CH2); 3.39–3.43 (m, MeN, ClCH2); 3.63 (s, MeN); 3.93
(s, MeN); 4.05 (s, MeO); 4.16 (t, J¼5.88, CH2O); 6.99 (d, Jo ¼ 8.87, 1 arom. H); 7.19 (dd, Jo ¼8.08, Jm ¼
2.0, 1 arom. H); 7.24 (d, Jm ¼1.95, 1 arom. H). Anal. calc. for C18H21ClN4O4 (392.83): C 55.03, H 5.39, N
14.26; found: C 54.98, H 4.82, N 14.06.
8-[3-(3-Chloropropoxy)phenyl]-3,7-dihydro-1,3-dimethyl-7-propyl-1H-purine-2,6-dione (10). Yield:
0.15 g (19.10%). M.p. 118–1208. FT-IR (KBr): 2957, 1698, 1659, 1540, 1432, 1289, 1222, 1044. 1H-NMR
(300 MHz, CDCl3): 0.87 (t, J ¼ 7.39, MeCH2); 1.71–1.90 (m, MeCH2); 2.23–2.39 (m, ClCH2CH2); 3.44 (s,
MeN); 3.61 (s, MeN); 3.77 (t, J¼6.38, ClCH2); 4.18 (t, J¼5.70, CH2O); 4.32 (t, J¼7.81, CH2N); 7.06 (dd,
Jo ¼7.79, Jm ¼1.66, 1 arom. H); 7.17–7.19 (m, 2 arom. H); 7.44 (t, Jo ¼8.20, 1 arom. H). Anal. calc. for
C19H23ClN4O3 (390.85): C 58.39, H 5.93, N 14.33; found: C 58.12, H 5.72, N 13.95.
General Procedure for the Synthesis of 8-[(Imidazolylalkoxy)phenyl]xanthines 5a–5d, 7, 9, 11, and
14a and 14b. The chloroalkoxy derivatives 4a–4d, 6, 8, 10, and 13a and 13b (0.40 g, 1.06 mmol) were
treated with powdered 1H-imidazole (0.40 g, 5.87 mmol) at 1408 for 3 h. The mixture was refluxed in dist.
H2O for 1 h to remove the excess imidazole and filtered. Ice-cold H2O was added to the mixture and
cooled in ice for complete precipitation. The precipitate thus obtained was filtered, washed with H2O, and
dried to obtain corresponding (imidazolyl)alkoxy compounds 5a–5d, 7, 9, 11, and 14a and 14b. The
compounds 5a–5d and 14a and 14b were recrystallized from a mixture DMF/MeOH, whereas 7, 9, and 11
were recrystallized from acetone.
3,7-Dihydro-8-{4-[3-(1H-imidazol-1-yl)propoxy]-3-methoxyphenyl}-1,3-dimethyl-1H-purine-2,6-di-
one (5a). Yield: 0.17 g (39.26%). M.p. 254–2588. IR (KBr): 3200, 2950, 1690, 1640, 1490, 1235. 1H-NMR
(300 MHz, CDCl3/(D6)DMSO): 2.20 (quint., CH2CH2CH2); 3.25 (s, MeN); 3.48 (s, MeN); 3.87 (s, MeO);
3.95 (t, J¼6.14, CH2N); 4.15 (t, J ¼ 6.84, CH2O); 6.91 (s, 1 imidazole H); 7.05 (d, Jo ¼8.40, 1 arom. H);
7.22 (s, 1 imidazole H); 7.64–7.69 (m, 2 arom. H); 7.73 (s, 1 imidazole H). Anal. calc. for C20H22N6O4
(410.43): C 58.52, H 5.40, N 20.47; found: C 58.47, H 5.22, N 20.29.
3,7-Dihydro-8-{4-[2-(1H-imidazol-1-yl)ethoxy]-3-methoxyphenyl}-1,3-dimethyl-1H-purine-2,6-dione
(5b). Yield: 0.32 g (74.07%). M.p. 288–2928. FT-IR (KBr): 3184, 2944, 1684, 1643, 1555, 1488, 1446, 1276,

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CHEMISTRY & BIODIVERSITY – Vol. 8 (2011)
1223, 1038. 1H-NMR (300 MHz, CDCl3/(D6)DMSO): 3.40 (s, MeN); 3.63 (s, MeN); 3.95 (s, MeO); 4.34
(t, J¼4.92, CH2N); 4.48 (t, J ¼ 4.77, CH2O); 6.91 (d, Jo ¼9.10, 1 arom. H); 7.10 (s, 1 imidazole H); 7.26 (s,
1 imidazole H); 7.73 (dd, Jo ¼8.44, Jm ¼1.76, 1 arom. H); 7.77 (d, Jm ¼1.6, 1 arom. H); 7.99 (s, 1 imidazole
H). Anal. calc. for C19H20N6O4 (396): C 57.57, H 5.09, N 21.20; found: C 57.74, H 4.84, N 21.09.
3,7-Dihydro-8-{3-[3-(1H-imidazol-1-yl)propoxy]-4-methoxyphenyl}-1,3-dimethyl-1H-purine-2,6-di-
one (5c). Yield: 0.3 g (69.23%). M.p. 2368. IR (KBr): 3320, 1680, 1620, 1480, 1200. 1H-NMR (300 MHz,
CDCl3/(D6)DMSO): 2.29 (quint., J¼6.15, CH2); 3.42 (s, MeN); 3.63 (s, MeN); 3.93 (s, MeO); 4.02 (t, J ¼
5.70, CH2); 4.25 (t, J¼6.65, CH2O); 6.97–7.03 (m, 2 imidazole H, 1 arom. H); 7.37 (s, 1 arom. H), 7.60 (s,
1 imidazole H); 12.85 (s, NH). Anal. calc. for C20H22N6O4 (410.43): C 58.52, H 5.40, N 20.48; found: C
58.48, H 5.12, N 19.98.
3,7-Dihydro-8-{3-[3-(1H-imidazol-1-yl)propoxy]phenyl}-1,3-dimethyl-1H-purine-2,6-dione (5d).
Yield: 0.16 g (36.69%). M.p. 218–2208. FT-IR (KBr): 3190, 2950, 1700, 1650, 1450, 1210. 1H-NMR
(300 MHz, CDCl3/(D6)DMSO): 2.26 (quint., J¼6.10, CH2CH2CH2); 3.32 (s, MeN); 3.56 (s, MeN); 4.00
(t, J¼5.65, CH2N); 4.22 (t, J¼6.70, CH2O); 6.85 (s, 2 imidazole H); 7.04 (s, 1 arom. H); 7.32 (t, Jo ¼7.90, 1
arom. H); 7.54 (s, 1 imidazole H); 7.70–7.74 (m, 2 arom. H). Anal. calc. for C19H20N6O3 (380.4): C 59.99,
H 5.30, N 22.09; found: C 59.94, H 5.30, N 21.87.
3,7-Dihydro-8-{4-[3-(1H-imidazol-1-yl)propoxy]-3-methoxyphenyl}-1,3-dimethyl-7-propyl-1H-pu-
rine-2,6-dione (7). Yield: 0.18 g (40.74%). M.p. 158–1608. FT-IR (KBr): 2963, 1690, 1655, 1537, 1477,
1418, 1227, 1033. 1H-NMR (300 MHz, CDCl3): 0.89 (t, J¼7.39, MeCH2); 1.84–1.91 (m, MeCH2); 2.29–
2.37 (m, CH2CH2CH2); 3.44 (s, MeN); 3.63 (s, MeN); 3.95 (s, MeO); 4.01 (t, J ¼ 5.77, CH2O); 4.26–4.33
(m, 2 CH2N); 6.92 (d, Jo ¼8.28, 1 arom. H); 7.00 (s, 1 imidazole H); 7.11–7.14 (m, 2 arom. H), 7.18 (s, 1
imidazole H); 7.75 (s, 1 imidazole H). Anal. calc. for C23H28N6O4 (452.5): C 61.05, H 6.24, N 18.57; found:
C 60.72, H 6.18, N 18.41.
3,7-Dihydro-8-{4-[3-(1H-imidazol-1-yl)propoxy]-3-methoxyphenyl}-1,3,7-trimethyl-1H-purine-2,6-
dione (9). Yield: 0.32 g (74.07%). M.p. 180–1848. FT-IR (KBr): 2959, 1694, 1649, 1538, 1488, 1426, 1259,
1034. 1H-NMR (300 MHz, CDCl3): 2.27–2.35 (m, CH2CH2CH2); 3.43 (s, MeN); 3.62 (s, MeN); 3.96 (s,
MeN); 4.00 (t, J¼5.74, CH2N); 4.05 (s, MeO); 4.26 (t, J¼5.84, CH2O); 6.91 (d, Jo ¼8.48, 1 arom. H);
6.99–7.02 (m, 1 arom. H); 7.09 (s, 1 imidazole H); 7.17 (dd, Jo ¼8.34, Jm ¼1.73, 1 arom. H); 7.22 (m, 1
imidazole H); 7.24 (s, 1 imidazole H). Anal. calc. for C21H24N6O4 (424.45): C 59.42, H 5.70, N 19.80;
found: C 59.12, H 5.41, N 19.47.
3,7-Dihydro-8-{3-[3-(1H-imidazol-1-yl)propoxy]phenyl}-1,3-dimethyl-7-propyl-1H-purine-2,6-dione
(11). Yield: 0.18 g (50.92%). M.p. 128–1308. FT-IR (KBr): 3114, 2954, 1694, 1652, 1539, 1459, 1223, 1045.
1H-NMR (300 MHz, CDCl3): 0.86 (t, J¼7.67, MeCH2); 1.79–1.86 (m, MeCH2); 2.25–2.33 (m,
CH2CH2CH2); 3.44 (s, MeN); 3.62 (s, MeN); 3.99 (t, J¼5.71, CH2O); 4.19–4.34 (m, 2 CH2N); 6.95 (s,
1 imidazole H); 7.02 (dd, Jo ¼8.23, Jm ¼2.63, 1 arom. H); 7.11 (s, 1 imidazole H); 7.16–7.20 (m, 2 arom.
H); 7.43 (t, Jo ¼7.98, 1 arom. H); 7.75 (s, 1 imidazole H). Anal. calc. for C22H26N6O3 (422.48): C 62.54, H
6.20, N 19.89; found: C 62.28, H 5.95, N 19.47.
3,7-Dihydro-8-{4-[3-(1H-imidazol-1-yl)propoxy]-3-methoxyphenyl}-3-methyl-1H-purine-2,6-dione
(14a). Yield: 0.32 g (73.66%). M.p. >3008. FT-IR (KBr): 3155, 3039, 2941, 1688, 1592, 1498, 1423, 1391,
1249, 1029. 1H-NMR (300 MHz, CDCl3/(D6)DMSO): 2.44–2.48 (m, CH2CH2CH2); 3.70 (s, MeN); 3.94
(s, MeO); 4.17 (t, J¼5.33, CH2N); 4.52 (t, J¼6.37, CH2O); 7.01 (d, Jo ¼8.58, 1 arom. H); 7.42 (s, 1 arom.
H, 1 imidazole H); 7.58 (s, 1 imidazole H); 7.67 (dd, Jo ¼8.61, Jm ¼1.67, 1 arom. H); 8.74 (s, 1 imidazole
H). Anal. calc. for C19H20N6O4 (396.4): C 57.57, H 5.08, N 21.20; found: C 57.85, H 4.78, N 20.98.
3,7-Dihydro-8-{4-[3-(1H-imidazol-1-yl)propoxy]-3-methoxyphenyl}-3-methyl-1-propyl-1H-purine-
2,6-dione (14b). Yield: 0.33 g (77.39%). M.p. 226–2308. FT-IR (KBr): 3120, 2959, 1694, 1652, 1567, 1488,
1319, 1265, 1076. 1H-NMR (300 MHz, CDCl3): 0.97 (t, J¼8.88, MeCH2); 1.67–1.74 (m, MeCH2); 2.30–
2.34 (m, ClCH2CH2); 3.68 (s, MeN); 3.98 (s, MeO); 4.03 (t, J ¼ 7.23, 2 CH2N); 4.28 (t, J¼6.31, CH2O);
6.87 (d, Jo ¼7.88, 1 arom. H); 6.97 (s, 1 imidazole H); 7.12 (s, 1 imidazole H); 7.68–7.76 (m, 3 arom. H, 1
imidazole H). Anal. calc. for C22H26N6O4 (438.48): C 60.26, H 5.98, N 19.17; found: C 60.14, H 5.73, N
18.88.
Biological Activity. Radioligand-binding assays of xanthine analogs were performed using cloned
human adenosine A1 and A2A receptors and [3H]DPCPX and [3H]ZM 241385 as radioligands, resp.
[3H]DPCPX from Tocris Cookson (Bristol, UK) with specific activity 3.8 TBq/mmol, concentration

CHEMISTRY & BIODIVERSITY – Vol. 8 (2011)
1299
37 MBq/ml, was used in assays at 20 nm. The receptor membrane preparation (human recombinant
adenosine A1 receptor – ES-010-M) was from Euroscreen (B-Brussels). The recombinant adenosine A1
receptor was stably expressed in CHOꢀK1 cells; the membrane suspensions (received as frozen aliquots
in 7.5 mm Tris·HCl pH 7.5; 12.5 mm MgCl2, 0.3 mm EDTA, 1 mm EGTA, and 250 mm sucrose) were
diluted in assay buffer on thawing. [3H]ZM 241385 from Tocris Cookson (UK-Bristol) with specific
activity 0.777 TBq/mmol, concentration 37 MBq/ml, was used in assays at 20 nm. Receptor membrane
preparation (human A2A receptor membrane RBHA2AM) was from Wallac (UK-Beaconsfield). The
human recombinant A2A receptor was expressed in HEK-293 cells. The membrane suspensions were
received as frozen aliquots in 50 mm Tris·HCl (pH 7.4) and 10% sucrose, and were diluted in assay buffer
on thawing. The assays were performed using a similar method as described in [7][14][15]. Binding
assays were performed using Milipore (Watford, UK) Multiscreen MHAF B3 H60 filter plates presoaked
in 0.3% polyethyleneimine (PEI). Ten mg of membrane protein was used in the final assay volume of
0.2 ml Tris·HCl buffer (50 mm Tris·HCl, 0.5 mm EDTA, and 10 mm MgCl2; pH 7.4) supplemented with
1 U/ml adenosine deaminase for A2A binding assays and HEPES buffer (20 mm HEPES, 10 mm NaCl,
10 mm MgCl2, pH 7.4) for A1 binding assays were used. Stock solns. of the compounds were prepared in
DMSO, the final concentration of DMSO in assays was ꢁ1%. Nonspecific binding was determined in the
presence of 100 mm known non-radioactive ligands and accounted for less than 5% of total binding. The
incubation time was 1 h at 258. Termination of the incubation was performed by rapid filtration using a
Millipore manifold at a pressure of 700 mbr. Filters were washed three times with 200 ml of the relevant
assay buffer. Scintillation fluid was added (100 ml/well), and bound radioactivity was counted in a Wallac
Microbeta scintillation counter (UK-Beaconsfield). Testing was performed in three stages: first, all of the
compounds were tested at 100 mm; second, those causing greater than 60% displacement of binding were
retested at three different concentrations from 1 to 100 mm; third, those that were consistently active in a
concentration-dependent manner were tested over a full concentration range to determine the IC50 and
Ki values. Data were analyzed using GraphPad Prism, Version 2.0 (GraphPad, San Diego, CA). For
nonlinear regression analysis, the ChengꢀPrusoff equation and KD values of 1.6 nm (human A1) for
[3H]DPCPX and 1 nm (human A2A) for [3H]ZM241385 were used to calculate Ki values from IC50 values.
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Received May 20, 2010