A mild approach to the synthesis of 4-amino-8-(arylamino)pyrimido[5,4-d] pyrimidine 3-oxides

Article abstract of DOI:10.1002/ejoc.200900216

The reaction of benzylhydroxylamine with 6-cyanopurines leads to the formation of 7-benzyloxy-8-imino-7, 8-di-hydropyrimido[5,4-d]pyrimidines. The hydrochloride of these compounds, isolated upon addition of aqueous hydro-chloric acid, is a convenient prec

Full text of DOI:10.1002/ejoc.200900216

FULL PAPER
DOI: 10.1002/ejoc.200900216
A Mild Approach to the Synthesis of 4-Amino-8-(arylamino)pyrimido[5,4-d]-
pyrimidine 3-Oxides
Alexandra Ribeiro,[a] M. Alice Carvalho,*[a] and M. Fernanda Proença[a]
Keywords: Nitrogen heterocycles / Nitrones / Purines / Pyrimido[5,4-d]pyrimidines / Rearrangment / Cleavage reactions
The reaction of benzylhydroxylamine with 6-cyanopurines
leads to the formation of 7-benzyloxy-8-imino-7,8-di-
hydropyrimido[5,4-d]pyrimidines. The hydrochloride of
these compounds, isolated upon addition of aqueous hydro-
chloric acid, is a convenient precursor of the pyrimido[5,4-d]-
fluxed in ethanol or acetonitrile. Refluxing a solution of the
same salt in ethanol, leads to the Dimroth-rearranged prod-
uct.
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
pyrimidine N-oxides when a suspension of the salt is re- Germany, 2009)
1[34,35] and studied their reactivity with amines. Our stud-
Introduction
ies[21,34] showed that nucleophilic attack of the amine to C8
of the purine ring was the first step of the cascade process
leading to the formation of pyrimido-pyrimidine 2 through
an ANRORC type mechanism (Scheme 1). Compounds 2
were isolated in very good yield, although in some cases the
rearrangement product 3 could also be identified as a minor
product. When hydroxylamine was used as nucleophile the
reaction followed a different pathway leading to the 6-ami-
dinopurine 4 as the only product. These results suggest that
the nature of the amine nucleophile is decisive for the
course of the reaction.
Pyrimido[5,4-d]pyrimidine is a fused heterocyclic system
of interest in the context of drug development because of
its structural similarity to purines. Dipyridamole, a 2,4,6,8-
tetrasubstituted pyrimido[5,4-d]pyrimidine is marketed
nowadays as a coronary vasodilator.[1,2] Many other deriva-
tives are biologically active as antiviral and antitumour
agents,[3–13] as inhibitors of tyrosine kinase,[7,9,12] or as
bronchodilators and antiallergic agents.[14] Several deriva-
tives were synthesised from the 2,4,6,8-tetrachloro deriva-
tive by nucleophilic substitution of the chlorine atoms by
the desired amine.[5,10,11,15] They were also synthesised from
substituted pyrimidines by reaction with convenient electro-
philes or nucleophiles.[12,16,17] The 6-cyanopurines were also
used as precursors of the pyrimido[5,4-d]pyrimidines by re-
action with an amine.[3,4,6,18–21] To the best of our knowl-
edge, pyrimido[5,4-d]pyrimidine N-oxides were never re-
ported in the literature. However, pyrimidine N-oxides are
known. Most of them were obtained by N-oxidation of the
appropriate pyrimidines, usually in low yields, using perox-
ides or peracids.[1,22–25] Pyrimidine N-oxides were also pre-
pared from carboxamide oximes by reaction with an appro-
priate electrophile[26–28] and from 1,2,4-oxadiazoles by ring
rearrangement/transformation.[29–33]
Scheme 1. Reaction of 6-cyanopurine with nucleophilic amines.
The present work reports the reaction of 6-cyanopurines
1 with benzylhydroxylamine (R1 = OCH2Ph), initially aim-
ing at the formation of pyrimido-pyrimidines 2 and 3, gen-
erated upon Dimroth rearrangement of 2, or 6-amidinopur-
ine 4 (R1 = OCH2Ph). It also describes the chemospecific
reaction conditions to convert the pyrimido[5,4-d]pyrimid-
ine 2 into the N-oxide derivative 5 or in the rearranged
product 6.
When purine 1c was treated with 7 equiv. of benzyl-
hydroxylamine, in the presence of a catalytic amount of
DBU, using diethyl ether as solvent, the poor solubility of
the reagents led to an exceptionally long reaction time. Af-
ter 35 d at room temperature, the solid suspension was fil-
tered and the 1H NMR of the product mixture showed that
Herein we report a simple, mild and efficient method to
synthesise the first example of pyrimido[5,4-d]pyrimidine
N-oxides and the corresponding precursor.
Results and Discussion
In recent years, our research group developed a mild and
efficient approach to 9-aryl- or 9-alkyl-6-cyanopurines
[a] Departamento de Química, Universidade do Minho, Campus
de Gualtar,
4710-057 Braga, Portugal
Fax: +351-253678983
E-mail: Mac@quimica.uminho.pt
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A. Ribeiro, M. A. Carvalho, M. F. Proença
FULL PAPER
the purine 1c was still the major compound, together with applied to the reaction of purines 1a, b and d with benzyl-
two new products, one of them identified as compound 2c hydroxylamine and the derivatives 2a, b and d were isolated
(Table 1, entry1). To overcome the solubility problem, THF in very good yield (Table 1, entries 6–8).
was selected as solvent (Table 1). A careful control of the
The formation of the pyrimido-pyrimidine 3 by Dimroth
reaction conditions allowed the isolation of pyrimido-pyr- rearrangement of 2 usually requires base or acid cataly-
imidine 2c in 83% yield after 4 d at room temperature sis.[36] Considering that compounds 2 were isolated from a
(Table 1, entry 5). The optimized reaction conditions were basic medium, in the presence of a large excess of nucleo-
phile (benzylhydroxylamine), hydrochloric acid was used to
induce the rearrangement.
When a concentrated aqueous hydrochloric acid solution
was added to an ethanolic suspension of compound 2a, an
exothermic reaction occurred generating a yellow solid.
This product was identified as the hydrochloride of 2a
(Table 2, entry 1). The 1H NMR spectrum was very similar
to that of the starting material, but the signals at δ =
10.95 ppm (1 H, 4-NH) and δ = 10.82 ppm (2 H, 4-NH +
8-NH) confirmed the formation of the salt 2a·HCl.
Table 1. Synthesis of pyrimido[5,4-d]pyrimidines 2 from 6-cyano-
purines 1.
In a different experiment, the reaction conditions leading
to the hydrochloride of 2a were reproduced, and the sus-
pension was refluxed for 90 min (Table 2, entry 2). A light
1
yellow solid was isolated and H NMR showed it to be a
Entry
1
Reaction conditions
Product
% Yield
mixture of two products. The minor component was iden-
tified as the rearranged product 6a (10%). Besides the ben-
zylic group at δ = 5.16 ppm, two signals at δ = 9.9 ppm and
11.6 ppm could be assigned to the acidic 8-NH and 3-NH
protons, respectively. The major compound showed an aro-
matic pyrimido-pyrimidine core (δCH Ն 8.7 ppm, 2-H and
6-H) and the benzylic group was no longer present. The
arylamine substituent and an amino group (δ = 8.97 and
[a]
1
2
3
4
5
6
7
8
c
c
c
c
c
a
b
d
Diethyl ether, 35 d, r.t.
2c
2c
2c
2c
2c
2a
2b
2d
THF(4 mL), DBU (75 µL), 9 d, r.t.
THF(8 mL), DBU (25 µL), 6.5 d, r.t.
THF(4 mL), DBU (20 µL), 9 d, r.t.
THF(4 mL), DBU (25 µL), 4 d, r.t.
THF(4 mL), DBU (25 µL), 22 h, r.t.
THF(4 mL), DBU (25 µL), 22 h, r.t.
THF(4 mL), DBU (25 µL), 26 h, r.t.
40
60
68
83
89
84
75
1
[a] Product 2c was detected by H NMR of the reaction mixture.
Table 2. Reaction of of pyrimido[5,4-d]pyrimidines 2 with aqueous hydrochloric acid.
Entry
Reagent
Reaction conditions
Product
% Yield
1
2
2a
2a
1. EtOH (4 mL), 2 (0.38 mmol), HCl (21 equiv.); 5 min, r.t.
1. EtOH (4 mL), 2 (0.40 mmol), HCl (12 equiv.); 15 min, r.t.
2. EtOH, reflux, 90 min
1. EtOH (2 mL), 2 (0.40 mmol), HCl (13 equiv.), 20 min, r.t.
2. EtOH, reflux, 30 min
1 EtOH (4 mL), 2 (0.27 mmol), HCl (17 equiv.), 5 min, r.t.
2. EtOH, reflux, 30 min
1. EtOH (4 mL), 2 (0.87 mmol), HCl (6.9 equiv.), 10 min, r.t.
2. EtOH, reflux, 60 min
1. EtOH (2 mL), 2 (0.32 mmol), HCl (15 equiv.), 5 min, r.t.
2. EtOH, reflux, 60 min
1. EtOH (4 mL), 2 (0.35 mmol), HCl (17 equiv.), 15 min, r.t.
2. EtOH, reflux, 35 min
2a·HCl
5a + 6a
96
90:10[a]
3
4
5
6
7
2c
2b
2d
2c
2c
5c + 6c
85:15[a]
5b + 2b·HCl + 6b 53:15:32[a]
74:26[a]
5c + 2c·HCl + 6c 19:13:68[a]
5d + 6d
5c + 6c
20:80[a]
8
9
10
2c
2c·HCl
2b
1. EtOH (4 mL), 2 (0.79 mmol), HCl (7.6 equiv.), 10 min, r.t.
2. EtOH (20 mL), reflux, 105 min
1. EtOH (4 mL), 2 (0.33 mmol), HCl (15 equiv.), 5 min, r.t.
2. EtOH, reflux, 60 min
2c·HCl
5c
92
73
80
6b
11
2b·HCl
2. EtOH (15 mL), reflux, 75 min
5b + 6b
80:20[a]
1
[a] Ratio of products based on H NMR of the mixture.
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Synthesis of Pyrimido[5,4-d]pyrimidine 3-Oxides
8.28 ppm) were also identified. Structure 5a was proposed Table 3. Optimized reaction conditions to generate selectively the
compounds 5 and 6.
for this compound, on the basis of analytic and spectro-
scopic data.
A detailed study on the reaction of compounds 2b–d with
aqueous hydrochloric acid/ethanol was performed (Table 2).
The use of a large volume of solvent and/or hydrochloric
acid solution improved the yield of the rearranged product
6 (entries 3, 6 and 7). A similar situation was observed when
the hydrochloride of 2 was more soluble in the reaction
mixture (entries 5, 10). When the hydrochloride of 2c was
isolated and then refluxed as a suspension in ethanol, where
it was practically insoluble, compound 5c was isolated ex-
clusively (entry 9).
In order to understand the role of water and hydrochloric
acid in the reaction pathway, an aqueous suspension of the
hydrochloride of 2b was stirred at room temperature for
24 h. The yellow colour of the salt gradually faded leading
to a white solid identified as 2b (85%). This result shows
that, in aqueous solution, the pyrimido-pyrimidine 2 and
its hydrochloride are present as an equilibrium mixture, and
the concentration of HCl in the reaction mixture may deter-
mine the concentration of each species.
Entry Reagent Reaction conditions
Product % Yield
1
2
3
2b
2c
2d
2a
1. EtOH, HCl (aq.), r.t., 15 min
2. reflux, 3 h 40 min
3. H2O, r.t., 20 min
1. EtOH, HCl (aq.), r.t., 15 min
2. reflux, 15 min
3. H2O, r.t., 20 min
1. EtOH, HCl (aq.), r.t., 15 min
2. reflux, 25 min
3. H2O, r.t., 20 min
4. EtOH, HCl (aq.), r.t., 15 min 2a·HCl
5a
4. EtOH, HCl (g), 0 °C, 15 min 2b·HCl
6b
6c
6d
82
85
83
This information enabled us to optimize the reaction
conditions in order to prepare selectively pyrimido-pyrimid-
ine 6 or the N-oxide 5 (Table 3).
A general approach to the synthesis of pyrimido-pyr-
imidine 6 required the addition of a large excess of aqueous
hydrochloric acid solution to a suspension of compound 2
in ethanol. The mixture was refluxed until the starting ma-
terial was totally consumed (by TLC). The yellow solid that
precipitates upon cooling corresponds to compound 6,
which could be contaminated with the corresponding hy-
drochloride. Neutralization was achieved simply by washing
with water and the product was isolated in 82–85% yield
(Table 3, entries 1–3). Product 6b was also obtained from
an ethanolic solution of the hydrogeno sulfate of 2b under
reflux conditions (entry 10). The non-aromatic structure of
the compound 6 was confirmed by 1H and 13C NMR spec-
troscopy. The chemical shift registered for C2-H (δH ≈ 7.64,
δC ≈ 144 ppm) and the coupling observed between C2-H
and N3-H (J ≈ 3 Hz) supports the non-aromatic structure
assigned to this pyrimidine ring.
4
5
6
7
8
82
92
77
63
75
2a·HCl 5. EtOH, reflux, 50 min
2b
2b·HCl 5. EtOH, reflux, 3 h 30 min
2c
2d
2b
5b
5c
6. CH3CN, HCl (g), 0 °C,
15 min
7. reflux, 15 min
6. CH3CN, HCl (g), 0 °C,
15 min
7. reflux, 1 h
8. EtOH, H2SO4, r.t., 5 min
9. reflux, 30 min
9
5d
6b
61
10
100
vated pyrimidine ring may occur (pathway a). Ring opening
followed by ring closure generates the final product 6, either
as the free base or as the hydrochloride. The acid-base equi-
librium can be displaced in water to generate the free base 6.
The formation of the N-oxide 5 (Table 3) occurred by
refluxing the hydrochloride of 2 in a solvent where it was
practically insoluble. The hydrochlorides of 2a and 2b were
prepared by adding aqueous or gaseous hydrochloric acid
to an ethanolic suspension of 2a or 2b. These salts were
isolated, dried and refluxed in absolute ethanol to generate
the corresponding N-oxide 5a and 5b (Table 3, entries 4–7).
Compounds 5c and 5d were obtained from 2c and 2d, using
acetonitrile as solvent. Gaseous hydrochloric acid was
bubbled for 15 min, at 0 °C, followed by reflux for 15 min
to 1 h (Table 3, entries 8, 9).
The mechanism for booth reactions involves the forma-
tion of the hydrochloride of pyrimido-pyrimidine
2
(Scheme 2). When this compound is partially soluble in a
polar protic solvent, both the anion and cation are solvated
and nucleophilic attack of water/ethanol to C2 of the acti- Scheme 2. Mechanism proposed for the reaction.
Eur. J. Org. Chem. 2009, 4867–4872
© 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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4869

A. Ribeiro, M. A. Carvalho, M. F. Proença
FULL PAPER
amount of DBU (25 µL, 0.18 mmol). The suspension was stirred
at room temperature until TLC showed the absence of the starting
material. The solid suspension was filtered and washed with etha-
nol followed by diethyl ether.
When the hydrochloride of 2 is insoluble in the solvent, a
tight ionic pair is formed (Figure 1). The repulsion between
the chloride anion and the non-bonding electrons of the oxy-
gen atom determines their opposite position in the molecule.
This geometry favours nucleophilic attack to the benzylic car-
bon which occurs upon heating, with subsequent elimination
of the benzyl group (Scheme 2, pathway b).
4-{[7-(Benzyloxy)-8-imino-7,8-dihydropyrimido[5,4-d]pyrimidin-4-
yl]amino}benzonitrile (2a): From 1a (0.83 g, 3.4 mmol) and benzyl-
hydroxylamine (2.95 g, 24 mmol), 2a was isolated as an off-white
solid (1.12 g, 3.0 mmol, 89 %); m.p. 211.8–212.2 °C. 1H NMR
(300 MHz, [D6]DMSO, 25 °C): δ = 10.13 (s, 1 H, 8-NH), 8.66 (s, 1
3
H, 6-H), 8.57 (s, 1 H, 4-NH), 8.21 (s, 1 H, 2-H), 8.19 (d, JH,H
=
3
8.7 Hz, 2 H, Ho), 7.79 (d, JH,H = 8.7 Hz, 2 H, Hm), 7.56 (m, 2 H,
Ho), 7.42 (m, 3 H, Hm+p), 5.35 (s, 2 H, OCH2) ppm. 13C NMR
(75 MHz, [D6]DMSO, 25 °C): δ = 156.21, 154.48, 151.99, 146.15,
142.77, 140.84, 133.69, 132.53, 129.69, 128.96, 128.35, 124.66,
Figure 1. Ionic pair responsible for the formation of the N-oxide.
120.90, 118.85, 104.80, 76.88 ppm. IR (nujol mull): ν = 3314, 3291,
˜
This mechanistic proposal is supported by the perception
that the nucleophilicity of the counterion is important for
the formation of the N-oxide. In a separate experiment,
compound 2b was combined with sulfuric acid in ethanol,
and the dihydrogen sulfate of 2b was isolated and dried. A
suspension of this salt in ethanol was refluxed until the
starting material was no longer present (17 d) and the solid
was filtered and analysed by H NMR spectroscopy. Com-
pound 6b was identified as a minor component in a com-
plex reaction mixture.
3273, 2223, 1644, 1601, 1585, 1556, 1521 cm–1. C20H15N7O
(369.41): calcd. C 65.02, H 4.10, N 26.55; found C 65.07, H 4.20,
N 26.67.
7-(Benzyloxy)-N-(4-fluorophenyl)-8-imino-7,8-dihydropyrimido-
[5,4-d]pyrimidin-4-amine (2b): From 1b (0.73 g, 3.0 mmol) and ben-
zylhydroxylamine (2.58 g, 21 mmol), 2b was isolated as an off-white
solid (0.92 g, 2.6 mmol, 84 %); m.p. 208.7–210.4 °C. 1H NMR
(300 MHz, [D6]DMSO, 25 °C): δ = 9.80 (s, 1 H, 8-NH), 8.53 (s, 1
H, 6-H), 8.47 (s, 1 H, 4-NH), 8.18 (s, 1 H, 2-H), 7.86 (m, 2 H, Ho),
1
3
7.56 (m, 2 H, Ho), 7.43 (m, 3 H, Hm+p), 7.18 (t, JH,H = 9.0 Hz, 2
H, Hm), 5.35 (s, 2 H, OCH2) ppm. 13C NMR (75 MHz, [D6]DMSO,
1
25 °C): δ = 159.47 (d, JC,F = 239.10 Hz), 156.70, 155.07, 152.34,
4
146.05, 140.43, 134.83 (d, JC,F = 2.55 Hz), 133.89, 130.03, 129.27,
Conclusions
3
2
128.62, 124.35, 123.74 (d, JC,F = 8.03 Hz), 115.03 (d, JC,F
=
21.98 Hz), 76.97 ppm. IR (nujol mull): ν = 3355, 3251, 3065, 3035,
˜
The treatment of pyrimido[5,4-d]pyrimidines 2 with hy-
drochloric acid leads to the selective formation of pyrim-
ido[5,4-d]pyrimidine 6 upon Dimroth rearrangement if the
1635, 1613, 1588, 1558, 1533, 1508 cm–1. C19H15FN6O (362.39):
calcd. C 62.97, H 4.18, N 23.20; found C 62.60, H 4.28, N 23.06.
hydrochloride of 2 is partially soluble in the solvent (water 7-(Benzyloxy)-8-imino-N-(4-methoxyphenyl)-7,8-dihydropyrimido-
[5,4-d]pyrimidin-4-amine (2c): From 1c (0.58 g, 2.3 mmol) and ben-
zylhydroxylamine (1.97 g, 16 mmol), 2c was isolated as an off-white
solid (0.73 g, 1.94 mmol, 83 %); m.p. 200.3–201.8 °C. 1H NMR
or ethanol) and the reaction mixture is heated under reflux
conditions. When the hydrochloride of 2 is insoluble in the
solvent (acetonitrile or ethanol) heating causes cleavage of
the benzyl group possibly as the alkyl chloride, leading to
the regiospecific formation of the pyrimido[5,4-d]pyrimid-
ine N-oxide 5.
(300 MHz, [D6]DMSO, 25 °C): δ = 9.61 (s, 1 H, 8-NH), 8.48 (s, 1
3
H, 6-H), 8.44 (s, 1 H, 4-NH), 8.15 (s, 1 H, 2-H), 7.71 (d, JH,H
=
9.00 Hz, 2 H, Ho), 7.56 (m, 2 H, Ho), 7.42 (m, 3 H, Hm+p), 6.91 (d,
3JH,H = 9.00 Hz, 2 H, Hm), 5.33 (s, 2 H, OCH2), 3.74 (s, 3 H, OMe)
ppm. 13C NMR (75 MHz, [D6]DMSO, 25 °C): δ = 156.59, 155.73,
155.05, 152.19, 145.70, 140.08, 133.82, 131.29, 129.81, 129.06,
128.44, 124.09, 123.32, 113.56, 76.89, 55.15 ppm. IR (nujol mull):
Experimental Section
ν = 3449, 3358, 3237, 3053, 1628, 1600, 1579, 1553, 1522, 1509
˜
General: 9-Substituted 6-cyanopurines 1 were prepared according
to previously reported procedures.[34,35] Solvents and other chemi-
cals commercially available were used as shipped. The melting
points were determined with a Gallenkamp melting point appara-
tus and are uncorrected. The reactions were monitored by thin-
layer chromatography (TLC) using Silica Gel 60 F254 (Merck) with
detection by UV light. The 1H and 13C NMR spectra were recorded
on a Varian Unit Plus for solutions in [D6]DMSO [residual [D6]-
DMSO (δH = 2.49 ppm) or [D6]DMSO (δC = 39.5 ppm) as internal
standard] at 298 K. IR spectra were recorded with a FT-IR Bomem
MB 104 using nujol mulls and NaCl cells. Elemental analyses were
performed with a LECO CHNS-932 instrument.
cm–1. C20H18N6O2 (374.42): calcd. C 64.15, H 4.86, N 22.45; found
C 64.28, H 5.03, N 22.64.
7-(Benzyloxy)-8-imino-N-phenyl-7,8-dihydropyrimido[5,4-d]pyr-
imidin-4-amine (2d): From 1d (0.40 g, 1.8 mmol) and benzylhydrox-
ylamine (1.55 g, 12.6 mmol), 2d was isolated as an off-white solid
(0.46 g, 1.4 mmol, 75%); m.p. 196.0–197.1 °C. 1H NMR (300 MHz,
[D6]DMSO, 25 °C): δ = 9.67 (s, 1 H, 8-NH), 8.54 (s, 1 H, 6-H),
3
8.48 (s, 1 H, 4-NH), 8.18 (s, 1 H, 2-H), 7.86 (d, JH,H = 7.80 Hz, 2
3
H, Ho), 7.57 (m, 2 H, Ho), 7.42 (m, 3 H, Hm+p), 7.34 (t, JH,H
=
7.80 Hz, 2 H, Hm), 7.09 (t, 3JH,H = 7.50 Hz, 1 H, Hp), 5.35 (s, 2 H,
OCH2) ppm. 13C NMR (75 MHz, [D6]DMSO, 25 °C): δ = 156.66,
155.06, 152.31, 146.02, 140.39, 138.44, 133.88, 129.97, 129.21,
128.58, 128.42, 124.36, 123.72, 121.63, 76.93 ppm. IR (nujol mull):
General Procedure for the Synthesis of Pyrimido[5,4-d]pyrimidines
2a–d from 1a–d: The O-benzylhydroxylamine hydrochloride was
neutralised with an aqueous sodium hydroxide solution (1 ) and
the solution was then extracted with diethyl ether (4ϫ35 mL). The
combined organic layers were dried (MgSO4), filtered and concen-
trated to give an oil. The oil was dissolved in THF (4 mL), the 6-
cyanopurine 1 was added to this solution together with a catalytic
ν = 3445, 3359, 3231, 1676, 1629, 1600, 1579, 1552, 1519 cm–1.
˜
C19H16N6O (344.40): calcd. C 66.26, H 4.69, N 24.40; found C
66.20, H 4.77, N 24.42.
General Procedure for the Synthesis of Pyrimido[5,4-d]pyrimidines
6b–d from 2b–d: A concentrated aqueous solution of hydrochloric
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Synthesis of Pyrimido[5,4-d]pyrimidine 3-Oxides
acid (16 equiv.) was added to a suspension of 2 in ethanol, kept
under magnetic stirring at room temperature. The resulting yellow
solution was stirred for 15 min and then was refluxed until TLC
showed the absence of the starting material. The reaction mixture
was then cooled in an ice bath leading to a yellow solid suspension
which was filtered and washed with ethanol followed by diethyl
ether. The isolated yellow solid was suspended in water (20 mL)
and kept under magnetic stirring until the yellow colour faded,
leading to a white solid. The solid was filtered, washed with water
and diethyl ether.
resulting suspension was cooled, and the light yellow solid was fil-
tered and washed with ethanol followed by diethyl ether. The iso-
lated compound was identified as N-oxide 5a (0.05 g, 0.18 mmol,
1
92%). M.p. Ͼ 300 °C. H NMR (300 MHz, [D6]DMSO, 25 °C): δ
= 10.58 (s, 1 H, 8-NH), 8.92 (s, 2 H, 2-H, 4-NH), 8.74 (s, 1 H, 6-
3
H), 8.29 (d, JH,H = 8.70 Hz, 2 H, Ho), 8.22 (s, 1 H, 4-NH), 7.84
3
(d, JH,H = 8.70 Hz, 2 H, Hm) ppm. 13C NMR (75 MHz, [D6]
DMSO, 25 °C): δ = 156.23, 154.80, 150.93, 145.48, 142.72, 132.60,
131.71, 124.08, 121.26, 118.83, 105.12 ppm. IR (nujol mull): ν =
˜
3361, 2226, 1669, 1599, 1573, 1538, 1524 cm–1. HRMS (ESI-TOF):
calcd. for C13H10N7O [M + 1]+ 280.0947; found 280.0943.
8-[(4-Fluorophenyl)amino]pyrimido[5,4-d]pyrimidin-4(3H)-one O-
Benzyloxime (6b): From 2b (0.11 g, 0.30 mmol), 6b was isolated as
a white solid (0.09 g, 0.25 mmol, 82%); m.p. 267.9–269.2 °C. 1H
NMR (300 MHz, [D6]DMSO, 25 °C): δ = 11.46 (s, 1 H, 3-H), 9.52
N-8-(4-Fluorophenyl)pyrimido[5,4-d]pyrimidine-4,8-diamine 3-Oxide
(5b): Dry hydrochloric acid was bubbled into a suspension of 2b
(0.15 g, 0.41 mmol) in ethanol (3 mL), kept under magnetic stirring
(s, 1 H, 8-NH), 8.34 (s, 1 H, 6-H), 7.88 (m, 2 H, Ho), 7.64 (d, 3JH,H at 0 °C, until a yellow solution was formed. From this solution, a
= 3.00 Hz, 1 H, 2-H), 7.37 (m, 5 H, Ph), 7.16 (m, 2 H, Hm), 5.15
yellow solid precipitated out and was filtered after 15 min. The
(s, 2 H, OCH2) ppm. 13C NMR (75 MHz, [D6]DMSO, 25 °C): δ = product was washed with ethanol followed by diethyl ether and was
1
1
158.17 (d, JC,F = 238.50 Hz), 155.40, 154.61, 144.81, 141.04,
identified as the hydrochloride of 2b (0.17 g, 0.41 mmol, 77%). H
138.90, 138.03, 135.07 (d, 4JC,F = 2.65 Hz), 128.26, 127.91, 127.68,
NMR (300 MHz, [D6]DMSO, 25 °C): δ = 10.74 (s, 2 H, 4-NH +
8-NH), 10.54 (s, 1 H, 4-NH), 8.86 (s, 1 H, 6-H), 8.76 (s, 1 H, 2-H),
7.86 (m, 2 H, Ho), 7.69 (m, 2 H, Ho), 7.47 (m, 3 H, Hm+p), 7.25 (t,
3
2
125.26, 123.15 (d, JC,F = 7.73 Hz), 114.97 (d, JC,F = 22.35 Hz),
75.42 ppm. IR (nujol mull): ν = 3356, 3264, 3235, 3066, 1635, 1611,
˜
1586, 1549, 1533, 1505 cm–1. C19H15FN6O (362.39): calcd. C 62.97, 3JH,H = 8.70 Hz, 2 H, Hm), 5.46 (s, 2 H, OCH2) ppm. A suspension
H 4.18, N 23.20; found C 62.77, H 4.20, N 23.09.
of 2b·HCl (0.17 g, 0.42 mmol) in ethanol (4 mL) was refluxed for
3.5 h and the mixture was cooled to room temperature. The yellow
solid suspension was filtered, washed with ethanol and diethyl ether
and identified as N-oxide 5b (0.07 g, 0.26 mmol, 63 %). m.p. Ͼ
300 °C. 1H NMR (300 MHz, [D6]DMSO, 25 °C): δ = 10.25 (s, 1 H,
8-NH), 8.88 (s, 2 H, 2-H + 4-NH), 8.60 (s, 1 H, 6-H), 8.16 8 (s, 1
H, 4-NH), 7.97 (m, 2 H, Ho), 7.22 (m, 2 H, Hm) ppm. 13C NMR
8-[(4-Methoxyphenyl)amino]pyrimido[5,4-d]pyrimidin-4(3H)-one O-
Benzyloxime (6c): From 2c (0.07 g, 0.19 mmol), 6c was isolated as
a white solid (0.07 g, 0.16 mmol, 85%); m.p. 239.8–241.2 °C. 1H
NMR (300 MHz, [D6]DMSO, 25 °C): δ = 11.43 (d, 3JH,H = 3.6 Hz,
1 H, 3-H), 9.30 (s, 1 H, 8-NH), 8.30 (s, 1 H, 6-H), 7.72 (m, 2 H,
3
Ho), 7.63 (d, JH,H = 3.6 Hz, 1 H, 2-H), 7.37 (m, 5 H, Ph), 6.90
1
(75 MHz, [D6]DMSO, 25 °C): δ = 158.55 (d, JC,F = 239.40 Hz),
(m, 2 H, Hm), 5.14 (s, 2 H, OCH2), 3.73 (s, 3 H, OMe) ppm. 13C
NMR (75 MHz, [D6]DMSO, 25 °C): δ = 155.45, 155.44, 154.79,
144.65, 141.11, 138.56, 138.07, 131.66, 128.28, 127.89, 127.67,
156.47, 155.34, 151.04, 145.36, 134.74 (d, 4JC,F = 2.55 Hz), 131.53,
3
2
124.14, 123.91 (d, JC,F = 7.73 Hz), 115.07 (d, JC,F = 22.28 Hz)
ppm. IR (nujol mull): ν = 3357, 3281, 3156, 1664, 1637, 1612, 1589,
˜
125.13, 122.96, 113.61, 75.37, 55.19 ppm. IR (nujol mull): ν = 3361,
˜
1567, 1553, 1533, 1504 cm–1. C12H9FN6O (272.26): calcd. C 52.93,
H 3.34, N 30.87; found C 52.90, H 3.50, N 30.91.
3220, 3077, 3031, 1636, 1603, 1581, 1550, 1531, 1507 cm–1
.
C20H18N6O2 (374.42): calcd. C 64.15, H 4.86, N 22.45; found C
64.16, H 4.90, N 22.35.
General Procedure for the Synthesis of Pyrimido[5,4-d]pyrimidines
3-Oxide 5c–d from 2c–d: Dry hydrochloric acid was bubbled into a
suspension of 2 in acetonitrile, kept under magnetic stirring at
room temperature, until a yellow solution was formed. Shortly af-
ter, a yellow solid precipitated out from the yellow solution. The
suspension was kept at room temperature for about 15 min and
was then refluxed until TLC showed the absence of the starting
material. The reaction mixture was cooled in an ice bath and the
light yellow solid precipitated out of solution, was filtered and
washed with ethanol followed by diethyl ether.
8-Anilinopyrimido[5,4-d]pyrimidin-4(3H)-one O-Benzyloxime (6d):
From 2d (0.11 g, 0.32 mmol), 6d was isolated as a white solid
(0.09 g, 0.27 mmol, 83 %); m.p. 260.0–261.4 °C. 1H NMR
(300 MHz, [D6]DMSO, 25 °C): δ = 11.46 (s, 1 H, 3-H), 9.38 (s, 1
3
H, 8-NH), 8.37 (s, 1 H, 6-H), 7.87 (d, JH,H = 7.80 Hz, 2 H, Ho),
3
7.65 (d, JH,H = 3.30 Hz, 1 H, 2-H), 7.38 (m, 7 H, Ph + Hm), 7.06
(t, 3JH,H = 7.50 Hz, 1 H, Hp), 5.15 (s, 2 H, OCH2) ppm. 13C NMR
(75 MHz, [D6]DMSO, 25 °C): δ = 155.39, 154.64, 144.87, 141.02,
138.68, 138.04, 128.47, 128.27, 127.90, 127.69, 125.32, 123.28,
121.11, 75.42 ppm. IR (nujol mull): ν = 3320, 3355, 3067, 3027,
N-8-(4-Methoxyphenyl)pyrimido[5,4-d]pyrimidine-4,8-diamine 3-
Oxide (5c): From 2c (0.13 g, 0.36 mmol), 5c was isolated as a light
˜
2954, 2925, 2854, 1637, 1604, 1581, 1548, 1533 cm– 1
.
C19H16N6O·0.1H2O (346.20): calcd. C 65.91, H 4.67, N 24.28; yellow solid (0.08 g, 0.27 mmol, 75%); m.p. Ͼ 300 °C. 1H NMR
found C 65.79, H 4.78, N 24.16.
(300 MHz, [D6]DMSO, 25 °C): δ = 10.06 (s, 1 H, 8-NH), 8.86 (s, 1
H, 2-H), 8.78 (s, 1 H, 4-NH), 8.55 (s, 1 H, 6-H), 8.10 (s, 1 H, 4-
4-[(8-Amino-7-oxido-pyrimido[5,4-d]pyrimidin-4-yl)amino]benzo-
nitrile (5a): A concentrated aqueous solution of hydrochloric acid
(5.88 mmol, 21 equiv., 12 ) was added to a suspension of 2a
(0.10 g, 0.28 mmol) in ethanol (4 mL), under magnetic stirring at
room temperature. The resulting yellow solution led to a yellow
solid which was filtered, washed with ethanol and diethyl ether and
identified by 1H NMR as the hydrochloride of 2a (0.09 g,
0.22 mmol, 82%). 1H NMR (300 MHz, [D6]DMSO, 25 °C): δ =
10.95 (s, 1 H, 4-NH), 10.82 (s, 2 H, 4-NH, 8-NH), 8.93 (s, 1 H, 6-
3
3
NH), 7.82 (d, JH,H = 9.30 Hz, 2 H, Ho), 6.95 (d, JH,H = 9.30 Hz,
2 H, Hm), 3.75 (s, 3 H, OMe) ppm. 13C NMR (100.62 MHz, [D6]-
DMSO, 25 °C): δ = 156.38, 155.90, 155.49, 150.96, 145.18, 131.31,
131.26, 124.11, 123.61, 113.63, 55.21 ppm. IR (nujol mull): ν =
˜
3367, 3125, 1664, 1615, 1603, 1555, 1507 cm–1. HRMS (ESI-TOF):
calcd. for C13H13N6O2 [M + 1]+ 285.1100; found 285.1101.
N-8-Phenylpyrimido[5,4-d]pyrimidine-4,8-diamine 3-Oxide (5d):
From 2d (0.28 g, 0.82 mmol), 5d was isolated as a light yellow solid
(0.22 g, 0.50 mmol, 61 %); m.p. Ͼ 300 °C. 1H NMR (300 MHz, [D6]-
3
H), 8.89 (s, 1 H, 2-H), 8.21 (d, JH,H = 8.70 Hz, 2 H, Ho), 7.88 (d,
3JH,H = 8.70 Hz, 2 H, Hm), 7.70 (m, 2 H, Ho), 7.47 (m, 3 H, Hm+p), DMSO, 25 °C): δ = 9.98 (s, 1 H, 8-NH), 9.00–7.50 (br. s, 2 H, 4-
5.46 (s, 2 H, OCH2) ppm. A suspension of the hydrochloride of 2a
(0.08 g, 0.20 mmol) in ethanol (4 mL) was refluxed for 50 min. The
NH), 8.87 (s, 1 H, 2-H), 8.62 (s, 1 H, 6-H), 7.96 (d, 3JH,H = 7.80 Hz,
3
3
2 H, Ho), 7.38 (dt, JH,H = 7.80, JH,H = 7.50 Hz, 2 H, Hm), 7.13
Eur. J. Org. Chem. 2009, 4867–4872
© 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
4871

A. Ribeiro, M. A. Carvalho, M. F. Proença
FULL PAPER
(t, JH,H = 7.50 Hz, 1 H, Hp) ppm. 13C NMR (75 MHz, [D6]-
3
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DMSO, 25 °C): δ = 156.33, 155.21, 150.85, 145.12, 138.13, 131.32,
128.20, 123.97, 123.77, 121.66 ppm. IR (nujol mull): ν = 3353,
˜
3285, 3159, 1667, 1638, 1608, 1582, 1567, 1556, 1533 cm–1
.
C12H10N6O (254.27): calcd. C 56.68, H 3.97, N 33.06; found C
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Treatment of the Hydrochloride of Pyrimido[5,4-d]pyrimidine 2b
with Water: A yellow suspension of 2b·HCl (0.12 g, 0.31 mmol) in
water (4 mL) was stirred at room temperature during 24 h. The
white solid in suspension was filtered, washed with ethanol fol-
lowed by diethyl ether and identified as 2b (0.09 g, 0.26 mmol,
85%).
Reaction of 2b with Sulfuric Acid
7-(Benzyloxy)-N-(4-fluorophenyl)-8-imino-7,8-dihydropyrimido-
[5,4-d]pyrimidin-4-amine Dihydrogen Sulfate (2b·H2SO4): Concen-
trated sulfuric acid (5.35 mmol) was added to an off-white suspen-
sion of 2b (0.10 g, 0.28 mmol) in acetonitrile (3 mL) and ethanol
(3 mL), kept under magnetic stirring at room temperature. A yellow
solution was formed and after 5 min the reaction mixture was par-
tially concentrated at 30 °C. The resulting solution was cooled to
0 °C leading to a yellow solid. The solid was filtered, washed with
ethanol and diethyl ether and identified as 2b·H2SO4 (0.07 g,
0.14 mmol, 52%). 1H NMR (300 MHz, [D6]DMSO, 25 °C): δ =
10.83 (s, 1 H, 4-NH), 10.68 (s, 1 H, 4-NH), 10.54 (s, 1 H, 8-NH),
8.83 (s, 1 H, 6-H), 8.78 (s, 1 H, 2-H), 7.87 (m, 2 H, Ho), 7.68 (m,
2 H, Ho), 7.48 (m, 3 H, Hm+p), 7.26 (m, 2 H, Hm), 5.43 (s, 2 H,
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Acknowledgments
The authors gratfully acknowledge the financial support by Uni-
versidade do Minho, Centro de Química and Fundação para a
Ciência e Tecnologia (FTC) (project PPCDT/QUI/59356/2004 and
PhD grant to A. R. SFRH/BD/24760/2005).
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Received: March 2, 2009
Published Online: August 18, 2009
4872
www.eurjoc.org
© 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Org. Chem. 2009, 4867–4872