Synthesis of some novel 7-(1H-benzimidazol-2-ylazo)-1,3- -dimethyl-6,8-disubstituted-1H-pyrimido[4,5-b] [1,4]diazepine-2,4-diones as potential anti-anxiety agents

Article abstract of DOI:10.2298/JSC120409082S

An efficient and mild method for the synthesis of some novel 7-(1H- -benzimidazol-2-ylazo)-1,3-dimethyl-6,8-disubstituted-1H-pyrimido[4,5-b]- [1,4]diazepine-2,4-diones in water has been developed. This method is a good option to obtain the title compounds in quantitative yields in a simple and inexpensive manner. Further, the NH of the title compounds was replaced by various substituents in an ionic liquid, [bmim]PF₆, a recyclable and environmentally benign solvent. The synthesized compounds were characterized by analytical and spectral (IR, ¹H-NMR, ¹³C-NMR and FAB mass) data and have been screened for their anti-anxiety activity in mice.

Full text of DOI:10.2298/JSC120409082S

J. Serb. Chem. Soc. 78 (2) 165–172 (2013)
JSCS–4405
UDC 547.532+547.78+547.853+
547.235:616.89–008.441
Original scientific paper
Synthesis of some novel 7-(1H-benzimidazol-2-ylazo)-1,3-
-dimethyl-6,8-disubstituted-1H-pyrimido[4,5-b]
[1,4]diazepine-2,4-diones as potential anti-anxiety agents
*
KANTI SHARMA and RENUKA JAIN
Department of Chemistry, University of Rajasthan, Jaipur 302 004, India
(Received 9 April, revised 29 July 2012)
Abstract: An efficient and mild method for the synthesis of some novel 7-(1H-
-benzimidazol-2-ylazo)-1,3-dimethyl-6,8-disubstituted-1H-pyrimido[4,5-b]-
[1,4]diazepine-2,4-diones in water has been developed. This method is a good
option to obtain the title compounds in quantitative yields in a simple and
inexpensive manner. Further, the NH of the title compounds was replaced by
various substituents in an ionic liquid, [bmim]PF6, a recyclable and environ-
mentally benign solvent. The synthesized compounds were characterized by
analytical and spectral (IR, 1H-NMR, 13C-NMR and FAB mass) data and have
been screened for their anti-anxiety activity in mice.
Keywords: 2-aminobenzimidazole, diketones, benzimidazolyl-pyrimido-diaze-
pines, aqueous/ionic liquid mediated synthesis; anti-anxiety activity.
INTRODUCTION
Nitrogen-containing heterocycles represent the central framework of many
1
biologically active compounds. Benzimidazoles show antifungal, anti-inflam-
1
1
2
3
4
5
matory, analgesic, CNS depressant, antitubercular, anticancer, anti-HIV,
6
etc. activities. Pyrimidines exhibit varied activities, such as anticancer and anti-
bacterial. 5-Fluorouracil is a well-known anticancer agent. Diazepine derivatives
are used as tranquilizers, anticonvulsant, anxiolytic, analgesics, sedatives, anti-
depressives and hypnotic agents. 1,5-Benzodiazepine shows anti-anxiety,
7
8
9
10
11
11
11
analgesic, anticonvulsant and anti-inflammatory activities.
Due to their wide range of pharmacological activity and industrial appli-
cations, the development of mild and efficient protocols for the preparation of
diazepines continues to be challenging endeavor in synthetic organic chemistry.
The common procedure for the synthesis of these compounds is a one-pot con-
12
densation between o-phenylenediamines and carbonyl compounds. However, a
*Corresponding author. E-mail: drkanti@gmail.com
doi: 10.2298/JSC120409082S
165
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SHARMA and JAIN
13
large number of modified methods reported in the literature suffer from several
drawbacks, such as the use of a large amount of catalysts, unsatisfactory product
yield and critical product isolation procedures. These disadvantages requires the
14
development of an efficient and practically useful process of preparation.
15–17
In continuation of work on bioactive heterocycles,
reported herein is for
the first time a rapid efficient, clean and environmentally benign exclusive syn-
thesis of 7-(1H-benzimidazol-2-ylazo)-1,3-dimethyl-6,8-disubstituted-1H-pyrimi-
do[4,5-b][1,4]diazepine-2,4-diones 4a–c by the reaction of 2-[(1H-benzimidazol-
-2-yl)hydrazono]-1,3-disubstituted-1,3-dione 3a–c and 5,6-diamino-1,3-dime-
thyl-2,4(1H,3H)-pyrimidinedione hydrochloride in aqueous medium in excellent
yields (92–97 %). The use of water as a solvent for organic transformations of-
18
fers green chemistry benefits and is widely used to enable and expedite the
synthesis of diverse heterocycles. Compounds 3a–c were prepared by coupling
19
reaction of a diazonium salt, formed by the diazotization of 2-aminobenzimi-
dazole 1 and 1,3-diketones 2 in presence of sodium acetate and ethanol. Further,
the NH of title compounds was substituted by various substituents by reacting
with methyl iodide, benzyl chloride, chloroacetyl chloride, formaldehyde and se-
condary amines, trifluoroacetic anhydride in [bmim]PF , an ionic liquid to give
6
the N-substituted derivatives 5a–i (Scheme 1). The synthesized compounds 4a–c
and 5a–i were screened for their anti-anxiety activity in mice and they exhibited
excellent results.
EXPERIMENTAL
Melting points are uncorrected and were taken in open glass capillaries using a Gallen-
kamp melting point apparatus. The IR spectra were recorded on a Shimadzu FTIR-8400S
spectrophotometer in KBr pellets and band positions are recorded in wavenumbers (cm-1).
The 1H- and 13C-NMR spectra were recorded on a Bruker DRX-300 instrument at 300.15 and
75 MHz, respectively. CDCl3 was used as the solvent and TMS as internal reference. The
mass spectra were recorded on a JEOL, SX 102 (FAB) mass spectrometer. The mass spectra
and elemental analyses were performed at the Central Drug Research Institute, Lucknow,
India. All employed chemicals were of analytical reagent grade, purchased from Acros and
used without further purification.
General method for the synthesis of compounds 3a–c
2-Aminobenzimidazole (0.02 mol) was dissolved in a mixture of conc. HCl and water
(20 mL; 1:1), then cooled to 0 °C and a cold aqueous solution of sodium nitrite (0.02 mol; 1.3
g in 10 mL water) was added slowly maintaining the temperature between 0–2 °C. The cold
diazotized solution was added dropwise to a cooled mixture of 1,3-diketone (0.02 mol) and
sodium acetate (10 g) in 20 mL of 50 % ethanol. The stirring was continued for 1 h and the
crystals that separated were filtered, washed with water, dried and crystallized from ethanol to
yield 3a–c.
Water mediated synthesis of compounds 4a–c
A solution of 5,6-diamino-1,3-dimethyl-2,4(1H,3H)-pyrimidinedione hydrochloride
(0.01 mol) in water (200 mL) was neutralized with ammonia to pH 7 then 3a–c (0.01 mol),
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BENZIMIDAZOLYL-PYRIMIDO-DIAZEPINES AS POTENTIAL ANTIANXIETY AGENTS
167
was added and the mixture heated under reflux for 30 min. The crystalline product, which
started to separate out just after cooling the reaction mixture, was washed with water and
found to be pure by TLC with no need for further purification. All compounds were syn-
thesized similarly to yield 4a–c.
5a–i
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168
SHARMA and JAIN
Compound
R1
R2
R3
Compound
R1
R2
R3
3a
3b
3c
4a
4b
4c
5a
5b
CH3
CH3
CF3
CH3
CH3
CF3
CH3
CH3
CH3
C6H5
C6H5
CH3
C6H5
C6H5
CH3
–
–
–
–
–
–
CH3
CH3
5c
5d
5e
5f
5g
5h
5i
CH3
CH3
CH3
CH3
CH3
CH3
CF3
CH3
C6H5
CH3
C6H5
C6H5
CH3
CH2Ph
CH2Ph
COCH2Cl
CH2NMe2
CH2NEt2
COCF3
C6H5
COCF3
C6H5
Scheme 1. Synthesis of compounds 3a–c, 4a–c and 5a–i.
Ionic liquid mediated synthesis of compounds 5a–i
A mixture of 4a–c (0.01 mol), methyl iodide/benzoyl chloride/chloroacetyl chloride/
/formaldehyde and secondary amine (1:1) or trifluoroacetic anhydride (0.01 mol) and ionic
liquid [bmim]PF6 (5 mL) were taken in a round bottom flask and heated at 60–70 °C under N2
protection for 1 h. The progress of the reaction was monitored by TLC. After completion of
reaction, the mixture was extracted with diethyl ether (6×10 mL). The organic extract was
washed with 5 % Na2CO3 (40 mL) and water (40 mL), dried with anhydrous sodium sulfate
and evaporated under vacuum. The residual product was purified by recrystallization from
AcOEt/petroleum ether (60–80 °C) or by column chromatography (silica gel, 60–120 mesh,
eluent petroleum ether/AcOEt = 4:1) to give 5a–i (the ionic liquid layer was washed with
water (3×5 mL) and kept for 2 h. at 80–85 °C under reduced pressure, the ionic liquid was
recycled).
Anti-anxiety activity in mice
Synthesized compounds 4a–c and 5a–i were screened for their anti-anxiety activity in
mice on a plus maze apparatus devised by Crawley and Godwin20 modified by Kilfoil21 using
sodium pentabarbitone as the standard.
The apparatus consisted of a Plexiglas base (102 cm×53 cm×53 cm) divided into two
chambers by a black Plexiglas partition. The box was placed within a layer of a soundproof
box, which was equipped with a one-way observation window. The partition dividing the sub
chamber had a 33 cm×13 cm opening, through which animal could easily pass. The dark
chamber (36 cm×53 cm×53 cm) was made up of dark Plexiglas except for the side ferry
observation window. This side was clear and covered with black plastic. The testing was
performed between 12.00 noon to 6.00 p.m. in an isolated darkened laboratory.
Mice weighing 25 g were chosen and were sorted into five animals in a group. They
were allowed free access to food and water ad libitum. Animals were given 60 min to accli-
matize to the environment prior to the administration of the drug. Drugs or test samples in
DMF were given at a dose of 20 mg kg-1 body weight, after which each animal was indivi-
dually placed in the centre of the light area of the apparatus and observed for 10 min. The total
amount of time spent in the dark area was measured.
RESULTS AND DISCUSSION
2-Aminobenzimidazole (1) was reacted with sodium nitrite and HCl at 0–5
°C. The so-formed diazonium salt was further reacted with 1,3-diketones (2) and
sodium acetate in ethanol to afford 2-[(1H-benzimidazole-2-yl)hydrazono]-1,3-
-disubstituted 1,3-diones 3a–c. The formation of compound 3 was confirmed by
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BENZIMIDAZOLYL-PYRIMIDO-DIAZEPINES AS POTENTIAL ANTIANXIETY AGENTS
169
–1
their IR spectra, in which band appeared at 3200, 3030, 1680 and 1620 cm due
1
to –NH benzo, NH hydrazono, C=O and C=N, respectively. The H-NMR spetra
showed peaks at δ 10.62 and 9.52 ppm due to the NH hydrazono and NH of
13
benzimidazolyl moieties, respectively. The C-NMR spectra showed peaks at δ
178.2, 180.1 and 162.4 ppm due to two C=O and the C=N, respectively. Further,
+
the mass spectrum shows M at m/z 244 (3a).
The advantage of using water as a solvent is its cost, safety (it is non-inflam-
mable and is devoid of any carcinogenic effects) and simple operation. It has the
highest specific heat value of all substances as well as unique enthalpic and en-
tropic properties, which have led us to its use. Water has abnormally low vola-
tility because its molecules are associated with each other by means of hydrogen
bonding.
In view of this, compounds 3a–c were reacted with 5,6-diamino-1,3-dime-
thyl-2,4(1H,3H)-pyrimidinedione hydrochloride in aqueous medium to afford
7-(1H-benzimidazol-2-ylazo)-1,3-dimethyl-6,8-disubstituted-1H-pyrimido[4,5-b]-
[1,4]diazepine-2,4-diones 4a–c.
The formation of compounds 4 was confirmed by their IR spectra, which
showed the disappearance of the peaks due to the hydrazono group and the ke-
tonic group of the 1,3-diketone part with the appearance of a peak due to the NH of
–1
–1
diazepine at 3340 cm and at 1720 and 1700 cm due to the C=O of the pyri-
1
midine ring. The H-NMR spectra showed peaks at δ 9.51 and 8.46 ppm due to
the NH of benzimidazole and diazepine moiety, respectively. The NCH methyl
3
groups present at the positions 4 and 6 appeared at δ 3.20 and 3.62 ppm, res-
13
pectively. The C-NMR spectra showed peaks at δ 33.5 and 34.8 ppm due to the
two NCH groups of the pyrimidine ring, while two C=O groups appeared at δ
3
+
190.2 and 195.1 ppm. Further, the mass spectrum shows M at m/z 378 (4a).
22
In recent years ionic liquids (ILs) have emerged, which are organic salts
the ions of which do not pack well and remain liquid at room temperature. They
have unique properties, such as, for example, a wide liquid range, good solvency,
tunable polarity, high thermal stability, negligible vapor pressure and recycl-
ability. Ionic liquids could be recycled and reused as opposed to traditional sol-
vent catalyst systems.
In view of the above, compounds 4a–c were reacted with: i) methyl iodide,
ii) benzyl chloride, iii) chloroacetyl chloride, iv) formaldehyde and a secondary
amine and v) trifluoroacetic anhydride in the ionic liquid [bmim]PF to give
6
N-benzimidazolyl substituted derivatives 5a–i.
Although compounds 4a–c have two NH groups, one in the benzimidazolyl
moiety and another in the diazepine ring, it was observed that only NH of benz-
imidazolyl moiety was replaced by various substituents. The NH of the diazepine
moiety could not be replaced due to hydrogen bonding with the C=O of the
pyrimidine ring.
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170
SHARMA and JAIN
The formation of compounds 5a–i were confirmed by the disappearance of
peak due to the NH of the benzimidazolyl moiety and the appearance of a peak
1
13
due to N-substituent in their IR, H-NMR and C-NMR spectra. The mass
+
spectrum also shows M at m/z 392 (5a).
Anti-anxiety activity in mice
Synthesized compounds 4a–c and 5a–i were screened for their anti-anxiety
20
activity in mice on a plus maze apparatus devised by Crawley and Godwin
21
modified by Kilfoil. The less time a mouse spent in the dark space, the greater
was the anti-anxiety activity of the drug. The results of the anti-anxiety activity
of the compounds are given in Table I. Compounds 4c, 5b, 5e, 5h and 5i showed
better activity than the standard, sodium pentabarbitone, and the other com-
pounds were moderately active. The good activity of these compounds is due to
presence of trifluoromethyl, chloro and trifluoroacetyl groups, which enhance
their activity.
TABLE I. Results of anti-anxiety activity in mice of compounds 4a–c and 5a–i
Compound
R1
R2
R3
Time spent in dark space, s
4a
4b
4c
5a
5b
5c
5d
5e
5f
5g
5h
5i
CH3
CH3
CF3
CH3
CH3
CH3
CH3
CH3
CH3
CH3
CH3
CF3
CH3
C6H5
C6H5
CH3
C6H5
CH3
C6H5
CH3
C6H5
C6H5
CH3
–
–
–
CH3
CH3
300±9.2
298±8.4
291±8.2
304±10.5
294±10.2
314±9.2
318±10.8
294±12.0
318±11.7
321±10.5
292±9.20
290±9.82
401.6±25.0
295±11.50
CH2Ph
CH2Ph
COCH2Cl
CH2NMe2
CH2NEt2
COCF3
COCF3
C6H5
Control
Pentabarbitone
CONCLUSIONS
Novel 7-(1H-benzimidazol-2-ylazo)-1,3-dimethyl-6,8-disubstituted-1H-pyri-
mido[4,5-b][1,4]diazepine-2,4-diones 4a–c were synthesized in aqueous medium
by the reaction of 2-[(1H-benzimidazol-2-yl)hydrazono]-1,3-disubstituted-1,3-di-
ones 3a–c and 5,6-diamino-1,3-dimethyluracil hydrochloride. Further, the title
compounds were reacted with various reagents/reactants in [bmim]PF to de-
6
termine whether all NH groups of the title compounds could be substituted. It
was observed that only monosubstitution was possible yielding 5a–i as the di-
azepine NH is involved in hydrogen bonding with the C=O of the pyrimidine
ring. Compounds 4a–c and 5a–i were screened for their anti-anxiety activity in
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BENZIMIDAZOLYL-PYRIMIDO-DIAZEPINES AS POTENTIAL ANTIANXIETY AGENTS
171
mice and the data revealed that the compounds showed good to moderate acti-
vity. Some of the compounds were more active than the standard and could act as
potential anti-anxiety agents.
SUPPLEMENTARY MATERIAL
Analytical and spectral data for the compounds 3a–c, 4a–c and 5a–i are available
electronically from http://www.shd.org.rs/JSCS/, or from the corresponding author on request.
Acknowledgements. One of the authors (KS) is grateful to UGC, New Delhi, India, for
granting a Research Award. We are also thankful to Central Drug Research Institute, Luck-
now, India for the elemental analysis and mass spectra and to the Principal, R. L. Saharia
Govt. P. G. College Kaladera, Jaipur, India, for providing the facilities for the activity
evaluations.
И З В О Д
СИНТЕЗА НОВИХ 6,8-ДИСУПСТИТУИСАНИХ 7-(1H-БЕНЗИМИДАЗОЛ-2-ИЛАЗО)-1,3-
-ДИМЕТИЛ-1H-ПИРИМИДО[4,5-b][1,4]ДИАЗЕПИН-2,4-ДИОНА КАО
ПОТЕНЦИЈАЛНИХ ЛЕКОВА ПРОТИВ АНКСИОЗНОСТИ
KANTI SHARMA и RENUKA JAIN
Department of Chemistry, University of Rajasthan, Jaipur 302 004, India
Развијен је ефикасан поступак синтезе нових 6,8-дисупституисаних 7-(1H-бензими-
дазол-2-илазо)-1,3-диметил-1H-пиримидо[4,5-b][1,4]диазепин-2,4-диона,
у води под
благим реакционим условима. Описани поступак је добра алтернатива за синтезу ових
деривата, у квантитативном приносу на једноставан и економски приступачан начин.
Осим тога, NH-група производа дериватизована је различитим супституентима, реак-
цијом у јонској течности, [bmim]PF6, растварачу који може да се рециклира и еколошки
је прихватљив. Синтетисана једињења су окарактерисана аналитичким и спектроскоп-
ским методама (IC, 1H-NMR, 13C-NMR и FAB масена спектрометрија) и испитана је
њихова антианксиозна активност на мишевима.
(Примљено 9. априла, ревидирано 29. јула 2012)
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