PEG-600-Mediated Efficient and Environmentally Sustainable Synthesis of N-[4-(1H-Benzazole-2-yl)phenyl]phthalimides

Article abstract of DOI:10.1134/S1070428020070246

Abstract: An economically and environmentally sustainable protocol has been proposed forthe synthesis of N-[4-(1H-benzazole-2-yl)phenyl]phthalimides via stepwiseand tandem reactions of phthalic anhydride, 4-aminobenzaldehyde, and o-phenylenediamine, N-met

Full text of DOI:10.1134/S1070428020070246

ISSN 1070-4280, Russian Journal of Organic Chemistry, 2020, Vol. 56, No. 7, pp. 1289–1294. © Pleiades Publishing, Ltd., 2020.
PEG-600-Mediated Efficient and Environmentally Sustainable
Synthesis of N-[4-(1H-Benzazole-2-yl)phenyl]phthalimides
P. Karunakara,b and C. N. S. S. P. Kumarb,*
a GVK Biosciences Private Limited, IDA Nacharam, Hyderabad, Telangana, 500076 India
b Division of Chemistry, Department of Sciences and Humanities, Vignan’s Foundation for Science, Technology & Research
(VFSTR) University, Vadlamudi, Andhra Pradesh, 522213 India
*e-mail: padurikarunakar123@gmail.com
Received July 8, 2019; revised February 10, 2020; accepted February 12, 2020
Abstract—An economically and environmentally sustainable protocol has been proposed for the synthesis of
N-[4-(1H-benzazole-2-yl)phenyl]phthalimides via stepwise and tandem reactions of phthalic anhydride,
4-aminobenzaldehyde, and o-phenylenediamine, N-methyl-o-phenylenediamine, 2-aminobenzenethiol, or
2-aminophenol in PEG-600 for 2–3 h in the presence of boric acid as catalyst. Binding of the synthesized
molecules to the tubulin protein (PDB ID: 5CA1) was simulated using AutoDock. All molecules showed good
binding energies as compared to Nocodazole. This simple methodology could be useful for environmentally
sustainable chemistry with the advantages including shorter reaction time, nontoxic and nonhazardous catalyst,
and mild conditions.
Keywords: PEG-600, phthalic anhydride, boric acid, environmentally sustainable synthesis
DOI: 10.1134/S1070428020070246
INTRODUCTION
a complexing solvent with unique properties of cationic
coordination ability [11].
The development of efficient, economical, and en-
vironmentally sustainable synthesis is of great impor-
tance for modern organic synthesis [1]. In many
synthetic organic processes organic solvents represent
the highest pollution problem. Therefore, replacement
of hazardous solvents by relatively green solvents or
elimination of solvents from chemical processes has
been one of the key areas of green chemistry [2]. Based
on the green chemistry principles, a green solvent
should meet numerous criteria such as low toxicity,
non-volatility, non-mutagenicity, non-flammability,
and widespread availability among others [3]. In the
past decade, water [4], glycerol [5], polyethylene
glycol [6], and ionic liquids [7] have been shown as the
most important options for current solvent innovation.
Phthalic anhydride is used in the manufacture of
dialkyl phthalates [12] which find application as
plasticisers for polymers like polyvinyl chloride (PVC),
polyvinyl acetate (PVA), etc. It is used in the manufac-
ture of phenolphthalein indicator [13], anthraquinone
[14] (a versatile raw material in dye industry [15]), and
metal phthalocyanines [16]. Phthalocyanines are used
as pigments, in paints [17], and in many types of dye-
stuffs. Phthalic anhydride derivatives have been widely
reported to possess beneficial pharmaceutical effects
like analgesic [18], anti-inflammatory [19], antiviral
[20], etc. On the other hand, benzimidazole derivatives
exhibit versatile pharmacological properties [21]; benz-
imidazole fragment is present in both clinical medi-
cines [22] and compounds of broad biological functions
[23]. In addition, the potency of benzimidazoles in the
treatment of diseases such as ischemia/reperfusion
injury [24], hypertension [25], obesity [26], etc., has
been recently reported.
Nowadays, PEG-600 has attracted much interest in
the context of environmentally sustainable synthesis.
Indeed, PEG-600 was considered an attractive green
solvent for various organic reactions [8]. The choice of
PEG-600 was provoked by several factors like its non-
volatility, thermal stability, and commercial availability
[9]. Additionally, PEG-600 is a reasonably priced, fully
non-halogenated, and easily degradable solvent that
possesses low toxicity [10]. PEG-600 has been used as
There are currently a number of synthetic method-
ologies available for the synthesis of 2-substituted
benzimidazoles. Generally, two protocols are com-
monly followed for this purpose. The first route is
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KARUNAKAR, KUMAR
1290
coupling of 1,2-phenylenediamines with carboxylic
acids or their derivatives [27], and the second route is
condensation of 1,2-phenylenediamines with alde-
hydes, followed by oxidative cyclodehydrogenation
[28]. However, these protocols do not meet the require-
ments of green chemistry due to long reaction times,
low yields, and the use of toxic solvents and catalysts.
condensation of 3 (1 mmol) with 4a (1 mmol) in dif-
ferent solvents at different temperatures in the presence
of different catalysts (Tables 3, 4). A reasonably high
yield (75%) of 5a was obtained in the reaction of 3
with 4a at 50–55°C for 30–60 min in PEG-600 (unlike
other solvents, namely DMF, DMSO, ethylene glycol,
and triethanolamine) in the presence of boric acid as
catalyst. After having optimized the reaction condi-
tions, the generality of the reaction was confirmed by
carrying out the condensation of several other aromatic
amines 4b–4e with 3 at 50–55°C for 30–60 min in
PEG-600 in the presence of boric acid. Compounds
5b–5e were thus obtained in good yields, and their
structure was confirmed on the basis of spectral and
analytical data.
Herein, we report the synthesis of N-[4-(1H-benz-
azole-2-yl)phenyl]phthalimides via stepwise and tan-
dem reactions of phthalic anhydride with 4-aminobenz-
aldehyde and o-phenylenediamine, N-methyl-o-phe-
nylenediamine, 2-aminobenzenethio, or 2-aminophenol
in PEG-600 for 2–3 h in the presence of boric acid as
catalyst.
RESULTS AND DISCUSSION
Encouraged by the above results, compounds 5a–5e
were also synthesized by a tandem reaction. In this
case, phthalic anhydride (1) was treated with aldehyde
2 in 20 ml of PEG-600 at 50–55°C for 2 h in the pres-
ence of boric acid until the reaction was complete as
checked by TLC. Compound 4 was then added to the
reaction mixture, and the mixture was heated at 50–
55°C for 30–60 min to give 5a–5e with good yields.
The products were identical to those isolated in the
stepwise reactions.
As illustrated in Scheme 1, the reaction of phthalic
anhydride (1) with 2-aminobenzaldehyde (2) was
initially optimized using 1 mmol of each reactant in
different solvents in the presence of 0.2 mmol of boric
acid as catalyst at different temperatures (Table 1).
Unlike other solvents such as DMF, DMSO, and
ethylene glycol, the reaction in PEG-600 at 50–55°C
for 120 min gave a reasonably high yield (85%) of
condensation product 3 (Table 1, entry no. 1). We then
tried different catalysts, including boric acid, p-toluene-
sulfonic acid, aluminum(III) chloride, and acetic acid,
at different amounts at 50–55°C (Table 2). It was
notable that the reaction of 1 with 2 at 50–55°C in the
presence of boric acid (0.2 mmol) as catalyst gave
a good yield (85%) in PEG-600 (Table 2, entry no. 1).
The possible mode of interaction between the syn-
thesized compounds and tubulin protein was investigat-
ed using molecular docking. AutoDock Vina (v.1.1.2)
was used to dock the ligands into protein binding site.
The crystal structure of tubulin protein co-crystallized
with Nocodazole (PDB ID: 5CA1) was downloaded
from the protein data bank. The ligand structures were
drawn in Chem3D Ultra 8.0, and the energies were
Then, a series of experiments were carried out to
check the feasibility for the synthesis of 5a by the
Scheme 1.
CHO
O
O
PEG-600, H3BO3
50–55°C, 2 h
+
O
N
CHO
NH2
O
Y
O
1
2
3
NH2
XH
4a–4e
PEG-600, H3BO3
50–55°C, 30–60 min
O
O
X
N
N
Y
5a–5e
4, 5, X = NH, Y = CH (a), X = O, Y = CH (b), X = S, Y = CH (c), X = NH, Y = N (d), X = NMe, Y = CH (e).
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PEG-600-MEDIATED EFFICIENT AND ENVIRONMENTALLY SUSTAINABLE SYNTHESIS
1291
Table 1. Effect of solvent and temperature on the reaction of 1with 2 to form 3 in the presence of boric acid
Entry no.
Solvent
PEG-600
Temperature, °C
50–55
Reaction time, h
Yield of 3, %
1
2
3
4
5
6
7
2
4
85
48
33
72
61
82
78
DMF
50–55
DMSO
50–55
4
Ethylene glycol
PEG-600
PEG-600
PEG-600
50–55
3
Room temp.
80–85
24
2
100
1.5
Table 2. Effect of catalyst and its amount on the yield of 3 in the reaction of 1 with 2 in PEG-600 at 50–55°C
Entry no.
Catalyst
Boric acid
TsOH
Amount of catalyst, mmol
Reaction time, h
Yield of 3, %
1
2
3
4
5
6
0.2
0.2
0.2
0.2
0.1
0.3
2
2
85
76
71
72
79
73
AlCl3
3
AcOH
2.5
3
Boric acid
Boric acid
1.5
Table 3. Effect of solvent and temperature on the reaction of 3 with 4a to form 5a in the presence of boric acid
Entry no.
Solvent
PEG-600
Temperature, °C
50–55
Reaction time, min
Yield of 5a, %
1
2
3
4
5
6
7
30
60
75
60
64
70
75
70
65
DMF
50–55
DMSO
50–55
60
Ethylene glycol
PEG-600
PEG-600
PEG-600
50–55
40
Room temp.
80–85
120
30
100
25
Table 4. Effect of catalyst and its amount on the yield of 5a in the reaction of 3 with 4a in PEG-600 at 50–55°C
Entry no.
Catalyst
Boric acid
TsOH
Amount of catalyst, mmol
Reaction time, h
Yield of 5a, %
1
2
3
4
5
6
0.2
0.2
0.2
0.2
0.1
0.3
30
55
40
45
60
25
75
66
65
63
74
73
AlCl3
AcOH
Boric acid
Boric acid
minimized using the MMFF94s force field. The protein
structure was then obtained by removing Nocodazole,
non-protein molecules, and other heteroatoms from the
binding groove. MGL Tools (v.1.5.6) were used for
processing the ligands and receptor for docking, and
the final outcomes were saved in .pdbqt format. The
grid box was generated, and the ligands were docked
with the receptor protein. All the docked poses were
examined manually, and the best pose was retained and
then analyzed with PyMol v.2.3 [29–31].
Amino acid residues in the active site of the tubulin
protein and the best docking poses of compounds 5a–
5e are shown in Fig. 1. Hydrogen bonds are represented
with yellow lines. Each compound interacted with
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KARUNAKAR, KUMAR
1292
Fig. 1. Binding interactions of molecules 5a–5e ligands with the tubulin protein (PDB ID: 5CA1). The surface cavity (blue) and
ribbon (gray) model was used to depict the protein-ligand interactions. Nocodazole (NCZ) was used as positive control. ChemDraw
Ultra was used for overlaying the ligands with one another.
different amino acid residues in the binding site. The
binding energies and protein–ligand interactions with
specific amino acid residues are given in Table 5.
Among all the screened compounds, 5e showed a good
binding affinity (–10.3 kcal/mol) toward the tubulin
protein. Compound 5c showed three hydrogen bonds
between the amino acid residues Arg46d, Gly244d, and
Leu246d, and Nocodazole was used as a positive
control.
they could be useful for environmentally sustainable
chemistry. Molecular docking of the synthesized mole-
cules to the tubulin protein revealed their good binding
affinities, which indicates their antimitotic nature.
Further in vitro evaluation should give more detailed
data on the inhibitory activity of the synthesized
compounds, which can help in rational drug design in
the future.
EXPERIMENTAL
In summary, practical and green synthetic methods
have been developed for the synthesis of
N-[4-(1H-benzazole-2-yl)phenyl]phthalimides in PEG-
600 through stepwise and tandem reactions with good
yields. The proposed procedures are advantageous due
to shorter reaction time, the use of nontoxic and non-
hazardous catalyst, and mild reaction conditions, and
The melting points were determined in open capil-
lary tubes in a sulfuric acid bath and are uncorrected.
Thin-layer chromatography was run on silica gel G,
and visualization was done using iodine vapor or UV
light. The IR spectra were recorded in KBr pellets
Table 5. Binding energies and protein–ligand interactions of ligands 5a–5e
Ligand
Ebind, kcal/mol
–9.4
Amino acid residues
Arg391b, Asp431c
5a
5b
5c
5d
5e
–9.4
Gln15c, Thr225c
–10.3
–8.9
Arg46d, Gly244d, Leu246d
Gln15c, Thr225c, Tyr224c
–9.1
Gly410a, Arg105a, Arg251a
Arg221c, Ser178c, Pro175c, Gln334d, Thr351d
Nocodazole
–7.9
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PEG-600-MEDIATED EFFICIENT AND ENVIRONMENTALLY SUSTAINABLE SYNTHESIS
1293
1
using a Perkin Elmer 1000 instrument. The H NMR
spectra were recorded in DMSO-d6 with tetramethyl-
silane as internal standard using a Bruker Avance III
400 MHz spectrometer. The mass spectra (chemical
ionization) were recorded on an Agilent LCMS 5975C
VL MSD instrument. Initial compounds 1, 2, and 4
were obtained from commercial sources and were used
without further purification.
150.1, 152.2, 166.1, 166.5. Mass spectrum: m/z 357.2
[M + H]+.
2-[4-(1H-imidazo[4,5-b]pyridin-2-yl)phenyl]-1,3-
dihydro-2H-isoindole-1,3-dione (5d). mp >220°C. IR
spectrum, ν, cm–1: 3319–3471 m.br (NH), 1722 s
1
(C=O). H NMR spectrum, δ, ppm: 7.0–7.9 m (11H,
H
arom), 12.3 s (1H, NH). 13C NMR spectrum, δC, ppm:
114.2, 121.2, 122.0, 122.9, 123.0, 124.3, 125.1, 126.1,
130.1, 130.3, 132.2, 134.1, 134.1, 135.6, 151.1, 151.3,
166.9, 167.1. Mass spectrum: m/z 341.3 [M + H]+.
4-(1,3-Dioxo-1,3-dihydro-2H-isoindol-2-yl)benz-
aldehyde (3). A mixture of 1 (10 mmol), 2 (10 mmol),
PEG-600 (10 mL), and boric acid (0.2 mmol) was
heated at 50–55°C for 2 h. The progress of the reaction
was monitored by TLC. After completion of the reac-
tion, water (50 mL) was added, and the colorless solid
was filtered off, washed with hexane (10 mL), dried,
and recrystallized from ethanol. Yield 85%, mp 345–
348°C [31].
2-[4-(1-Methyl-1H-benzimidazol-2-yl)phenyl]-
1,3-dihydro-2H-isoindole-1,3-dione (5e). mp >220°C.
IR spectrum, ν, cm–1: 3311–3478 m.br (NH), 1711 s
1
(C=O). H NMR spectrum, δ, ppm: 3.1 s (3H, CH3),
7.0–7.9 m (12H, Harom), 12.4 s (1H, NH). 13C NMR
spectrum, δC, ppm: 31.2, 114.1, 122.1, 122.4, 122.9,
123.9, 124.3, 124.9, 125.9, 127.3, 131.4, 131.3, 134.0,
134.7, 135.2, 151.5, 151.9, 167.9, 168.4. Mass spec-
trum: m/z 354.1 [M + H]+.
General procedures for the synthesis of com-
pounds 5a–5e. a. Stepwise procedure. A mixture of 3
(10 mmol), 4a–4e (10 mmol), PEG-600 (10 mL), and
boric acid (0.2 mmol) was heated at 50–55°C for
30–40 min. After completion of the reaction (TLC),
water (50 mL) was added, and the mixture was
extracted with ethyl acetate (3×50 mL). The combined
organic layers were washed with brine, dried over
anhydrous sodium sulfate, and evaporated under
reduced pressure, and the residue was purified by
column chromatography using ethyl acetate–hexane
(2:8) as eluent. Yield 70–75%.
b. Tandem reaction. A mixture of 1 (10 mmol), 2
(10 mmol), PEG-600 (10 mL), and boric acid
(0.2 mmol) was heated at 50–55°C for 2 h. After com-
pletion of the reaction (TLC), amine 4a–4e was added,
and the mixture was heated at 50–55°C for 30–
40 min. The mixture was then treated as described
above in a. Yield 60–63%. The products were identical
to samples prepared as described in a.
ACKNOWLEDGMENTS
2-[4-(1H-Benzimidazol-2-yl)phenyl]-1,3-dihydro-
2H-isoindole-1,3-dione (5a). mp >220°C. IR spec-
trum, ν, cm–1: 3279–3479 m.br (NH), 1727 s (C=O).
1H NMR spectrum, δ, ppm: 7.0–7.9 m (12H, Harom),
12.6 s (1H, NH). 13C NMR spectrum, δC, ppm: 114.9,
120.3, 121.7, 122.3, 127.3, 128.5, 128.8, 129.5, 129.9,
130.3, 130.5, 132.5, 136.7, 137.9, 138.9, 150.3, 151.3,
168.0, 168.5. Mass spectrum: m/z 340.3 [M + H]+.
The authors thank GVK Biosciences Private Limited
(IDA Nacharam, Hyderabad, Telangana, India) and Division
of Chemistry, Department of Sciences and Humanities,
Vignan’s Foundation for Science, Technology & Research
(VFSTR) University (Vadlamudi, Guntur, Andhra Pradesh,
India).
2-[4-(1,3-Benzoxazol-2-yl)phenyl]-1,3-dihydro-
2H-isoindole-1,3-dione (5b). mp >220°C. IR spec-
trum: ν 1722 cm–1, s (C=O). 1H NMR spectrum: δ 7.0–
7.9 ppm, m (12H, Harom). 13C NMR spectrum, δC,
ppm: 113.1, 121.2, 121.9, 122.5, 127.7, 128.0, 128.9,
129.8, 129.9, 131.3, 131.9, 132.8, 134.2, 136.6, 137.1,
153.3, 155.3, 166.0, 166.1. Mass spectrum: m/z 341.5
[M + H]+.
CONFLICT OF INTEREST
No conflict of interest is declared by the authors.
SUPPLEMENTARY MATERIALS
Supplementary materials are available for this article at
https://doi.org/10.1134/S1070428020070246 and are acces-
sible for authorized users.
2-[4-(1,3-Benzothiazol-2-yl)phenyl]-1,3-dihydro-
2H-isoindole-1,3-dione (5c). mp >220°C. IR spec-
trum: ν 1732 cm–1, s (C=O). 1H NMR spectrum: δ 7.0–
7.9 ppm, m (12H, Harom). 13C NMR spectrum, δC,
ppm: 110.2, 119.0, 120.1, 122.3, 124.5, 125.1, 126.2,
127.2, 128.1, 131.0, 131.2, 132.4, 134.2, 136.8, 137.2,
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