Synthesis of novel 1-alkyl-8-substituted-3-(3-methoxypropyl) xanthines as putative A2B receptor antagonists

Article abstract of DOI:10.1016/j.bmc.2009.03.029

In order to identify a high-affinity, selective antagonist for the A_2B subtype adenosine receptor, more than 40 1,8-disubstituted-3-(3-methoxypropyl) xanthines were prepared and evaluated for their binding affinity at recombinant human adenosin

Full text of DOI:10.1016/j.bmc.2009.03.029

Bioorganic & Medicinal Chemistry 17 (2009) 3426–3432
Contents lists available at ScienceDirect
Bioorganic & Medicinal Chemistry
journal homepage: www.elsevier.com/locate/bmc
Synthesis of novel 1-alkyl-8-substituted-3-(3-methoxypropyl) xanthines
as putative A2B receptor antagonists
María Isabel Nieto a, María Carmen Balo b, José Brea c,d, Olga Caamaño b,d, María Isabel Cadavid c,d
,
Franco Fernández b,d, , Xerardo García Mera b,d, Carmen López b,d, José Enrique Rodríguez-Borges e
*
a Departamento de Química Fundamental, Facultade de Química, Universidade de A Coruña, Campus da Zapateira, 15071 A Coruña, Spain
b Departamento de Química Orgánica, Facultade de Farmacia, Universidade de Santiago de Compostela, E-15782 Santiago de Compostela, Spain
c Departamento de Farmacología, Facultade de Farmacia, Universidade de Santiago de Compostela, E-15782 Santiago de Compostela, Spain
d Instituto de Farmacia Industrial, Facultade de Farmacia, Universidade de Santiago de Compostela, E-15782 Santiago de Compostela, Spain
e CIQ-Departamento de Química, Facultade de Ciencias, Universidade de Porto, Rua do Campo Alegre, 687, 4169-007 Porto, Portugal
a r t i c l e i n f o
a b s t r a c t
Article history:
In order to identify a high-affinity, selective antagonist for the A2B subtype adenosine receptor, more than
40 1,8-disubstituted-3-(3-methoxypropyl) xanthines were prepared and evaluated for their binding
affinity at recombinant human adenosine receptors, mainly of the A2A and A2B subtypes. Some of the
1-ethyl-3-(3-methoxypropyl)-8-aryl substituted derivatives 15(a–m) showed moderate-to-high affinity
at human A2B receptors, with compound 15d showing A2B selectivity over the other A receptors assayed
(A1, A2A, A3) of 34-fold or over.
Received 22 January 2009
Revised 4 March 2009
Accepted 14 March 2009
Available online 21 March 2009
Keywords:
Ó 2009 Elsevier Ltd. All rights reserved.
1-Alkyl-8-substituted-3-(3-methoxypropyl)
xanthines
Adenosine receptor antagonist
hA2B and hA2A Binding affinities
Structure–affinity relationships
1. Introduction
activation of potassium and inhibition of calcium channels in car-
diac muscles and neurons (A1-AdoR); stimulation of phospholipase
Adenosine is an endogenous purine nucleoside that is present in
every cell of the human body and has a wide variety of well-docu-
mented regulatory functions and physiological effects. Four dis-
tinct adenosine receptor subtypes have been identified to date
(A1, A2A, A2B and A3), all of which belong to the rhodopsin family
of G-protein-coupled receptors (GPCRS) that are characterized by
7-transmembrane-spanning helical domains (TM I–VII) as well as
extracellular (EL) and intracellular (IL) loops.1–4 Interaction of
adenosine with its receptors initiates signal transduction path-
ways, including the classical adenylate cyclase effector system that
utilizes cyclic adenosine monophosphate (cAMP) as a second mes-
senger. Activation of the A1 and A3 adenosine receptors (A1-AdoR
and A3-AdoR) inhibits adenylate cyclase activity through activation
of pertussis-sensitive Gi proteins and results in a decrease in intra-
cellular levels of cAMP. On the other hand, activation of the A2A and
A2B adenosine receptors (A2A-AdoR and A2B-AdoR) stimulates
adenylate cyclase by activation of Gs proteins and this leads to
intracellular accumulation of cAMP. Coupling of adenosine recep-
tors to other second messenger systems has also been described:
C (A1-, A2B-, and A3-AdoRs); mobilization of intracellular calcium
(A3-AdoR).3,5,6
Adenosine receptors have been recognized as playing an impor-
tant role in chronic inflammatory airway conditions such as asth-
ma, chronic obstructive pulmonary disease (COPD) and fibrosis.7,8
Experimental evidence, such as the increase in the adenosine con-
centration in hypoxia and cellular inflammation in bronchoalveolar
fluids of asthmatics and in plasma (upon a contact with allergens),
has highlighted the key role that adenosine and its A2B receptors
play in asthma.9–11 Moreover, adenosine, in the form of AMP, in-
duces bronchoconstriction in asthmatics but not in healthy indi-
viduals.12 The bronchodilating effect of theophylline (1) and its
analogue enprophylline (2) has been attributed to a selective
antagonism of A2B-AdoR.11 The discovery that A2B-AdoRs are func-
tionally active on human airway smooth muscle cells and lung
fibroblast cells provides further support for the role of A2B
-
AdoR.11,13 Antagonists of A2B-AdoR would therefore represent a no-
vel approach for the management and treatment of asthma and
COPD.
Even though a number of high-affinity A2B-AdoR antagonists
have been reported, only a few have shown high affinity and selec-
tivity for A2B-AdoR relative to A1-, A2- and A3-AdoRs.14–18 Several
* Corresponding author. Tel.: +34 981 563100 14942; fax: +34 981 594912.
E-mail address: franco.fernandez@usc.es (F. Fernández).
0968-0896/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmc.2009.03.029

M. I. Nieto et al. / Bioorg. Med. Chem. 17 (2009) 3426–3432
3427
research groups have synthesized 8-phenyl-substituted xanthines
that have high A2B affinity and selectivity against the other AdoR
subtypes.19–21 Jacobson and co-workers demonstrated that the
introduction of a para-substituted phenyl derivative at the 8-posi-
tion of the xanthine core increases the A2B-AdoR affinity and selec-
tivity in comparison to the other AdoRs, as exemplified by
compound 3 (see Chart 1).20,21
imide (DIC) in MeOH, and subsequent cyclization by heating under
reflux with 2.5 N NaOH in MeOH afforded the xanthines 8.
The 7-methylated derivatives 9a and 9b were obtained by
methylation of 8b and 8d with excess methyl iodide in DMF in
the presence of K2CO3. Oxidation of the xanthines 8q and 8r
(Scheme 2) to the corresponding sulfoxides (10a and 10b) and sulf-
ones (11a and 11b) was achieved by following previously reported
literature procedures.25
For this reason, some years ago we began a systematic study to
identify an easy, efficient and rapid synthetic pathway to fully func-
tionalized xanthines and followed this with an investigation into the
biological and pharmacological properties of the resulting com-
pounds. Our goal was the discovery of a selective, high-affinity
A2B-AdoR antagonist through the preparation of xanthines bearing
appropriate substituents in several positions of the xanthine
nucleus, with the aim of exploring the structure–affinity (SAR) and
structure–selectivity relationships (SSR). Exploratory tests made to
evaluate the effect of replacement of each alkyl group (methyl, ethyl,
propyl) of very simple 1,3,8-trialkylxanthines by slightly more polar,
oxygen bearing alkyl groups showed a noticeable increase in the
affinity towardsA2B-AdoR receptors whena 3-methoxypropyl group
wasplaced at theposition3 of thexanthine. Thus, thestructuralvari-
ations at the xanthine nucleus initially envisaged at this work were
made at position 1 (alkyl or functionalized alkyl substituents) and
position 8 (aryl or heteroaryl substituents), with the 3-methoxypro-
pyl substituent kept at position 3 (Schemes 1 and 2). Also, on the ba-
sis of the interesting results obtained by Jacobson et al.,12 an oxy
acetamido group linked through its nitrogen atom to a variety of aro-
matic, heteroaromatic and cycloaliphatic substituents was placed in
the para-positionof the 8-phenylring in a secondseries of xanthines,
while keeping the 3-methoxypropyl group at position 3 (Scheme 3).
Compound 7a was also converted into the series of xanthines
15a–15m, which bear an amide group linked to the position 8 of
the xanthine through
a
para-phenyloxymethyl unit. This
transformation was achieved in a one-pot procedure, by reacting
p-formylphenoxyacetic acid (12) with the corresponding amine
in the presence of DIC, followed by treatment of the resulting,
not isolated amides 13 with diamine 7a in MeOH/AcOH at room
temperature for 1 h, and then by oxidative cyclization of the
benzylidene adduct 14 to form the imidazole ring present in
15a–15m (Scheme 3). At our hands, this procedure proved to be
not more time consuming than the alternative of preparing first
a common intermediate, 2-{4-[1-ethyl-3-(3-methoxypropyl)-2,6-
dioxo-2,3,6,7-tetrahydro-1H-purin-8-yl]phenoxy}acetic acid (16)
to be condensed with each of a variety of amines, while these last
condensations resulted in very low yields of compounds 15.
3. Results and discussion
The affinity (pKi or displacement percentage) values of the 3-(3-
methoxypropyl)xanthine derivatives 8a–8s, 9a, 9b, 10a, 10b, 11a,
11b, and cloned human A2A and A2B receptors, expressed in HeLa
(A2A) and HEK-293 (A2B) cells, are given in Table 1.26 These com-
pounds showed moderate-to-low affinity for human A2A and A2B
receptors expressed. The affinity values of compounds that did
not fully displace specific radioligand binding are given only in
terms of displacement percentage. However, the results shown in
Table 1 enable certain trends to be deduced concerning the SAR
in this group of xanthines. For example, in order for these com-
pounds to show some level of affinity for the receptors in question
they must contain a heteroaromatic unit in position 8 of the xan-
thine nucleus (compounds 8a, 8l–8q, in which a phenyl radical is
in position 8, completely lack activity for both receptors). Similarly,
the introduction of a methylene group between the heteroaromatic
unit and the carbon at position 8 of the xanthine, cf. compounds
8b/8e and 8r/8s, leads to the loss of affinity for both receptors.
Comparison of the affinity results for the pairs of compounds 8c/
8k and 8d/8l shows that lengthening of the carbon chain in posi-
tion 1 also leads to the loss of affinity.
2. Chemistry
3-Alkyl-1-(3-methoxypropyl)-5,6-diaminouracils (7) were syn-
thesized according to previously described methods. Thus, 1-(3-
methoxypropyl)urea was condensed with cyanoacetic acid,22,23 to
give uracil 4 (75%). Direct alkylation of this uracil was performed
with 15% aqueous NaOH and the appropriate alkyl halide,22,24 to
give the corresponding 1,3-disubstituted-6-aminouracil, 5. Stan-
dard nitrosation of 5(a–g), with sodium nitrite in acetic acid, was
followed by reduction with sodium dithionite to achieve diamino-
uracils 7(a–g). Finally, condensation of diaminouracils 7 with an
appropriate carboxylic acid in the presence of diisopropylcarbodi-
O
O
H
H
The affinity (pKi or displacement percentage) values of the 1-
ethyl-3-(3-methoxypropyl)-9-susbtituted xanthine derivatives
15a–15m at cloned human adenosine receptors expressed in HeLa
(A2A and A3), and HEK-293 (A2B) cells, and at rat A1 receptors, are
given in Table 2. The radioligand [3H]DPCPX was used for compe-
tition binding assays on A1 and A2B receptors whereas
[3H]ZM241385 was used for A2A, and [3H]NECA for A3.26
N
N
N
N
N
N
N
O
N
O
1 Theophylline
2 Enprophylline
1-Ethyl-3-(3-methoxypropyl)-9-subtituted xanthine deriva-
tives showed moderate-to-high affinity at human A2B receptors
with selectivity over A2A receptors of up to 33.8-fold (compound
15d). The affinity at rat A1 and human A3 receptors for most of
these compounds showed lower pKi values than those observed
at A2B receptors. It should be noted that several compounds were
selective for A2B receptors over A2A, A1 and A3 receptors with selec-
tivity indexes higher than 10-fold over each of the adenosine
receptors evaluated (compounds 15b, 15d, 15g, 15h and 15k).
Compounds 15b and 15d proved to be excellent A2B selective com-
pounds with pKi values higher than 8.5 at human A2B receptors and
a good selectivity profile over the other adenosine receptors. These
O
H
O
N
N
O
HN
CN
N
N
O
ki (hA2B)- 2 nM
3 MRS-1754c
ki (hA1)- 400 nM
ki (hA2A)- 500 nM
ki (hA3)- 570 nM
Chart 1. Xanthines with antiasthmatic (1 and 2) and selective A2B AdoR antago-
nistic activities (3).

3428
M. I. Nieto et al. / Bioorg. Med. Chem. 17 (2009) 3426–3432
O
N
O
H
N
ii
i
O
NH2
NH2
O
O
NH
4
O
NH2
iii
O
O
N
O
NO
NH2
R1
O
R1
R1
v
iv
N
N
N
N
NH2
O
NH2
N
NH2
O
O
O
O
5a R = Et
7a R = Et
6a R = Et
5b R = n-Pr
5c R= i-Bu
5d R = n-Pen
7b R = n-Pr
6b R = n-Pr
6c R= i-Bu
6d R = n-Pen
7c R= iso-Bu
7d R = n-Pen
5e R = 2-(MeO)Et
5f R = 2-(EtO)Et
5g R = 2-(EtS)Et
7e R = 2-(MeO)Et
7f R = 2-(EtO)Et
7g R = 2-(EtS)Et
6e R = 2-(MeO)Et
6f R = 2-(EtO)Et
6g R = 2-(EtS)Et
8a R1 = Et, R2 = Ph
vi
8b R1 = Et, R2 = furan-2-yl
8c R1= Et, R2 = thiophen-2-yl
8d R1 = Et, R2 = biphenyl-4-yl
8e R1 = Et, R2 = furfuryl
8f R1= Et, R2 = pyrrol-2-yl
8g R1 = n-Pr, R2 = Ph
8h R1 = n-Pr, R2 = c-Pen
8i R1 = n-Pr, R2 = 3-PhPr
8j R1= n-Pr, R2 = pyrrol-2-yl
O
O
H
R1
R1
N
N
N
N
R2
R2
vii
N
N
N
O
N
O
8k R1 = n-Pr, R2 = thiophen-2-yl
8l R1= n-Pr, R2 = biphenyl-4-yl
8m R1= iso-But, R2 = Ph
O
O
8n R1 = n-Pen, R2 = Ph
9a R1 = Et, R2 = furan-2-yl
9b R1 = Et, R2 = biphenyl-4-yl
8o R1 = 2-(MeO)Et, R2 = Ph
8p R1 = 2-(EtO)Et , R2 = Ph
8q R1 = 2-(EtS)Et, R2 = Ph
8r R1 = 2-(EtS)Et , R2 = furan-2-yl
8s R1 = 2-(EtS)Et, R2 = furfuryl
Scheme 1. Reagents and conditions: (i) KOCN, H2SO4, 85 °C, 1 h; (ii) (a) NCCH2CO2H, Ac2O, 85 °C, 3 h, (b) 10% NaOH, EtOH, 80 °C, 1 h; (iii) RX, NaOH, EtOH; (iv) NaNO2, AcOH
rt, 2-24 h; (v) Na2S2O4, NH4OH, 60 °C, 1 h; (vi) (a) R2CO2H, DIC, MeOH, rt, 0.5 h, (b) 2.5 N NaOH, MeOH, reflux, 10 min–9 h; (vii) CH3I, K2CO3, DMF, 100 °C, 5 h.
results indicate that, of the compounds synthesized, the best affin-
ity and selectivity indexes are obtained when R is a phenylamino
or bromophenylamino radical (15b, 15d). The presence of other
substituents on the phenylamino group, such as fluoro or cyano
(15d, 15k), leads to compounds that maintain similar affinity levels
on the three types of receptor under investigation, albeit with a
significant decrease in selectivity.
5. Experimental
All chemicals used were of reagent grade and were obtained
from Aldrich Chemical Co. and used without further purification.
When necessary, solvents were dried by standard techniques and
distilled. All air-sensitive reactions were carried out under argon.
Flash chromatography was performed on silica gel (Merck 60,
230–240 mesh) and analytical TLC was carried out on pre-coated
silica gel plates (Merck 60 F254, 0.25 mm) type E. Chromatographic
spots were visualized by UV light or with Hanessian reagent.27
Melting points (uncorrected) were measured in glass capillary
tubes on a Stuart Scientific electro thermal apparatus SMP3. Infra-
red spectra were recorded on a Perkin–Elmer 1640 FTIR spectro-
photometer. 1H NMR and 13C NMR spectra were recorded on a
Bruker AMX 300 spectrometer at 300 and 75.47 MHz, respectively,
using TMS as internal standard (chemical shifts in d values, J in
Hertz). All of the observed signals are consistent with the proposed
structures. Chemical shifts (d scale) are reported in parts per mil-
lion (ppm) relative to the centre of the solvent peak. Coupling con-
stants (J values) are given in Hertz (Hz). Spin multiplicities are
4. Conclusions
In summary, compounds 8a–8s, 9a, 9b, 10a, 10b, 11a, 11b and
15a–15m have been synthesized and their biological properties
evaluated. Even though low affinity binding results were obtained
for 3-(3-methoxypropyl)xanthine compounds (8a–11b), the 1-
ethyl-3-(3-methoxypropyl)-9-subtituted xanthine derivatives
15a–15m showed moderate-to-high affinity at human A2B recep-
tors with selectivity over A2A receptors of up to 33.8-fold (com-
pound 15d). Even though low affinity binding results were
obtained for these compounds, an important pool of compounds
has been synthesized and characterized.

M. I. Nieto et al. / Bioorg. Med. Chem. 17 (2009) 3426–3432
3429
O
O
O
N
O
N
O
O
H
N
H
N
S
H
N
S
S
N
N
N
i)
R
R
ii)
R
N
N
N
N
O
O
O
O
O
O
10a R = phenyl
10b R = furfuryl
11a R = phenyl
11b R = furfuryl
8q R = phenyl
8s R = furfuryl
Scheme 2. Reagents and conditions: (i) OxoneR, aliquat, CH3CN, H2O, CH2Cl2, rt; (ii) AcOH, 30% H2O2, rt.
CO2H
O
CONHR
O
O
N
O
N
i)
N
ii)
O
NHR
NH2
O
CHO
CHO
O
13a-13m
12
14a-14m
iii)
a R = c-Pen
b R = C6H5
c R = 4-C6H4F
O
O
d R = 4-C6H4Br
e R = pyridin-2-yl
f R = pyridin-3-yl
g R = 4-C6H4OH
h R = 4-C6H4COCH3
H
N
N
NHR
O
N
N
O
i R = 1H-benzo[d]imidazol-2-y
j R = 4-C6H4NMe2
k R = 4-C6H4CN
O
15a-15m
l R = isoxazol-3-yl
m R = 3,4-dihydroisoquinolin-2(1H)-yl
Scheme 3. Reagents and conditions: (i) RNH2, DIC, THF, 0 °C, 15 min; (ii) 7a, MeOH, AcOH, rt, 1 h; (iii) m-CPBA, CH3CN, rt, 24 h.
given as s (singlet), d (doublet), dd (double doublet), t (triplet), m
(multiplet), bs (broad singlet), dt (double triplet), q (quadruplet).
Mass spectra were recorded on Hewlett–Packard HP5988A or
Micromass Autospec spectrometers. Elemental analyses were per-
formed in a FISONS EA 1108 Elemental Analyser at the University
of Santiago Microanalysis Service; all results shown are within
0.4% of the theoretical values (C, S, N, H).
further purification. Unreacted starting material was recovered
from the aqueous phase by evaporation to dryness.
5.1.1. 6-Amino-3-ethyl-1-(3-methoxypropyl)uracil (5a)
Alkylating agent: ethyl iodide; reaction time 5 h; thick oil, yield
65%. 1H NMR (CDCl3): 5.43 (br s, 2H, D2O exchan, NH2), 4.93 (s, 1H,
5-H), 3.99–3.89 (m, 4H, 30-H2 and 1-H2 C2H5), 3.42 (t, 2H, J = 5.6 Hz,
10-H2), 3.35 (s, 3H, OCH3), 1.99 (qt, 2H, J = 5.7 Hz, 20-H2), 1.17 (t, 3H,
J = 7.0 Hz, CH3). 13C NMR and DEPT (CDCl3): 163.18 (C4), 154.97
(C6), 150.02 (C2), 78.76 (C5), 68.81 (C30), 58.80 (OCH3), 40.05 (C1
C2H5), 36.45 (C10), 28.43 (C20), 13.61 (CH3). HRMS m/z calcd for
C9H15N3O3, 213.1113; found, 213.1130.
5.1. General procedure for the preparation of 3-substituted
6-amino-1-(3-methoxypropyl)uracils 5a–5g
A mixture of 6-amino-1-(3-methoxypropyl)uracil (1 mmol),
NaOH 15% (0.3 mL) and 95% EtOH (0.62 mL) was heated under re-
flux for 15 min and the corresponding alkylating agent RX
(2 mmol) was added dropwise. The resulting solution was heated
under reflux for 0.25–48 h, except in the preparation of 5g, which
was carried out at room temperature. The solvents were removed
under reduced pressure and the residue was partitioned between
CHCl3/H2O (2/1, 6.3 mL). The organic layer was washed with water
H2O, dried (Na2SO4) and evaporated to dryness to give the corre-
sponding 3-substituted 6-amino-1-(3-methoxypropyl)uracils 5a–
5g, which in most cases were used in subsequent steps without
5.2. General procedure for the preparation of 3-substituted
6-amino-1-(3-methoxypropyl)-5-nitrosouracils 6a–6g
A solution of NaNO2 (18 mmol) in H2O (7 mL) was added slowly
over 15 min to a solution of the corresponding 3-substituted 6-
amino-1-(3-methoxypropyl)uracil
5 (6 mmol) in 50% AcOH
(30 mL) at 80 °C. The reaction mixture was stirred at room temper-
ature. In cases where a precipitate was formed the solid was fil-
tered off, washed with water and dried under vacuum. In cases

3430
M. I. Nieto et al. / Bioorg. Med. Chem. 17 (2009) 3426–3432
Table 1
volume of the filtrate was reduced to give a second crop of the cor-
responding diaminouracil.
Chemical structures and binding affinitiesa at human hA2A and hA2B AdoRs of 3-(3-
methoxypropyl) xanthine derivatives
R3
O
5.3.1. 5,6-Diamino-3-ethyl-1-(3-methoxypropyl)uracil (7a)
Yield 76%, white solid, mp = 170–172 °C (recrystallized from
EtOAc). 1H NMR (CDCl3): 5.56 (br s, 2H, D2O exchan, NH2), 4.02–
3.93 (m, 4H, 30-H2 and 1-H2 C2H5), 3.42 (t, 2H, J = 5.5 Hz, 10-H2),
3.38 (s, 3H, OCH3), 2.26 (br s, 2H D2O exchan, NH2), 2.01 (qt, 2H,
J = 5.9 Hz, 20-H2), 1.20 (t, 3H, J = 7.2 Hz, CH3). 13C NMR and DEPT
(CDCl3): 161.83 (C4), 150.67 (C6), 150.51 (C2), 95.47 (C5), 68.74
(C30), 58.80 (OCH3), 40.63 (C1 C2H5), 36.88 (C10), 28.33 (C20),
13.65 (CH3). Anal. Calcd for C9H16N4O3 (228.25): C, 47.36; H,
7.07; N, 24.55. Found: C, 47.62; H, 7.31; N, 24.89.
R1
N
N
N
R2
N
O
O
General Structure of compounds 8a–8s, 9a, 9b, 10a, 10b, 11a, 11b.
R1
R2
R3
hA2B
hA2A
5.4. General procedure for the preparation of the xanthines
8a–8s
8a
8b
8c
8d
8e
Ethyl
Ethyl
Ethyl
Ethyl
Ethyl
Ethyl
Propyl
Propyl
Propyl
Propyl
Propyl
Propyl
Isobutyl
Pentyl
2-Methoxyethyl
2-Ethoxyethyl
2-(Ethylthio)ethyl
2-(Ethylthio)ethyl
2-(Ethylthio)ethyl
Ethyl
Phenyl
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
67%
6.86
6.90
7.15
17%
7.07
6.49
62%
70%
6.83
77%
66%
30%
49%
70%
21%
41%
6.31
5.7
49%
6.31
6.66
6.28
11%
6.68
6.47
84%
26%
6.40
25%
14%
25%
0%
54%
51%
8%
5.30
1%
14%
5%
18%
12%
20%
6%
Furan-2-yl
Thiophen-2-yl
Biphenyl-4-yl
Furfuryl
Pyrrol-2-yl
Phenyl
Cyclopentyl
3-Phenylpropyl
Pyrrol-2-yl
Thiophen-2-yl
Biphenyl-4-yl
Phenyl
Phenyl
Phenyl
Phenyl
Phenyl
Furan-2-yl
Furfuryl
Furan-2-yl
Biphenyl-4-yl
Phenyl
Diisopropylcarbodiimide (1 mmol) was added to a solution or
suspension of the corresponding carboxylic acid (1 mmol) in anhy-
drous MeOH (2 mL) and this was followed by the addition of the
appropriate diaminouracil 7a–7g (1 mmol). The reaction mixture
was stirred at room temperature for 30 min. The solvent was re-
moved under reduced pressure and the sticky residue was tritu-
rated with H2O. The resulting yellow solid was filtered off and
mixed with MeOH (3.5 mL) and 2.5 N NaOH (5 mL). The mixture
was heated under reflux for the appropriate time in each case
and allowed to cool down to room temperature. The solid was fil-
tered off and the filtrate was adjusted to pH 6 by the addition of
2 N HCl. The corresponding 1H-purine-2,6(3H,7H)-dione 8 precip-
itated and was filtered off, washed with H2O and purified by crys-
tallization or washing with the appropriate solvent.
8f
8g
8h
8i
8j
8k
8l
8m
8n
8o
8p
8q
8r
H
H
H
8s
9a
Methyl
Methyl
H
H
H
H
1%
6%
11%
0%
0%
9b
10a
10b
11a
11b
Ethyl
2-(Ethylsulfinyl)ethyl
2-(Ethylsulfinyl)ethyl
2-(Ethylsulfonyl)ethyl
2-(Ethylsulfonyl)ethyl
5.4.1. 1-Ethyl-3-(3-methoxypropyl)-8-phenyl-1H-purine-
2,6(3H,7H)-dione (8a)
Furfuryl
Phenyl
Furfuryl
Reflux 10 min, yield 30%, white solid, mp = 239–241 °C (washed
15%
from MeOH). IR (KBr)
m
(cmꢀ1): 3188, 2975, 2807, 1701, 1655,
a
Binding affinity is expressed as pKi or displacement percentage at 1
l
M where
1588, 1555, 1521, 1469, 1279, 1236, 1110, 1013, 818, 779, 762,
748, 520. 1H NMR (CDCl3): 13.08 (br s, 1H, D2O exchan, NH),
8.35–8.32 (m, 2H, 2-H and 6-H C6H5), 7.51–7.48 (m, 3H, 3-H, 4-H
and 5-H C6H5), 4.35 (t, J = 7.0 Hz, 2H, 30-H2), 4.24 (t, J = 7.0 Hz,
2H, 1-H2 C2H5), 3.53 (t, J = 6.2 Hz, 2H, 10-H2), 3.34 (s, 3H, OCH3),
2.19–2.10 (m, 2H, 20-H2), 1.37 (t, J = 7.1 Hz, 3H, CH3). 13C NMR
and DEPT (CDCl3): 156.11 (C8), 152.04 (C6), 151.00 (C2), 150.25
(C4), 131.01 (C3 and C5 C6H5), 129.21 (C1 C6H5), 129.19 (C4
C6H5), 127.40 (C2 and C6 C6H5), 108.56 (C5), 70.66 (C30), 58.99
(OCH3), 41.29 (C10), 37.55 (C1 C2H5), 28.50 (C20), 13.81 (CH3). MS
(EI) m/z (%): 328 (M, 41), 313 (51), 296 (10), 270 (17), 257 (74),
242 (23), 241 (15), 225 (14), 212 (34), 199 (17), 198 (100), 193
(12), 149 (12), 104 (67), 103 (13), 77 (15), 71 (14), 67 (14), 58
(12). Anal. Calcd for C17H20N4O3 (328.37): C, 62.18; H, 6.14; N,
17.06. Found: C, 62.55; H, 6.49; N, 17.02.
indicated. pKi and displacement percentage values had an SEM <10%.
where precipitation was only slight the aqueous solution was ex-
tracted with EtOAc. The organic phase was dried (Na2SO4), the sol-
vent was removed under reduced pressure and the residue was
crystallized from the appropriate solvent.
5.2.1. 6-Amino-3-ethyl-1-(3-methoxypropyl)-5-nitrosouracil
(6a)
Reaction time 24 h; yield 76%, violet solid, mp = 150–152 °C
(recrystallized from EtOAc). 1H NMR (CDCl3), 13.42 (br s, 2H D2O
exchan, NH2), 4.14 (q, 2H, J = 7.0 Hz, 30-H2), 4.0 (t, 2H, J = 6.3 Hz,
1-H2 C2H5), 3.45 (t, 2H, J = 5.6 Hz, 10-H2), 3.41 (s, 3H, OCH3), 2.04
(qt, 2H, J = 5.9 Hz, 20-H2), 1.31 (t, 3H, J = 7.1 Hz, CH3). 13C NMR
and DEPT (CDCl3): 160.58 (C4), 149.63 (C6), 146.62 (C2), 138.79
(C5), 68.81 (C30), 59.14 (OCH3), 39.10 (C10), 37.58 (C1 C2H5),
27.98 (C20), 13.58 (CH3). Anal. Calcd for C10H16N4O4 (256.26): C,
46.87; H, 6.29; N, 21.86. Found: C, 46.99; H, 6.43; N, 22.10.
5.5. General procedure for the preparation of 8-{4-
[(substituted)carbamoyl]methoxy]phenyl}-1-ethyl-3-(3-
methoxypropyl)-1H-purine-2,6(3H,7H)-diones 15
Diisopropylcarbodiimide (4 mmol) was added to a solution of 4-
formylphenoxyacetic acid (12, 4 mmol) in dry THF (30 mL) and the
mixture was stirred for 10 min. The solution was cooled to 0 °C and
the corresponding amine (4 mmol) was added in one portion and
the mixture was stirred at this temperature for 15 min. The result-
ing N,N0-diisopropylurea was filtered off and the filtrate was evap-
orated to dryness. The residue was washed with hot H2O. The solid
was dissolved in MeOH (12 mL) and this solution was added to a
solution of the diamine 7a (4 mmol) in AcOH (2.5 mL) and MeOH
(15 mL). The mixture was stirred for 1 h. The solid was filtered
5.3. General procedure for the preparation of 3-substituted 5,6-
diamino-1-(3-methoxypropyl)uracils 7a–7g
Na2S2O4 (16 mmol) was added in small portions to a well-stir-
red suspension of the corresponding 6-amino-1-(3-methoxypro-
pyl)-5-nitrosouracil 6 (8 mmol) in 30% NH4OH (25 mL) at 60 °C.
On completion of the addition the reaction mixture was heated
for 1 h and then cooled to 4–5 °C for 18 h. The resulting precipitate
was filtered off, washed with H2O and dried under vacuum. The

M. I. Nieto et al. / Bioorg. Med. Chem. 17 (2009) 3426–3432
3431
Table 2
Chemical structures and binding affinitiesa at human A2B, A2A, A3 and rat A1 AdoRs of 1-ethyl-3-(3-methoxypropyl)-8-susbtituted xanthine derivatives
O
O
H
N
Et
N
R
O
N
N
O
O
General Structure of compounds 15a–15m
c
R
hA2B
hA2A
rA1
hA3
A2B/A2A ratio
A2B/A1 ratio
A2B/A3 ratio
15a
15b
15c
15d
15e
15f
15g
15h
15i
Cyclopentylamino
Phenylamino
4-Fluorophenylamino
4-Bromophenylamino
Pyridin-2-ylamino
7.11
8.52
8.52
8.54
7.81
7.47
7.81
7.93
7.03
6.76
8.04
7.68
7.31
7.02
7.06
7.25
7.01
6.77
6.12
6.38
6.83
6.69
5.75
6.95
6.67
6.64
n.d.b
7.44
7.68
7
n.d.
5.60
6.02
5.89
69%
7.02
65%
6.15
n.d.
6.68
6.4
1.20
28.8
18.6
33.8
11.0
22.4
26.9
12.6
2.20
10.0
12.3
10.2
5.00
n.c.d
13.8
6.92
34.7
n.c.
4.79
11.2
n.c.
n.c.
832
316
447
n.c.
2.82
n.c.
60.3
n.c.
1.20
44.7
n.c.
2%
Pyridin-3-ylamino
6.79
6.76
68%
n.d.
6.06
6.17
7.02
7.0
4-Hydroxyphenylamino
4-Acetylphenylamino
1H-benzo[d]imidazol-2-ylamino
4-(Dimethylamino)phenylamino
4-Cyanophenylamino
Isoxazol-3-ylamino
n.c.
15j
15k
15l
5.05
74.1
n.c.
6.03
n.d.
15m
3,4-Dihydroisoquinolin-2(1H)-yl
n.c.
n.c.
a
Binding affinity is expressed as pKi or displacement percentage at 1
n.d.: not determined.
Affinity ratios were calculated on the basis of Ki values.
lM where indicated. pKi and displacement percentage values had an SEM <10%.
b
c
d
n.c.: not calculated due to the low affinity of the compounds at one of the receptors assayed.
off in cases where the amide precipitated from the reaction med-
ium. In cases where a precipitate did not form, the solvents were
removed under reduced pressure. In all cases, the residue (14) ob-
tained (1 mmol) was suspended in CH3CN (25 mL) and MCPBA
(1 mmol) was added. The reaction mixture was stirred for 24 h
and the resulting solid was filtered off.
Isidro Parga Pondal’. J.B. thanks the Xunta de Galicia for financial
support under ‘Isabel Barreto Contract’.
Supplementary data
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.bmc.2009.03.029.
5.5.1. 8-{4-[(Cyclopentylcarbamoyl)methoxy]phenyl}-1-ethyl-
3-(3-methoxypropyl)-1H-purine-2,6(3H,7H)-dione (15a)
Yield 20%, beige solid, mp = 274–275 °C (recrystallized from
References and notes
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m
(cmꢀ1): 3287, 3166, 2970, 2871, 1699, 1655,
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1612, 1560, 1478, 1366, 1236, 1183, 835, 763. 1H NMR (DMSOd6):
13.43 (br s, 1H, D2O exchan, NH), 10.24 (br s, 1H, NH), 8.08 (d,
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(C5), 69.32 (C30), 66.98 (OCH2), 58.42 (OCH3), 41.22 (C10), 40.65
(C1 c-C5H9), 36.05 (C1 C2H5), 32.43 (C2 and C5 c-C5H9), 28.90 (C3
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H, 6.85; N, 15.13.
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