New fluorescent 3H-imidazo[4,5-e][2,1]benzoxazoles: Synthesis, spectroscopic characterization, and antibacterial activity

Article abstract of DOI:10.1007/s10593-015-1796-0

The new fluorophores of the 3H-imidazo[4,5-e][2,1]benzoxazoles series were synthesized by the regioselective nitration of 3-alkyl-8-phenyl-3H-imidazo[4,5-e][2,1]benzoxazoles. The latter compounds were obtained from the reaction of 1-alkyl-5-nitro-1H-benzi

Full text of DOI:10.1007/s10593-015-1796-0

DOI 10.1007/s10593-015-1796-0
Chemistry of Heterocyclic Compounds 2015, 51(10), 918–922
New fluorescent 3H-imidazo[4,5-e][2,1]benzoxazoles:
synthesis, spectroscopic characterization, and antibacterial activity
Mitra Rezazadeh1, Mehdi Pordel1*, Abolghasem Davoodnia1, Sattar Saberi2
1 Department of Chemistry, Mashhad Branch, Islamic Azad University,
Mashhad, Iran; e-mail: mehdipordel58@mshdiau.ac.ir
2 Buali Research Institute,
Ferdowsi Blvd. Buali Sq., Mashhad, Iran; e-mail: satar.saberi@gmail.com
Submitted September 9, 2015
Accepted after revision October 17, 2015
Published in Khimiya Geterotsiklicheskikh Soedinenii,
2015, 51(10), 918–922
The new fluorophores of the 3H-imidazo[4,5-e][2,1]benzoxazoles series were synthesized by the regioselective nitration of 3-alkyl-
8-phenyl-3H-imidazo[4,5-e][2,1]benzoxazoles. The latter compounds were obtained from the reaction of 1-alkyl-5-nitro-1H-
benzimidazoles with benzyl cyanide in basic MeOH solution. The structures of synthesized compounds were established using spectral
(UV-vis, IR, 1H NMR, 13C NMR, and NOESY) and analytical data. Furthermore, it was found that these fluorophores underwent thermal
rearrangement to new 5H-imidazo[4,5-f][2,1,3]benzoxadiazole 3-oxides in AcOH in moderate yields. The fluorescence properties and
antibacterial activities of new compounds against Gram-positive and Gram-negative bacterial species were also studied.
Keywords: 3H-imidazo[4,5-e][2,1]benzoxazole, 5H-imidazo[4,5-f][2,1,3]benzoxadiazole 3-oxide, emission and absorption spectra,
fluorescence, NOESY.
A huge number of heterocyclic systems which include
mainly five- and six-membered compounds represent a
diverse group of molecular scaffolds. Several of such
heterocyclic scaffolds have been successfully incorporated
into novel drug leads and therapeutic agents.1–3
Imidazoles exhibit wide ranges of biological activities
and optical properties as fluorescence compounds, dyes,
and TPA (Two-photon absorption) materials4–6 making
them attractive compounds for organic chemists. Also,
many commercial fluorescent brighteners for application to
synthetic fibers contain an imidazole moiety.7
On the other hand, 2,1-benzoxazole derivatives, e.g.,
risperidone, are prescribed as antipsychotic drugs.8
Compounds of this class play a key role in many organic
reactions,9 notably those leading to anthranilic acids.
A combination of the 2,1-benzoxazole moiety with the
imidazole nucleus may enhance optical and biological
properties. Taking this fact into consideration and in
continuation of our studies on the synthesis of new
bioactive10–12 and fluorescent heterocyclic compounds,13–15
we have synthesized some new fluorescent 3H-imidazo-
[4,5-e][2,1]benzoxazoles. Spectroscopic characteristics of
these compounds were studied and the structure of the
major products was established by Nuclear Overhauser
Effect Spectroscopy (NOESY) experiment. Furthermore, it
was found that these fluorophores can be converted to new
5H-imidazo[4,5-f][2,1,3]benzoxadiazole 3-oxides in AcOH.
Antibacterial activities of the new compounds against
Gram-positive and Gram-negative bacterial species were
also studied.
The key compounds 3a–e were obtained from the
reaction of 1-alkyl-5-nitro-1H-benzimidazoles 1a–e with
benzyl cyanide (2) in basic MeOH solution.16 Nitration of
compounds 3a–e was carried out using a mixture of sulfuric
and nitric acids and led to the formation of new 3-alkyl-
5-nitro-8-(4-nitrophenyl)-3H-imidazo[4,5-e][2,1]benzoxazoles
4a–e in excellent yields (Scheme 1).
Structural assignments of the new compounds 4a–e were
based on their spectral and microanalytical (C, H, and N)
1
data. For example, in the aromatic region of the H NMR
spectrum of compound 4e there are two doublet signals at
9.21 and 8.51 ppm attributed to four aromatic protons of
4-nitrophenyl ring and two singlet signals at 8.76 and
8.21 ppm assignable to two aromatic protons of imidazole
0009-3122/15/5110-0918©2015 Springer Science+Business Media New York
918

Chemistry of Heterocyclic Compounds 2015, 51(10), 918–922
Scheme 1
O
N
KOH
MeOH
O2N
N
N
N
N
+
, 4 h
R
R
CH2CN
2
1a e
–
3a e
–
NO2
O
N
HNO3
a
R = Me
H2SO4
N
b R = Et
0°C to rt
1 h
80–90%
c
R = Pr
d R = n-Bu
e R = i-Bu
N
R
O2N
4a–e
and phenyl rings, respectively. Also, there are 15 different
carbon atom signals in the 13C NMR spectrum of
compound 4e. Moreover, the IR spectrum of compound 4e
in KBr showed two absorption bands at 1546 and 1356 cm–1
corresponding to the nitro group. All this evidence taken in
conjunction with molecular ion peak at m/z 381 [M]+
support the structure of compound 4e. However, these
spectral characteristics are consistent with another possible
structure which is isomeric with compounds 4a–e.
Nitration of compounds 3a–e can lead to addition of the
nitro group at C-4 or C-5 position. Fortunately, there are
some convincing reasons that confirm the structure of
compounds 4a–e as the major products of the nitration of
compounds 3a–e. For example, electron release from the
imidazole ring would make C-5 position more susceptible
to electrophilic attack than C-4 position. Furthermore the
latter is also hindered by its proximity to the alkyl group at
N-3 position.
The data of NOESY experiment for compound 4d
showed a massive cross peak between the singlet of the
H-4 proton at 8.23 ppm and the signal of NCH2 protons of
N-3 alkyl substituent at 4.41 ppm (t, J = 7.2 Hz),
confirming that the nitration of compound 3d resulted in
the formation of 5-nitro-substituted compound 4d (Fig. 1).
Compounds 4a,e underwent thermal rearrangement in
AcOH17 giving new 5-alkyl-8-(4-nitrobenzoyl)-5H-imidazo-
[4,5-f][2,1,3]benzoxadiazole 3-oxides 5a,b in moderate
yields (Scheme 2). Structural assignments of compounds
5a,b were based on their spectral and microanalytical data.
Figure 1. NOESY spectrum of compound 4d.
Compounds 3a–e, 4a–e, and 5a,b were spectrally
characterized by UV-Vis and fluorescence spectrophoto-
metry methods in the wavelength range of 200–1000 nm.
The emission spectra of the precursors 3a–e don't show any
fluorescence emission peak (λabs 282–285 nm) and the
emission spectra of compounds 5a,b show weak emission
peak at high concentration (10–2 mol·l–1). The fluorescence
absorption and emission spectra of compounds 4a–e were
recorded in chloroform at concentrations of 5·10–5 and
1·10–6 mol·l–1, respectively. The absorbance and fluores-
cence spectral properties of compounds 4a–e are similar to
each other, and the numerical data are presented in Table 1.
Values of extinction coefficient (ε) were calculated as
the slope of the plot of absorbance vs concentration. The
fluorescence excitation (λex) wavelengths at 425 and
390 nm (λex/nm) were used for compounds 4a–e and 5a,b,
respectively. The fluorescence quantum yields (ΦF) of
compounds 4a–e and 5a,b were determined via comparison
methods, using fluorescein as a standard sample in 0.1 M
NaOH and MeOH solution.18 The used value of the
Table 1. Spectroscopic data for compounds 4a–e, 5a,b at 298 K
Com-
pound
ε·10–4,
λabs, nm*
λex, nm*** λflu, nm*4
ΦF*5
l·(mol·cm)–1**
420
425
425
425
425
390
390
1.30
1.50
1.60
1.70
1.60
0.08
0.09
425
425
425
425
425
390
390
515
520
525
525
525
480
480
0.39
0.43
0.42
0.47
0.45
0.07
0.09
4a
4b
4c
4d
4e
5a
5b
Scheme 2
* Wavelengths of maximum absorbance.
** Extinction coefficient.
*** Wavelengths of fluorescence excitation.
*4 Wavelengths of fluorescence emission.
*5 Fluorescence quantum yield.
919

Chemistry of Heterocyclic Compounds 2015, 51(10), 918–922
Table 2. Spectroscopic data for compound 4a at 298 K
fluorescein emission quantum yield is 0.79. As can be seen
in different solvents
in Table 1, the ΦF of compound 4d (R = n-Bu) is the
highest and the lowest ΦF is attributed to compound 5a.
The fluorescence absorptions of compounds 4a–e at 420–
425 nm can be corresponded to π–π* transitions from N-3
atom in the imidazole cycle and oxygen atom in the
isoxazole cycle to the nitro group.
Solvent
n-Hexane
1,4-Dioxane
EtOAc
λabs, nm
405
λflu, nm
475
410
495
415
510
The relatively high fluorescence emission of the new
compounds 4a–e can be explained in terms of the extended
conjugation pathway.14,15 Comparing the resonance
structures of compounds 4a–e and 5a,b demonstrates that
π-electron delocalization in compounds 4a–e can occur
much more easily compared to compounds 5a,b.
The solvatochromic properties of compound 4a were
deduced from the fluorescence absorption and emission
spectra (Fig. 2). The fluorescence absorption and emission
spectra of compound 4a in polar solvents undergo a
relatively modest red shift. Increasing the solvent polarity
stabilizes the molecule excited state comparative to the
molecule ground state with the red shift of the absorption
maximum as the experimentally observed result (Table 2).
For example, in the absorption and emission spectra of
compound 4a, λabs and λflu shift from 405 to 430 nm and
475 to 555 nm, respectively, as the solvent changes from
n-hexane to methanol (Table 2).
The antibacterial activity of compounds 3a–e, 4a–e, and
5a,b was tested against standard strains of two Gram-
positive (Staphylococcus aureus ATCC 29213 and Bacillus
subtilis ATCC 6633) and two Gram-negative bacteria
(Escherichia coli ATCC 10538 and Salmonella
typhimurium ATCC 14028) species (Table 3), using the
broth microdilution method as previously described.19 We
used amoxicillin and ciprofloxacin as reference compounds
in the evaluation of antibacterial activity. The lowest
concentration of the antibacterial agent that prevents
Acetone
DMF
425
530
425
545
MeOH
430
555
growth of the test organism, as detected by lack of visual
turbidity (matching the negative growth control), is
assigned the minimum inhibitory concentration (MIC).
Experimental details of the tests can be found in our earlier
study.12
The antimicrobial tests performed with compounds 3a–e,
4a–e, and 5a,b confirmed that only compounds 4a–e and
5a,b were effective against both Gram-positive and Gram-
negative bacteria. Also, the results revealed that new
compound 5b displayed greater antibacterial activity
against the Staphylococcus aureus ATCC 29213, Bacillus
subtilis ATCC 6633, Escherichia coli ATCC 10538, and
Salmonella typhimurium ATCC 14028 species compared to
the well-known antibacterial agent amoxicillin (Table 3).
In conclusion, we have synthesized some new donor–
acceptor fluorescent heterocyclic compounds in excellent
yields. These compounds were obtained from regio-
selective nitration of 3-alkyl-8-(4-nitrophenyl)-3H-imidazo-
[4,5-e][2,1]benzoxazoles at room temperature. Confirma-
tion of the correct structure has been achieved by NOESY
experiment. The fluorescence properties of the obtained
nitro derivatives together with high antibacterial activity,
Figure 2. Visible absorption (left) (5·10–5 mol·l–1) and emission spectra (right) (1·10–6 mol·l–1) of compound 4a in different solvents.
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Chemistry of Heterocyclic Compounds 2015, 51(10), 918–922
Table 3. Inhibitory activity (MIC, μg·ml–1) of compounds 4a–e, 5a,b against bacteria
Staphylococcus aureus
Bacillus subtilis
(ATCC 6633)
Escherichia coli
(ATCC 10538)
Salmonella typhimurium
Compound
(ATCC 29213)
(ATCC 14028)
125
95
80
55
75
5
125
95
150
115
115
110
90
150
115
115
100
85
4a
4b
80
4c
55
4d
60
4e
5a
5
15
15
3
3
10
5
5b
Ciprofloxacin
Amoxicillin
16
25
0.05
0.06
1
>128
>128
150
can offer an excellent opportunity for the study of
physiological functions of bacteria at single-cell level.19 In
addition, the new synthesized compounds can play a key
role in many organic reactions, leading to the formation of
the other significant compounds. For example, rearrange-
ment of the nitrated 3H-imidazo[4,5-e][2,1]benzoxazoles in
boiling AcOH leads to the new 5H-imidazo[4,5-f][2,1,3]-
benzoxadiazole 3-oxide derivatives.
3-Methyl-5-nitro-8-(4-nitrophenyl)-3H-imidazo[4,5-e]-
[2,1]benzoxazole (4a). Yield (85%), shiny yellow powder,
mp 295–297°C. IR spectrum, ν, cm–1: 1353, 1546 (NO2).
1H NMR spectrum, δ, ppm (J, Hz): 4.57 (3H, s, NCH3);
8.27 (1H, s, H Ar); 8.51 (2H, d, J = 8.8, H Ar); 8.80 (1H, s,
H Ar); 9.21 (2H, d, J = 8.8, H Ar). 13C NMR spectrum,
δ, ppm: 46.6; 112.4; 119.5; 125.5; 128.2; 129.7; 131.5;
133.8; 140.9; 146.0; 149.6; 152.2; 164.7. Mass spectrum,
m/z (Irel, %): 339 [M]+ (7), 338 (30), 294 (35), 249 (37),
235 (43), 91 [C7H7]+ (100). Found, %: C 52.90; H 2.64;
N 20.51. C15H9N5O5. Calculated, %: C 53.10; H 2.67;
N 20.64.
3-Ethyl-5-nitro-8-(4-nitrophenyl)-3H-imidazo[4,5-e]-
[2,1]benzoxazole (4b). Yield (80%), shiny yellow powder,
mp 287–289°C. IR spectrum, ν, cm–1: 1353, 1546 (NO2).
1H NMR spectrum, δ, ppm (J, Hz): 1.77 (3H, t, J = 7.2,
NCH2CH3); 4.49 (2H, t, J = 7.2, NCH2CH3); 8.25 (1H, s,
H Ar); 8.52 (2H, d, J = 8.8, H Ar); 8.79 (1H, s, H Ar); 9.21
(2H, d, J = 8.8, H Ar). 13C NMR spectrum, δ, ppm: 16.0;
43.2; 112.4; 119.6; 125.5; 128.1; 129.8; 131.6; 133.9;
140.7; 146.2; 149.6; 152.2; 164.8. Mass spectrum, m/z (Irel, %):
353 [M]+ (5), 352 (21), 308 (39), 263 (34), 235 (41), 91
[C7H7]+ (100). Found, %: C 54.18; H 3.13; N 20.01.
C16H11N5O5. Calculated, %: C 54.40; H 3.14; N 19.82.
5-Nitro-8-(4-nitrophenyl)-3-propyl-3H-imidazo[4,5-e]-
[2,1]benzoxazole (4c). Yield (87%), shiny yellow powder,
mp 283–285°C. IR spectrum, ν, cm–1: 1357, 1543 (NO2).
1H NMR spectrum, δ, ppm (J, Hz): 1.10 (3H, t, J = 7.2,
NCH2CH2CH3); 2.05–2.11 (2H, m, NCH2CH2CH3); 4.39 (2H,
t, J = 7.2, NCH2CH2CH3); 8.24 (1H, s, H Ar); 8.51 (2H, d,
J = 8.8, H Ar); 8.78 (1H, s, H Ar); 9.21 (2H, d, J = 8.8,
H Ar). 13C NMR spectrum, δ, ppm: 12.4; 25.1; 49.1; 112.3;
119.7; 125.5; 128.3; 129.9; 131.5; 133.6; 140.7; 146.1;
149.8; 152.3; 164.9. Mass spectrum, m/z (Irel, %): 367 [M]+
(3), 366 (15), 322 (30), 280 (25), 235 (37), 91 [C7H7]+
(100). Found, %: C 55.43; H 3.54; N 19.23. C17H13N5O5.
Calculated, %: C 55.59; H 3.57; N 19.07.
Experimental
Absorption and fluorescence spectra were recorded on a
Varian 50-bio UV-Visible spectrophotometer and a Varian
Cary Eclipse spectrofluorophotometer, respectively. UV-vis
and fluorescence scans were recorded from 200 to 1000 nm.
IR spectra were obtained on a Tensor 27 spectrometer in KBr
1
pellets; only noteworthy absorption bands are listed. H and
13C NMR spectra were recorded on a Bruker Avance DRX-
400 FT spectrometer (400 and 100 MHz, respectively) in
CDCl3, internal standard TMS. Mass spectra were recorded
on a Varian Mat, CH-7 mass spectrometer at 70 eV.
Elemental analysis was performed on a Thermo Finnigan
Flash EA microanalyzer. Melting points were measured on
an Electrothermaltype-9100 melting point apparatus. All
measurements were carried out at room temperature.
All reagents and solvents used in this work were
purchased from Merck. Amoxicillin and ciprofloxacin were
purchased from Sigma–Aldrich. All solvents were dried
according to standard procedures. Compounds 1a–e20 and
3a–e10 were synthesized by the literature methods. The
microorganisms Staphylococcus aureus ATCC 29213,
Bacillus subtilis ATCC 6633, Escherichia coli ATCC
10538, Salmonella typhimurium ATCC 14028 were
purchased from Pasteur Institute of Iran.
Synthesis of compounds 4a–e by nitration of com-
pounds 3a–e (General method). To a solution of com-
pound 3a–e (20 mmol) in concd. H2SO4 (4 ml), 65% HNO3
(1 ml) was added with stirring over 0.5 h period keeping
the temperature at 0–5°C. After the addition was
completed, the mixture was allowed to warm to room
temperature with stirring for 0.5 h. Then the mixture was
poured into crushed ice and water (200 ml), the precipitated
solid was filtered off, washed with water, and dried to give
the crude compounds 4a–e. More purification was achieved
by recrystallization from acetone.
3-Butyl-5-nitro-8-(4-nitrophenyl)-3H-imidazo[4,5-e]-
[2,1]benzoxazole (4d). Yield (85%), shiny yellow powder,
mp 277–279°C. IR spectrum, ν, cm–1: 1356, 1541 (NO2).
1H NMR spectrum, δ, ppm (J, Hz): 1.06 (3H, t, J = 7.2,
NCH2CH2CH2CH3); 1.42– 1.52 (2H, m, NCH2CH2CH2CH3);
1.99 (2H, t, J = 7.2, NCH2CH2CH2CH3); 4.41 (2H, t,
J = 7.2, NCH2CH2CH2CH3); 8.23 (1H, s, H Ar); 8.51 (2H,
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Chemistry of Heterocyclic Compounds 2015, 51(10), 918–922
d, J = 8.8, H Ar); 8.77 (1H, s, H Ar); 9.21 (2H, d, J = 8.8,
m, NCH2CH(CH3)2); 4.25 (2H, d, J = 7.2, NCH2CH(CH3)2);
6.93 (1H, s, H Ar); 8.09 (2H, d, J = 8.9, H Ar); 8.25 (2H, d,
J = 8.9, H Ar); 8.41 (1H, s, H Ar). 13C NMR spectrum, δ,
ppm: 21.6; 30.5; 55.9; 106.2; 117.5; 129.4; 129.9; 131.9;
132.5; 133.6; 139.3; 145.7; 150.9; 159.6; 187.8. Mass
spectrum, m/z (Irel, %): 381 [M]+ (3), 380 (12), 325 (19),
309 (33), 264 (58), 91 [C7H7]+ (100). Found, %: C 56.53;
H 3.93; N 18.15. C18H15N5O5. Calculated, %: C 56.69;
H 3.96; N 18.36.
H Ar). 13C NMR spectrum, δ, ppm: 12.5; 21.4; 32.9; 54.6;
112.5; 119.8; 125.5; 128.1; 130.1; 131.6; 133.6; 140.5;
146.3; 149.7; 152.5; 164.6. Mass spectrum, m/z (Irel, %):
381 [M]+ (2), 380 (13), 336 (35), 291 (23), 235 (43), 91
[C7H7]+ (100). Found, %: C 56.49; H 3.94; N 18.19.
C18H15N5O5. Calculated, %: C 56.69; H 3.96; N 18.36.
3-Isobutyl-8-(4-nitrophenyl)-5-nitro-3H-imidazo[4,5-e]-
[2,1]benzoxazole (4e). Yield (90%), shiny yellow powder,
mp 265–267°C. IR spectrum, ν, cm–1: 1356, 1546 (NO2).
1H NMR spectrum, δ, ppm (J, Hz): 1.07 (6H, d, J = 6.9,
NCH2CH(CH3)2); 2.29–2.36 (1H, m, NCH2CHMe2); 4.21
(2H, d, J = 7.2, NCH2CHMe2); 8.21 (1H, s, H Ar); 8.51
(2H, d, J = 8.8, H Ar); 8.76 (1H, s, H Ar); 9.21 (2H, d,
J = 8.8, H Ar). 13C NMR spectrum, δ, ppm: 21.1; 31.0;
54.8; 112.3; 119.8; 125.5; 128.5; 129.9; 131.6; 133.6;
140.6; 146.4; 149.8; 152.2; 164.9. Mass spectrum, m/z (Irel, %):
381 [M]+ (5), 380 (19), 336 (31), 291 (27), 235 (43), 91
[C7H7]+ (100). Found, %: C 56.51; H 3.95; N 18.49.
C18H15N5O5. Calculated, %: C 56.69; H 3.96; N 18.37.
Synthesis of compounds 5a,b by thermal rearrange-
ment of compounds 4a,e (General method). Compounds 4a,e
(10 mmol) were refluxed for 1 h in glacial AcOH (10 ml). The
solution was cooled, and an equal volume of water was added.
The precipitated solid was filtered off, washed with water, and
dried to give the crude compounds 5a,b. More purification
was achieved by recrystallization from EtOH.
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(NO2), 1687 (C=O). H NMR spectrum, δ, ppm (J, Hz):
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5-Isobutyl-8-(4-nitrobenzoyl)-5H-imidazo[4,5-f][2,1,3]-
benzoxadiazole 3-oxide (5b). Yield (40%), yellow
powder, mp > 300°C (decomp.). IR spectrum, ν, cm–1:
1
1353, 1546 (NO2), 1687 (C=O). H NMR spectrum, δ, ppm
(J, Hz): 1.11 (6H, d, J = 6.9, NCH2CH(CH3)2); 2.33–2.40 (1H,
922