A new route to pyrimido[1,6-a]benzimidazole derivatives

Article abstract of DOI:10.1002/jhet.5570450534

(Chemical Equation Presented) Cyclocondensation of orthophenylendiamines with 3-bromopropionic acid in PPA afforded 2-vinyl-benzimidazoles which were subsequently converted to their corresponding 2-(1,2-dibromoethyl)-1H- benzimidazoles on treatment with bromine. Reaction of these compounds with aryl nitriles or aryl isocyanates in basic chloroform yielded 1-arylpyrimido[1,6-a] benzimidazoles and 2-arylpyrimido[1,6-a]-benzimidazol-3-ones respectively.

Full text of DOI:10.1002/jhet.5570450534

Sep-Oct 2008
1465
A New Route to Pyrimido[1,6-a]benzimidazole Derivatives
M. Bakavolia, b*, M. Nikpourc, A. R. Ebrahimia, A. Taghizadehb,
M. Rahimizadeha and A. Davoodniab
a:Department of Chemistry, School of Sciences, Ferdowsi University, Mashhad 91775-1436, Iran
b:Department of Chemistry, School of Sciences, Islamic Azad University, Mashhad , Iran
c: Department of Chemistry, School of Sciences, Islamic Azad University, Ahvaz Branch,
Ahvaz, Iran
*Corresponding author: email:mbakavoli@yahoo.com
Received June 26, 2007
Ar
O
Ph
N
N
H
N
N
N
N
N
TEA, ArCN
TEA, ArNCO
R
Br
R
N
Br
R
Cyclocondensation of orthophenylendiamines with 3-bromopropionic acid in PPA afforded 2-vinyl-
benzimidazoles which were subsequently converted to their corresponding 2-(1,2-dibromoethyl)-1H-
benzimidazoles on treatment with bromine. Reaction of these compounds with aryl nitriles or aryl
isocyanates in basic chloroform yielded 1-arylpyrimido[1,6-a]benzimidazoles and 2-arylpyrimido[1,6-a]-
benzimidazol-3-ones respectively.
J. Heterocyclic Chem., 45, 1465 (2008).
INTRODUCTION
[8-10] and methyl 2-(bromomethyl)-1H-benzimidazole-
1-carboxylate with tosylmethylisocyanides [15-17],
Pyrimido[1,6-a]benzimidazoles are a class of fused
heterocycles with a wide spectrum of biological activities.
These compounds have been described as being DNA
cleaving agents [1], DNA gyrase inhibitors [2-7], anti-
inflammatory [8-10], antiamoebic and analgesic [8,9],
herbicidal [11], antimicrobial and antifungal [12],
antiulcer [13], polymer dye additive [14], variolin
alkaloids analogues [15-17], and nucleoside analogues
agents [18,19]. Studies on benzene carcinogenesis and
mutagenesis showed that p-benzoquinone which is the
oxidation product of benzene reacts with deoxycitidine in
the human body [20-27] to produce a pyrimido[1,6-
a]benzimidazole structure as shown in Figure 1.
reaction
of
4-O-[(triisopropylphenyl)sulfonyl]-
pyrimidine nucleosides with orthophenylenediamine
[18-19] and direct or biomedia reaction of
deoxycytidine with p-benzoquinone [20-27]. Other
routes include condensation of 2-(3-aminopropyl)-
benzimidazole with carbon disulfide [28], 2-(2-benz-
imidazolyl)acetonitrile and 2-(2-benzimidazolyl)ethyl-
acetate [29-36], ring construction of pyrimido[4,5-b]-
[1,5]benzodiazepines [37-39], reaction of 6-chloro[1,3]-
oxazine-2,4-diones with orthophenylenediamine [40]
and cyclocondensation of (benzimidazol-2-ylmethyl)-
triphenylphosphonium chloride [41-43].
In connection with our work on the synthesis of
heterocyclic compounds with potential biological
activities [44] and exploration of their synthetic pathways
[45], we report herein a general methodology for the
preparation of pyrimido[1,6-a]benzimidazoles 4 through
heterocyclization of 2-(1, 2-dibromoethyl)-1H-benzimida-
zoles 3a,b with aromatic nitriles and isocyanates.
In view of all these activities we were intrigued by
the possibility of preparing various pyrimido[1,6-a]-
benzimidazole derivatives. These derivatives can be
prepared by cyclocondensation of 1-(2-aminoaryl) urea
with diethyl malonate, 2-substituted benzimidazolyl-
acetates with carbonate esters [1-6], condensation of
ketoisothiocyanates with orthophenylenediamines
OH
O
NH2
N
N
O
N
N
H
O
N
O
HO
+
HO
O
O
H
H
H
H
H
H
H
H
H
OH
OH
Figure 1

1466
M. Bakavoli, M. Nikpour, A. R. Ebrahimi, A. Taghizadeh, M. Rahimizadeh and A. Davoodnia
Vol 45
Scheme 1
H
N
Br
N
2'
R
t
a
e
h
,
A
PP
NH2
NH2
O
Br
+
P
P
R
OH
A,
heat
H
N
1a,b a: R=H, b: R= Me
N
R
2a,b a: R=H, b: R= Me
Cl 3
H
, C
2
Br
Ar
O
N
Ar
N
H
N
N
N
TEA, ArCN
TEA, ArNCO
Br
N
R
N
Br
N
R
R
4d-f
4a-c
3a,b a: R=H, b: R= Me
d:R= H, Ar= C6H5
a:R= Me, Ar= C6H5
e:R= H, Ar= 3,4-di(Cl)-C6H3
f:R= Me, Ar= 3,4-di(Cl)-C6H3
b:R= Me, Ar= 4-Br-C6H4
c:R= Me, Ar= 3,5-di(OMe)-C6H3
RESULTS AND DISCUSSION
Scheme 2
Hb
H
As shown in Scheme 1, reaction of orthophenyl-
enediamines 1a,b with 3-bromopropionic acid in PPA
failed to produce the bromobenzimidazole derivatives 2'
as have been claimed by an earlier worker [46] but instead
afforded their corresponding 2-vinylbenzimidazoles 2a,b.
The structures assigned to these compounds were
substantiated by their spectral and analytical data. For
example, the IR spectrum of 2a exhibited two vibrational
bands at 1650 cm-1 and 3300cm-1 due to C=C and NH
bonds respectively. The 1H NMR spectrum of this
compound showed four signals at ꢀ 5.6, 6.0, 6.7 and 8.5
N
Ha
N
Hc
R
subjected to base catalyzed heterocyclization with aryl
nitriles or aryl isocyanates and gave the desired 1-aryl-
pyrimido[1,6-a]benzimidazoles 4a-c and 2- arylpyrimido-
[1,6-a]benzimidazol-3-ones 4d-f respectively.
A
plausible mechanism is depicted for the formation of
selected compounds 4a-c (Scheme 3). Heterocyclization
seems to start with the base catalyzed nucleophilic attack
of the benzimidazole ring nitrogen onto the nitrile
functionality of the arylnitrile with subsequent
nucleophilic attack on the bromomethylene moiety which
was followed by dehydrobromination to furnish the
tricyclic structure.
2
2
ppm due to Ha (3J= 10Hz, J= 2Hz), Hb (3J= 18Hz, J=
3
2Hz), Hc (3JHbHc
=
18Hz, JHaHc
=
10Hz) and NH
respectively (Scheme 2). The mass spectrum showed a
molecular ion peak at m/z 144, which confirms the
proposed structure.
Bromination of the 2-vinylbenzimidazoles 2a,b with
Br2/CHCl3 afforded the dibromo derivatives 3a,b on the
basis of their spectral and microanalytical data (data in the
experimental section). The latter compounds were
The structures of compounds 4a-f were confirmed
from analytical data. For example, the IR spectra of
compounds 4a-c were devoid of the stretching vibration
Scheme 3
N
C
Ar
Ar
Ar
N
N
H
N
H
TEA, ArCN
N
N
N
N
TEA, ArCN
Br
N
Br
Br
R
R
R

Sep-Oct 2008
A New Route to Pyrimido[1,6-a]benzimidazole Derivatives
1467
bands at 3300cm-1 and 2250 cm-1 due to NH and CN
functional groups while compounds 4d-f only exhibited a
vibrational band for C=O group at 1650 cm-1.The 1H
NMR spectra of compounds 4a-f did not exhibit signals
attributed to protons of CH2Br, CHBr and NH groups of
their precursors but instead the appearance of two signals
for the representative compounds 4c,f around ꢀ, 8.0 and
9.0 ppm for C4H and C3H respectively is a good
indication of the cyclization process. Further proof came
from the mass spectra of compounds 4a-f which showed
their expected molecular ion peacks with the exclusion of
isotopic pattern for two bromine atoms but in case of 4b
and 4e,f isotopic patterns were observed for one bromine
and two chlorine atoms respectively.
2-(1,2-Dibromoethyl)-1H-benzimidazole (3a). This
compound was obtained as a red powder in 65% yield , mp 174
°C; IR: 3300 cm-1 (NH) ;1H NMR: (CDCl3) ꢀ, 4.2 (d, 2H,
CH2Br), 6.5 (t, 1H, CHBr), 7.0-7.4 (m, 4H, aromatic), 13.6
(broad,1H, NH); ms: m/z, 284 (100%), 302 (25%), 304 (49%),
306 (24%). Anal. Calc. for C9H8Br2N2 (%):C, 35.56; H, 2.65; N,
9.22. Found (%): C, 35.24; H, 2.71; N, 9.43.
2-(1, 2-Dibromoethyl)- 5- methyl-1H-benzimidazole (3b).
This compound was obtained as a red powder in 65% yield, mp
153 °C; IR: 3320 cm-1 (NH) ;1H NMR: (CDCl3) ꢀ, 2.3 (s, 3H,
CH3), 3.7 (d, 2H, CH2Br), 6.0 (t, 1H, CHBr), 6.5- 7.1 (m, 3H,
aromatic), 14.0 (broad, 1H, NH); ms: m/z, 298 (100%), 316
(20%), 318 (39%), 320 (19%). Anal. Calc. for C10H10Br2N2
(%):C, 37.77; H, 3.17; N, 8.81. Found (%): C, 38.04; H, 2.95; N,
9.03.
General procedure for the reaction of 2-(1,2-dibromo-
ethyl)- 5- methyl-1H-benzimidazole with nitriles. A solution
of 2-(1,2-dibromoethyl)-5-methyl-1H-benzimidazole 3b (10
mmol), aromatic nitrile (20 mmol) and triethylamine (3mL) in
chloroform (20mL) was heated under reflux for 6 hours. The
solvent was removed under reduced pressure and the residue
was washed with water and then recrystallized from ethanol to
give compounds 4a-c.
In conclusion, treatment of 2-(1,2-dibromoethyl)-1H-
benzimidazoles 3a,b with aromatic nitriles and
isocyanates in basic chloroform is a new, efficient and
general access to 1-arylpyrimido[1,6-a]benzimidazole
derivatives.
7-Methyl-1-phenylpyrimido[1,6-a]benzimidazole (4a). This
compound was obtained as a yellow powder in 75% yield , mp
275 °C; IR: 2850, 2900 cm-1 (CH3); 1H NMR: (CDCl3) ꢀ, 2.4 (s,
3H, CH3), 7.0- 7.8 (m, 9H, aromatic), 8.2 (d, 1H, C3H); ms: m/z,
259. Anal. Calc. for C17H13N3 (%):C, 78.74; H, 5.05; N, 16.20.
Found (%): C, 78.56; H, 5.24; N, 16.41.
1-(4-Bromophenyl)-7-methylpyrimido[1,6-a]benzimidazole
(4b). This compound was obtained as a brown powder in 85%
yield , mp 282 °C; IR: 2850, 2900 cm-1 (CH3); 1H NMR:
(CDCl3) ꢀ, 2.2 (s, 3H, 7-CH3), 7.1- 7.85 (m, 8H, aromatic), 8.25
(d, 1H, C3H); ms: m/z, 172 (100%) , 337 (55%), 339 (54.3%).
Anal. Calc. for C17H12BrN3 (%):C, 60.37; H, 3.58; N, 12.42.
Found (%): C, 60.12; H, 3.72; N, 12.34.
1-(3,5-Dimethoxyphenyl)-7-methylpyrimido[1,6-a]benz-
imidazole (4c). This compound was obtained as a brown
powder in 72% yield , mp 215-217 °C; IR: 2900, 2950 cm-1
(CH3); 1H NMR: (CDCl3) ꢀ, 2.4 (s, 3H, 7-CH3), 3.8 (s, 6H,
2OCH3), 6.8- 7.7 (m, 8H, aromatic),; ms: m/z, 319. Anal. Calc.
for C19H17N3O2 (%):C, 71.46; H, 5.37; N, 13.16. Found (%): C,
71.27; H, 5.21; N, 12.95.
EXPERIMENTAL
Melting points were recorded on an Electrothermal type 9100
melting point apparatus. The IR spectra were obtained on a 4300
1
Shimadzu Spectrometer. The H NMR (100 MHz) spectra were
recorded on a Bruker AC 100 spectrometer. The mass spectra
were scanned on a Varian Mat CH-7 instrument at 70 eV.
Elemental analysis was obtained on a Thermo Finnigan Flash
EA microanalyzer.
General procedure for the preparation of 2-vinylbenz-
A mixture of orthophenylenediamine or 4-
imidazoles.
methylorthophenylendiamine (10 mmol), 3-bromopropionic acid
(10 mmol) in PPA (5 mL) was heated with stirring for 4 hours at
100-120 °C. Then the mixture was poured onto ice and
neutralized with concentrated ammonia solution and kept over 8
hours at 5-10 °C. The yellow residue was recrystallized from
ethanol.
2- Vinylbenzimidazole (2a). This compound was obtained as
a yellow powder in 40% yield , mp 159 °C; IR:3280 cm-1 (NH)
;1H NMR: (CDCl3) ꢀ, 5.6 (dd,1H, Ha) (3J= 10Hz, J= 2Hz), 6.0
(dd, 1H, Hb) (3J= 18Hz, 2J= 2Hz), 6.7 (dd, 1H, Hc) (3J= 18Hz, 3J=
10Hz), 7.1-7.6 (m, 4H, aromatic), 8.5 (broad,1H, NH); ms: m/z,
144. Anal. Calc. for C9H8N2 (%):C, 74.98; H, 5.59; N, 19.43.
Found (%): C, 75.20; H, 5.79; N, 19.23.
2
General procedure for the reaction of 2-(1,2-
Dibromoethyl)-5-methyl-1H-benzimidazole with aryl
isocyanates.
A solution of each 2-(1,2-dibromoethyl)-1H-
benzimidazoles 3a,b (10 mmol), aromatic isocyanate (20 mmol)
and triethylamine (3mL) in chloroform (30mL) was heated
under reflux for 5 hours. The solvent was removed under
reduced pressure and the residue was washed with water and
then crystallized from ethanol to give compounds 4d-f.
2-Phenylpyrimido[1,6-a]benzimidazole-1-one (4d). This
compound was obtained as an orange powder in 67% yield , mp
235 °C; IR: 1650 (C=O), 2850, 2900 cm-1 (CH3); 1H NMR: (d6-
DMSO) ꢀ, 7.0 - 7.8 (m, 9H, aromatic), 7.87 (d, 3J = 12Hz, 1H,
C4H), 8.2 (d, 3J = 12Hz, 1H, C3H); ms: m/z, 261. Anal. Calc. for
C16H11N3O (%):C, 73.55; H, 4.24; N, 16.08. Found (%): C,
73.37; H, 4.13; N, 15.89.
5- Methyl- 2- vinylbenzimidazole (2b). This compound was
obtained as a yellow powder in 46% yield , mp 125 °C; IR:3300
cm-1 (NH) ;1H NMR: (CDCl3) ꢀ, 2.3 (s, 3H, CH3), 5.6 (dd,1H,
Ha) (3J = 10Hz, 2J = 2Hz), 6.2 (dd, 1H, Hb) (3J = 17Hz, 2J = 2Hz),
6.8 (dd, 1H, Hc) (3J = 17Hz, 3J = 10Hz), 7 -7.45 (m, 3H,
aromatic), 9.1 (broad,1H, NH); ms: m/z, 158. Anal. Calc. for
C10H10N2 (%):C, 75.92; H, 6.37; N, 17.71. Found (%): C, 76.22;
H, 6.18; N, 17.53.
General procedure for the bromination of 2-vinylbenz-
imidazoles. To a solution of 2-vinylbenzimidazoles 2a or 2b (10
mmole) in chloroform (10 mL), bromine (10 mmol) was added
dropwise and stirred for half an hour. Then the solvent was
removed under reduced pressure and the residue was
recrystallized from ethanol.
2-(3,4-Dichlorophenyl)-pyrimido[1,6-a]benzimidazole-1-
one (4e). This compound was obtained as an orange powder in
80% yield , mp 220 °C; IR: 1600 (C=O), 2880, 2950 cm-1 (CH3);
3
1H NMR: (CDCl3) ꢀ, 7.2- 7.7 (m, 7H, aromatic), 7.9 (d, J =

1468
M. Bakavoli, M. Nikpour, A. R. Ebrahimi, A. Taghizadeh, M. Rahimizadeh and A. Davoodnia
Vol 45
3
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(100%), 329 (9%), 331 (6%), 333 (1%). Anal. Calc. for
C16H9Cl2N3O (%):C, 58.20; H, 2.75; N, 12.73. Found (%): C,
57.92; H, 2.89; N, 12.94.
2-(3,4-Dichlorophenyl)-7-methylpyrimido[1,6-a]benzimi-
dazole-1-one (4f). This compound was obtained as an orange
powder in 70% yield , mp 278 °C; IR: 750 (C-Cl), 1600 (C=O),
2850, 2900 cm-1 (CH3); 1HNMR: (CDCl3) ꢀ, 2.4 (s, 3H, 7-CH3),
7.3- 7.6 (m, 6H, aromatic), 7.9 (d, 1H, C4H), 9.2 (d, 1H, C3H);
ms: m/z, 173 (100%), 343(12%), 345(8%), 347(1%). Anal. Calc.
for C17H11Cl2N3O (%):C, 59.32; H, 3.22; N, 12.21. Found (%):
C, 59.12; H, 2.96; N, 12.41.
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