Synthesis of 2-amino-s-triazino[1,2-a]benzimidazoles as potential antifolates from 2-guanidino- and 2-guanidino-5-methylbenzimidazoles

Article abstract of DOI:10.1002/jhet.5570430115

The syntheses of 2-amino-s-triazino[1,2-a]benzimidazoles from 2-guanidinobenzimidazoles were successfully carried out by a ring annelation reaction. The regiochemistry of the ring closure of 5-methyl-2- guanidinobenzimidazole with diethyl azodicarboxylate, aldehydes, acetone, diethyl ethoxymethylene-malonate and orthoesters, leading to the formation of s-triazine ring was studied. High regioselectivity was not observed in any of these reactions. However, the synthesis of s-triazino[1,2-a]benzimidazole system was found to be more regioselective than its 3,4-dihydro analogue. NOESY experiment indicated that the compound, 2-amino-4,4-dimethyl-3,4-dihydro-s- triazino[1,2-a]benzimidazole existed predominantly as the 3,4-dihydro tautomer in dimethyl sulfoxide. It was found to inhibit bovine dihydrofolate reductase with IC₅₀ 10.9 μM.

Full text of DOI:10.1002/jhet.5570430115

Jan-Feb 2006
Synthesis of 2-Amino-s-triazino[1,2-a]benzimidazoles as Potential
Antifolates from 2-Guanidino- and 2-Guanidino-5-methylbenzimidazoles
Anton V. Dolzhenko and Wai-Keung Chui*
95
Department of Pharmacy, Faculty of Science, National University of Singapore, 18 Science Drive 4, Singapore
117543, Singapore E-mail: phacwk@nus.edu.sg
Received June 2, 2005
The syntheses of 2-amino-s-triazino[1,2-a]benzimidazoles from 2-guanidinobenzimidazoles were suc-
cessfully carried out by a ring annelation reaction. The regiochemistry of the ring closure of 5-methyl-2-
guanidinobenzimidazole with diethyl azodicarboxylate, aldehydes, acetone, diethyl ethoxymethylene-
malonate and orthoesters, leading to the formation of s-triazine ring was studied. High regioselectivity was
not observed in any of these reactions. However, the synthesis of s-triazino[1,2-a]benzimidazole system was
found to be more regioselective than its 3,4-dihydro analogue. NOESY experiment indicated that the com-
pound, 2-amino-4,4-dimethyl-3,4-dihydro-s-triazino[1,2-a]benzimidazole existed predominantly as the 3,4-
dihydro tautomer in dimethyl sulfoxide. It was found to inhibit bovine dihydrofolate reductase with IC50
1
0.9 µM.
J. Heterocyclic Chem., 43, 95 (2006).
Our laboratory has been working on the s-triazine class
of dihydrofolate reductase (DHFR) inhibitors in the search
for potential antibacterial and anticancer agents [1]. It is
therefore of interest to us to explore the synthesis of fused
s-triazines in an attempt to discover new heterocyclic
structures that may inhibit DHFR. It has been noted from
some recent reports [2,3] that compounds having the core
structure of s-triazino[1,2-a]benzimidazole (1), with par-
ticular reference to 2-amino-4,4,7,8-tetramethyl-3,4-dihy-
dro-s-triazino[1,2-a]benzimidazole (2), have demonstrated
inhibitory activity against the plasmodial DHFR.
of unsymmetrical 2-guanidinobenzimidazoles, with
respect to 5-methyl-2-guanidinobenzimidazole (5b).
However, the regiochemistry of the reactions has not yet
been investigated thoroughly and the existing reports
appear to be incomplete and controversial. For example, 2-
amino-7(8)-methyl-s-triazino[1,2-a]benzimidazoles and 2-
amino-7-methyl-4-oxo-4,10-dihydro-s-triazino[1,2-a]ben-
zimidazole have been shown to be the products when 5b
was allowed to react with triethyl orthoformate [8,9] or
diethyl azodicarboxylate [12], respectively. In the first
case [8,9] the analysis of the mixture of isomers has not
been described fully; while in the second case [12], no evi-
dence to support the participation of N-3 instead of N-1
atom of 5b in the annulation step has been demonstrated.
Therefore, this study is designed with the objective to
synthesize and investigate the nature of the ring closure
step during the formation of a series of 2-amino-s-tri-
azino[1,2-a]benzimidazoles. In addition, the inhibitory
activity of the series of analogues against the bovine
DHFR will be evaluated. This study will report for the first
time, a simple and effective method for the preparation of
compounds related to fused structure 2 and having two
geminal methyl groups at position 4. At the same time the
regiochemistry of the ring closure of 5b, with the forma-
tion of the fused s-triazine ring will also be discussed.
Figure 1
The synthesis of the 2-amino-s-triazino[1,2-a]benzimi-
dazoles has been achieved through the ring closure of 2-
guanidinobenzimidazoles [4-13]. To date, only a limited
number of reports [8,9,12] have discussed the ring closure

9
6
A. V. Dolzhenko and W.-K. Chui
Vol. 43
Table 1
Guanidinobenzimidazoles (5a,b) were prepared from a
2
-Amino-s-triazino[1,2-a]benzimidazoles
reaction between an appropriate o-phenylenediamine (3)
with cyanoguanidine (4) as described in previous general
method [14]. The synthesized 2-guanidinobenzimidazoles
Compound R1
R2
R3
Ratio of regioisomers (b / c), %
(
5) were then cyclized using different reagents to give 2-
6a
H
-
-
H
H
H
H
-
-
H
H
Ph
Ph
-
6
7
7
8
8
b,c
a
b,c
a
b,c
7(8)-Me
H
7(8)-Me
H
55 / 45
-
50 / 50
-
55 / 45 (method A), 50 / 50
(methods B and C)
-
amino-s-triazino[1,2-a]benzimidazoles with different sub-
stitutions at position 4 (Scheme 1). The structures of the
synthesized 2-amino-[1,3,5]triazino[1,2-a]benzimidazoles
(
6-11) were elucidated with the help of ir and nmr spectral
and elemental analyses. The ratios of the isomeric products
6-11b,c) prepared from 5b were determined, without sep-
aration of mixtures, using nmr spectroscopic techniques.
7(8)-Me
9
9
1
1
a
b,c
0a
H
Me Me
Me Me 54 / 46
H
H
(
7(8)-Me
H
7(8)-Me
-
-
-
1
13
The assignment of H and C nmr signals for the 7(8)-
methyl substituted compounds (6-11b,c) in the mixtures
was made based on the data for analogous compounds (6-
0b,c
20 / 80 (method A), 17 / 83
(method B)
11a
11b,c
H
Me
Me
-
-
-
1
1a) prepared from the unsubstituted 2-guanidinobenzimi-
7(8)-Me
65 / 35
dazole (5a) with the help of COSY, DEPT90 and
DEPT135 experiments.
Scheme 1
1
2
3
2
Reagents and reaction conditions: R = H, Me (i) diethyl azodicarboxylate, EtOH, reflux, 3 h; (ii) R = R = H, 37% HCHO, dioxane, reflux, 2 h; R = H,
3
2
3
R = Ph, PhCHO, KOH (method A), piperidine (method B) or N-methylpiperazine (method C), reflux, 3-5 h; R = R = Me, acetone, piperidine, reflux, 7 h;
iii) R = H, EtOCH=C(COOEt) , MeCN, reflux, 5 h (method A) or HC(OMe) , DMF, reflux, 3 h (method B); R = Me, MeC(OEt) , DMF, reflux, 18 h.
2
2
(
2
3
3
Contrary to the previously reported information [12], it
(6). These findings confirmed the s-triazine ring formation
and are in agreement with literature data [15] for related
fused heterocyclic systems with 4-oxo-s-triazine ring.
Reactions of 5 with aldehydes afforded 2-amino-3,4-
dihydro-s-triazino[1,2-a]benzimidazoles (7-8). Refluxing
of 2-guanidinobenzimidazoles (5) in acetone under piperi-
dine catalysis led to the formation of 2-amino-3,4-dihydro-
s-triazino[1,2-a]benzimidazoles (9) with two geminal
methyl groups. Cycloadditions of these carbonyl reagents
to 5b did not show regioselectivity and gave the isomeric
compounds 7-9b and 7-9c in equal or almost equal propor-
tion (Table 1). The methyl group of 5b did not have an
influence on the ring closure taking place at N-1 and N-3.
was found that the ring closure of 5b with diethyl azodi-
carboxylate did not proceed regioselectively and afforded
a mixture of 2-amino-7(8)-methyl-4-oxo-3,4-dihydro-s-
triazino[1,2-a]benzimidazoles (6b,c) that could be clearly
1
observed from the H nmr spectrum. Regioisomers (6b,c)
were formed in almost equal proportion (Table 1).
The presence of the carbonyl group at C4 in the structure
was supported with a significant stretching absorption sig-
nal at 1717 cm- (νC=O) in the ir spectra of compound
1
(
6a).The anisotropic effect of the oxygen atom of this car-
bonyl group led to the downfield shift of the signal of H-6
1
to 7.92-8.09 ppm in the H nmr spectra of the compounds

Jan-Feb 2006
Synthesis of 2-Amino-s-triazino[1,2-a]benzimidazoles as Potential Antifolates
97
The nature of catalyst used did not have a significant
impact on the ratio of isomeric forms. Thus, it was
observed that with the use of potassium hydroxide as a cat-
alyst, the reaction resulted in the formation of 55% of 8b
and 45% of 8c; while reactions catalyzed separately by
piperidine and N-methylpiperazine afforded a mixture of
isomers (8b,c) of 50% each.
azino[1,2-a]benzimidazoles (10b,c). The 8-methyl substi-
tuted isomeric form 10c was predominant (80-83%) in the
mixture with 17-20% of the minor compound 10b.
Interestingly, almost an inverse ratio (65% and 35%) of 4-
methyl substituted analogues (11b,c) was observed in the
result of the reaction when triethyl orthoacetate was used
for cycloaddition instead.
It should be noted that several signals in the 13C nmr
spectrum of 10a have not been correctly assigned in a pre-
vious report [13]. For example, the signal of methine car-
bone C(4)H could not be located at 163 ppm as was indi-
cated in [13]. A signal was not found in this region in
DEPT90 experiment, at the same time the C-4 signal was
observed at 148.8 ppm. The corrected assignments for 10a
are presented below in experimental section.
DHFR inhibitory activity of the individual compounds
6-10a was evaluated using bovine DHFR (Fluka Chemie)
according to a previously described method [1]. The com-
pounds for the DHFR inhibition bioassays were dissolved
in dimethyl sulfoxide. In order to ensure that the solvent
per se did not have an effect on the enzymatic activity,
negative control test was performed using dimethyl sulfox-
ide at the same concentration. 2-Amino-4-methyl-s-tri-
azino[1,2-a]benzimidazole (11a) was not tested because of
its very poor solubility in the bioassay medium. The most
active compound in the series was 2-amino-4,4-dimethyl-
3,4-dihydro-s-triazino[1,2-a]benzimidazole (9a). This
compound was found to inhibit DHFR with IC50 value
10.9 µM. All the other compounds in the series possessed
IC50 values more than 100 µM and were therefore consid-
ered as inactive.
The formation of the dihydro-s-triazine ring with a sp3
hybridized C-4 was confirmed by the signals of C-4 in sep-
1
3
arate C nmr spectra, namely the signals at 53.0, 65.7-
5.8 and 69.3-69.4 ppm for secondary, tertiary and quater-
6
nary carbon atom of compounds (7-9), respectively. The
2
signals of the other two sp carbon atoms of the formed s-
triazine ring (C-2 and C-10a), surrounded by three nitro-
gen atoms each, were located at 155.2-156.5 and 153.0-
1
3
1
53.7 ppm, respectively. The C nmr spectral data were in
good agreement with the work carried out on the formation
of the spiro- analogue, as described in another report (7,
2
3
R R = C H [8]).
5
10
Evidence indicating that the dihydro-s-triazino[1,2-a]-
benzimidazoles (6-9) could exist in 3,4-dihydro-, 1,4-dihy-
dro- and 4,10-dihydro- tautomeric forms was observed
(
Figure 2). The prototropic interconversion between these
tautomeric forms led to the broadening of the signals of C-
1
3
2
, C-4, C-10a atoms in the C nmr spectra that was
observed for the compounds 6-9. In a NOESY experiment
conducted on compound 9, strong cross-peaks were
observed for the signal at 1.82 ppm and the signals of 6-H
as well as the N-H protons. The close spatial relationship
of the geminal methyl groups and proton at the annular
nitrogen atom corresponded to the 3,4-dihydrotautomeric
Figure 2. Annular tautomerism in 2-amino-dihydro-s-triazino[1,2-a]benzimidazoles.
form. In the condition of the experiment, no cross-peak
was found for N-H and 9-H that indicated the predomi-
nance of 3,4-dihydro- tautomeric form in the dimethyl sul-
foxide solution.
The completely conjugated 2-amino-s-triazino[1,2-a]-
benzimidazoles (10-11) was prepared from the reaction of
EXPERIMENTAL
Melting points (uncorrected) were determined on a Gallenkamp
melting point apparatus. The infrared spectra were performed on a
Jasco FT-IR-430 spectrophotometer in potassium bromide pellets.
1
13
H and C nmr spectra were recorded on a Bruker DPX-300
spectrometer in the dimethyl sulfoxide-d solution using TMS as
6
5
with diethyl ethoxymethylenemalonate or orthoesters.
These types of annulations when applied to unsymmetrical
b seemed to be more regioselective than the other reac-
an internal reference. The samples prepared in the reaction of 2-
guanidino-5-methylbenzimidazole (5b) described below were
analyzed by nmr spectroscopy without recrystallization in order to
avoid changes in the isomers ratio. The satisfied purity of the sam-
ples was achieved by careful washing with ethanol (acetone) that
confirmed by nmr spectral data and elemental analyses.
5
tions described above. Thus, reaction of 5b with diethyl
ethoxymethylenemalonate or trimethyl orthoformate led to
the formation of a mixture of 7(8)-methyl-2-amino-s-tri-

98
A. V. Dolzhenko and W.-K. Chui
Vol. 43
General Procedure for Preparation of 2-Benzimidazolyl-
guanidines (5).
(20 mL) was heated under reflux for 2 h. After cooling, the prod-
uct was filtered, washed with ethanol and dried.
The mixture of the appropriate o-phenylenediamine 3 (0.1
mol), cyanoguanidine 4 (8.4 g, 0.1 mol) and conc. hydrochloric
acid (20 mL, 0.2 mol) in 200 mL of water was heated under
reflux for 1 h, cooled to 0° and 50 mL of 10% solution of potas-
sium hydroxide was added slowly. Precipitated 2-guanidinobenz-
imidazole 5 was filtered, washed with water, dried and used in
subsequent steps of the reactions without further purification.
2-Amino-3,4-dihydro-s-triazino[1,2-a]benzimidazole (7a).
Compound 7a was obtained in 68% yield; mp 284-285°
(
DMF), (Lit. mp 304-305° [8], mp 307-308° [9]); ir (potassium
bromide): NH 3323, NH 3144, CH (aromatic) 3058, CH
(
1
aliphatic) 2898, C=N 1649, NH 1620, 1588, 1526, 1467, 1439,
2
375, 1279, 1244, 758, 740 cm ; H nmr (dimethyl sulfoxide-
-1 1
d ): δ 5.40 (s, 2H, CH ), 6.83 (s, 2H, NH , deuterium oxide-
6
2
2
2
-Guanidinobenzimidazole (5a).
exchangeable), 6.97 (td, 1H, 7-H, J = 7.3, 1.3 Hz), 7.02 (td, 1H,
-H, J = 7.3, 1.3 Hz), 7.17 (dd, 1H, 9-H, J = 7.3, 1.1 Hz), 7.29
dd, 1H, 6-H, J = 7.3, 1.1 Hz), 7.89 (br s, 1H, NH, deuterium
8
Compound 5a was obtained in 78% yield; mp 243-244 °, (Lit.
(
[
14] mp 245 °).
-Guanidino-5-methylbenzimidazole (5b).
Compound 5b was obtained in 82% yield; mp 220-221 °; H
1
3
oxide-exchangeable); C nmr (dimethyl sulfoxide-d ): δ 53.0
(C-4), 107.3 (C-6), 115.5 (C-9), 119.0 (C-8), 120.7 (C-7), 131.5
6
2
(
C-5a), 142.7 (C-9a), 153.7 (C-10a), 156.5 (C-2).
Anal. Calcd. for C H N : C, 57.74; H, 4.85; N, 37.41. Found:
1
9
9 5
nmr (dimethyl sulfoxide-d ): δ 2.32 (s, 3H, Me), 6.72 (dd, 1H, 6-
6
C, 57.62; H, 4.92; N, 37.24.
H, J = 7.9, 0.8 Hz), 6.7 (br s, 4H, NH-C(=NH)NH , deuterium
2
oxide-exchangeable), 6.96 (s, 1H, 4-H), 7.03 (d, 1H, 7-H, J = 7.9
Hz), 10.84 (br s, 1H, N1(3)-H, deuterium oxide-exchangeable).
2-Amino-7(8)-methyl-3,4-dihydro-s-triazino[1,2-a]benzimida-
zoles (7b,c).
General Procedure for Preparation of 2-Amino-4-oxo-3,4-dihy-
dro-s-triazino[1,2-a]benzimidazoles (6).
The mixture of the regioisomers 7b,c was obtained in 70%
1
yield; mp 253-254°; H nmr (dimethyl sulfoxide-d ): δ 7b: 2.35
6
(
s, 3H, Me), 5.30 (s, 2H, CH ), 6.40 (s, 2H, NH , deuterium
2 2
To a solution of the appropriate 2-guanidinobenzimidazole 5
oxide-exchangeable), 6.81 (d, 1H, 8-H, J = 7.9 Hz), 7.00 (d, 1H,
-H, J = 7.9 Hz), 7.06 (s, 1H, 6-H), 7.36 (br s, 1H, NH, deuterium
oxide-exchangeable); 7c: 2.33 (s, 3H, Me), 5.30 (s, 2H, CH2),
(0.01 mol) in ethanol (15 mL), diethyl azodicarboxylate (1.74 g,
9
0
.01 mol) was added dropwise with stirring and the mixture was
heated under reflux for 3 h. After cooling, the product was fil-
tered, washed with ethanol and dried.
6
7
.38 (s, 2H, NH , deuterium oxide-exchangeable), 6.77 (d, 1H,
2
-H, J = 7.9 Hz), 6.94 (s, 1H, 9-H), 7.13 (d, 1H, 6-H, J = 7.9 Hz),
2
-Amino-4-oxo-3,4-dihydro-s-triazino[1,2-a]benzimidazole
7.36 (br s, 1H, NH, deuterium oxide-exchangeable).
Anal. Calcd. for C10H N : C, 59.69; H, 5.51; N, 34.80.
(6a).
11
5
Found: C, 59.34; H, 5.29; N, 34.45.
Compound 6a was obtained in 76% yield; mp >360° (DMF)
Lit. mp >360° [5,8,10], mp > 300° [11]); ir (potassium bromide):
(
General Procedure for Preparation of 2-Amino-4-phenyl-3,4-
NH 3415, NH 3219, NH 3123, 2997, 2886, 2800, C=O 1717,
dihydro-s-triazino[1,2-a]benzimidazoles (8).
-
1 1
NH 1616, 1601, 1526, 1457, 1405, 774, 736 cm ; H nmr
2
A solution of the appropriate 2-guanidinobenzimidazole 5
(
dimethyl sulfoxide-d ): δ 7.19 (br s, 1H, NH , deuterium oxide-
6 2
(
(
0.01 mol), 1.00 mL (0.01 mol) benzaldehyde and 0.4 g KOH
method A) or 0.50 mL piperidine (method B) or 0.50 ml N-
exchangeable), 7.25 (td, 1H, 7-H, J = 7.7, 1.1 Hz), 7.34 (td, 1H,
8
6
-H, J = 7.7, 1.5 Hz), 7.42 (d, 1H, 9-H, J = 7.2 Hz), 8.09 (d, 1H,
-H, J = 7.5 Hz), 12.20 (br s, 1H, NH, deuterium oxide-
methylpiperazine (method C) in ethanol (20 mL) was heated
under reflux for 5 h (A) or 3 h (B and C). After cooling, the prod-
uct was filtered, washed with ethanol and dried.
13
exchangeable); C nmr (dimethyl sulfoxide-d ): δ 112.6 (C-6),
6
113.9 (C-9), 121.8 (C-7), 124.7 (C-8), 126.6 (C-5a), 133.1 (C-
9
a), 150.3 (C-10a), 153.7 (CO), 162.9 (C-2).
2-Amino-4-phenyl-3,4-dihydro-s-triazino[1,2-a]benzimidazole
(8a).
Anal. Calcd. for C H N O: C, 53.73; H, 3.51; N, 34.81.
9
7 5
Found: C, 53.68; H, 3.60; N, 34.72.
Compound 8a was obtained in 85% yield (method B); mp 292-
293° (DMF/ethanol), (Lit. [4,8] mp 294-295°); ir (potassium bro-
mide): NH 3407, NH 3321, NH 3224, CH (aromatic) 3052, C=N
2
-Amino-7(8)-methyl-4-oxo-3,4-dihydro-s-triazino[1,2-a]benz-
imidazoles (6b,c)
1
6
660, NH 1613, 1590, 1571, 1459, 1423, 1400, 1279, 1246, 740,
2
The mixture of the regioisomers 6b,c was obtained in 58%
-
1 1
99 cm ; H nmr (dimethyl sulfoxide-d ): δ 6.58 (s, 2H, NH ,
1
6
2
yield; mp >360 °; H nmr (dimethyl sulfoxide-d ): δ 6b: 2.42 (s,
6
deuterium oxide-exchangeable), 6.75 (dd, 1H, 9-H, J = 7.2, 1.5
Hz), 6.77 (s, 1H, H-4), 6.79 (td, 1H, 8-H, J = 7.3, 1.1 Hz), 6.93
3
H, C(7)Me), 7.08 (br s, 2H, NH , deuterium oxide-exchange-
2
able), 7.15 (d, 1H, 8-H, J = 8.3 Hz), 7.28 (d, 1H, 9-H, J = 8.3 Hz),
(
td, 1H, 7-H, J = 7.3, 1.5 Hz), 7.22 (d, 1H, 6-H, J = 7.5 Hz), 7.33-
7
8
7
.92 (s, 1H, 6-H); 6c: 2.41 (s, 3H, C(8)Me), 7.07 (d, 1H, 7-H, J =
7
.42 (m, 5H, phenyl protons), 8.20 (s, 1H, NH, deuterium oxide-
.3 Hz), 7.08 (br s, 2H, NH , deuterium oxide-exchangeable),
2
13
exchangeable); C nmr (dimethyl sulfoxide-d ): δ 65.8(C-4),
1
6
.20 (s, 1H, 9-H), 7.93 (d, 1H, 6-H, J = 8.3 Hz).
08.1 (C-6), 115.8 (C-9), 118.8 (C-8), 120.7 (C-7), 126.2 (C-2'
Anal. Calcd. for C10H N O: C, 55.81; H, 4.22; N, 32.54.
9
5
and -6'), 128.8 (C-3' and -5'), 129.1 (C-4'), 131.2 (C-5a), 140.4
Found: C, 55.84; H, 4.27; N, 32.15.
(
C-1'), 143.2 (C-9a), 153.4 (C-10a), 155.2 (C-2).
General Procedure for Preparation of 2-Amino-3,4-dihydro-s-tri-
azino[1,2-a]benzimidazoles (7a-c).
Anal. Calcd. for C15H13N5: C, 68.42; H, 4.98; N, 26.60.
Found: C, 68.26; H, 5.14; N, 26.42.
A solution of the appropriate 2-guanidinobenzimidazole 5
0.01 mol) and 1.00 mL (0.01 mol) 37% formaldehyde in dioxane
2-Amino-7(8)-methyl-4-phenyl-3,4-dihydro-s-triazino[1,2-a]-
benzimidazoles (8b,c).
(

Jan-Feb 2006
Synthesis of 2-Amino-s-triazino[1,2-a]benzimidazoles as Potential Antifolates
99
The mixture of the regioisomers 8b,c was obtained in 76%
Anal. Calcd. for C12H15N : C, 62.86; H, 6.59; N, 30.54.
5
(
2
method A), 85% (method B) and 81% (method C) yields; mp
Found: C, 62.73; H, 6.80; N, 30.24.
1
84-286°; H nmr (dimethyl sulfoxide-d ): δ 8b: 2.26 (s, 3H,
6
General Procedures for Preparation of 2-Amino-s-triazino[1,2-
a]benzimidazoles (10).
Me), 6.36 (s, 2H, NH , deuterium oxide-exchangeable), 6.56-
6
2
.62 (m, 2H, 8- and 9-H), 7.02 (s, 1H, 6-H), 7.30-7.42 (m, 5H,
phenyl protons), 7.98 (s, 1H, NH, deuterium oxide-exchange-
Method A. A solution of the appropriate 2-guanidinobenzimi-
dazole 5 (0.005 mol) and 1.00 ml (0.005 mol) diethyl
ethoxymethylenemalonate in acetonitrile (20 mL) was heated
under reflux for 5 h. After cooling, the product was filtered,
washed with ethanol and dried.
Method B. A solution of the appropriate 2-guanidinobenzimi-
dazole 5 (0.005 mol) and 0.55 ml (0.005 mol) trimethyl orthofor-
mate in DMF (15 mL) was heated under reflux for 3 h. After
cooling, the product was filtered, washed with ethanol and dried.
able); 8c: 2.20 (s, 3H, Me), 6.35 (s, 2H, NH , deuterium oxide-
2
exchangeable), 6.60 (s, 1H, 9-H), 6.72 (s, 1H, 4-H), 6.75 (d,
1
H, J = 7.9 Hz, 7-H), 7.11 (d, 1H, J = 7.9 Hz, 6-H), 7.30-7.42
(
m, 5H, phenyl protons), 7.95 (s, 1H, NH, deuterium oxide-
1
3
exchangeable); (8b+8c) C nmr (dimethyl sulfoxide-d ): δ
6
2
1
1
8
1
1
2
1.1 (Me), 21.2 (Me), 65.7 (C-4), 65.8 (C-4), 107.8 (C-6 8c),
08.4 (C-6 8b), 115.4 (C-9 8b), 116.0 (C-9 8c), 119.9 (C-8 8b),
21.9 (C-7 8c), 126.1 (C-2'), 126.2 (C-2' and -6'), 127.8 (C-8
c), 128.8 (C-3' and -5'), 129.1 (C-7 8b), 129.5 (C-5a 8c),
31.3 (C-5a 8b), 140.4 (C-1'), 140.6 (C-1'), 140.9 (C-9a 8b),
43.3 (C-9a 8c), 153.0 (C-10a 8c), 153.5 (C-10a 8b), 155.2 (C-
), 155.3 (C-2).
2
-Amino-s-triazino[1,2-a]benzimidazole (10a).
Compound 10a was obtained in 95% yield (method A); mp
301° (DMF) (Lit. mp 305-306° [6], mp 323-324° [8,9], mp 300-
301° [13]); ir (potassium bromide): NH 3304, NH 3157, CH (aro-
Anal. Calcd for C H N : C, 69.29; H, 5.45; N, 25.25. Found:
1
6 15 5
C, 68.93; H, 5.39; N, 24.90.
matic) 3046, CH (aromatic) 3015, C=N 1685, NH 1632, 1603,
2
1
480, 1450, 1343, 1306, 1275, 1243, 1184, 1091, 779, 758, 741
General Procedure for Preparation of 2-Amino-4,4-dimethyl-3,4-
dihydro-s-triazino[1,2-a]benzimidazoles (9).
-
1 1
cm ; H nmr (dimethyl sulfoxide-d ): δ 7.21 (td, 1H, 7-H, J =
7
6
.4, 1.1 Hz), 7.36 (td, 1H, 8-H, J = 7.7, 1.1 Hz), 7.53 (d, 1H, 9-H,
A solution of the appropriate 2-guanidinobenzimidazole 5
0.01 mol) and 0.50 mL piperidine in acetone (20 mL) was heated
J = 7.9 Hz), 7.69 (br s, 2H, NH , deuterium oxide-exchangeable),
8.01 (d, 1H, 6-H, J = 7.9 Hz), 9.60 (s, 1H, 4-H); C nmr
2
1
3
(
under reflux for 7 h. After cooling, the product was filtered,
washed with acetone and dried.
(dimethyl sulfoxide-d ): δ 110.9 (C-6), 117.4 (C-9), 120.0 (C-7),
6
125.5 (C-8), 126.4 (C-5a), 144.1 (C-9a), 148.8 (C-4), 152.0 (C-
1
0a), 161.1 (C-2).
Anal. Calcd. for C H N : C, 58.37; H, 3.81; N, 37.82. Found:
2
-Amino-4,4-dimethyl-3,4-dihydro-s-triazino[1,2-a]benzimi-
9
7 5
dazole (9a).
C, 58.56; H, 3.72; N, 37.90.
Compound 9a was obtained in 84% yield; mp 295-296°; ir
potassium bromide): NH 3285, NH 3132, CH (aliphatic) 2980,
2
-Amino-7(8)-methyl-s-triazino[1,2-a]benzimidazoles (10b,c)
(
CH (aliphatic) 2929, C=N 1659, NH 1616, 1584, 1539, 1456,
The mixture of the regioisomers 10b,c was obtained in 95%
2
-1
1
1
1
406, 1387, 1284, 1250, 762, 747, 549 cm ; H nmr (dimethyl
(method A) and 86% (method B) yields; mp 306-308 °; H nmr
sulfoxide-d ): δ 1.82 (s, 6H, Me ), 6.93 (s, 2H, NH , deuterium
(dimethyl sulfoxide-d ): δ 10b: 2.44 (s, 3H, Me), 7.18 (d, 1H, 8-
6
2
2
6
oxide-exchangeable), 6.95 (t, 1H, 7-H, J = 7.9 Hz), 7.02 (t, 1H, 8-
H, J = 7.5 Hz), 7.29 (d, 1H, 9-H, J = 7.5Hz), 7.39 (d, 1H, 6-H, J =
H, J = 8.3 Hz), 7.40 (d, 1H, 9-H, J = 8.3 Hz), 7.62 (s, 2H, NH ,
2
deuterium oxide-exchangeable), 7.82 (s, 1H, 6-H), 9.53 (s, 1H, 4-
H); 10c: 2.42 (s, 3H, Me), 7.02 (d, 1H, 7-H, J = 7.5 Hz), 7.32 (s,
1
3
7
.9 Hz), 8.17 (br s, 1H, NH, deuterium oxide-exchangeable);
C
nmr (dimethyl sulfoxide-d ): δ 28.5 (Me ), 69.4 (C-4), 109.7 (C-
1H, 9-H), 7.62 (s, 2H, NH , deuterium oxide-exchangeable), 7.87
(d, 1H, 6-H, J = 8.3 Hz), 9.52 (s, 1H, 4-H); C nmr (dimethyl sul-
6
2
2
1
3
6
9
), 115.8 (C-9), 119.0 (C-8), 120.9 (C-7), 130.5 (C-5a), 143.3 (C-
a), 153.5 (C-10a), 155.3 (C-2).
foxide-d ): δ 10b: 21.1 (Me), 110.8 (C-6), 117.0 (C-9), 126.4 (C-
6
Anal. Calcd. for C11H13N : C, 61.38; H, 6.09; N, 32.54.
5a), 126.6 (C-8), 129.3 (C-7), 142.0 (C-9a), 148.5 (C-4), 151.6
5
Found: C, 61.34; H, 6.36; N, 32.20.
(C-10a), 160.9 (C-2); 10c: 21.4 (Me), 110.4 (C-6), 117.4 (C-9),
1
4
21.1 (C-7), 124.3 (C-5a), 134.9 (C-8), 144.4 (C-9a), 148.6 (C-
), 152.1 (C-10a), 160.9 (C-2).
2
-Amino-4,4,7(8)-trimethyl-3,4-dihydro-s-triazino[1,2-a]benz-
imidazoles (9b,c).
Anal. Calcd. for C H N : C, 60.29; H, 4.55; N, 35.16. Found:
1
0 9 5
The mixture of the regioisomers 9b,c was obtained in 72%
C, 60.25; H, 4.49; N, 35.02.
1
yield; mp 284-286°; H nmr (dimethyl sulfoxide-d ): δ 9b 1.84
6
General Procedures for Preparation of 2-Amino-4-methyl-s-tri-
azino[1,2-a]benzimidazoles (11).
(
s, 6H, C(4)Me ), 2.39 (s, 3H, C(7)Me), 6.86 (d, 1H, 8-H, J = 7.9
2
Hz), 7.13 (s, 2H, NH , deuterium oxide-exchangeable), 7.18 (d,
2
1
H, 9-H, J = 7.9 Hz), 7.24 (s, 1H, 6-H), 8.32 (br s, 1H, NH, deu-
A solution of the appropriate 2-guanidinobenzimidazole 5
(0.005 mol) and 0.90 ml (0.005 mol) triethyl orthoacetate in
DMF (15 mL) was heated under reflux for 18 h. After cooling,
the product was collected by filration, washed with ethanol and
dried.
terium oxide-exchangeable); 9c 1.82 (s, 6H, C(4)Me ), 2.34 (s,
2
3
7
H, C(8)Me), 6.78 (d, 1H, 7-H, J = 7.9 Hz), 7.10 (s, 1H, 9-H),
.13 (s, 2H, NH , deuterium oxide-exchangeable), 7.26 (d, 1H, 6-
2
H, J = 8.3 Hz), 8.32 (br s, 1H, NH, deuterium oxide-exchange-
able); 13C nmr (dimethyl sulfoxide-d ): δ 9b 21.1 (Me), 28.5
6
2
-Amino-4-methyl-s-triazino[1,2-a]benzimidazole (11a).
(
Me ), 69.3 (C-4), 110.1 (C-6), 115.4 (C-9), 120.1 (C-8), 128.5
2
(
C-7), 130.7 (C-5a), 141.1 (C-9a), 153.2 (C-10a), 155.2* (C-2);
Compound 11a was obtained in 76% yield; mp 324-325° (Lit.
9
1
1
c 21.3 (Me), 28.6 (Me ), 69.3 (C-4), 109.3 (C-6), 116.0 (C-9),
[8] mp 314-315°); ir (potassium bromide): NH 3329, NH 3166,
2
21.7 (C-7), 128.1 (C-8), 129.5 (C-5a), 143.5 (C-9a), 153.7 (C-
0a), 155.3* (C-2). * - assignments may be reversed.
CH (aliphatic) 2966, CH (aliphatic) 2923, C=N 1667, NH 1635,
2
1598, 1512, 1449, 1357, 1307, 1284, 1251, 1220, 1147, 779, 745,

100
A. V. Dolzhenko and W.-K. Chui
Vol. 43
-
1
1
[2] T. Toyoda, R. K. B. Brobey, G. Sano, T. Horii, N. Tomioka,
and A. Itai, Biochem. Biophys. Res. Comm., 235, 515 (1997).
7
26, 567 cm ; H nmr (dimethyl sulfoxide-d ): δ 2.92 (s, 3H,
6
Me), 7.20 (t, 1H, 7-H, J = 7.7 Hz), 7.37 (t, 1H, 8-H, J = 7.8 Hz),
.51 (s, 1H, NH , deuterium oxide-exchangeable), 7.54 (d, 1H, 9-
H, J = 7.9 Hz), 7.86 (d, 1H, 6-H, J = 7.9 Hz).
Anal. Calcd. for C H N : C, 60.29; H, 4.55; N, 35.16. Found:
[
3] A. Itai and T. Toyota, Japanese Patent 10,310,526 (1998);
Chem. Abstr., 130, 62956 (1999).
4] K. Nagarajan, V. R. Rao, and A. Venkateswarlu, Indian J.
Chem., 8, 126 (1970).
7
2
[
1
0 9 5
C, 60.34; H, 4.60; N, 35.04.
[
5] L. Capuano, H. J. Schrepfer, M. E. Jaeschke, and H. Porschen,
Chem. Ber., 107, 62 (1974).
2
-Amino-4,7(8)-dimethyl-s-triazino[1,2-a]benzimidazoles
[
[
6] A. Kreutzberger, Arch. Pharm., 309, 794 (1976).
7] I. Lalezary and S. Nabahi, J. Heterocyclic Chem., 17, 1121
(11b,c).
The mixture of regioisomers 11b,c was obtained in 65% yield;
(1980).
[8] D. Martin, H. Graubaum, G. Kempter, and W. Ehrlichmann, J.
Prakt. Chem., 323, 303 (1981).
9] W. Ehrlichmann, G. Kempter, and A. Jumar, East German
Patent 149,938 (1981); Chem. Abstr., 96, 85594 (1982).
10] D. Martin, H. Graubaum, and W. Kochmann, East German
Patent, 149,937 (1981); Chem. Abstr., 96, 52339 (1982).
[11] M. Furukawa, K. Kawanabe, A. Yoshimi, T. Okawara, and Y.
Noguchi, Chem. Pharm. Bull., 31, 2473 (1983).
1
mp 319°; H nmr (dimethyl sulfoxide-d ): δ 11b: 2.46 (s, 3H,
6
C(7)Me), 2.90 (s, 3H, C(4)Me), 7.19 (d, 1H, 8-H, J = 8.3 Hz),
[
7
.41 (d, 1H, 9-H, J = 8.3 Hz), 7.46 (s, 2H, NH , deuterium oxide-
2
exchangeable), 7.67 (s, 1H, 6-H); 11c: 2.43 (s, 3H, C(8)Me), 2.88
s, 3H, C(4)Me), 7.01 (d, 1H, 7-H, J = 8.3 Hz), 7.33 (s, 1H, 9-H),
.46 (s, 2H, NH , deuterium oxide-exchangeable), 7.71 (d, 1H, 6-
[
(
7
2
H, J = 8.3 Hz).
Anal. Calcd. for C H N : C, 61.96; H, 5.20; N, 32.84.
11 11 5
[12] Y. Kihara, S. Kabashima, T. Yamasaki, T. Ohkawara, and M.
Furukawa, J. Heterocyclic Chem., 27, 1213 (1990).
Found: C, 62.05; H, 5.25; N, 32.87.
[
13] E. A. M. Badawey and T. Kappe, Arch. Pharm., 330(3), 59
(
1997).
REFERENCES AND NOTES
[
14] F. E. King, R. M. Acheson, and P. C. Spensley, J. Chem. Soc.,
1
366 (1948).
[
1] H. K. Lee and W. K. Chui, Bioorg. Med. Chem., 7, 1255
[15] D. V. Krylsky, Kh. S. Shikhaliev, A. S. Solovyev, Chem.
Heterocyclic Compd., 37, 524 (2001).
(
1999).