Three-component condensation of 5-aminoimidazole derivatives with aldehydes and Meldrum's acid. Synthesis of 3,4,6,7-tetrahydroimidazo[4,5-b]pyridin-5-ones

Article abstract of DOI:10.1007/s11172-012-0211-6

A convenient method for the synthesis of the earlier unknown substituted 3,4,6,7-tetrahydroimidazo[4,5-b]pyridin-5-ones was developed based on a three-component condensation of 5-aminoimidazole derivatives, aldehydes, and Meldrum's acid. Unstable aminoimidazole derivatives were readily formed in situ by decarboxylation of 5-amino-4-imidazolecarboxylic acids in acidic medium at room temperature, whose sodium salts were obtained by the alkaline hydrolysis of the corresponding, readily available esters.

Full text of DOI:10.1007/s11172-012-0211-6

Russian Chemical Bulletin, International Edition, Vol. 61, No. 8, pp. 1591—1595, August, 2012
1591
Threeꢀcomponent condensation of 5ꢀaminoimidazole derivatives
with aldehydes and Meldrum´s acid.
Synthesis of 3,4,6,7ꢀtetrahydroimidazo[4,5ꢀb]pyridinꢀ5ꢀones
B. V. Lichitsky, A. N. Komogortsev, A. A. Dudinov, and M. M. Krayushkin
N. D. Zelinsky Institute of Organic Chemistry, Russian Academy of Sciences,
47 Leninsky prosp., 119991 Moscow, Russian Federation.
Eꢀmail: mkray@ioc.ac.ru
A convenient method for the synthesis of the earlier unknown substituted 3,4,6,7ꢀtetraꢀ
hydroimidazo[4,5ꢀb]pyridinꢀ5ꢀones was developed based on a threeꢀcomponent condensation
of 5ꢀaminoimidazole derivatives, aldehydes, and Meldrum´s acid. Unstable aminoimidazole
derivatives were readily formed in situ by decarboxylation of 5ꢀaminoꢀ4ꢀimidazolecarboxylic
acids in acidic medium at room temperature, whose sodium salts were obtained by the alkaline
hydrolysis of the corresponding, readily available esters.
Key words: 5ꢀaminoꢀ4ꢀimidazolecarboxylic acids, threeꢀcomponent condensation, Melꢀ
drum´s acid, 5ꢀaminoimidazoles, decarboxylation, 3,4,6,7ꢀtetrahydroimidazo[4,5ꢀb]pyridinꢀ
5ꢀones.
Earlier,1—8 we have developed a general approach to
the synthesis of fused heterocyclic systems 1 containing
a dihydropyridinꢀ2ꢀone fragment based on the condensaꢀ
tion of labile heterocyclic amines 2 with carbonyl comꢀ
pounds 3 and the Meldrum´s acid 4 (Scheme 1). The
unstable aminoheterocycles 2 were generated directly in
the reaction medium either from vicinal aminocarboxylic
acids 5 obtained by the alkaline hydrolysis of the correꢀ
sponding, readily available esters 6, or as a result of neuꢀ
tralization of stable hydrochlorides 7 with anhydrous soꢀ
dium acetate.
In the present work, we broadened the approach develꢀ
oped on the synthesis of the earlier unknown substituted
3,4,6,7ꢀtetrahydroimidazo[4,5ꢀb]pyridinꢀ5ꢀones 8 (X = Z =
= N, Y = CH). The free forms of 5ꢀaminoimidazoles are
known to be extremely unstable.9 We suggested to use as
their precursors sodium salts 9, obtained by alkaline hyꢀ
drolysis of the corresponding esters 10 (Scheme 2). We
showed that the reaction carried out in acetic acid at room
temperature during 24 h resulted in the synthesis of the
earlier unknown imidazopyridinones 8 in 54—80% yields
(Table 1). An increase in the reaction temperature led to
Scheme 1
X
S
N
CR1
CR1
CR1
N
Y
Z
CR2
S
S
N
S
Se
CR1
CR1
CR1
N
CR2
CR2
CR1
N
N
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 8, pp. 1575—1579, August, 2012.
1066ꢀ5285/12/6108ꢀ1591 © 2012 Springer Science+Business Media, Inc.

1592
Russ.Chem.Bull., Int.Ed., Vol. 61, No. 8, August, 2012
Lichitsky et al.
Scheme 2
ester 11 with ethyl orthoformate 12 and subsequent conꢀ
densation with anilines (Scheme 3).
Scheme 3
However, from our point of view this approach is not
preparatively useful because of high reactivity of imino
ester 13, which is the reason, in particular, for a possibility
of the condensation of the latter with the starting 2ꢀamiꢀ
noꢀ2ꢀcyanoacetate 11, leading to various side products.12
We elaborated a convenient general method for the
preparation of 5ꢀaminoꢀ4ꢀimidazolecarboxylic esters 10
with the aryl substituent on the ring nitrogen atom. The
literature describes an approach to the synthesis of 5ꢀaminoꢀ
imidazole derivatives based on the reaction of cyclic imidꢀ
ates with 2ꢀaminocyanoacetamide.13 We suggested that
9, 10: R = Ph (a), 4ꢀMeOꢀC6H4 (b), 3ꢀCF3ꢀC6H4 (c), 3ꢀClꢀC6H4 (d)
resinification of the reaction mixture and decrease in the
yields of the target compounds.
It should be noted that the preparation of the starting
vicinal amino esters 10 posed a certain problem. The synꢀ
thesis of esters 10 containing an aryl substituent on the
ring nitrogen atom was described in the literature.10—12 It
is based on the reaction of labile 2ꢀaminoꢀ2ꢀcyanoacetic
Table 1. Yields, melting points, and elemental analysis data for compounds 8a—k
Comꢀ
pound
R
(%)
Ar
M.p./C
Yield
Found
Calculated
Molecular formula
(%)
C
H
N
8a
8b
8c
8d
8e
8f
Ph
3,4ꢀ(MeO)2ꢀC6H3
Ph
211—212
221—222
201—202
233—234
239—240
220—221
203—204
222—223
224—225
212—213
227—228
80
72
78
61
70
56
66
79
54
55
63
68.51 5.60
68.75 5.48
74.50 5.34
74.72 5.23
71.68 5.25
71.46 5.37
64.72 4.66
64.50 4.56
65.90 4.83
66.11 4.72
62.36 4.82
62.58 4.73
60.11 3.79
60.35 3.66
62.29 3.96
62.05 3.84
60.66 4.46
60.43 4.35
58.01 3.45
58.25 3.34
60.06 3.43
59.85 3.52
12.28
12.03
14.37
14.52
13.31
13.16
11.73
11.88
11.69
11.56
10.75
10.95
11.91
11.73
11.25
11.42
9.92
C20H19N3O3
C18H15N3O
Ph
Ph
4ꢀMeOꢀC6H4
3ꢀClꢀC6H4
C19H17N3O2
4ꢀMeOꢀC6H4
4ꢀMeOꢀC6H4
3ꢀClꢀC6H4
3ꢀClꢀC6H4
3ꢀClꢀC6H4
3ꢀCF3ꢀC6H4
3ꢀCF3ꢀC6H4
3ꢀCF3ꢀC6H4
C19H16ClN3O2
C20H17N3O4
3,4ꢀOCH2OꢀC6H3
3,4ꢀ(MeO)2ꢀC6H3
3ꢀClꢀC6H4
C20H18ClN3O3
C18H13Cl2N3O
C19H14ClN3O3
C21H18F3N3O3
C19H13ClF3N3O
C20H14F3N3O3
8g
8h
8i
3,4ꢀOCH2OꢀC6H3
3,4ꢀ(MeO)2ꢀC6H3
3ꢀClꢀC6H4
10.07
10.58
10.73
10.61
10.47
8j
8k
3,4ꢀOCH2OꢀC6H3

Condensation of 5ꢀaminoimidazole derivatives
Russ.Chem.Bull., Int.Ed., Vol. 61, No. 8, August, 2012
1593
Scheme 5
a similar reaction of readily available and stable Nꢀarylꢀ
substituted imino esters 14 with 2ꢀaminocyanoacetic ester
11 would lead to imidazoles 10. In fact, compounds 10
were obtained in good yields as a result of such a condenꢀ
sation (Scheme 4).
Scheme 4
10, 14: Ar = Ph (a); 4ꢀMeOꢀC6H4 (b); 3ꢀCF3ꢀC6H4 (c); 3ꢀClꢀC6H4 (d)
An important problem to be solved during the present
investigation was the isolation and confirmation of the
structure of a key intermediate product 15 (Scheme 5) of
the threeꢀcomponent condensation under study. We have
chosen trifluoroacetic aldehyde hemiacetal as an aldehyde
component to effect these studies, since the presence of
a trifluoromethyl group significantly simplified confirmaꢀ
tion of the structure of intermediate 15 by NMR spectroꢀ
scopy. The threeꢀcomponent condensation of salts 9, the
Meldrum´s acid, and trifluoroacetic aldehyde hemiacetal
at room temperature in acetic acid during 3 h allowed us
to obtain the intermediate compound 15 in moderate yield,
which was isolated and completely characterized by NMR
spectroscopy, mass spectrometry, and elemental analysis.
The 1H NMR spectrum of the compound obtained exhibꢀ
its the signal pattern for the methine proton characteristic
of the splitting on the fluorine atoms of the trifluoromethꢀ
yl group with the spinꢀspin coupling constant of 12 Hz,
that unambiguously confirms the structure of compound
15. The subsequent cyclization of compound 15 upon heatꢀ
ing in alcohol in the presence of excess acetic acid led to
the target imidazopyridinone 16 (Scheme 5).
To sum up, taking 5ꢀaminoimidazole derivatives as
examples we for the first time synthesized the intermediꢀ
ate 15, whose intramolecular cyclization with elimination
of the CO2 and acetone molecules led to the target biꢀ
cyclic products. The isolation of compound 15 and its
transformation to imidazopyridinone 16 confirmed the
scheme suggested by us earlier1—8 for the threeꢀcompoꢀ
nent condensation of heterocyclic amines with aldehydes
and Meldrum´s acid.
Ar = 4ꢀMeOꢀC6H4
i. 20 C, 3 h.
good agreement with the literature data reported for analꢀ
ogous objects.1—8
In conclusion, we have developed a convenient methꢀ
od for the synthesis of the earlier unknown substituted
3,4,6,7ꢀtetrahydroimidazo[4,5ꢀb]pyridinꢀ5ꢀones, which
was based on a threeꢀcomponent condensation of 5ꢀaminoꢀ
imidazoles, aldehydes, and Meldrum´s acid. Sodium salts
of 5ꢀaminoꢀ4ꢀimidazolecarboxylic acids were shown to
serve as synthetic equivalents of 5ꢀaminoimidazoles. It
was found that the reaction proceeded through the formaꢀ
tion of an intermediate, whose subsequent cyclization led
to the target products.
Experimental
1
H NMR spectra were recorded on a Bruker Avance II 300
(300 MHz) spectrometer in DMSOꢀd6. Mass spectra were obꢀ
tained on a FINNIGAN MAT INCOS 50 instrument (direct
injection, electron impact, 70 eV). Melting points were meaꢀ
sured on a Boetius heating stage and were not corrected.
Reagents and solvents used in the work were commercially
available (Acros Organics) and used without additional puriꢀ
fication.
Ethyl 2ꢀaminocyanoacetate (11)14 and imino esters 14a
(R = Ph),15 14b (R = 4ꢀMeOC6H4),16 14c (R = 3ꢀCF3C6H4
obtained similarly to 14a), and 14d (R = 3ꢀClC6H4)17 were obꢀ
tained according to the methods described in the literature.14—17
Ethyl 5ꢀaminoꢀ1ꢀphenylꢀ1Hꢀimidazoleꢀ4ꢀcarboxylate (10a).
A mixture of aminocyanoacetate 11 (12.8 g, 0.1 mmol) and imiꢀ
no ester (14.9 g, 0.1 mmol) 14a in acetonitrile (75 mL) was
refluxed for 30 min, then, the reaction mixture was cooled,
a precipitate formed was filtered off and washed with ethanol on
the filter. The yield was 15.1 g (65%). M.p. 203—204 C. 1H NMR
(DMSOꢀd6), : 1.28 (t, 3 H, CH3, J = 7.1 Hz); 4.22 (q, 2 H,
CH2, J = 7.1 Hz); 5.91 (s, 2 H, NH2); 7.35 (s, 1 H, CH);
The synthesized products 8 are the solid crystalline
compounds, whose structure was confirmed by the eleꢀ
mental analysis data and 1H NMR spectroscopy. The
1H NMR spectra of the products exhibit signals characꢀ
teristic of the protons of the methine fragment in the reꢀ
gion  4.15—4.30 and of the nonequivalent protons of the
methylene group in the region  2.60—3.12, which is in

1594
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Lichitsky et al.
7.43—7.62 (m, 5 H, C6H5). Found (%): C, 62.54; H, 5.58;
N, 18.32. C12H13N3O2. Calculated (%): C, 62.33; H, 5.67;
N, 18.17. Esters 10b—d were obtained similarly.
CH2, J = 7.1 Hz); 6.01 (s, 2 H, NH2); 7.40 (s, 1 H, CH); 7.43—7.70
(m, 4 H, C6H4). Found (%): C, 54.02; H, 4.66; N, 15.03.
C12H12ClN3O2. Calculated (%): C, 54.25; H, 4.55; N, 15.81.
Synthesis of 3,7ꢀdiarylꢀ3,4,6,7ꢀtetrahydroimidazo[4,5ꢀb]ꢀ
pyridinꢀ5ꢀones (8a—k) (general procedure). A mixture of the corꢀ
responding ester 10 (5 mmol) and NaOH (0.4 g, 10 mmol) in
ethanol (3 mL) and water (3 mL) was refluxed for 4 h, then, the
solution was cooled, a salt formed was filtered off and washed
with ethanol on the filter. The salt obtained was further used
without additional purification. A mixture of the previously obꢀ
tained salt (4 mmol), Meldrum´s acid (0.64 g, 4.4 mmol), and
the corresponding aldehyde (4.2 mmol) in acetic acid (12 mL)
was kept for 24 h at room temperature, then, the solvent was
evaporated in vacuo, the residue was recrystallized from aqueꢀ
ous ethanol. Characteristics of compounds 8a—k are given in
Tables 1 and 2.
Ethyl 5ꢀaminoꢀ1ꢀ(4ꢀmethoxyphenyl)ꢀ1Hꢀimidazoleꢀ4ꢀcarbꢀ
1
oxylate (10b). The yield was 63%. M.p. 223—224 C. H NMR
(DMSOꢀd6), : 1.28 (t, 3 H, CH3, J = 7.1 Hz); 3.82 (s, 3 H,
OCH3); 4.22 (q, 2 H, CH2, J = 7.1 Hz); 5.83 (s, 2 H, NH2); 7.11
(d, 2 H, C6H4, J = 8.1 Hz); 7.28 (s, 1 H, CH); 7.39 (d, 2 H,
C6H4, J = 8.1 Hz). Found (%): C, 60.01; H, 5.67; N, 18.24.
C13H15N3O3. Calculated (%): C, 59.76; H, 5.79; N, 16.08.
Ethyl 5ꢀaminoꢀ1ꢀ(3ꢀtrifluoromethylphenyl)ꢀ1Hꢀimidazoleꢀ4ꢀ
carboxylate (10c). The yield was 57%. M.p. 190—191 C.
1H NMR (DMSOꢀd6), : 1.28 (t, 3 H, CH3, J = 7.1 Hz); 4.22
(q, 2 H, CH2, J = 7.1 Hz); 6.08 (s, 2 H, NH2); 7.45 (s, 1 H, CH);
7.75—7.93 (m, 4 H, C6H4). Found (%): C, 51.97; H, 3.93; N, 14.18.
C13H12F3N3O2. Calculated (%): C, 52.18; H, 4.04; N, 14.04.
Ethyl 5ꢀaminoꢀ1ꢀ(3ꢀchlorophenyl)ꢀ1Hꢀimidazoleꢀ4ꢀcarboxylꢀ
ate (10d). The yield was 68%. M.p. 199—200 C. 1H NMR
(DMSOꢀd6), : 1.28 (t, 3 H, CH3, J = 7.1 Hz); 4.22 (q, 2 H,
5ꢀ{1ꢀ[5ꢀAminoꢀ1ꢀ(4ꢀmethoxyphenyl)ꢀ1Hꢀimidazolꢀ4ꢀyl]ꢀ
2,2,2ꢀtrifluoroethyl}ꢀ2,2ꢀdimethylꢀ1,3ꢀdioxaneꢀ4,6ꢀdione (15).
A mixture of ester 10b (1.3 g, 5 mmol) and NaOH (0.4 g,
Table 2. 1H NMR spectra (DMSOꢀd6) of compounds 8a—k
Comꢀ
pound
H—C—H
H—C—H
(dd, 1 H)
CH
, J/Hz
R
CHimide
Ar
NH
8a
2.65 (J = 7.1,
J = 16.1)
3.07 (J = 7.1,
J = 16.1)
4.15 (J = 7.1,
J = 7.1)
7.41—7.62 (m, 6 H)
4.71 (s, 6 H, 2 OMe);
6.69 (m, 1 H, C6H3);
6.89 (m, 2 H, C6H3)
7.18—7.38 (m, 5 H)
10.11
8b
8c
2.65 (J = 7.1,
J = 16.1)
2.62 (J = 7.1,
J = 16.1)
3.12 (J = 7.1,
J = 16.1)
3.08 (J = 7.1,
J = 16.1)
4.23 (J = 7.1,
J = 7.1)
4.17 (J = 7.1,
J = 7.1)
7.41—7.62 (m, 6 H)
7.41—7.62 (m, 6 H)
10.12
10.12
6.87 (d, 2 H, C6H4,
J = 8); 6.12 (d, 2 H,
C6H4, J = 8); 4.71
(s, 3 H, OMe)
8d
8e
2.66 (J = 7.1,
J = 16.1)
2.60 (J = 7.1,
J = 16.1)
3.10 (J = 7.1,
J = 16.1)
3.05 (J = 7.1,
J = 16.1)
4.28 (J = 7.1,
J = 7.1)
4.15 (J = 7.1, 4.81 (s, 3 H, OMe);
7.04—7.51 (m, 9 H); 4.81 (s, 3 H, OMe)
10.15
7.49
(s, 1 H)
5.98 (s, 2 H, OCH2O); 10.09
6.65 (d, 1 H, C6H3);
J = 7.1)
7.09 (d, 2 H, C6H4,
J = 8.1); 7.40 (d,
6.76 (s, 1 H, C6H3);
2 H, C6H4, J = 8.1)
6.83 (d, 1 H, C6H3)
8f
2.65 (J = 7.1,
J = 16.1)
3.07 (J = 7.1,
J = 16.1)
4.18 (J = 7.1,
J = 7.1)
7.42—7.68 (m, 5 H)
3.72 (s, 6 H, 2 OMe);
6.65 (m, 1 H, C6H3);
6.88 (m, 2 H, C6H3)
7.18—7.40 (m, 4 H,
C6H4)
10.25
10.32
8g
8h
2.70 (J = 7.1,
J = 16.1)
2.60 (J = 7.1,
J = 16.1)
3.10 (J = 7.1,
J = 16.1)
3.05 (J = 7.1,
J = 16.1)
4.30 (J = 7.1,
J = 7.1)
4.16 (J = 7.1,
J = 7.1)
7.42—7.70 (m, 5 H)
7.42—7.72 (m, 5 H)
5.98 (s, 2 H, OCH2O); 10.24
7.65 (d, 1 H, C6H3);
7.76 (s, 1 H, C6H3);
7.83 (d, 1 H, C6H3)
8i
2.67 (J = 7.1,
J = 16.1)
3.09 (J = 7.1,
J = 16.1)
4.18 (J = 7.1, 7.72—7.93 (m, 4 H,
J = 7.1) C6H4)
7.69
(s, 1 H)
3.72 (s, 6 H, 2 OMe);
6.68 (d, 1 H, C6H3,
J = 7.1);6.88 (d, 2 H,
C6H3, J = 7.1)
10.30
8j
2.70 (J = 7.1,
J = 16.1)
2.60 (J = 7.1,
J = 16.1)
3.10 (J = 7.1,
J = 16.1)
3.08 (J = 7.1,
J = 16.1)
4.30 (J = 7.1, 7.75—7.95 (m, 4 H,
J = 7.1) C6H4)
4.19 (J = 7.1, 7.75—7.92 (m, 4 H,
J = 7.1) C6H4)
7.70
(s, 1 H)
7.70
7.19—7.40 (m, 4 H,
C6H4)
5.98 (s, 2 H, OCH2O); 10.30
7.65 (d, 1 H, C6H3);
10.38
8k
(s, 1 H)
7.76 (s, 1 H, C6H3);
7.83 (d, 1 H, C6H3)

Condensation of 5ꢀaminoimidazole derivatives
Russ.Chem.Bull., Int.Ed., Vol. 61, No. 8, August, 2012
1595
10 mmol) in ethanol (3 mL) and H2O (3 mL) was refluxed for
4 h, then, the solution was cooled, a salt formed was filtered off
and washed with ethanol on the filter. A mixture of the salt
obtained (4 mmol), Meldrum´s acid (0.64 g, 4.4 mmol), and
trifluoroacetic aldehyde hemiacetal (0.86 g, 6 mmol) in acetic
acid (15 mL) was stirred for 3 h at room temperature until
a precipitate was completely dissolved, then, concentrated until
the weight was constant, the residue obtained was recrystallized
from aq. ethanol. The yield was 0.43 g (25%). M.p. 201—203 C.
1H NMR (DMSOꢀd6), : 1.55 (s, 6 H, 2 CH3); 4.82 (s, 3 H,
OMe); 5.18 (q, 1 H, CH—CF3, J = 12.1 Hz); 5.52 (s, 2 H, NH2);
7.16 (d, 2 H, C6H4, J = 8.1 Hz); 7.58 (d, 2 H, C6H4, J = 8.1 Hz);
8.61 (s, 1 H, CHimid); 15.79 (s, 1 H, CHmeldr). Found (%):
C, 52.53; H, 4.51; N, 10.01. C18H18F3N3O5. Calculated (%):
C, 52.30; H, 4.39; N, 10.17. MS (EI, 70 eV) m/z: 413 [M]+.
3ꢀ(4ꢀMethoxyphenyl)ꢀ7ꢀtrifluoromethylꢀ3,4,6,7ꢀtetrahydroꢀ
imidazo[4,5ꢀb]pyridinꢀ5ꢀone (16). Acetic acid (0.24 g, 4 mmol)
was added to a solution of compound 15 (0.41 g, 1 mmol) in
ethanol (2 mL), the mixture was refluxed for 2 h, then another
portion of acetic acid (0.24 g, 4 mmol) was added followed by
reflux for 6 h. The solution was cooled and diluted with water
(10 mL). A precipitate formed was filtered off and washed with
water on the filter. The yield was 0.22 g (72%). M.p. 225—227 C.
1H NMR (DMSOꢀd6), : 2.59 (m, 1 H, H—C—H); 3.15 (m, 1 H,
H—C—H); 3.88 (m, 1 H, CH—CF3); 7.09 (d, 2 H, C6H4,
J = 8.1 Hz); 7.38 (d, 2 H, C6H4, J = 8.1 Hz); 7.51 (s, 1 H, CHimid);
10.21 (s, 1 H, NH). Found (%): C, 54.25; H, 4.01; N, 13.65.
C14H12F3N3O2. Calculated (%): C, 54.02; H, 3.89; N, 13.50.
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Received January 19, 2012;
in revised form April 25, 2012