Cu(I)-Catalysis of One-Pot Synthesis of Some Novel Regioselective Isoxazole-Benzimidazole Hybrids and Their In Vitro Anti-Cancer Evaluation

Article abstract of DOI:10.1134/S1070363219120296

Regioselective synthesis of some novel isoxazole-benzimidazole hybrids in high yields via Cu(I)-catalyzed tandem one-pot reaction of aromatic aldehydes with 1-prop-2-ynylbenzimidazole is developed. Structures of the synthesized compounds are confirmed by

Full text of DOI:10.1134/S1070363219120296

ISSN 1070-3632, Russian Journal of General Chemistry, 2019, Vol. 89, No. 12, pp. 2512–2515. © Pleiades Publishing, Ltd., 2019.
Cu(I)-Catalysis of One-Pot Synthesis of Some Novel
Regioselective Isoxazole-Benzimidazole Hybrids
and Their In Vitro Anti-Cancer Evaluation
B. Ashok Kumara,b* and V. Shanmukha Kumar Jagarlapudibb**
a Department of Chemistry, Sreenidhi Institute of Science and Technology, Ghatkesar, Telangana State, 501301 India
*e-mail: mailme2ashokkumar@gmail.com
b Department of Chemistry, Deemed to be University, Guntur, Andhra Pradesh, 522502 India
**e-mail: jshanmukh22@gmail.com
Received July 19, 2019; revised August 6, 2019; accepted August 6, 2019
Abstract―Regioselective synthesis of some novel isoxazole-benzimidazole hybrids in high yields via Cu(I)-
catalyzed tandem one-pot reaction of aromatic aldehydes with 1-prop-2-ynylbenzimidazole is developed. Structures
of the synthesized compounds are confirmed by NMR and mass spectra. All the synthesized compounds have been
tested for their in vitro anticancer activity against three human cancer cell lines including ACHN, MCF-7, and
A375. Among these, three compounds demonstrate anticancer activity (0.54 to 4.21 μM) higher than the positive
control doxorubicin.
Keywords: one-pot method, isoxazole-benzimidazole hybrids, anticancer activity
DOI: 10.1134/S1070363219120296
Benzimidazole has been recognized as an important
pharmacophore that imparts some valuable biological
activity features to a vast number of natural and syn-
thetic compounds. Isoxazole ring has been of special
interest to synthetic and medicinal chemists, due to its
low cytotoxicity, comparatively easy synthetic approach,
interesting chemical properties, and biological potential
[1, 2]. Cu(I)-Catalyzed 1,3-dipolar cycloaddition of ni-
trile oxides to alkynes was determined to be an efficient
synthetic approach to 3,5-disubstituted isoxazoles [3].
pot process that proceeded via the reaction of aromatic
aldehydes with the mixture of NH2OH.HCl and NaOH
(1 : 1) at ambient temperature that gave corresponding
aldoximes. The latter intermediates under the action of
chloramine T-trihydrate were immediately converted
into the respective aromatic nitrile oxides that reacted
further with 1-prop-2-ynylbenzimidazole (3) under Cu(I)
catalysis to give regioselective 3,5-disubstituted isoxazole
derivatives 5a–5l in 81–89% yields.
According to the accumulated data, the aromatic
aldehydes bearing electron withdrawing groups led to
slightly higher yields of the corresponding isoxazoles than
benzaldehyde itself and its derivatives bearing electron
donating groups.
In continuation of our group research effort in syn-
thesis of new hybrid compounds consisting of two dif-
ferent heterocyclic pharmacophores, we describe herein
the Cu(I)-catalyzed tandem one-pot reaction of 1-prop-
2-ynylbenzimidazole with aromatic aldehydes that leads
to isoxazole-benzimidazole hybrids and their in vitro
anti-cancer activity.
In 1
spectra, absence of the singlet of the al-
H NMR
kyne terminal hydrogen at 2.47 ppm and the recorded new
singlet in the range of 6.83–6.89 ppm that corresponded to
the 4th C–H proton of isoxazole, indicated the regioselec-
tive synthesis of 3,5-disubstituted isoxazoles. The same
RESULTS AND DISCUSSION
13C
was supported by
NMR spectra, that demonstrated
Synthesis of the desired regioselective isoxazole-
benzimidazole hybrids 5a–5l (Scheme 1) was carried
out via two steps. Initially, 1-prop-2-ynylbenzimidazole
(3) was accumulated in the reaction of benzimidazole (1)
with propargyl bromide (2) in dry acetone in the presence
of K2CO3 at 60°C. The second step was a tandem one-
a new signal in the range of 97.41–98.17 ppm attributed
to the 4th carbon atom of isoxazole ring formed upon
cycloaddition.
In vitro cytotoxicity. The newly synthesized isoxa-
zole-benzimidazole hybrid compounds 5a–5l were tested
2512

Cu(I)-CATALYSIS OF ONE-POT SYNTHESIS
2513
Scheme 1. Cu(I)-Catalyzed one-pot synthesis of isoxazole-benzimidazole hybrids 5a–5l.
(1) NH2OHÂHCl, NaOH
(2) Chloramine-T trihydrate
N
(3)
N
N
CHO
N
3
+
R
CuSO4Â5H2O/Cu0
(1 : 1) t-BuOH/H2O
R
O
N
4a–4l
5a–5l
N
N
(1) NH2OHÂHCl, NaOH
(1 : 1) t-BuOH/H2O, 60 min
3
CuSO4Â5H2O/Cu0
6–7 h
CH=NOH
+
–
CŁN–O
(2) Chloramine-T trihydrate
15 min
R
R
R = C6H5 (4a, 5a); 4-CH3C6H4 (4b, 5b); 3,5-2CH3C6H3 (4c, 5c); 4-OCH3C6H4 (4d, 5d); 4-ClC6H4 (4e, 5e); 4-NO2C6H4 (4f, 5f);
4-FC6H4 (4g, 5g); 4-BrC6H4 (4h, 5h); 4-CNC6H4 (4i, 5i); 3,5-2ClC6H3 (4j, 5j); 3-ClC6H4 (4k, 5k); 4-CF3C6H4 (4l, 5l).
In vitro cytotoxicity of the synthesized compounds 5a–5l
for in vitro anticancer activity against a panel of three
human cancer cell lines including ACHN (renal), MCF-
7 (breast) and A375 (melanoma) using doxorubicin as
a standard drug by the MTT assay. The compounds 5d,
5g, and 51 exhibited higher anticancer activity against all
three cell lines than doxorubicin (see the table).
IC50, μM
Compound
ACHN
11.76
6.44
5.32
0.73
2.41
5.19
0.69
6.24
2.47
1.98
3.16
0.54
0.79
MCF-7
10.38
11.76
10.24
1.94
A375
18.94
13.85
12.67
4.21
7.82
7.24
3.93
9.43
8.16
6.47
7.49
3.42
5.51
5a
5b
EXPERIMENTAL
5c
All chemicals were purchased from S.D. Fine
Chemicals and used without purification. TLC was per-
formed using Merck silica gel 60F-254 pre-coated plates
(0.25 mm). Column chromatography was carried out
using silica gel 60–120 mesh size. Melting points were
5d
5e
4.17
5f
3.38
5g
1.79
determined on a Casia-Siamia (VMP-AM) melting point
1
13C
apparatus. H and
NMR spectra were measured on
5h
4.32
a 400 and 100 MHz spectrometers, respectively, using
CDCl3 as a solvent and TMS as a reference. EI (electron
impact) mass spectra (at an ionising voltage of 70 eV)
were measured on a Shimadzu QP5050A quadrupole-
based mass spectrometer.
5i
5.29
5j
2.38
5k
5l
4.26
1.68
Synthesis of 5-{(1H-benzo[d]imidazol-1-yl)-
methyl}-3-phenylisoxazole (5a). The mixture of benzal-
dehyde 4a (1.0 mmol) with hydroxylamine hydrochloride
Doxorubicin
2.02
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 89 No. 12 2019

2514
ASHOK KUMAR, SHANMUKHA KUMAR JAGARLAPUDIB
.
(1.2 mmol) and NaOH (1.2 mmol) in 5 mL of
t-BuOH:H2O (1 : 1) was stirred for 60 min at ambient
temperature. Upon formation of benzaldoxime, as moni-
tored by TLC, Chloramine-T trihydrate (1.2 mmol) was
added in small portions within 15 min. This was followed
by addition of CuSO4·5H2O (0.03 mmol), Cu-turnings
(10 mg), and 1-prop-2-ynylbenzimidazole (3) (1 mmol)
and stirring for 8 h. Upon completion of the process,
the crude reaction mixture was decanted into ice cold
water (10 mL), 5 mL of dilute NH4OH were added for
removing copper salts. The raw product 5a was filtered
off and purified by column chromatography on silica gel
60–120 mesh size using ethyl acetate–hexane (3 : 7) as an
eluent to afford 5-{(1H-benzo[d]imidazol-1-yl)methyl}-
3-phenylisoxazole (5a).
122.23, 121.97, 119.11, 115.33, 97.83, 57.4, 55.62 MS:
306
]+.
m/z:
[M + H
5-{(1H-Benzo[d]imidazol-1-yl)methyl}-3-(4-
chlorophenyl)isoxazole(5e). Y 87% mp 150 152°C.
1H NMR spectrum
ield
,
–
, δ, ppm: 8.08 s (1H), 7.78 d.d (J =
7.8, 2.3 Hz, 1H), 7.62–7.54 m (3H), 7.31 d (J = 7.8 Hz,
2H), 7.24–7.16 m (2H), 6.88 s (1H), 5.57 s (2H).
170.12, 161.98, 143.92, 138.46,
137.74, 135.17, 134.32, 128.93, 124.62, 120.34, 116.74,
13C
NMRspectrum δ ppm:
, ,
115.82, 97. 89, 57.81. MS: m/z: 311 [M + H]+.
5-{(1H-Benzo[d]imidazol-1-yl)methyl}-3-(4-
nitrophenyl)isoxazole (5f). Y
ield
89%
spectrum, δ, ppm: 8.14 s (1H), 7.88 d (1H,
J = 8.0 Hz, Ar), 7.62–7.53 m (3H), 7.32–7.24 m (2H),
13C
, mp 159–161°C.
1H NMR
6.94 d (2H, J = 8.7 Hz), 6.89 s (1H), 5.61 s (2H).
The products 5b–5l were synthesized and isolated
as above.
NMR spectrum, δ, ppm: 171.08, 162.22, 143.43, 138.24,
137.57, 135.46, 134.63, 128.36, 122.98, 120.19, 116.81,
98.12, 56.18. MS: m/z: 321 [M + H]+.
5-{(1H-Benzo[d]imidazol-1-yl)methyl}-3-phe-
H NMR
Yield 84%, mp129–
131°C. 1
nylisoxazole (5a).
spectrum
5-{(1H-Benzo[d]imidazol-1-yl)methyl}-3-(4-flu-
, δ, ppm: 8.07 s (1H), 7.79 m (1H), 7.61 m (2H),
7.55–7.51 m (1H), 7.36 m (3H), 7.24–7.20 m (2H), 6.86
C NMR spectrum
orophenyl)isoxazole (5g). Y
1H NMR
ie
ld
88%
, mp 138–140°C.
spectrum, δ, ppm: 8.13 s (1H ), 7.84 d (J =
8.3 Hz, 1H), 7.51–7.47 m (3H), 7.32–7.24 m (2H), 7.11
d (J = 8.1, 2H), 6.89 s (1H ), 5.53 s (2H). 13 NMR spec-
trum, δ, ppm: 171.08, 162.22, 143.83, 138.26, 137.54,
135.37, 134.24, 127.62, 123.43, 119.84, 115.78, 98.14,
56.24. MS: m/z: 294 [M + H]+.
s (1H), 5.57 s (2H). 13
, δ, ppm: 170.39,
161.79, 143.74, 138.29, 135.55, 130.29, 129.89, 128.16,
127.24, 123.19, 122.19, 118.79, 115.21, 97.91, 57.21.
C
276
]+.
[M + H
MS: m/z:
5-{(1H-Benzo[d]imidazol-1-yl)methyl}-3-(p-
1H
tolyl)isoxazole (5b).
Y
i
el
d
84%
, mp 134–136°C.
5-{(1H-Benzo[d]imidazol-1-yl)methyl}-3-(4-bro-
NMR spectrum
, δ, ppm: 8.08 s (1H), 7.74–7.48 m (4H),
mophenyl)isoxazole (5h). Y
i
eld
86%
, mp 141–143°C.
spectrum, δ, ppm: 8.13 s (1H), 7.84 d (J = 9.0
Hz, 1H), 7.51–7.46 m (2H), 7.35–7.29 m (1H), 7.22–7.18
m (2H), 6.97 d (2H, J = 9.0 Hz), 6.86 s (1H), 5.53 s (2H).
NMR spectrum, δ, ppm: 171.06, 161.87, 143.84,
138.47, 137.62, 134.46, 132.12, 128.7, 127.34, 123.24,
119.92, 115.76, 97.41, 57.56. MS: m/z: 355 [M + H]+.
.
7.31–7.22 m (4H), 6.86 s (1H), 5.43 s (2H), 2.31 s (3H)
1H NMR
13C NMR spectrum
, δ, ppm: 170.19, 161.68, 144.41,
138. 73, 134.61, 129.81, 123.42, 122.19, 121.79, 119.23,
. MS 290
]+.
115.26, 97.89, 57.4, 21.73
: m/z: [M + H
13C
5-{(1H-Benzo[d]imidazol-1-yl)methyl}-3-(3,5-
dimethylphenyl)isoxazole (5c). Y 83% mp 140
H NMR spectrum
i
e
l
d
,
–
142°C. 1
, δ, ppm: 8.10 s (1H), 7.67–7.64
4-(5-{(1H-Benzo[d]imidazol-1-yl)methyl}isoxa-
m (1H), 7.57–7.53 m (1H), 7.32 d (J = 7.2 Hz, 2H), 6.98
d (2H, J = 8.6 Hz), 6.84 s (1H), 5.64 s (2H), 2.81 s (3H),
zol-3-yl)benzonitrile (5i). Y
ie
ld
86%
, mp 135–137°C.
spectrum, δ, ppm: 8.12 s (1H), 7.81–7.76 m
(2H), 7.66–7.62 m (2H), 7.58 d.d (J = 8.1, 1.9 Hz, 1H),
7.45–7.41 m (1H), 7.31–7.20 m (2H), 6.87 s (1H), 5.54
1H NMR
2.39 s (1H). 13
, δ, ppm: 170.28, 161.79,
C NMR spectrum
144.21, 138.34, 137.29, 134.63, 129.24, 124.33, 122.26,
119.34, 115.27, 97. 92, 56.62, 24.9, 21.96. MS: m/z: 304
]+.
[M + H
13C
s (2H).
NMR spectrum, δ, ppm: 170.17, 161.74,
144.71, 138.78, 135.57, 130.27, 129.76, 128.11, 127.23,
5-{(1H-Benzo[d]imidazol-1-yl)methyl}-3-(4-
methoxyphenyl)isoxazole (5d). 81% mp
1H NMR spectrum δ ppm:
123.18, 122.17, 119. 23,118.35, 115.27, 114. 46, 97.47,
Y
i
e
l
d
,
+
301
56.36. MS: m/z:
[M + H] .
147
–
149°C.
, , 8.06 s (1H),
5-{(1H-Benzo[d]imidazol-1-yl)methyl}-3-(3,5-
7.81 d (J = 8.3 Hz, 1H), 7.52–7.48 m (3H), 7.32 t (J =
Hz
,
) 6.98 d ( 8.8 Hz 2H)
J = , 6.83 s (1H),
5.58 s
dichlorophenyl)isoxazole (5j). Yield 89%, mp 167–
4.2 , 2H ,
,
169°C. 1
spectrum, δ, ppm: 8.19 s (1H), 7.84 d
(2H) 3.82 s (3H) 13C NMR spectrum δ ppm:
, ,
H NMR
.
170.13,
161.71, 160.74, 143.87, 141.49, 134.72, 129.72, 123.17,
(1H, J = 8.4 Hz), 7.54 s (1H,), 7.42–7.34 m (3H), 7.06 d
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 89 No. 12 2019

Cu(I)-CATALYSIS OF ONE-POT SYNTHESIS
2515
(2H, J = 4.9 Hz), 6.89 s (1H), 5.51 s (2H). 13 NMR spec-
[3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium
bromide] (5 mg/mL) was introduced into each well and
the plates were incubated for additional 4 h. The super-
natant from each well was prudently removed, formazan
crystals were dissolved in 100 mLof DMSO.Absorbance
was recorded at 540 nm wavelength.
C
trum, δ, ppm : 171.07, 162.11, 143.84, 137.82, 135.83,
131.74, 128.93, 123.44, 122.72, 120.21, 115.78, 98.17,
56.71. MS: m/z: 345 [M + H]+.
5-{(1H-Benzo[d]imidazol-1-yl)methyl}-3-(3-
chlorophenyl)isoxazole (5k). Y
i
e
l
d
87%
, mp 150–
152°C. 1
(2H, J = 8.3Hz), 7.51–7.47 m (2H), 7.29–7.24 m (2H),
13C
spectrum, δ, ppm: 8.13 s (1H), 7.62 d
H NMR
CONCLUSIONS
6.98 d (2H, J = 5.4 Hz), 6.87 s (1H), 5.51 s (2H).
A simple and efficient Cu(I)-catalysed one-pot regi-
oselective synthesis of some new isoxazole-benzimid-
azole hybrids with high yields is developed. Three of
the synthesized compounds at micro molar concentration
have demonstrated in vitro anticancer activity against
three human cell lines includingACHN, MCF-7 andA375
superior than the standard doxorubicin.
NMR spectrum, δ, ppm : 171.12, 162. 08, 143.24, 138.75,
137.81, 134.74, 133.44, 131.73, 128.24, 127.14, 126.23,
122.12, 121.54, 116.78, 97.89, 56.23. MS: m/z: 310
[M + H]+.
5-{(1H-Benzo[d]imidazol-1-yl)methyl}-3-(3-
(trifluoromethyl)phenyl)isoxazole (5l). Y
mp168–170°C. 1
7.64 d (2H, J = 8.3 Hz), 7.52–7.48 m (2H), 7.29–7.23 m
(2H), 6.99 d (J = 5.4 Hz, 2H), 6.84 s (1H), 5.51 s (2H).
i
e
l
d
88%
,
H NMR
spectrum, δ, ppm: 8.11 s (1H),
CONFLICT OF INTEREST
No conflict of interest was declared by the authors.
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13C
NMR spectrum, δ, ppm: 171.08, 161. 83, 143.48,
138.42, 137.34, 134.73, 133.44, 131.62, 128.16, 127.19,
126.82, 123.37, 122.42, 121.24, 116.74, 97.49, 56.41.
MS: m/z: 344 [M + H]+.
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