Facile DES-mediated synthesis and antioxidant potency of benzimidazoquinazolinone motifs

Article abstract of DOI:10.1007/s11164-016-2734-1

Abstract: One-pot multicomponent synthesis of benzimidazoquinazolinone scaffolds was achieved by reaction between 2-aminobenzimidazole, aldehyde, and 1,3-cyclohexadione in choline chloride:glycerol as deep eutectic solvent (DES). The developed methodology offers mild and faster reaction conditions with excellent product yield. The synthesized compounds were screened for their antioxidant potency using 1,1-diphenyl-2-picrylhydrazyl (DPPH) radical scavenging, ferric reducing antioxidant power (FRAP), 2,2′-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS), and metal chelating assay. Most of the compounds were found to exhibit good to comparable antioxidant activity with respect to standards. Use of this inexpensive and biodegradable solvent makes this methodology a greener approach compared with other methods reported in literature. Graphical Abstract: Greener, one-pot multicomponent synthesis of benzimidazoquinazolinone scaffolds using DES (ChCl:glycerol) as efficient, recyclable, and biodegradable solvent has been achieved. The synthesized compounds show good to comparable antioxidant activity compared with standards.[Figure not available: see fulltext.]

Full text of DOI:10.1007/s11164-016-2734-1

Res Chem Intermed
DOI 10.1007/s11164-016-2734-1
Facile DES-mediated synthesis and antioxidant potency
of benzimidazoquinazolinone motifs
Vilas N. Mahire1 Vijay E. Patel2 Pramod P. Mahulikar1
•
•
Received: 30 May 2016 / Accepted: 6 September 2016
Ó Springer Science+Business Media Dordrecht 2016
Abstract One-pot multicomponent synthesis of benzimidazoquinazolinone scaffolds
was achieved by reaction between 2-aminobenzimidazole, aldehyde, and 1,3-cyclohexa-
dione in choline chloride:glycerol as deep eutectic solvent (DES). The developed
methodology offers mild and faster reaction conditions with excellent product yield. The
synthesized compounds were screened for their antioxidant potency using 1,1-diphenyl-2-
picrylhydrazyl (DPPH) radical scavenging, ferric reducing antioxidant power (FRAP),
2,20-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS), and metal chelating assay.
Most of the compounds were found to exhibit good to comparable antioxidant activity with
respect to standards. Use of this inexpensive and biodegradable solvent makes this
methodology a greener approach compared with other methods reported in literature.
Graphical Abstract Greener, one-pot multicomponent synthesis of benzimidazo-
quinazolinone scaffolds using DES (ChCl:glycerol) as efficient, recyclable, and
biodegradable solvent has been achieved. The synthesized compounds show good to
comparable antioxidant activity compared with standards.
Electronic supplementary material The online version of this article (doi:10.1007/s11164-016-2734-1)
contains supplementary material, which is available to authorized users.
&
Pramod P. Mahulikar
mahulikarpp@gmail.com
1
2
School of Chemical Sciences, North Maharashtra University, Jalgaon, Maharashtra, India
School of Life Sciences, North Maharashtra University, Jalgaon, Maharashtra, India
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V. N. Mahire et al.
Keywords Benzimidazoquinazolinone Á Deep eutectic solvent (DES) Á Choline
chloride:glycerol Á Antioxidants
Introduction
In green chemistry, considerable attention is focused on development of clean, high-
yielding, and environmentally friendly chemical processes and technologies [1, 2].
Over the last two decades, use of ionic liquids has attracted enormous attention from
the scientific community for a variety of applications including catalysis, CO2
absorption, etc. [3]. However, ionic liquids suffer from the disadvantages of high
price, poor biodegradability, biocompatibility, toxicity, and environmental persis-
tence [4, 5]. The cost and availability as well as tuning of anions always represent
problems for synthetic research. From this point of view, alternative solvent systems
are being investigated. In this respect, deep eutectic solvents (DESs) have been
considered as alternatives to ionic liquids (ILs) [6, 7]. Generally, DESs are mixtures
of a quaternary ammonium salt such as choline chloride with hydrogen-bond donors
such as urea, carboxylic acid, and glycerol. DESs are environmentally benign,
cheap, nontoxic, and biodegradable [8]. Their negligible vapor pressure, non-
flammability, nonreactiveness with water, and synthesis from readily available raw
materials makes them promising alternatives to ionic liquids (ILs) [9–11].
Over recent decades, multicomponent reactions (MCRs) have also gained
significant importance in the field of medicinal chemistry as a tool for synthesis of a
wide variety of biologically relevant scaffolds for generation of libraries of
compounds [12–14]. Furthermore, MCRs using environmentally benign solvents
such as DESs represent one of the most suitable strategies for green chemical
processes and development of libraries of medicinal scaffolds [15–17]. Quinazo-
lines and condensed quinazolines occupy a prominent position in the realm of
medicinal chemistry because of their diverse medicinal properties, such as
anticancer [18], histamine H1-receptor blocking [19], analgesic, antiinflammatory,
antibacterial [20], antihypertensive [21], etc. actions. The general method for
123

Facile DES-mediated synthesis and antioxidant potency…
synthesis of benzimidazoquinazolinone involves reaction of 2-aminobenzimidazole,
aldehydes, and 1,3-cyclohexadione or dimedone. In literature, catalysts such as
silica gel [22], p-TsOHÁH2O [23], I2 [24], H6P2W18O62Á18H2O [25], and NH2SO3H
[26], and solvents such as dimethylformamide (DMF) [27, 28], ionic liquids such as
[bmim?][BF-4 ] [29] etc., have been reported for synthesis of benzimidazoquina-
zolinones. However, some of these methods are associated with limitations such as
use of toxic solvent, drastic reaction conditions, long reaction time, and low product
yield. Our group is constantly working on synthesis and bioactivity evaluation of
benzimidazole compounds and development of greener synthetic approaches
[30–32]. We report herein a catalyst-free approach for synthesis of benzimidazo-
quinazolinones using deep eutectic solvent (choline chloride:glycerol). Figure 1
shows important bioactive drugs containing benzimidazole, quinazolinone, and
pyrimidine as core moieties in their structure. The synthetic strategies for synthesis
of DES and the target derivatives (4a–p) are depicted in Schemes 1 and 2,
respectively.
Fig. 1 Benzimidazole, quinazolinone and pyrimidine ring containing important bioactive molecules
Scheme 1 Preparation of deep eutectic solvent (ChCl:glycerol)
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V. N. Mahire et al.
Scheme 2 General scheme for synthesis of benzimidazoquinazolinones in ChCl:glycerol
Results and discussion
Glycerol is a cheap, renewable, nonflammable, and biocompatible liquid, and
choline chloride (quaternary ammonium salt) is a member of the vitamin B family,
serving as a dietary supplement in animal feeds. Therefore, we decided to prepare
DES (Scheme 1) from these two components (1:2 ratio) and study its applications as
solvent for greener organic synthesis (Fig. 2).
Optimization of reaction conditions
The model reaction of 2-aminobenzimidazole, 3-nitrobenzaldehyde, and 1,3-
cyclohexadione in ChCl:glycerol at 80 °C was carried out for synthesis of
benzimidazoquinazolinone derivatives. The reaction was found to complete within
30 min to give the 3-nitrophenyl benzimidazoquinazolinone 4b as product in
quantitative yield (Table 2, entry 10). Encouraged by this result, we studied the
effect of various solvents on the rate of the model reaction. Solvents such as ethanol,
acetonitrile, DMF, chloroform, tetrahydrofuran (THF), glycerol, etc. were used for
optimization of reaction conditions (Table 1). ChCl:glycerol was found to be an
efficient solvent for synthesis of benzimidazoquinazolinone derivatives. However,
reactions using other deep eutectic solvents such as ChCl:urea and ChCl:oxalic acid
did not provide satisfactory results (Table 1). ChCl:glycerol has low viscosity and
Fig. 2 Preparation of deep
eutectic solvent (ChCl:glycerol)
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Facile DES-mediated synthesis and antioxidant potency…
Table 1 Reaction of 2-aminobenzimidazole 1, 3-nitrobenzaldehyde 2b, and 1,3 cyclohexadione 3 in
different solvents
Entry
Solvent
Temp. (°C)
Time (min)
Yield of 4b (%)a
1
Neat
80
180
60
31
2
Ethanol
Reflux
Reflux
120 °C
Reflux
Reflux
90
33
3
Acetonitrile
DMF
180
180
240
240
180
180
180
30
56
4
75
5
Chloroform
THF
70
6
57
7
Glycerol
77
8
ChCl:glycerol
ChCl:glycerol
ChCl:glycerol
ChCl:urea
ChCl:oxalic acid
RT
39
9
60
74
10b
11
12
80
91, 74, 63
80
120
120
41
42
80
Conditions: 2-aminobenzimidazole 1 (2 mmol), 3-nitrobenzaldehyde 2b (2 mmol), 1,3-cyclohexadione
(2 mmol), and DES (5 mL)
a
Isolated yield
b
DES was reused three times
low freezing point depression compared with ChCl:urea or ChCl:oxalic acid.
Furthermore, the hydroxyl groups of glycerol impart more hydrogen bonding with
choline chloride, hence it seems to promote reaction with higher product yield. The
model reaction was performed at ambient temperature, but incomplete conversion
of reactant into product was observed (Table 2, entry 8). When studied at elevated
temperature of 80 °C, the model reaction afforded 91 % yield of 4b within 30 min.
To determine the efficiency and limitations of the DES in this multicomponent
reaction, we applied the optimized reaction conditions (Table 2, entry 10) to a series
of aldehydes. Aldehydes containing electron-donating as well electron-withdrawing
groups such as hydroxyl, methoxy, and nitro could react efficiently to give
corresponding products in good to quantitative yield (Table 2, entries 1–16).
Furthermore, the efficiency of DES was evaluated by recycling it for subsequent
reaction. Decrease in yield was observed after the second cycle. After reaction
completion, the reaction mass was poured into crushed ice to obtain a solid product,
which was filtered and dried. Water was removed from filtrate to obtain DES, which
was then used directly as solvent in the next cycle. Thus, it was found that
ChCl:glycerol plays a benign role of accelerator for reducing the reaction time and
promoting effective product formation. The synthesized compounds were isolated,
purified, and characterized by Fourier-transform infrared (FT-IR), 1H nuclear
magnetic resonance (NMR), and mass spectrometric techniques.
Study of Table 3 enables comparison of the efficiency of ChCl:glycerol (DES)
versus reported methods. It is clear that the reported methodologies are associated
with issues such as long reaction time, use of hazardous solvents, and reflux
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V. N. Mahire et al.
References
Table 2 Synthesis of benzimidazoquinazolinones in deep eutectic solvent
Entry
R
Product Time (min) Yield (%)a M.p. (°C)
Found
Reported
1
2-NO2C6H4
3-NO2C6H4
4-NO2C6H4
4-CH3C6H4
4-OCH3C6H4
4a
4b
4c
4d
4e
30
30
30
30
20
20
30
30
30
30
20
20
30
30
30
45
52
91
78
84
81
71
81
85
78
63
78
74
89
75
85
60
[300
[300
–
2
[300
[300
[33]
3
212
–
4
[300
[300
[33]
[33]
5
[300
[300
6
4-OH,3-OCH3C6H3 4f
3-OH,4-OCH3C6H3 4g
[300
–
7
238–240
[300
–
8
1-Naphthyl
4-BrC6H4
4h
4i
–
9
[300
–
10
11
12
13
14
15
16
2,3,4-(OCH3)3C6H2 4j
248–250
[300
–
4-SCH3C6H4
C6H5
4k
4l
–
[300
[300
[33]
2,5-(OCH3)2C6H3
3,4-(OCH3)2C6H3
4-OHC6H4
4m
4n
4o
4p
260 (dec.)
[300
–
–
–
–
[300
3,4-(OH)2C6H3
262 (dec.)
General reaction conditions: 2-aminobenzimidazole 1 (2 mmol), aldehydes 2a–p (2 mmol), 1,3-cyclo-
hexadione 3 (2 mmol), and ChCl:glycerol (5 mL). All products were characterized by IR, 1H and 13C
NMR, and mass spectrometry
a
Isolated yield
Table 3 Comparison of ChCl:glycerol with some reported methods
Entry
Catalyst
Condition
Time (min)
Solvent
Yield (%)
1
NH2SO3H
I2
Reflux
Reflux
90 °C
Reflux
MW
20
10
360
5
CH3CN
CH3CN
–
90 [26]
2
84 [24]
3
Ionic liquid
84 [29]
4
DMF
58 [27]
5
–
–
–
5
DMF
89 [28]
6
7a, b
Reflux
80 °C
360
30
DMF
65 [28]
ChCl:glycerol
91 [This work]
a
General reaction conditions: 2-aminobenzimidazole 1 (2 mmol), 3-nitrobenzaldehyde 2b (2 mmol),
1,3-cyclohexadione 3 (2 mmol), and DES (5 mL)
b
Isolated yield
conditions. These conditions are not preferable for green chemistry protocols.
Hence, it is worth mentioning that the present work overcomes the drawbacks of
reported methods.
We propose a mechanism for the synthesis of benzimidazoquinazolinones in
Scheme 3. It is seen that DES plays an important role in promoting the reaction by
123

Facile DES-mediated synthesis and antioxidant potency…
Scheme 3 Plausible mechanism for synthesis of benzimidazoquinazolinone derivatives
activation of carbonyl carbon of aldehyde (2) through hydrogen bonding and
promoting Knoevenagel condensation between aldehyde (2) and 1,3-cyclohexa-
dione (3) to form a Schiff base intermediate. In the next step, 2-aminobenzimidazole
(1) reacts with the intermediate through Michael addition followed by cyclization
and dehydration to give the desired product.
Antioxidant study
Measurement of antioxidant capacity is an easy task; however, determination of its
mechanism is rather difficult. Antioxidants can act via many different mechanisms,
such as hydrogen donation, electron donation, free-radical scavenging, and
chelation of metal ions which can initiate free-radical reactions. Therefore, a single
test for antioxidant capacity is not adequate [34].
DPPH radical scavenging activity
DPPH free-radical scavenging is a method regularly used to evaluate the antioxidant
property of natural or synthetic compounds. This method is based on reaction of
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V. N. Mahire et al.
DPPH with hydrogen donors to produce a stable product, causing a color change
from purple to faint yellow [35]. The DPPH radical scavenging activity of the
benzimidazoquinazolinone derivatives 4a–p is depicted in Fig. 3. L-Ascorbic acid
was used as positive control, and compounds 4a, b, d, f, g, i showed excellent DPPH
radical scavenging activity.
Ferric reducing antioxidant power (FRAP) assay
The benzimidazoquinazolinone derivatives 4a–p were also evaluated using ferric
reducing antioxidant power (FRAP). This reducing power assay monitors the
electron-donating capacity of reducing agents (i.e., antioxidants) that cause
reduction of the ferricyanide complex (Fe3?) to ferrous (Fe2?) ions, thereby
generating chromogenic complex [36]. The spectral absorbance of the resultant
blue–green-colored solution measured at 700 nm is proportional to the amount of
Fe2? in the mixture. The standard L-ascorbic acid showed 100 % reducing ability.
Compounds 4a, f, g showed moderate ferric reducing capacity, while 4p exhibited
excellent activity (Fig. 3).
Fig. 3 In vitro antioxidant activity of benzimidazoquinazolinones, a DPPH radical scavenging assay,
b ferric reducing antioxidant power (FRAP) assay, c ABTS, and d metal chelation assay (where AA
ascorbic acid, TC a-tocopherol)
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Facile DES-mediated synthesis and antioxidant potency…
ABTS assay
In this study, the ABTSÁ? radical cation scavenging capacity of the benzimidazo-
quinazolinone derivatives 4a–p was evaluated. Stable radicals of ABTS are
generated when ABTS is mixed with potassium persulfate and incubated in dark
condition, as estimated spectrophotometrically at the characteristic wavelength of
734 nm for analysis of ABTS radicals [37]. Compounds 4f, g showed comparable
radical scavenging ability, while compounds 4a, b, c were found to exhibit better
activity. In all these analyses, a-tocopherol was used as standard.
Metal chelating ability
This assay studies complex formation between Fe2? and ferrozine by measuring the
decrease in violet color intensity when the metal chelating compound is present in
solution. The decrease in the concentration of ferrous ions by benzimidazoquina-
zolinones provides protection against oxidative destruction of free radicals. The
percentage iron chelating ability of a-tocopherol and the benzimidazoquinazoli-
nones is shown in Fig. 3. Benzimidazoquinazolinone compounds 4i, m, n, o,
p showed significant metal chelating capacity compared with the standard a-
tocopherol, while compounds 4e, j exhibited moderate metal chelation activity.
All the radical scavenging activities are related to the ability of each antioxidant
to neutralize oxidative stress created by oxidants. This can be achieved through
various mechanisms such as hydrogen donation, electron donation, free-radical
scavenging, and chelation of metal ions. In the present study, the antioxidant
activity of the benzimidazoquinazolinones 4a–p was studied by using DPPH,
FRAP, ABTS, and metal chelating assays. From the obtained results it is noted that
the compounds having hydroxyl, methoxy, and electron-withdrawing substituents in
their scaffold showed good to comparable antioxidant activity with respect to
standards. Figure 4 shows the correlation between the substituents of the
benzimidazoquinazolinones and their antioxidant properties.
Experimental
Materials and methods
All solvents and reagents were purchased from commercial suppliers Sigma-
Aldrich, S.D. Fine Chemicals Ltd., and Spectrochem India Ltd. and used without
further purification. Reaction progress was monitored by thin-layer chromatography
(TLC) on 0.2-mm precoated silica-gel 60 F254 plates (Merck, Germany), locating
the spots using ultraviolet (UV) light as visualizing agent. Melting points were
obtained by open capillary method and are uncorrected. IR spectra (KBr pellets)
1
were recorded on a Shimadzu FT-IR-8400 spectrophotometer. H and 13C NMR
spectra were recorded on a FT-NMR Bruker Avance-II Spectrometer at 400 and
100 MHz, respectively, using dimethyl sulfoxide (DMSO)-d6 as solvent. Mass
spectra were recorded on Waters 2795 micromass Q-TOF micro and Waters 2995
123

V. N. Mahire et al.
Fig. 4 Schematic representation of structure–activity correlation of benzimidazoquinazolinones
micromass quattro micro API mass spectrometer in electrospray ionization (ESI)
mode. Antioxidant assays were performed using Shimadzu UV mini-1240 and
Agilent Technologies Cary 60 UV–Vis spectrophotometers.
General procedure for preparation of deep eutectic solvent (DES)
Choline chloride (100 g, 0.71 mol) and glycerol (131.9 g, 1.43 mol) were placed in
a 500-mL round-bottomed flask and stirred at 80 °C. After 15–20 min, a
homogeneous colorless liquid solution was formed; this was cooled to room
temperature and used in reactions without any purification (Scheme 1).
General procedure for synthesis of benzimidazoquinazolinone scaffolds
In a 50-mL round-bottomed flask, a mixture of 2-aminobenzimidazole 1 (2 mmol),
aldehyde 2a–p (2 mmol), 1,3-cyclohexadione 3 (2 mmol), and deep eutectic solvent
(5 mL) was stirred at 80 °C for specified time (Scheme 2, Table 2). After reaction
completion (monitored by TLC), the reaction mixture was cooled to room
temperature then poured into crushed ice to obtain solid product, which was filtered,
dried, and recrystallized from ethanol to afford pure product.
Spectral data of representative compounds
12-(2-Nitrophenyl)-3,4,5,12-tetrahydrobenzo[4,5]imidazo[2,1-b]quinazolin-1(2H)-
one (4a)
Isolated as yellow powder; FT-IR (KBr) cm-1: 3231, 3042, 2956, 2878, 2805, 1585,
1556, 1447, 1361, 1279, 1175, 1036, 806, 729; 1H NMR (400 MHz, DMSO-d6) dH:
1.98–1.79 (m, 2H, CH2), 2.28–2.11 (m, 2H, CH2), 2.68–2.65 (m, 2H, CH2),
123

Facile DES-mediated synthesis and antioxidant potency…
6.99–6.95 (m, 1H, ArH), 7.09 (s, 1H, CH), 7.12–7.07 (m, 2H, ArH), 7.33–7.31 (d,
J = 7.84 Hz, 1H, ArH), 7.43–7.38 (m, 2H, ArH), 7.51–7.49 (m, 1H, ArH),
7.85–7.83 (d, J = 8.12 Hz, 1H, ArH), 11.32 (brs, 1H, NH); 13C NMR (100 MHz,
DMSO-d6) dC: 196.03, 168.34, 142.16, 140.40, 133.78, 128.92, 124.06, 123.59,
122.96, 122.08, 117.34, 116.41, 114.41, 109.49, 96.73, 86.53, 57.84, 49.67, 30.07,
20.95; ESI MS [M ? H]?; m/z calcd. for C20H16N4O3: 361; found 361.
12-(3-Nitrophenyl)-3,4,5,12-tetrahydrobenzo[4,5]imidazo[2,1-b]quinazolin-1(2H)-
one (4b)
Isolated as white powder; FT-IR (KBr) cm-1: 3257, 3061, 2952, 2875, 1672, 1541,
1454, 1401, 1275, 1192, 1106, 877, 732; 1H NMR (400 MHz, DMSO-d6) dH:
2.01–1.83 (m, 2H, CH2), 2.37–2.19 (m, 2H, CH2), 2.73–2.70 (m, 2H, CH2), 6.66 (s,
1H, CH), 7.08–6.94 (m, 2H, ArH), 7.27–7.26 (d, J = 7.88 Hz, 1H, ArH), 7.41–7.39
(d, J = 7.84 Hz, 1H, ArH), 7.56–7.52 (m, 1H, ArH), 7.71–7.69 (d, J = 7.88 Hz,
1H, ArH), 8.05–8.02 (m, 1H, ArH), 8.26 (s, 1H, ArH), 11.33 (brs, 1H, NH); 13C
NMR (100 MHz, DMSO-d6) dC: 193.04, 147.57, 144.96, 143.66, 133.42, 131.63,
130.05, 122.68, 122.08, 121.75, 120.74, 117.09, 109.99, 106.55, 53.38, 36.24,
26.54, 20.66; ESI–MS [M ? H]?; m/z calcd. for C20H16N4O3: 361; found 361, 379
[M ? H2O].
12-(p-Tolyl)-3,4,5,12-tetrahydrobenzo[4,5]imidazo[2,1-b]quinazolin-1(2H)-one
(4d)
Isolated as light-yellow powder; FT-IR (KBr) cm-1: 3222, 3027, 2879, 2804, 1584,
1440, 1361, 1262, 1195, 1012, 756; 1H NMR (400 MHz, DMSO-d6) dH: 2.02–1.85
(m, 2H, CH2), 2.19 (s, 3H, CH3), 2.35–2.22 (m, 2H, CH2), 2.74–2.62 (m, 2H, CH2),
6.35 (s, 1H, CH), 6.95–6.91 (m, 1H, ArH), 7.05–7.01 (m, 3H, ArH), 7.20–7.17 (m,
3H, ArH), 7.36–7.34 (d, J = 7.80 Hz, 1H, ArH), 11.06 (brs, 1H, NH); 13C NMR
(100 MHz, DMSO-d6) dC: 193.38, 138.69, 136.68, 131.29, 128.72, 127.48, 126.80,
124.67, 121.59, 120.33, 116.88, 113.80, 109.82, 107.82, 105.92, 87.25, 53.64,
28.99, 27.54, 20.66; ESI–MS [M ? H]?; m/z calcd. for C21H19N3O: 330; found
330.
12-(4-Methoxyphenyl)-3,4,5,12-tetrahydrobenzo[4,5]imidazo[2,1-b]quinazolin-
1(2H)-one (4e)
Isolated as white powder; FT-IR (KBr) cm-1: 3217, 3032, 2890, 2814, 1584, 1445,
1361, 1256, 1032, 819, 742; 1H NMR (400 MHz, DMSO-d6) dH: 2.02–1.84 (m, 2H,
CH2), 2.35–2.19 (m, 2H, CH2), 2.73–2.62 (m, 2H, CH2), 3.66 (s, 3H, OCH3), 6.35
(s, 1H, CH), 6.77–6.75 (d, J = 8.72 Hz, 2H, ArH), 6.96–6.92 (m, 1H, ArH),
7.05–7.01 (m, 1H, ArH), 7.25–7.19 (m, 3H, ArH), 7.36–7.34 (d, J = 7.80 Hz, 1H,
ArH), 11.05 (brs, 1H, NH); 13C NMR (100 MHz, DMSO-d6) dC: 192.75, 168.15,
167.39, 158.45, 145.37, 133.75, 131.96, 128.07, 121.58, 120.24, 113.48, 109.83,
107.68, 54.86, 53.41, 36.40, 26.49, 20.75; ESI–MS [M ? H]?; m/z calcd. for
C21H19N3O2: 346; found 346.
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V. N. Mahire et al.
12-(4-Hydroxy-3-methoxyphenyl)-3,4,5,12-tetrahydrobenzo[4,5]imidazo[2,1-
b]quinazolin-1(2H)-one (4f)
Isolated as white powder; FT-IR (KBr) cm-1: 3541, 3129, 3046, 2986, 2902, 2815,
1
1578, 1441, 1371, 1273, 1138, 1038, 757; H NMR (400 MHz, DMSO-d6) dH:
2.01–1.87 (m, 2H, CH2), 2.31–2.23 (m, 2H, CH2), 2.71–2.66 (m, 2H, CH2), 3.72 (s,
3H, OCH3), 6.32 (s, 1H, CH), 6.59 (m, 2H, ArH), 7.06–6.94 (m, 3H, ArH),
7.30–7.28 (d, J = 7.80 Hz, 1H, ArH), 7.36–7.34 (d, J = 7.84 Hz, 1H, ArH), 8.87
(brs, 1H, OH), 11.02 (brs, 1H, NH); 13C NMR (100 MHz, DMSO-d6) dC: 192.84,
146.96, 145.93, 145.27, 141.96, 132.68, 131.96, 121.56, 120.21, 119.09, 116.77,
115.21, 111.74, 110.01, 107.75, 55.61, 53.66, 36.45, 26.52, 20.77; ESI–MS
[M ? H]?; m/z calcd. for C21H19N3O3: 362; found 362.
12-(3-Hydroxy-4-methoxyphenyl)-3,4,5,12-tetrahydrobenzo[4,5]imidazo[2,1-
b]quinazolin-1(2H)-one (4g)
Isolated as white powder; FT-IR (KBr) cm-1: 3485, 3305, 3226, 2963, 2886, 1719,
1
1625, 1585, 1443, 1359, 1273, 1033, 761; H NMR (400 MHz, DMSO-d6) dH:
2.02–1.82 (m, 2H, CH2), 2.35–2.14 (m, 2H, CH2), 2.74–2.61 (m, 2H, CH2), 3.67 (s,
3H, OCH3), 6.28 (s, 1H, CH), 6.68 (s, 1H, ArH), 6.75–6.71 (m, 2H, ArH), 6.97–6.93
(m, 1H, ArH), 7.06–7.02 (m, 1H, ArH), 7.20–7.18 (d, J = 7.76 Hz, 1H, ArH),
7.37–7.35 (d, J = 7.84 Hz, 1H, ArH), 8.79 (brs, 1H, OH), 11.06 (brs, 1H, NH); 13
C
NMR (100 MHz, DMSO-d6) dC: 192.73, 151.66, 146.97, 146.19, 145.33, 141.95,
134.19, 131.92, 121.57, 120.22, 117.92, 116.79, 111.93, 111.47, 109.83, 107.75,
55.40, 53.47, 36.43, 26.53, 20.73; ESI–MS [M ? H]?; m/z calcd. for C21H19N3O3:
362; found 362.
12-(Naphthalen-1-yl)-3,4,5,12-tetrahydrobenzo[4,5]imidazo[2,1-b]quinazolin-
1(2H)-one (4h)
Isolated as white powder; FT-IR (KBr) cm-1: 3227, 3039, 2956, 2889, 2808, 1625,
1586, 1443, 1361, 1258, 1175, 763; 1H NMR (400 MHz, DMSO-d6) dH: 2.01–1.84
(m, 2H, CH), 2.35–2.11 (m, 2H, CH), 2.79–2.70 (m, 2H, CH), 6.81–6.73 (m, 2H,
ArH), 6.95 (s, 1H, CH), 6.97–6.93 (m, 1H, ArH), 7.22 (brs, 1H, ArH), 7.34–7.32 (d,
J = 7.96 Hz, 1H, ArH), 7.41–7.40 (m, 1H, ArH), 7.55–7.51 (m, 2H, ArH), 7.65
(brs, 1H, ArH), 7.79–7.76 (d, J = 8.44 Hz, 1H, ArH), 7.91–7.89 (d, J = 7.96 Hz,
1H, ArH), 11.27 (brs, 1H, NH); 13C NMR (100 MHz, DMSO-d6) dC: because of less
solubility of compound, 13C NMR could not be obtained; ESI–MS [M ? H]?;
m/z calcd. for C24H19N3O: 366.43; found 366.26.
12-(2,3,4-Trimethoxyphenyl)-3,4,5,12-tetrahydrobenzo[4,5]imidazo[2,1-
b]quinazolin-1(2H)-one (4j)
Isolated as white powder; FT-IR (KBr) cm-1: 3221, 2947, 2827, 1584, 1442, 1359,
1273, 1102, 1030, 895, 750; 1H NMR (400 MHz, DMSO-d6) dH: 2.06–1.81 (m, 2H,
CH), 2.37–2.15 (m, 2H, CH), 2.75–2.63 (m, 2H, CH), 3.67 (s, 3H, OCH3), 3.71
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Facile DES-mediated synthesis and antioxidant potency…
(s, 3H, OCH3), 3.84 (s, 3H, OCH3), 6.47 (s, 1H, CH), 6.70–6.68 (d, J = 8.76 Hz,
1H, ArH), 7.05–6.92 (m, 3H, ArH), 7.28–7.26 (d, J = 7.84 Hz, 1H, ArH),
7.34–7.32 (d, J = 7.76 Hz, 1H, ArH), 11.05 (brs, 1H, NH); 13C NMR (100 MHz,
DMSO-d6) dC: because of less solubility of compound, 13C NMR could not be
obtained; ESI–MS [M ? H]?; m/z calcd. for C23H23N3O4: 406.45; found 406.29.
Experimental protocols for antioxidant activity
DPPH radical scavenging activity
The 1,1-diphenyl-2-picrylhydrazyl (DPPH) free radical scavenging method
described by Wu et al. [38] was used to assay the free radical scavenging ability
of benzimidazoquinazolinone derivatives 4a–p with modifications. One milliliter of
0.2 mM DPPH reagent prepared in methanol was added in tubes containing 0.8 mL
of each compound (1 mg mL-1 in DMSO), and the mixture was allowed to stand
for 30 min in the dark at room temperature. Similarly, the same protocol was
performed for L-ascorbic acid as standard antioxidant. The absorbance of the
resulting mixture was measured at 517 nm by UV–Vis spectrophotometer. Control
was prepared by adding only DMSO to DPPH reagent, and the analysis was
performed as described above. The % scavenging activity was determined as
follows:
Radical scavenging activity ð%Þ ¼ ½ðA0 À A1Þ=A0Š  100
where A0 is the absorption of control (blank, only DMSO) and A1 is the absorption
of compound 4a–p.
Ferric reducing antioxidant power (FRAP) assay
The FRAP ability of the compounds was determined using the protocol described by
Udayaprakash et al. [39] with modifications. One milliliter of compound
(1 mg mL-1), 2.5 mL phosphate buffer (0.1 M, pH 7), and 1 % potassium
ferricyanide (2.5 mL) were mixed together and incubated at 50 2 °C for
30 min. To the solution, 2.5 mL of 10 % trichloroacetic acid was added and
centrifuged at 7000 rpm for 10 min. Distilled water (2.5 mL) and 0.5 mL of 0.1 %
FeCl3 were added to 2.5 mL supernatant. The absorbance of the solution was
measured at 700 nm using UV–Vis spectrophotometer. In all experiments, L-
ascorbic acid was used as standard. The percentage reduction of the compound as
compared with standard was calculated using the following equation:
Percentage of reduction power ¼ ½1 À ð1 À As=Acފ  100
where Ac is the spectral absorption of the standard at the maximum concentration
tested and As is the absorption measured for the compound.
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V. N. Mahire et al.
ABTS activity
The ABTS ability of the compounds was determined using the protocol described
by Udayaprakash et al. [39] with modifications. Solution of 7 mM 2,2-azino-bis(3-
ethylbenzothiazoline-6-sulfonic acid) (ABTS) and potassium persulfate (2.45 mM)
was prepared and incubated in the dark for 14 h, after which the solution was
diluted with absolute ethanol until the optical absorbance at 734 nm reached
0.7 0.001. In a fresh test tube, one milliliter of diluted solution was mixed with
100 lL compound (1 mg mL-1), and after 5 min the absorbance was measured at
734 nm. a-Tocopherol was used as standard. The percentage reduction (I %) against
ABTS was calculated using the following equation:
Percentage reduction ðI %Þ ¼ ½ðA0 À A1Þ=A0Š  100
where A0 is the optical absorption of control (blank, only DMSO) and A1 is the
absorption obtained for the compound.
Metal chelating activity
The metal chelating ability of the synthesized compounds 4a–p was determined
using the protocol described by Nagulendran et al. [40] with modifications. Ferrous
chloride (50 lL of 2 mM) was added into test tubes containing 400 lL of
compound (1 mg mL-1). The reaction was started by adding ferrozine (200 lL of
5 mM), and the final volume of the mixture was adjusted to 4.0 mL using absolute
ethanol. The mixture was shaken and incubated at RT for 10 min. To determine the
ferrous chelating activity of compounds, the spectral absorbance of the mixture was
measured by UV–Vis spectrophotometer at 562 nm. a-Tocopherol was used as
standard. The percentage inhibition of ferrozine–Fe2? complex formation (I %) was
calculated using the following equation:
Radical scavenging activity ðI %Þ ¼ ½ðA0 À A1Þ=A0Š  100
where A0 is the absorption of control (blank, only DMSO) and A1 is the absorption
of the compound.
Conclusions
We successfully developed a catalyst-free protocol for environmentally benign
synthesis of benzimidazoquinazolinones using ChCl:glycerol as solvent. The
novelty of this work is the use of a nontoxic, low-cost, and environmentally benign
solvent. The methodology offers benefits such as clean and mild reaction conditions,
short reaction time, and quantitative product yield. Furthermore, it is noteworthy
that most of the synthesized compounds exhibited acceptable antioxidant activity.
Acknowledgments The authors are grateful to UGC, New Delhi for financial assistance under One Time
Grant [F. No. 19-110/2013 (BSR)] and Major Research Project [F. No. 41-302/2012 (SR)], and SAIF,
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Facile DES-mediated synthesis and antioxidant potency…
Panjab University, Chandigarh, for providing instrumental facilities. V.N.M. is very grateful to the UGC,
New Delhi for awarding a Rajiv Gandhi National Fellowship.
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