New application of heterocyclic diazonium salts. Synthesis of pyrazolo[3,4-d][1,2,3]triazin-4-ones and imidazo[4,5-d][1,2,3]triazin-4-ones

Article abstract of DOI:10.1016/j.tetlet.2011.01.040

The pyrazolo[3,4-d][1,2,3]triazin-4-ones 3 and imidazo[4,5-d][1,2,3] triazin-4-ones 4 are analogs structurally related to purines that have showed a wide and significant variety of biological activity. These compounds were synthesized by one-pot diazotization of 5-amino-1H-pyrazole-4-carbonitriles 1 and 5-amino-1H-imidazole-4-carbonitriles 2, respectively.

Full text of DOI:10.1016/j.tetlet.2011.01.040

Tetrahedron Letters 52 (2011) 1561–1565
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Tetrahedron Letters
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New application of heterocyclic diazonium salts. Synthesis of pyrazolo
[3,4-d][1,2,3]triazin-4-ones and imidazo[4,5-d][1,2,3]triazin-4-ones
⇑
Juan Pablo Colomer, Elizabeth Laura Moyano
Departamento de Química Orgánica, Facultad de Ciencias Químicas, Universidad Nacional de Córdoba, Córdoba 5016, Argentina
a r t i c l e i n f o
a b s t r a c t
Article history:
The pyrazolo[3,4-d][1,2,3]triazin-4-ones 3 and imidazo[4,5-d][1,2,3]triazin-4-ones 4 are analogs struc-
turally related to purines that have showed a wide and significant variety of biological activity. These
compounds were synthesized by one-pot diazotization of 5-amino-1H-pyrazole-4-carbonitriles 1 and
5-amino-1H-imidazole-4-carbonitriles 2, respectively.
Received 16 September 2010
Revised 3 January 2011
Accepted 10 January 2011
Available online 2 February 2011
Ó 2011 Elsevier Ltd. All rights reserved.
Keywords:
Diazonium ions
Pyrazolotriazines
Imidazotriazines
Aminopyrazoles
Aminoimidazoles
1. Introduction
zyl-4H-pyrazolo[3,4-d][1,2,3]triazin-4-one 12 by diazotization of
5. In order to explore the scope of this synthetic strategy, the utili-
Heterocyclic compounds structurally related to purine rings as
pyrazolo[3,4-d][1,2,3]triazin-4-ones and imidazo[4,5-d][1,2,3]tria-
zin-4-ones have shown important anticonvulsant activity,1 they
have also been reported as antifungal, antiviral, and anticancer
agents.2–6 Numerous synthetic compounds containing the 1,2,3 tri-
azine ring are also used as pharmaceuticals, herbicides, pesticides,
dyes, etc.7–9 Consequently, there is a continuous widespread inter-
est in the design and synthesis of novel purine analogs and hetero-
cyclic compounds containing the triazine moiety because of the
potential biological activities associated with these systems.
In general, the synthesis of purine-type compounds from simple
or non commercial molecules involves several reactions, leading to
different azolotriazines in moderate to low yields.6,10 Diazonium
ion condensation with an adjacent nucleophilic function to form
a five- or six-membered ring has proved a valuable tool for the syn-
thesis of various nitrogen fused-heterocycles. Thus, we have pre-
pared new derivatives of pyrazolo[3,4-d][1,2,3]triazin-4-ones in a
convenient one-pot diazotization of 5-amino-1H-pyrazole-4-
carbonitriles.11
zation of 5-amino-1H-imidazole-4-carbonitriles precursors 2a–c
for the synthesis of new imidazo[4,5-d][1,2,3]triazin-4-ones 4a–c
was studied (Fig. 1). As a part of the analysis of the whole synthetic
sequence, we also discuss the effect of the reaction conditions in
the preparation of aminopyrazole using hydrazine hydrochloride
salts as starting material.
2. Results and discussion
2.1. Synthesis of 5-amino-1H-pyrazole-4-carbonitriles
Initially, our work was conducted with the synthesis of 5-ami-
no-1H-pyrazole-4-carbonitriles 1a–d using ethoxymethylene-mal-
O
CN
NH2
NH
N
N
Y
X
Y
X
Diazotization
N
R
N
R
In our attempt to obtain heterocyclic compounds of biological
interest and following our work on the synthesis of azolotriazines,
we prepared different 7-aryl-4H-pyrazolo[3,4-d][1,2,3]triazin-4-
ones 3b–d by diazotization of aminopyrazoles 1b–d and 6-ben-
1a X= N, Y= C, R= Ph
3a-d X= N, Y= C
4a-c X= C, Y= N
1b X= N, Y= C, R= -OCH Ph
p
3
1c X= N, Y= C, R= o-FPh
1d X= N, Y= C, R= Bn
2a X= C, Y= N, R=p-CH3Ph
2b X= C, Y= N, R= -OCH Ph
p
2c X= C, Y= N, R= p-ClPh3
⇑
Corresponding author. Tel.: +54 351 433 4170; fax: +54 351 433 4170.
E-mail addresses: lauramoy@fcq.unc.edu.ar, lauramoy99@hotmail.com (E.L.
Moyano).
Figure 1. Diazotization of aminoazoles 1and 2 to give azolotriazines 3 and 4.
0040-4039/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2011.01.040

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NC
ononitrile and monosubstituted hydrazines as in a previously re-
ported method, which is the general procedure that make refer-
ence to the synthesis of these aminopyrazoles.12 The preparation
of pyrazole 1a was optimized in our previous work.11 Synthesis
of 1b and 1c was carried out using hydrazine monohydrochloride
while in the case of 1d benzylhydrazine dihydrochloride was used
as source of hydrazine. In these cases a previous step of neutraliza-
tion with sodium methoxide solution (25% w/v) was needed. Thus,
using one equivalent of a solution of NaOMe as a base, the com-
pound 1c was obtained in good yields; however, in the synthesis
of 1b a high decomposition reaction mixture was obtained without
the possibility of isolating the desired product. In order to improve
the yields and decrease the decomposition observed in the meth-
oxide neutralization step, triethylamine (TEA) was used as a base.
Thus, using one equivalent of TEA, both pyrazoles (1b and 1c) were
obtained in high yields as compared to those obtained by the
methodology described above (Scheme 1 and Table 1).
Particularly, when the synthesis of compound 1d was carried
out using two equivalents of solution of NaOMe as a base to neu-
tralize the benzylhydrazine dihydrochloride, a mixture of two
aminopyrazoles was obtained: the expected and main product
1d, and the isomer and secondary product 5 (Scheme 1). The for-
mation of 5 is believed to take place by the initial reaction of the
N-2 nucleophilic center of the benzylhydrazine following by cycli-
zation. It is known that under neutral or acidic conditions and if the
nucleophilicity of the two nitrogen atoms of the hydrazine is quite
different the most abundant isomer corresponds to the 1,2-addi-
tion at the terminal NH2 group of the hydrazine to the most
reactive center in the 1,3-difunctional compound.13 However,
when N-1 and N-2 of the hydrazine present similar nucleophilicity
a mixture of regioisomers is obtained. Said situation is observed in
our reactions using benzylhydrazine. As can be seen from the re-
sults outlined in Table 1, the formation of 1d was favoured over
NC
NH2
N
H2N
N
N
N
5
NOESY
NOESY
1d
Figure 2. NOE correlation 2D observed for isomers 1d and 5.
isomer 5 under all evaluated conditions, while 5 was obtained
when two equivalents of base were used. The amount of 5 was
slightly increased when NaOMe was used for the neutralization
step. A similar behavior was observed in the synthesis of benzyl-
pyrazoloquinolinones derivatives.14 On the other hand, it should
be mentioned that isomer 1d was exclusively obtained when pure
and distilled benzylhydrazine was used in this reaction.15
The structural assignment of 1d and 5 was made on the basis of
their 1D and 2D NMR spectra. The 1H NMR experiment of 1d
showed the typical signal for the vinylic proton at C-3 at
7.58 ppm, while in the case of proton at C-5 in 5 appeared as a
low field signal at 8.25 ppm. 13C NMR analysis showed similar sig-
nals for both isomers and were not useful to distinguish between
them. Conversely, NOESY experiments were crucial since in 1d
the correlation of NH2 protons with methylene and aryl protons
were observed, while in the case of 5, the correlation of the C-5
proton with methylene and aryl protons was achieved (Fig. 2).
2.2. Synthesis of pyrazolo[3,4-d][1,2,3]triazin-4-ones
Compounds 3a–d were synthesized by diazotization reaction
involving 5-amino-1H-pyrazole-4-carbonitriles 1a–d and aqueous
NaNO2 in a mixture of HCl/AcOH (3:1).16 In all cases, the new azol-
otriazinones 3b–d were obtained in good to very good yields
depending on the substituent at N-1 (Scheme 2 and Table 2). The
reaction is assumed to take place by the intermediacy of a diazo-
carboxamide 6 and/or a diazoic acid 7 which could give an intra-
molecular triazine ring closure.11 In the first case the amide
group in the species 6 could be obtained by ‘in situ’ hydrolysis of
the cyano group. In the second case the diazohydroxide form 7 is
a tautomer of the initial nitrosoamine formed under diazotization
conditions where cyano group could be also protonated.
CN
H
N
NH2
RHN
N
H
CN
CN
N
N-1 Addition
R
1a R= Ph
1b R= -OCH Ph
p
3
CN
CN
EtO
H
1c R= o-FPh
RHN NH2
+
1d R= Bn
R
NH2
NC
As it was found in the literature, the cyclization of pyra-
zolediazonium ion highly depends on the electrophilicity of the
diazo group and on the stability of the diazonium salt under the
reaction conditions described.17 Thus, this stability is affected by
N-2 Addition
H2N
N
CN
CN
N
N
R
H
5 R= Bn
Scheme 1. Synthesis of aminopyrazole-carbonitriles.
CONH2
Cl
N
N2
Table 1
N
R
Effects of reaction conditions on the synthesis of aminopyrazoles
6
O
Conditions base, equivalents
Time (h)
Yield (%)
1c
1d
5
CN
1b
NaNO2
NH
N
HCl/AcOH
NaOMe, 1
TEA, 1
2
4
2
4
NDa
69b
—
76b
91b
47b
100c
49b
100c
34b
72c
—
N
N
NH2
—
0-5 °C to r.t.
N
N
R
N
R
—
3a-d
—
1a-d
NaOMe, 2
TEA, 2
13b
28c
9b
CNH
H2O
—
41b
82c
N
N=NOH
18c
N
R
a
b
c
Not done due to high decomposition.
Isolated yields.
7
Relative yields of isomers determined by 1H NMR.
Scheme 2. Synthesis of pyrazolo[3,4-d][1,2,3]triazin-4-ones.

J. P. Colomer, E. L. Moyano / Tetrahedron Letters 52 (2011) 1561–1565
1563
Table 2
activated the aromatic ring towards this type of reaction. Thus,
the formation of azines 11 was promoted in two cases: (a) when
there was not substitution at the phenyl ring, it means in amino-
pyrazole 1a; and (b) when benzylpyrazole 1d was used.
Yields of formation of pyrazolotriazines 3
Compd
R
Yielda (%)
3a
3b
3c
3d
3e
3f
Ph
77b
88
64
In order to test the reactivity of isomers 1d and 5 towards the
diazotation reaction, a mixture of these pyrazoles was subjected
to NaNO2 in acid media (Scheme 4). The corresponding pyrazolotri-
azines 3d and 12 could be obtained and isolated from the reaction
crude. This experiment clearly showed that the diazo-intermediate
arised from 5 also proceed to the triazine ring closure as well as
from 5-aminopyrazoles 1. The structure of 12 was confirmed by
its NMR spectra. Particularly, in the homonuclear ROESY experi-
ment, the correlation of the methylene protons could be observed
when the C-5 proton irradiated.
p-OCH3Ph
o-FPh
Bn
tert-Bu
p-FPh
50
47b
70b
a
Isolated yields.
Ref. 11.
b
the nature of different substituents attached to the N-1 of the pyr-
azole ring. To evidence this effect, the results obtained for all pyr-
azolotriazinones (synthesized here and in a previous work) are
shown in Table 2. It can be seen that the presence of fluorophenyl
groups as well as alkyl groups in the starting pyrazole resulted in
lower yields in the synthesis of 3c–f than in the synthesis of 3a.
On the contrary, a p-methoxyphenyl group in N-1 position favored
the reaction and 3b was obtained with the highest yield. Therefore,
the presence of an electron releasing group in N-1 of the pyrazole
ring may promote the stabilization of the diazo-intermediates (6
and/or 7) and their cyclization reaction, while a withdrawing or
an alkyl substituent did not facilitate such kind of reaction and
the yields obtained were lower.
Apart from pyrazolotriazinones, side products (1–10% yields)
were obtained in the reaction mixture: 5-chloropyrazole-4-carbo-
nitriles 9, pyrazole-4-carbonitriles 10 and fused-pyrazoles 11. For-
mation of these products could be explained by reactions of the
diazonium ion 8 (Scheme 3). Compounds 9 are the expected prod-
ucts when the diazotization solution is stirred in the presence of
concentrated HCl or in diluted acid for longer periods; under these
conditions 8 can be substituted by chlorine.17 It is well known that
this substitution takes place by different mechanisms and that this
is strongly dependant on the reaction conditions.18 The formation
of pyrazoles 10 is thought to proceed by a hydro-dediazoniation
process. The mechanism of this process is unknown, but it may
be that diazonium intermediate is trapped by the solvent.19 On
the other hand; the formation of bicyclic pyrazoles 11 from phenyl
and benzyl derivatives 1a and 1d could be explained in terms of an
intramolecular coupling of the diazonium intermediates 8 with the
aromatic rings via an electrophilic aromatic substitution (Scheme
3). According to the results here obtained, it was found that N-1
substitution affected this reaction. Thus, the presence of a fluoro-
phenyl group avoided the cyclization of diazonium ion 8 onto the
aryl ring. Clearly, the electrophilic substitution was not promoted
by the inductive withdrawing effect of fluorine. Contrary to what
was expected, in the synthesis of 3b, the product formed by elec-
trophilic aromatic substitution was not even obtained with an
electron releasing group, as a methoxy group, that should have
2.3. Synthesis of 5-amino-1H-imidazole-4-carbonitriles
Afterward, 5-amino-1H-imidazole-4-carbonitriles 2a–c were
obtained by a sequence of reactions described in the literature.20
Initially, diaminomaleonitrile (13) reacted with triethyl orthofor-
mate in refluxing dry dioxane to give imidate 14 in moderate yields
(45%). Then formamidines 15 were obtained by nucleophilic sub-
stitution reaction of imidate 14 with different anilines in ethanol
at room temperature. In the last step, the cyclization to give 5-ami-
no-1H-imidazole-4-carbonitriles 2 was achieved by treatment of
formamidines 15 in aqueous potassium hydroxide 1 M. All amino-
imidazoles are not described in the literature and were synthesized
in good to excellent yields (Scheme 5).
2.4. Synthesis of imidazo[4,5-d][1,2,3]triazin-4-ones
As outlined in Scheme 6 and Table 3, imidazotriazinones 4a–c
were subsequently prepared by diazotization of 5-amino-1H-imid-
azole-4-carbonitriles 2a–c with NaNO2 in a mixture of aqueous
HCl/AcOH (3:1) as described above for the synthesis of pyrazolotri-
azinones 3a–d. The complete conversion of starting imidazoles
was obtained in all cases and yields of isolated products (41–
90%) were strongly dependant on the success of the workup. The
formation of side products was not observed in these reactions,
contrary to what was found in the reactions of 1a–d to obtain
3a–d were competitive reactions as displacement of nitrogen elim-
ination and substitutions did not take place.
Regarding the stability of diazo intermediates involved in the
diazotization of structural isomers pyrazole and imidazole, the dia-
zonium group would be better stabilized by the imidazole ring
than the pyrazole ring. In recent computational studies of the
bonding and the stability of the intermediates generated from 1b
and 2b, the electron-density distribution analyzes reveal a mayor
stabilization of diazonium ion by the imidazole nucleus. These
findings could explain the absence of the dediazoniation process
and the favored cyclization of imidazole-diazo intermediates to tri-
azinone rings. In this sense, it was found that the diazo compound
obtained by diazotization of 5-aminoimidazole-4-carboxamide
readily cyclized to 2-azahypoxanthine, and this cyclization was
more rapid than photofluorodediazonization, avoiding the synthe-
sis of 5-fluoroimidazole-4-carboxamide.21
CN
CN
CN
NaNO2
HCl/AcOH
24h
N
N
N
N
N
NH2
N
N2
N
R
1a-d
N
R
(CH2)n
8
-N2
Cl
-N2
N
[H]
CN
CN
Cl
3. Conclusions
11a n= 0
11d n= 1
N
New pyrazolotriazines 3b–d and imidazotriazines 4a–c were
obtained from aminopyrazoles 1b–d and aminoimidazoles 2a–c
in a ‘one-step’ process. In both cases, different azolotriazines could
be synthesized from good to very good yields in a simple and effi-
cient methodology.
H
N
N
R
10a-d
R
9a-d
Scheme 3. Formation of side products 9, 10 and 11.

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J. P. Colomer, E. L. Moyano / Tetrahedron Letters 52 (2011) 1561–1565
O
HN
N
CN
NC
NH2
O
N
NH
N
NaNO2
HCl/AcOH
N
N
N
+
+
N
NH2
N
N
N
5
N
N
0-5 °C to r.t.
24h
ROESY
Ph
1d
Ph
3d
Ph
Ph
12
Scheme 4. Diazotization of a mixture of 3d and 5.
R
H
OEt
CN
CN
CN
H
NH
H2N
H2N
CN
CN
HC(OEt)3
H2N
R
N
KOH, 1M
N
N
CN
NH2
Dioxane
EtOH
N
R
r.t.
H2N
H2N
CN
2a (90%)
2b (98%)
2c (74%)
14
15a R= p-CH3Ph (52%)
15b R= p-OCH3Ph (80%)
15c R= p-ClPh (39%)
13
Scheme 5. Synthesis of 5-amino-1H-imidazole-4-carbonitriles.
O
Compounds 4a–c and their precursors were synthesized by
methodologies described in the literature.19
NaNO2
CN
NH2
NH
N
N
HCl/ AcOH
0-5 ºC to r.t.
N
N
Acknowledgments
N
R
N
R
Financial support from CONICET and SECyT-UNC is gratefully
acknowledged. The authors are grateful for the contribution of
Ph.D. Walter J. Peláez.
4a-c
2a R= p-CH3Ph
2b R= p-OCH3Ph
2c R= p-ClPh
Scheme 6. Synthesis of imidazo[4,5-d][1,2,3]triazin-4-ones.
Supplementary data
Supplementary data (experimental procedure and analyzes
data of compounds 1b–d, 2a–c, 3b–d, 4a–c, 5, 9a–d, 10a–d, 11a,
11d and 12 are presented) associated with this article can be
found, in the online version, at doi:10.1016/j.tetlet.2011.01.040.
Table 3
Yields of formation of imidazotriazinones 4
Compd
R
Yielda (%)
4a
4b
4c
p-CH3Ph
p-OCH3Ph
p-ClPh
56
90
41
References and notes
a
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was neutralized with only one equivalent of base disregarding
the nature of the base used. Alternatively, the regioisomer 5 was
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in the neutralization of hydrochloride salt. Aminopyrazole 5 also
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conditions expanding the application of the methodology here
described.
4. Experimental section
General: NMR spectra were recorded with Bruker AC 200 and
400 spectrometers. Chemical shifts are reported in ppm relative
to internal TMS. All chemicals were of reagent grade and used
without purification. High-resolution mass spectra were recorded
with an Agilent LCTOF instrument. Column chromatography was
carried out on grade 60 silica gel (70–230). Compounds 3a–d were
prepared from monosubstituted hydrazine (3a), monosubstituted
hydrazine hydrochlorides (3b and 3c), or monosubstituted hydra-
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16. General procedure for diazotization of aminopyrazoles 1a–d and
aminoimidazoles 2a–c: Aqueous NaNO2 (1.8 mmol, 1 mL) was added to a
well-stirred and cooled solution (0–5 °C) of pyrazole or imidazole (1 mmol) in
a mixture of HCl/AcOH (3:1, 20 mL) over a period of 10 min. The reaction
mixture was allowed to warm at room temperature and was stirred for 20 h.

J. P. Colomer, E. L. Moyano / Tetrahedron Letters 52 (2011) 1561–1565
1565
The precipitate of pyrazolotriazinone or imidazotriazinone was then filtered
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