Benzimidazoles and related heterocycles 10. A novel acid-catalyzed rearrangement in the system 3-(α-aminobenzyl)quinoxalin-2(1H)-one-ethyl acetoacetate as a simple and efficient method for synthesizing 2-(pyrrol-3-yl)benzimidazoles

Article abstract of DOI:10.1007/s11172-011-0059-1

A reaction of 3-(α-aminobenzyl)quinoxalin-2(1H)-one with ethyl acetoacetate in boiling acetic acid is accompanied by contraction of the pyrazine ring. A rearrangement involving the fragment C(2)-C(3)-C(NH ₂)Ph of the quinoxaline system and the

Full text of DOI:10.1007/s11172-011-0059-1

Russian Chemical Bulletin, International Edition, Vol. 60, No. 2, pp. 368—372, February, 2011
368
Benzimidazoles and related heterocycles
10.* A novel acidꢀcatalyzed rearrangement in the system
3ꢀ(αꢀaminobenzyl)quinoxalinꢀ2(1H)ꢀone—ethyl acetoacetate
as a simple and efficient method for synthesizing 2ꢀ(pyrrolꢀ3ꢀyl)benzimidazoles
V. A. Mamedov, E. A. Khafizova, A. T. Gubaidullin, A. M. Murtazina, D. I. Adgamova,
A. I. Samigullina, and I. A. Litvinov
A. E. Arbuzov Institute of Organic and Physical Chemistry,
Kazan Research Center of the Russian Academy of Sciences,
8 ul. Akad. Arbuzova, 420088 Kazan, Russian Federation.
Fax: +7 (843) 273 2253. Eꢀmail: mamedov@iopc.ru
A reaction of 3ꢀ(αꢀaminobenzyl)quinoxalinꢀ2(1H)ꢀone with ethyl acetoacetate in boiling
acetic acid is accompanied by contraction of the pyrazine ring. A rearrangement involving the
fragment C(2)—C(3)—C(NH2)Ph of the quinoxaline system and the fragment C(2)—C(3) of
ethyl acetoacetate yields 2ꢀ(4ꢀethoxycarbonylꢀ5ꢀmethylꢀ2ꢀphenylpyrrolꢀ3ꢀyl)benzimidazole.
Possible pathways of this reaction are considered.
Key words: 3ꢀ(αꢀaminobenzyl)quinoxalinꢀ2(1H)ꢀone, ethyl acetoacetate, acidꢀcatalyzed
rearrangement, spiro[pyrrolineꢀ3,2´ꢀquinoxalin]ꢀ3´(4´H)ꢀone, 2ꢀ(pyrrolꢀ3ꢀyl)benzimidazole,
Xꢀray diffraction analysis.
The benzimidazole system is found in many physioꢀ
analysis data; its structure was proved by IR and 1H NMR
spectroscopy and Xꢀray diffraction.
logically active compounds, including vitamin B12,2 antiꢀ
viral3,4 and antitumor agents,5—8 drugs used in veterinary
medicine,9—12 and fungicides.13 Two classic methods most
commonly employed for the synthesis of the benzimidꢀ
azole system are the Phillips—Ladenburg synthesis from
diaminobenzenes and carboxylic acid derivatives and the
Weidenhagen reaction of orthoꢀphenylenediamine and its
derivatives with aldehydes.14—16 However, the scope of
these methods is limited by low yields of the target prodꢀ
ucts and the impossibility of introducing the benzimidꢀ
azole fragment into various heterocyclic systems. Hetarylꢀ
benzimidazoles can be obtained by the classic methods via
introduction of either a hetaryl fragment into the benzimidꢀ
azole system or a benzimidazole fragment into a desired
heterocyclic system, which must be preceded by developꢀ
ment of methods for the synthesis of the heterocyclic sysꢀ
tem. This results in lowered yields of the target products.
In the present work, we propose a simple and effiꢀ
cient route to 2ꢀ(pyrrolꢀ3ꢀyl)benzimidazoles via a novel
rearrangement occurring in the system ethyl acetoaceꢀ
tate—3ꢀ(αꢀaminobenzyl)quinoxalinꢀ2(1H)ꢀone (1).17
A reaction of quinoxalinꢀ2(1H)ꢀone hydrochloride 1
with ethyl acetoacetate (2) in boiling acetic acid for 15 h gave
2ꢀ(pyrrolꢀ3ꢀyl)benzimidazole 3 in 62% yield (Scheme 1).
Its molecular formula was determined from elemental
Scheme 1
The formation of the benzimidazole system was conꢀ
firmed by the 1H NMR spectrum of the reaction product:
the signals for the protons of the phenylene fragment resoꢀ
nate, owing to benzimidazole tautomerism, as symmetriꢀ
cal multiplets of the AA´XX´ system1,18 at δ 7.55—7.57
and 7.79—7.81 (rather than as multiplet signals of the
ABCD system as for the protons of the phenylene ring in
quinoxaline derivatives19—20). In addition, the spectrum
shows multiplets for the phenyl protons at δ 7.30—7.33
* For Part 9, see Ref. 1.
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 2, pp. 359—363, February, 2011.
1066ꢀ5285/11/6002ꢀ368 © 2011 Springer Science+Business Media, Inc.

Synthesis of 2ꢀ(pyrrolꢀ3ꢀyl)benzimidazoles
Russ.Chem.Bull., Int.Ed., Vol. 60, No. 2, February, 2011
369
and 7.35—7.38, a triplet and a quartet for the protons of
the ethoxy group at δ 0.87 and 4.02 (3J = 7.2 Hz), respecꢀ
tively, and a singlet for the methyl protons at δ 2.62.
According to Xꢀray diffraction data, benzimidazole 3
crystallizes with a solvate methanol molecule in a ratio of
1 : 1 to form triclinic crystals (space group Pꢀ1). The moꢀ
lecular structure of compound 3 is shown in Fig. 1.
O(13´)
In structure 3, the angle between the plane of the pheꢀ
nyl substituent and the plane of the central pyrrole ring is
21.3° and the plane of the benzimidazole fragment is nearly
orthogonal to the pyrrole ring, making a dihedral angle of
77.3°. The planar ethoxycarbonyl group is also noncoplaꢀ
nar with the central pyrrole ring: the dihedral angle beꢀ
tween their planes is 7.2°. Such relative positions of the
above fragments preclude intramolecular hydrogen bondꢀ
ing N—H...O between the H atom of the amino group of
the benzimidazole fragment and the O atom of the ethꢀ
oxycarbonyl group. This gives rise to a centrosymmetric
Hꢀdimer due to intermolecular pair interactions of
this type (Fig. 2). The parameters of the hydrogen bond
N(1)—H(1)...O(13´) (–x, 1 – y, 1 – z) are as follows:
N(1)...O(13´), 2.959(3) Å; H(1)...O(13´), 2.09(3) Å; the
angle N(1)—H(1)...O(13´), 168(2)°. The H(8) atom of
the amino group of the pyrrole ring forms a classic hydroꢀ
gen bond to the O(23) atom of the solvate methanol moleꢀ
cule (N(8)...O(23), 2.854(3) Å; H(8)...O(23), 1.98(3) Å;
the angle N(8)—H(8)...O(23), 170(3)°.
An analysis of the intermolecular interactions suggests
a crucial role of solvate methanol molecules in the formaꢀ
tion of a supramolecular structure in the crystal of benzꢀ
imidazole 3. Having a hydroxy group, the methanol moleꢀ
cule acts as both a donor and an acceptor in classic hydroꢀ
gen bonding. One molecule of benzimidazole 3 is hydroꢀ
genꢀbonded to methanol according to the earlier described
type of bonding (N—H...O) and the O(23) atom of the
methanol molecule acts as a proton acceptor; for a second
O(23)
N(8)
O(13)
O(14)
N(1)
N(3)
Fig. 2. Dimerization of benzimidazole 3 in the crystal through
N—H...O hydrogen bonding (the bonds are indicated with
dashed lines).
molecule of benzimidazole 3, methanol acts as a proton
donor (O—H...N). Thus, methanol molecules bridge the
Hꢀdimers of benzimidazole 3. This gives rise to a supramoꢀ
lecular structure as zigzag chains of Hꢀbonded molecules of
benzimidazole 3 and methanol along the crystallographic
axis 0b (Fig. 3). The parameters of the bond O—H...N
between the H(23) atom of the hydroxy group of MeOH
and the N(3″) atom of the imidazole ring are as follows:
O(23)...N(3″), 2.816(3) Å; H(23)...N(3″), 1.91(4) Å; the
angle O(23)—H(23)...N(3″), 168(4)° (1 – x ,1 – y, 1 – z).
The supramolecular structure is additionally stabilized
by pair π—πꢀcontacts between the aromatic systems
of the pyrrole ring C(9)—N(8) and the benzene ring
C(17)—C(22) of symmetrically related molecules
(–x, 1 – y, 1 – z). The shortest distance between the
planes of the corresponding rings is 2.88 Å; the dihedral
angle between these planes is 21.2°.
Surprisingly, although the fragments of the molecules
are not disordered, the calculated packing factor (0.62)
is appreciably lower than the lower bound of the range
characteristic of the crystals of organic compounds
(0.65—0.75). The free volume, which is potentially accesꢀ
sible in the crystal to solvent molecules, is small (51.8 Å3).
With free quinoxalinꢀ2(1H)ꢀone instead of its hydroꢀ
chloride 1 in a reaction with ethyl acetoacetate, the yield
of the rearrangement product remains virtually unchanged;
apparently, this is due to various acidꢀcatalyzed reactions
involving ethyl acetoacetate.
To increase the yield of the rearrangement product, we
recurred to our hypothesis21 that any spiro derivative of
quinoxalinꢀ3(4H)ꢀone with at least one acidic H atom in its
spiroꢀfused component tends to rearrange itself into a benzꢀ
imidazole derivative spiroꢀfused at position 2 (Scheme 2).
According to this hypothesis, our aim was to obtain
a spiro[4ꢀpyrrolineꢀ3,2´ꢀquinoxalin]ꢀ3´(4´H)ꢀone derivꢀ
C(20)
C(19)
C(21)
C(4)
C(22)
C(18)
C(17)
C(5)
C(6)
C(3A)
C(12)
N(8)
N(3)
C(11)
C(2)
N(1)
C(7)
C(7A)
C(15)
C(9)
O(14)
C(10)
O(13)
C(13)
C(16)
C(14)
Fig. 1. Molecular structure of benzimidazole 3 in the crystal with
thermal displacement ellipsoids for the nonꢀhydrogen atoms
(p = 50%). The hydrogen atoms are represented by spheres of
arbitrary radii. The solvate methanol molecule is omitted.

370
Russ.Chem.Bull., Int.Ed., Vol. 60, No. 2, February, 2011
Mamedov et al.
N(3)
N(1)
H(1)
O(13)
N(8)
H(8)
N(3″)
H(23)
Fig. 3. Hydrogenꢀbonded chains in the crystal of compound 3 (hydrogen bonds are indicated with dashed lines).
Scheme 2
that the reaction takes place both at room temperature
(12 h) and in boiling ethanol (4 h).
1
The H NMR spectrum of spiro compound 4 shows
multiplet signals for the phenyl and phenylene protons at
δ 7.26—7.83, a triplet and a quartet for the protons of the
ethoxy group at δ 1.19 and 4.05 (3J = 7.2 Hz), respectiveꢀ
ly, two doublets of the AB system for the CH—NH fragꢀ
ment at δ 6.26 and 9.91 (3J = 8.6 Hz), and singlets for the
methyl and two NH protons at δ 1.90, 4.47, and 12.56,
respectively.
Reflux of compound 4 in acetic acid for 1 h gave the
expected benzimidazole 3 in 95% yield. Apparently, the
reaction of 3ꢀ(αꢀaminobenzyl)quinoxalinꢀ2(1H)ꢀone (1)
with ethyl acetoacetate (2) initially produces spiro comꢀ
pound 4, which undergoes in situ acidꢀcatalyzed rearrangeꢀ
ment into benzimidazole 3. The rearrangement steps inꢀ
clude (1) opening of the pyrazine ring of the quinoxaline
system through cleavage of the N(1)—C(2) bond in interꢀ
mediate salt A and (2) closure of the imidazole ring by
intramolecular cyclization involving the newly formed
amino group and the carbamoyl CO group in pyrrole deꢀ
rivative B (Scheme 4).
ative. Considering 3ꢀ(αꢀaminobenzyl)quinoxalinꢀ2(1H)ꢀ
one (1) to be a hetero analog to the αꢀamino carbonyl
component of the Knorr reaction22—24 (synthesis of pyrꢀ
roles by condensation of αꢀamino ketones with active meꢀ
thylene ketones), we carried out a reaction of quinoxaꢀ
linone 1 with ethyl acetoacetate (2) in EtOH in the
presence of KOH (Scheme 3). The reaction occurred
smoothly to give the desired spiro compound 4ꢀethoxyꢀ
carbonylꢀ5ꢀmethylꢀ2ꢀphenylꢀ1´Hꢀspiro[4ꢀpyrrolineꢀ3,2´ꢀ
quinoxalin]ꢀ3´(4´H)ꢀone (4) in high yield (88%). Note
Scheme 3
It should be noted that substituted 3ꢀ(αꢀaminobenzyl)ꢀ
quinoxalinꢀ2(1H)ꢀones successfully react with both ethyl
acetoacetate and other βꢀoxo acid esters to give the correꢀ
sponding 2ꢀ(pyrrolꢀ3ꢀyl)benzimidazoles in high yields.
These data suggest the general character of the reaction
under study and will be discussed elsewhere.

Synthesis of 2ꢀ(pyrrolꢀ3ꢀyl)benzimidazoles
Russ.Chem.Bull., Int.Ed., Vol. 60, No. 2, February, 2011
371
Scheme 4
The residue was treated with 5% aqueous HCl. The crystals that
formed were filtered off, washed with water, and dried in air.
The yield of compound 4 was 0.22 g (88%), m.p. 113—115 °C.
Found (%): C, 69.09; H, 5.88; N, 11.54. C21H21N3O3. Calculatꢀ
ed (%): C, 69.41; H, 5.82; N, 11.56. IR, ν/cm–1: 3437, 2974,
2928, 1664, 1595, 1494, 1448, 1293, 1256, 1170, 1122, 1098,
1026, 786, 758, 699, 583. 1H NMR, δ: 1.19 (t, 3 H, OCH2CH3,
J = 7.2 Hz); 1.90 (s, 3 H, CH3); 4.05 (q, 2 H, OCH2CH3,
J = 7.2 Hz); 4.47 (s, 1 H, NHquinox); 6.26 (d, 1 H, CH,
J = 8.6 Hz); 7.26 (dd, 1 H, H(6), J = 7.2 Hz, J = 7.2 Hz);
7.31—7.36 (m, 4 H); 7.45 (d, 2 H, oꢀHPh, J = 7.7 Hz); 7.54
(dd, 1 H, H(7), J = 6.8 Hz, J = 7.2 Hz); 7.83 (d, 1 H, H(5),
J = 8.1 Hz); 9.91 (d, 1 H, NH, J = 8.6 Hz); 12.56 (br.s, 1 H,
NHcarbamoyl).
Singleꢀcrystal Xꢀray diffraction study of compound 3 was
performed at the Crystallographic Division of the Collective Use
Center of the Spectroanalytical Center based on the Diffraction
Investigations Laboratory of the A. E. Arbuzov Institute of Orꢀ
ganic and Physical Chemistry (Kazan Research Center, Russian
Academy of Sciences).
The crystals of compound 3, C21H19N3O2•CH4O, are triꢀ
clinic. The unit cell parameters at 23 °C: a = 9.526(1) Å,
b = 10.954(1) Å, c = 11.833(2) Å, α = 111.561(2)°, β = 94.151(2)°,
Experimental
3
γ = 105.140(2)°, V = 1088.7(3) Å , Z = 2, dcalc = 1.151 g cm–3
,
M = 377.43, space group Pꢀ1. The intensities of 7529 reflections
(Rint = 0.0445) were measured on a Bruker Smart Apex II CCD
diffractometer (MoꢀKα radiation, graphite monochromator,
λ = 0.71073 Å, ω scan mode, scan ranges: –11 < h < 10,
–13 < k < 13, –14 < l < 13, 1.88 < θ < 25.50°). The number
of independent reflections was 3813 (2329 reflections with
I > 2σ(I)). An absorption correction was applied semiempiricalꢀ
ly with the SADABS program25 (μMo = 0.78 cm–1). The strucꢀ
ture was solved by the direct method and refined by the leastꢀ
squares method first isotropically and then anisotropically for all
nonꢀhydrogen atoms. The H atoms at the N(1), N(8), and O(23)
atoms were located from difference electronꢀdensity maps and
refined isotropically. The positions of the other H atoms were
calculated from stereochemical considerations and refined using
appropriate riding models. The final residuals are R1 = 0.0530
and wR2 = 0.1362 for 2329 independent reflections with I > 2σ(I)
and R1 = 0.0983 and wR2 = 0.1646 for all reflections; GOOF =
Melting points were determined on a Boetius instrument. IR
spectra were recorded on a Vectorꢀ22 FTIR spectrometer (Bruker)
in KBr pellets. The 1H NMR spectra of compounds 3 and 4 were
recorded on a Bruker Avanceꢀ600 spectrometer (600.13 MHz)
in DMSOꢀd6. Chemical shifts δ are referenced to the signals of
residual solvent protons (δH 2.50).
2ꢀ(4ꢀEthoxycarbonylꢀ5ꢀmethylꢀ2ꢀphenylpyrrolꢀ3ꢀyl)benzꢀ
imidazole (3). A. Quinoxalinꢀ2(1H)ꢀone hydrochloride 1 (0.2 g,
0.7 mmol) and ethyl acetoacetate (2) (0.1 mL, 0.77 mmol) were
refluxed in acetic acid (6 mL) for 15 h and then kept at room
temperature for 12 h. The crystals that formed were filtered off
and dried in air. Compound 3 was analytically pure. To collect
an additional crop of product 3, the reaction mixture was evapoꢀ
rated to dryness in a water aspirator vacuum and the residue was
treated with water. The resulting crystals were filtered off, washed
with 5% aqueous NaHCO3 (3×10 mL), dried in air, and recrysꢀ
tallized from AcOH. The total yield of compound 3 was 0.15 g
(62%), m.p. 270—272 °C. Found (%): C, 72.92; H, 5.60; N, 12.08.
C21H19N3O2. Calculated (%): C, 73.03; H, 5.54; N, 12.17. IR,
ν/cm–1: 3208, 2818, 2729, 2638, 2589, 1704, 1630, 1581, 1458,
1446, 1385, 1301, 1262, 1208, 1140, 1095, 895, 867, 780, 761,
738, 694, 683, 618. 1H NMR, δ: 0.87 (t, 3 H, OCH2CH3,
J = 7.2 Hz); 2.62 (s, 3 H, CH3); 4.02 (q, 2 H, OCH2CH3,
J = 7.2 Hz); 7.30—7.33 (m, 3 H); 7.35—7.38 (m, 2 H); 7.55—7.57
(m, 2 H, benzimidazole); 7.79—7.81 (m, 2 H, benzimidazole).
B. A solution of spiroquinoxaline 4 (0.1 g, 0.28 mmol) in
acetic acid (10 mL) was refluxed for 6 h, kept at ~20 °C for 18 h,
and concentrated in vacuo. The resulting crystalline precipitate
was dried in air. The yield of compound 3 was 0.09 g (95%).
4ꢀEthoxycarbonylꢀ5ꢀmethylꢀ2ꢀphenylꢀ1´Hꢀspiro[4ꢀpyrroꢀ
lineꢀ3,2´ꢀquinoxalin]ꢀ3´(4´H)ꢀone (4). Ethyl acetoacetate (2)
(0.1 mL, 0.77 mmol) and a solution of KOH (0.08 g, 1.4 mmol)
in EtOH (1 mL) were added to a solution of quinoxalinꢀ2(1H)ꢀ
one hydrochloride 1 (0.2 g, 0.7 mmol) in EtOH (6 mL). The
reaction mixture was stirred at room temperature for 12 h or
refluxed for 4 h and then concentrated to half its initial volume.
= 1.005, the number of parameters refined is 268. The maximum
–3
and minimum residual electron densities are 0.189 and –0.177 e Å
,
respectively. Reflection intensity data were collected and editꢀ
ed, and the unit cell parameters were refined, with the APEX2
program.26 All calculations were performed with the SHELXTL27
and WinGX programs.28 Intermolecular interactions were anaꢀ
lyzed and the molecules were imaged with the PLATON proꢀ
gram.29 The atomic coordinates and thermal parameters for
structure 3 have been deposited with the Cambridge Crystalꢀ
lographic Data Center (http://www.ccdc.cam.ac.uk; CCDC
No. 773036).
This work was financially supported by the Russian
Foundation for Basic Research (Project No. 10ꢀ03ꢀ00413ꢀa).
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Received May 28, 2010;
in revised form December 23, 2010