Formation, structure and heterocyclization of aminoguanidine and ethyl acetoacetate condensation products

Article abstract of DOI:10.1134/S1070363209060309

Condensation of aminoguanidine hydrochloride and ethyl acetoacetate results in 2,3-diamino-6-methylpyrimidin-4(3H)-one, 5-hydroxy-1-carboxamidino-3- methylpyrazole or ethyl N-[(5-hydroxy-3-methylpyrazol-1-yl)imidoyl] aminocrotonoate depending on the type of the base. Formation of pyrazole derivatives occurs in the case of dequaternized substrate imine-group protonating by the acids formed as a result of ion exchange reaction. Chelate fragment of amidinohydroxypyrazole structure provides stabilization of this compound and stipulates its inertness towared the heterocycle closure.

Full text of DOI:10.1134/S1070363209060309

ISSN 1070-3632, Russian Journal of General Chemistry, 2009, Vol. 79, No. 6, pp. 1204–1209. © Pleiades Publishing, Ltd., 2009.
Original Russian Text © A.V. Erkin, V.I. Krutikov, 2009, published in Zhurnal Obshchei Khimii, 2009, Vol. 79, No. 6, pp. 1032–1037.
Formation, Structure and Heterocyclization
of Aminoguanidine and Ethyl Acetoacetate
Condensation Products
A. V. Erkin and V. I. Krutikov
St. Petersburg State Institute of Technology, Moskovskii pr. 26, St. Petersburg, 190013 Russia
e-mail: kruerk@yandex.ru
Received October 16, 2008
Abstract—Condensation of aminoguanidine hydrochloride and ethyl acetoacetate results in 2,3-diamino-6-
methylpyrimidin-4(3Н)-one, 5-hydroxy-1-carboxamidino-3-methylpyrazole or ethyl N-[(5-hydroxy-3-methyl-
pyrazol-1-yl)imidoyl]aminocrotonoate depending on the type of the base. Formation of pyrazole derivatives
occurs in the case of dequaternized substrate imine-group protonating by the acids formed as a result of ion
exchange reaction. Chelate fragment of amidinohydroxypyrazole structure provides stabilization of this
compound and stipulates its inertness towared the heterocycle closure.
DOI: 10.1134/S1070363209060309
Condensation of aminoguanidine and β-keto-
carboxylates as bifunctional reagents gives rise to 2,3-
diaminopyrimidin-4(3Н)-ones (I) and 1-carboxamidino-
pyrazol-5(4Н)-ones (II) mainly. Reaction of amino-
guanidine salts with β-ketocarboxylates in the presence
of organic base yields diaminopyridines I [1] and in
the absence of neutralizing agent forms amidino-
pyrazolones II [2, 3].
with ethyl acetoacetate in the water–alkali solutions
and to study heterocyclization of the products formed.
The studied reaction direction is appreciably
determined by the base reagent type. 2,3-Diamino-6-
methylpyrimidin-4(3Н)-one IV is formed as a result of
the subsequent action of sodium hydroxide and ethyl
acetoacetate on the compound III. The structure of
diamine IV obtained is confirmed by 1Н NMR
spectrum, where signals of the exocyclic amino group
protons are observed at 5.2 and 6.9 ppm. The IR and
The aim of this work is to establish the direction of
the condensation of aminoguanidine hydrochloride III
Me
O
H2N
H2N
NH2
NH2+
AcCH2COOEt
AcCH2COOEt
NaOH
N
Cl−
N
H2N
HO
HN
N
NaOAc (Na3PO4)
N
N
Me
H
NH2
IV
III
V
Na2CO3
AcCH2COOEt
Me
NH2
NH2+
N
HO
AcCH2COOEt
HCO3−
N
Me
O
H2N
N
H
H2N
OEt
N
VII
VI
1204

FORMATION, STRUCTURE AND HETEROCYCLIZATION
1205
UV spectra analysis of compound IV allows to exclude
alternative structure of 1,2-diamino-6-methylpyrimidin-
4-(1Н)-one from the consideration. In the IR spectrum
there is intensive absorption band at 1679 cm–1
belonging to the carbonyl group stretching vibrations
and characteristic of aminopyrimidines having 4-oxo-
3,4-dihydro-configuration [4]. The UV spectrum
contains the absorption bands with the maxima at 222
(log ε = 3.89) and 280 (log ε = 3.99) nm, which are
close by location and intensity with the absorption
bands of 2-amino-3-methylpyrimidin-4(3Н)-one [5].
methylene protons signals and the appearance of a
broad singlet at 7.7 ppm with integral intensity cor-
responding to four protons indicate that the compound
V exists predominantly in the lactime form Va with an
intramolecular hydrogen bond. This is confirmed by its
IR spectrum, which is characterized by the presence of
broad overlapped intensive absorption bands with
maxima at 3237 and 2967 cm–1 belonging to stretching
vibrations of the bonded amino (imino) and hydroxyl
groups. In the spectrum the low intensity absorption
band partially masked by these bands was also
observed at 2738 cm–1. Appearance of the two latter
bands, which are absent in the spectrum of amidine V
sodium salt, is conditioned by the strong association of
hydroxyl group proton with π- and n-electrons of the
imino group nitrogen atom. It leads to the electron
density redistribution in the initial molecule and to
formation of resonance structures Vb and Vc hybrid.
When sodium hydroxide is replaced by the less
strong base sodium acetate (pathway а), other con-
densation product, 5-hydroxy-1-carboxamidino-3-
methylpyrazole V, is formed. The latter is identified by
the presence of the imino group proton singlet at
1
4.4 ppm in the Н NMR spectrum. The absence of the
Me
Me
Me
O−
O−
N
N
N
O
N
N
N
H
H
H
V
+
N
H
NH2+
N
NH2
N
H
NH2
H
Vb
Vc
Vа
Intramolecular hydrogen bond provides the
amidine V structure stabilization. Similar conclusion
follows from comparison of the UV spectra of
compound V neutral (pH 7) and anionic (рН 12)
forms. In these spectra, apart from coincidence of the
absorption bands with maximum at 227 nm, occur
hypsochromic shifts of the long-wave absorption band
maximum of nonionized form, corresponding to the
process of charge transfer, and the accompanying its
low hyperchromic effect (Δε ~500) on increasing of
medium pH (Fig. 1).
reaction mixture. Phosphoric acid is
pronounced proton-donor than acetic acid.
a
more
At the treatment of hydrochloride III with sodium
carbonate (pathway a), the unexpected direction of
condensation of III and ethyl acetoacetate is realized.
In this case the amidine V functional derivative, ethyl
14000
12000
10000
8000
Change in direction of the reaction of hydrochloride
III with ethyl acetoacetate in the presence of sodium
acetate to amidine V formation forces us to suppose
that acetic acid (рKа 4.75), obtaining as a result of ion
exchange, protonates the more basic imino group of
substrate dequaternizated in situ and thus it prevents
reaction involving this reaction centre. The proof of
this is obtaining of compound V at the sequential
reaction of hydrochloride III with sodium phosphate
and ethyl acetoacetate (pathway b), which results in
the appearance of phosphoric acid (рKа 2.12) in the
6000
2
4000
2000
1
200
250
300
350
400
λ, nm
Fig. 1. UV spectra of (1) neutral and (2) anionic forms of
amidine V and ethyl crotonoate VI.
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 79 No. 6 2009

1206
ERKIN, KRUTIKOV
3-[(5-hydroxy-3-methylpyrazol-1-yl)imidoyl]aminocroto-
noate VI is formed. In the 1Н NMR spectra of the latter
is absent a singlet of imino group proton at 4.4 ppm,
but there are signals of ester group protons at 1.2 and
4.0 ppm and also singlet of exocyclic methyne group
protons at 6.2 ppm. The IR spectrum contains a low-
frequency shift at 1672 cm–1 originating from
stretching vibrations of the carbonyl group. It is
anomalous for aliphatic unsaturated acids and their
esters [7], but it is inherent in the structurally close
ethyl 3-(5-hydroxypyrazol-4-yl)-3-methyl-2-propenoate
[8]. From this fact one can conclude that the resonance
component VIа brings some contribution into the elec-
tron structure of ethyl crotonoate VI.
sodium carbonate [9]. Owing to this, the amount of
free aminoguanidine undergoing further protonation
with carbon acid involving imino group decrease
considerably. Therefore excess of ethyl acetoacetate
arises in the reaction mixture. Carbon dioxide dilutetd
in water is transformed partially into carboxylic acid
[10], which couples with aminoguanididne to form
aminoguanidine carbonate VII. The latter reacts
sequentially with ketoester excess resulting in ethyl
crotonoate VI. In order to confirm this hypothesis we
carried out condensation of carbonate VII and ethyl
acetoacetate in the molar ratio 1:2 (pathway b) and
obtain product similar by physico-chemical charac-
teristics to compound VI obtained by way а.
Me
Compound IV undergoes heterocyclization by
reaction with the lower carboxylic acids. We
established that formation of 2,5-dimethyl[1,2,4]
triazolo[1,5-a]pyrimidine VIII occurs actually on
heating of diamine IV with excess of acetic acid. This
reaction requires longer contact of reagents in
comparison with the outlined in literature [11]. The
reaction time decrease results in the incomplete
conversion of compound IV. Therefore the target
product VIII is impossible to isolate from the
unreacted diamine IV admixture even by the repeated
N
HO
Me O −
N
VI
+
H2N
OEt
N
VIа
The unsaturated character of ethyl crotonate VI is
illustrated by absorption bands at 325 nm in the UV
spectrum. The heterocycle lactime form involving into
the general conjugation system is proved by the
presence in this spectrum of a broad band of π → π*
transition with the maxima at 230 and 242 nm, which
un-dergoes bathochromic shifting relative to the analo-
gous band in the spectrum of amidine V (Fig. 1).
1
crystallization. In the Н NMR spectrum of bicycle
VIII there are signals of methyl group protons in the
region 2.3 ppm as well as a signal of one of the ring
imine centers near 13 ppm. The correctness of descrip-
tion of compound VIII structure by the respective
formula is dictated by the work [12] attesting its
stabilization through proton localization on the atom
N4 of the annelated system.
Ethyl crotonoate VI formation becomes
explainable, if we take into account the hydrochloride
III tendency of decomposing in the presence of
Cl
NHR
O
N
N
N
N
N
N
POCl3
RNH2
AcOH
N
N
N
IV
Me
Me
Me
N
Me
N
Me
N
Me
H
IX
Xа, Xb
VIII
R = PhCH2 (a), 4-MeC6H4 (b).
The exchange chlorination of bicycle VIII with
phosphorus oxotrichloride results in 2,5-dimethyl-7-
chloro[1,2,4]triazolo[1,5-a]pyrimidine IX. We em-
phasize that the crystallization of chloride IX from
chloroform by procedure [13] fails to realize due to
high product solubility in this solvent. Replacing
chloroform by cyclohexanone, we obtain compound
IX with the yield 33% only. We attained the almost
twofold increase in chloride IX yield by the continual
extraction of the uncrystallized product with minimal
amount of solvent in the Soxhlet apparatus.
The compound IX have a halogen atom capable of
substitution with amino-group. Its reaction with the
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 79 No. 6 2009

FORMATION, STRUCTURE AND HETEROCYCLIZATION
1207
equimolar amounts of some aralkyl- and arylamines
under rigid conditions produces 7-amino-2,5-dimethyl-
[1,2,4]triazolo[1,5-a]pyrimidines hydrochlorides Xa
and Xb. The latter compounds transform into the free
bases by workup with aqueous ammonia solution. The
1Н NMR spectra of aminotriazolopyrimidines Xa and
Xb contain signals of the aromatic fragments protons
at 7 ppm and signals of the second amino group in the
region of 8–10 ppm.
tion of diketone molecule with amidine V anion acting
as a base.
Me
Me
Me
O−
HN
N
Me
Me
OH
O
N
V +
O−
O
NH2
Just diketone serves as a proton source since the
compound V sodium salt is the only product
chromatographically detected under conditions of its
producing by addition of substrate to solution of
sodium ethoxide in absolute ethanol.
The amidine chelate structure determines its
inertness toward heterocycle closure. We could not
obtain 5-hydroxy-3-methyl-1-[N-(4-oxopent-2-en-2-yl)
carboxamidino]pyrazole XI or 2-(5-hydroxy-3-methyl-
pyrazol-1-yl)-4,6-dimethylpyrimidine by heating of
substrate V with 2,4-pentadione under various condi-
tions: in toluene using a water separator or in the
presence of phosphorus pentaoxide as dehydrating
agent. In the first case the initial compound V
regenerated, and in the second the reaction mixture
tarring occurs. Oxopentene XI is formed only at reflux
EXPERIMENTAL
1
The Н NMR spectra were registered on a Bruker
WM-400 spectrometer (400.13 МHz) in DMSO-d6
relatively to the signals of residual DMSO protons as
internal reference. The IR spectra were taken on a
Shimadzu FTIR-8400S spectrophotometer from KBr
pellets. The UV spectra were recorded on a SF-26
spectrophotometer at the substances concentration
~1×10–4 M in water except for the spectrum of XI
registered in ethanol. The individuality of compound
obtained was monitored by TLC method on Silufol
UV-254 plates in the systems 1-butanol–acetic acid–
water, 1:1:1 (А), 2-propanol–25% aqueous ammonia,
3:1 (B), acetone–heptane, 3:1 (C) and 2-propanol–25%
aqueous ammonia, 3:2 (D); UV detection. The
elemental analysis was performed on a Leco CHNS-
932 analyser.
1
of amidine with the diketone excess. In the Н NMR
spectrum of compound XI there are singlets of methyl
groups in the region 2.0–2.5 ppm and of exocyclic
fragment methyne group at 6.7 ppm. The UV spectrm
contains high-intensive absorption band with the
maximum at 367 nm (log ε = 4.19) belonging to
electron transitions in the conjugatied bond system
involving heterocycle.
Me
N
HO
Ac2CH2
N
Me
O
V
2,3-Diamino-6-methylpyrimidine-4(3Н)-one (IV).
To a solution of 11.05 g of hydrochloride III in 40 ml
of water were sequentially added dropwise a solution
of 4 g of sodium hydrochloride in 10 ml of water and
13 g of ethyl acetoacetate in 15 ml of ethanol, under
vigorous stirring. The mixture was stirred for 3 h; then
precipitate was filtered off, washed with water,
recrystalled from water and dried at 80°C during 8 h.
Yield 3 g (21%), mp 280°C (decomp.) (published data:
mp 283–285°C [1], 283°C [11]), Rf 0.50 (А). 1Н NMR
spectrum, δ, ppm: 1.99 s (3Н, Ме), 5.24 s (2Н, NH2),
5.48 s (1Н, CH), 6.96 br.s (2Н, NH2).
H2N
Me
N
XI
The low yield of oxopentene XI is indicative of
high strength of the intramolecular hydrogen bond in
amidine V. To prevent chelation influence on the
reactivity of the amidine group in this compound, we
carried out reaction of amidine V sodium salt with 2,4-
pentanedione in absolute ethanol. This allows to
localize anionic center on the oxygen atom through
hydrogen bonds formation and thus to exclude imine
group from potential process of proton transfer. In
spite of the reasonable choice of solvent the similar
pathway of heterocycle closure also did not give rise to
the expected result: we isolated compound V from the
reaction mixture not undergone preliminary neutraliza-
tion. This circumstance attests clearly the deprotona-
5-Hydroxy-1-carboxamidino-3-methylpyrazole (V).
а. To a solution of 5.53 g of hydrochloride III in 25 ml
of water was sequentially added 4.1 g of sodium
acetate, and then dropwise added 6.5 g of ethyl
acetoacetate under vigorous stirring. The mixture was
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 79 No. 6 2009

1208
ERKIN, KRUTIKOV
stirred for 3 h and kept for 15 h at room temperature.
The precipitate formed was filtered off, washed with
water, recrystalled from water and then from 50%
ethanol and dried at 80°C for 8 h. Yield 2.35 g (34%),
mp 235°C (decomp.) (published data: mp 235°C [2]),
Rf 0.61 (B). Н NMR spectrum, δ, ppm: 1.95 s (3H,
Me), 4.39 s (1H, NH), 7.69 br.s (4H, NH2, CH, OH).
2,5-Dimethyl-7-chloro[1,2,4]triazolo[1,5-a]pyrimi-
dine (IX). A mixture of 2.41 g of bicycle VIII and
15 ml of the freshly distilled phosphorus oxotrichloride
was refluxed for 1 h. The excess of POCl3 was distilled
off in vacuum. To the reaction residue were added
ground ice and then 25% aqueous ammonia to appe-
arance of the stable alkalescent reaction. The pre-
cipitate formed was filtered off, washed with water and
dried under vacuum over phosphorus pentaoxide. The
dry product was extracted with 35 ml of cyclohexane
and dried in vacuum. Yield 1.69 g (63%), mp 149°C
1
b. To a solution of 5.53 g of hydrochloride III in
25 ml of water were sequentially added 6.34 g of
sodium phosphate and dropwise 6.5 g of ethyl
acetoacetate, under vigorous stirring. The mixture was
stirred for 3 h and kept for 15 h at room temperature.
The precipitate formed was filtered off, washed with
water, recrystalled from water and dried at 80°C for 8 h.
Yield 2.34 g (34%).
1
(published data: mp 149–150°C [13]), Rf 0.59 (C). Н
NMR spectrum, δ, ppm: 2.48 s (3H, Me), 2.61 s (3H,
Me), 7.47 s (1H, CH).
7-Benzylamino-2,5-dimethyl[1,2,4]triazolo[1,5-a]
pyrimidine (Xa). A mixture of 0.5 g of chloride IX
and 0.29 g benzylamine freshly distilled was refluxed
at 130–140°C for 30 min. On cooling to room
temperature, the caked reaction residue was mecha-
nically crushed, suspended in 10 ml of water and
alkalified with 25% aqueous ammonia to appearance
of the stable alkaline reaction. After 30 min the
precipitate was filtered off, washed with water and
dried in vacuum over phosphorus pentaoxide. The dry
product was crystalled from benzene and the sus-
pension formed was diluted with equal volume of
heptane to precipitating completeness. Yield 0.32 g
Ethyl N-[(5-hydroxy-3-methylpyrazol-1-yl)imido-
yl]aminocrotonoate (VI). а. A mixture of 5.53 g of
hydrochloride III and 2.65 g of sodium carbonate was
dissolved in 25 ml of water. To the filtered solution
was dropwise added 6.5 g of ethyl acetoacetate at
vigorous stirring. The reaction mixture was stirred for
2 h and kept for 3 days at room temperature. The
precipitate formed was filtered off, recrystalled from
10% ethanol, and dried at 80°C for 8 h. Yield 1.52 g
(12%), mp 180°C (decomp.) (published data: mp 180°C
1
[2]), Rf 0.67 (B). Н NMR spectrum, δ, ppm: 1.19 t
(3H, OCH2Me), 2.22 s (3H, Me), 2.44 s (3H, Me), 3.99
q (2H, OCH2Me), 6.22 s (1H, CH), 7.90 br.s (2H,
NH2), 8.86 br.s (2H, CH, OH).
1
(46%), mp 165°C, Rf 0.24 (C). Н NMR spectrum, δ,
ppm: 2.34 s (3H, Me), 2.43 s (3H, Me), 4.55 d (2H,
CH2), 6.06 s (1H, CH), 7.32 m (5H, Ph), 8.61 t (1H,
NH). Found, %: С 66.05; Н 5.68; N 27.52. C14H15N5.
Calculated, %: С 66.38; Н 5.97; N 27.65.
b. To a suspension of 6.8 g of carbonate VII in 40 ml
of water was dropwise added 13 g of ethyl acetoacetate
ander vigorous stirring. The mixture was stirred for 2 h
and kept at room temperature for 2 days. Then pre-
cipitate was separated and filtrate was kept to full
crystallization of the oil released from them. The
precipitate formed was filtered off, twice recrystalled
from water and then from 30% ethanol and dried at
80°C for 8 h. Yield 0.4 g (3.2%).
2,5-Dimethyl-7-(4-methylphenyl)amino[1,2,4]tri-
azolo[1,5-a]pyrimidine (Xb). A mixture of 0.5 g of
chloride IX and 0.293 g of 4-methylphenylamine was
held at 130–140°C for 1 h. On cooling to room
temperature, the caked reaction residue was mecha-
nically crushed, suspended in 10 ml of water and
alkalified with 25% aqueous ammonia to appearance
of the stable alkaline reaction. The precipitate was
filtered off, washed with water and dried in vacuum
over phosphorus pentaoxide. The dry product was
recrystalled from benzene:cyclohexene (3:7) mixture
and dried under vacuum. Yield 0.29 g (42%), mp 160°C,
2,5-Dimethyl[1,2,4]triazolo[1,5-a]pyrimidin-7(4Н)-
one (VIII). A mixture of 1 g of diamine IV and 12 ml
of glacial acetic acid was refluxed for 16 h. The
precipitate formed was filtered off, recrystalled from
nitromethane-DMFA (1:1) mixture, washed with
diethyl ether and dried under vacuum. Yield 0.62 g
(53%), mp > 280°C (decomp.) (literary data: mp 310–
1
Rf 0.22 (C). Н NMR spectrum, δ, ppm: 2.35 s (3H,
Me), 2.37 s (3H, Me), 2.48 s (3H, Me), 6.19 s (1H,
CH), 7.25 m (4H, Ar), 9.92 s (1H, NH). Found, %: С
66.12; Н 5.81; N 27.47. C14H15N5. Calculated, %: С
66.38; Н 5.97; N 27.65.
1
313°C [13]), Rf 0.70 (А). Н NMR spectrum, δ, ppm:
2.28 s (3H, Me), 2.33 s (3H, Me), 5.68 s (1H, CH),
12.93 br.s (1H, NH).
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 79 No. 6 2009

FORMATION, STRUCTURE AND HETEROCYCLIZATION
1209
4. Short, L.N. and Thompson, H.W., J. Chem. Soc., 1952,
5-Hydroxy-3-methyl-1-[N-(4-oxopent-2-en-2-yl)-
carboxamidino]pyrazole (XI). A mixture of 0.6 g of
amididne V and 5 ml of 2,4-pentandione was refluxed
at 150–160°C to homogenizing. The excess of
diketone was distilled off under vacuum, and the
residue was crystalled by grinding with 5 ml of
acetone. The precipitate formed was filtered off,
washed with water and dried in vacuum over
phosphorus pentaoxide. The dry product was
recrystalled from acetonitrile and dried under vacuum
over phosphorus pentaoxide. Yield 95 mg (10%), mp
no. 1, p. 168.
5. Brown, D.J. and Teitei, T., Austral. J. Chem., 1965,
vol. 18, no. 4, p. 559.
6. Keiko, N.A., Mamashvili, T.N., Rassolova, G.V.,
Kashik, T.V., Kalikhman, I.D., and Voronkov, M.G.,
Izv. Akad. Nauk SSSR Ser. Khim., 1984, no. 3, p. 608.
7. Bellami, L., Infrakrasnye spektry slozhnykh molekul
(Complex Molecules IR Spectra), Мoscow: Inostran-
naya Literatura, 1963, p. 590.
8. Evans, N.A., Whelan, D.J., and Johns, R.B., Tetra-
hedron, 1965, vol. 21, no. 12, p. 3351.
1
156°C (comp.), Rf 0.68 (D). Н NMR spectrum, δ,
ppm: 2.03 s (3Н, Ме), 2.24 s (3H, Me), 2.43 s (3H,
Me), 6.73 s (1H, CH), 7.92 br.s (2H, NH2), 8.84 br.s
(2H, CH, OH). Found, %: С 53.45; Н 5.81; N 24.72.
C10H14N4O2. Calculated, %: С 54.04; Н 6.35; N 25.21.
9. Lieber, E. and Smith, G.B.L., Chem. Rev., 1939, vol. 25,
no. 2, p. 213.
10. Cotton, F.A. and Wilkinson, G., Sovremennaya neorga-
nicheskaya khimiya (Advanced Inorganic Chemistry),
Moscow: Mir, 1969, vol. 1, p. 223.
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