Iodine (III)-mediated synthesis of some 2-aryl/ hetarylbenzoxazoles as antibacterial/antifungal agents

Article abstract of DOI:10.1007/s00044-009-9211-y

Ten 2-aryl/hetarylbenzoxazoles (5a, 5b, and 6a-h) were synthesized via oxidative cyclization of Schiff bases (3a, 3b, and 4a-h) with 1.1 equivalent of iodobenzene diacetate (IBD) in methanol. All of these 2-aryl/hetarylbenzoxazoles (5a, 5b, and 6a-h) were tested in vitro for their antibacterial and antifungal activities against Bacillus subtilis, Bacillus stearothermophilus, Escherichia coli, and Pseudomonas putida. These compounds also were screened for their antifungal activity against Aspergillus flavus and Aspergillus niger. Biological activity of these compounds was compared with those of commercially available antibiotics, chloramphenicol and antifungal agent cycloheximide. Most of these compounds, 5a, 5b, 6a, 6b, 6d, 6e, 6g, 6h, were equipotent or more potent than these commercial drugs at concentration 100 μg/ml. Birkhaeuser Boston 2009.

Full text of DOI:10.1007/s00044-009-9211-y

MEDICINAL
CHEMISTRY
RESEARCH
Med Chem Res (2010) 19:541–550
DOI 10.1007/s00044-009-9211-y
ORIGINAL RESEARCH
Iodine (III)-mediated synthesis of some 2-aryl/
hetarylbenzoxazoles as antibacterial/antifungal agents
Ravi Kumar Æ Reshmi R. Nair Æ
Saurabh Sudha Dhiman Æ Jitender Sharma Æ
Om Prakash
Received: 15 December 2008 / Accepted: 15 April 2009 / Published online: 29 May 2009
¨
Ó Birkhauser Boston 2009
Abstract Ten 2-aryl/hetarylbenzoxazoles (5a, 5b, and 6a–h) were synthesized via
oxidative cyclization of Schiff bases (3a, 3b, and 4a–h) with 1.1 equivalent of
iodobenzene diacetate (IBD) in methanol. All of these 2-aryl/hetarylbenzoxazoles
(5a, 5b, and 6a–h) were tested in vitro for their antibacterial and antifungal
activities against Bacillus subtilis, Bacillus stearothermophilus, Escherichia coli,
and Pseudomonas putida. These compounds also were screened for their antifungal
activity against Aspergillus flavus and Aspergillus niger. Biological activity of these
compounds was compared with those of commercially available antibiotics,
chloramphenicol and antifungal agent cycloheximide. Most of these compounds, 5a,
5b, 6a, 6b, 6d, 6e, 6g, 6h, were equipotent or more potent than these commercial
drugs at concentration 100 lg/ml.
Keywords Iodobenzene diacetate ꢀ Schiff bases ꢀ 2-hetarylbenzoxazoles ꢀ
Antibacterial activity ꢀ Antifungal activity
Introduction
Organohypervalent iodine (III) reagents find wide utility in organic synthesis
(Varvoglis, 1981; Merkushev, 1987; Moriarty et al., 1990; Moriarty and Prakash,
1998, 2001; Prakash, 1995; Stang and Zhdankin, 1996; Koser, 2002; Zhdankin and
Stang, 2002; Prakash et al., 1994a, b) Many of these reactions lead to heterocyclic
compounds, such as benzoxazoles, thiazoles, and pyrazoles, which are associated
R. Kumar (&) ꢀ R. R. Nair ꢀ O. Prakash
Department of Chemistry, Kurukshetra University, Kurukshetra 136119, India
e-mail: ravi.dhamija@rediffmail.com
S. S. Dhiman ꢀ J. Sharma
Department of Biotechnology, Kurukshetra University, Kurukshetra 136119, India

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Med Chem Res (2010) 19:541–550
with significant biological properties (Grimmet et al., 1979; Deluca and Kerwin,
1997). Organoiodine (III)-mediated oxidative approach is preferred to other existing
oxidative methods because of the low toxicity, simple experimentation, and easy
handling of these reagents compared with other oxidants, such as DDQ (Chang
et al., 2002), PCC (Praveen et al., 2008). In view of the various biological
properties, such as cholesteryl ester transfer protein inhibitors (Harikrishnan et al.,
2008), transthyretin amyloloidogenesis inhibitors (Johnson et al., 2008) are
associated with 2-substituted benzoxazoles and encouraged by promising results
on the iodine (III)-mediated oxidative cyclization of Schiff’s bases (Prakash et al.,
1997, 2003, 2006). We synthesized some 2-aryl/hetarylbenzoxazoles (5a–5b and
6a–h) from Schiff bases (3a–3b and 4a–h) to evaluate their antifungal and
antibacterial activities.
Results and discussion
Chemistry
2-Aryl/hetaryl benzoxazoles were prepared from oxidation of Schiff’s bases with
IBD according to our recently disclosed procedure (Prakash et al., 1997, 2003,
2006). It has been found that the reaction between Schiff’s bases and 1.1 equivalent
of IBD in methanol occurs smoothly to afford the corresponding 2-substituted
benzoxazoles (5a, 5b, and 6a–h) (Scheme 1 and Table 1). The structures of Schiff
bases (3a, 3b and 4a–h) and 2-aryl/hetarylbenzoxazoles (5a–5b and 6a–h) were
elaborated by their spectral data and elemental analysis.
OH
OH
ArCHO
MeOH relux
N=CHAr
X
X
NH2
3,4
1,2
IBD/MeOH
X
N
Ar
O
5,6
2,4,6
X
1,3,5
X
Ar
Ar
2ꢀthienyl
Cl 3ꢀmethylꢀ2ꢀthienyl
a
b
c
d
e
Cl
a
H
H
2ꢀthienyl
5ꢀmethylꢀ2ꢀthienyl
b
5ꢀmethylꢀ2ꢀthienyl
3ꢀpyridyl
Cl
Cl
Cl
C H
6
5
f
Cl 4ꢀMeC H
6
4
g
Cl
4ꢀNO C H
2
6
4
Cl
3ꢀOMeꢀ4ꢀOHC H
h
6
3
Scheme 1

Med Chem Res (2010) 19:541–550
543
Table 1 In vitro antifungal
activity
Compound
Concentration
(lg/ml)
% Age inhibition
A flavus A niger
5a
5b
6a
6b
6c
6d
6e
6f
10 lg
50 lg
100 lg
10 lg
50 lg
100 lg
10 lg
50 lg
100 lg
10 lg
50 lg
100 lg
10 lg
50 lg
100 lg
10 lg
50 lg
100 lg
10 lg
50 lg
100 lg
10 lg
50 lg
100 lg
10 lg
50 lg
100 lg
10 lg
50 lg
100 lg
10 lg
50 lg
100 lg
21.05
63.15
88.47
24.31
51.61
87.99
21.05
47.36
84.24
21.05
36.84
84.21
13.74
24.92
37.47
15.78
26.31
78.94
29.04
51.42
86.23
20.11
43.23
66.91
19.94
49.61
78.54
29.04
51.42
86.23
15.78
63.15
78.94
41.71
64.70
82.35
04.89
05.23
16.64
04.89
5.88
11.76
17.47
47.05
76.49
12.44
63.87
71.88
17.64
41.71
76.47
08.93
05.88
21.86
04.19
07.47
21.38
43.19
66.93
73.62
08.93
05.88
21.86
11.76
23.52
35.29
6g
6h
Cycloheximide
Experimental
Chemical synthesis
Melting points were determined in open capillaries in electrical melting point
apparatus and are uncorrected. The IR (KBr) and 1HNMR spectra were recorded on
Buck Scientific IR M-500 and Bruker (300 MHz) spectrophotometers, respectively.

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Med Chem Res (2010) 19:541–550
The compounds 5b, 6a–d gave satisfactory analytical results (within 0.4 of the
theoretical values).
Schiff Bases 3a, 3b, and 4a–h
General procedure Schiff bases were prepared by refluxing 10 mmol of o-amino/
p-chloro-o-aminophenol and 15 mmol of appropriate aldehyde in methanol. The
Schiff bases precipitated on cooling the reaction mixture and were recrystallized
from methanol.
Characterization data of Schiff bases 3a, 3b, and 4a–d is given 3b: m.p. 60–61°C;
yield 50%; IR (KBr) cm-1 1589, 2930, 3343 (O–H) str. 1H NMR (CDCl3); d 2.50 (s,
3H, –CH3), 6.79–6.81 (m, 1H), 6.84–6.90 (m, 1H), 6.969–7.00 (dd, 1H,
J1 = 1.5 Hz, J2 = 8.1 Hz), 7.12–7.15 (m, 1H), 7.15–7.18 (m, 1H), 7.30–7.31 (m,
1H), 8.72 (s, 1H, = C–H).
1
4a: m.p. 90–91°C; yield 84%; IR (KBr) cm-1 1589, 2924, 3360 (O–H) str. H
NMR (CDCl3); d 6.93–6.96 (m, 1H), 7.13–7.27 (m, 4H), 7.55–7.58 (m, 2H), 8.82 (s,
1H, = C–H).
4b: m.p. 130–131°C; yield 89%; IR (KRr) cm-1 1589, 2930, 3327 (O–H) str. 1H
NMR (CDCl3); d 2.51 (s, 3H, –CH3), 6.92–6.97 (m, 1H), 7.12–7.16 (dd, 1H), 7.26–
7.28 (m, 2H), 7.45–7.49 (m, 1H), 8.71 (s, 1H, = C–H).
1
4c: m.p. 96–98°C; yield 65%; IR (KBr) cm-1 1590, 2925, 3357 (O–H) str. H
NMR (CDCl3); d 2.53 (s, 3H, –CH3), 6.83–6.94 (m, 2H), 7.10–7.13 (dd, 1H), 7.24–
7.36 (m, 2H), 8.63 (s, 1H, = C–H).
4d: m.p. 135–137°C; yield 87%; IR (KBr) cm-1 1571, 2900, 3333 (O–H) str. 1H
NMR (CDCl3); d 6.97–7.00 (d, 1H, J = 8.7 Hz), 7.19–7.22 (dd, 1H, J1 = 2.1 Hz,
J2 = 8.7 Hz), 7.45–7.49 (m, 1H), 8.28–8.29 (m, 1H), 8.72 (s, 1H, = C–H), 8.75–
8.76 (m, 1H), 9.07–9.09 (m, 1H).
4e: m.p. 81–82°C; lit m.p. 81–82°C (Prakash et al., 1997, 2006); yield 70%.
4f: m.p. 93–94°C; lit m.p. 92°C (Prakash et al., 1997, 2006); yield 76%.
4g: m.p. 184–185°C; lit m.p. 186°C (Prakash et al., 1997, 2006); yield 60%.
4h: m.p. 130–131°C; lit m.p. 133°C (Prakash et al., 1997, 2006); yield 63%.
2-aryl/hetarylbenzoxazoles (5a, 5b, and 6a–h)
General procedure To a solution of Schiff’ base (1 mmol) in 10 ml, methanol
was added IBD (1.1 mmol) and the mixture was stirred for 15–20 minutes. Solvent
was evaporated in vacuo to bring the volume to one-third. Crystalline solids,
separated out on keeping the reaction mixture at room temperature. Solids, thus
obtained, were filtered and washed with methanol followed by petroleum ether to
get 2-aryl/hetarylbenzoxazoles (5a, 5b, and 6a–h).
Characterization data of 2-aryl/hetarylbenzoxazoles (5a, 5b, and 6a–h) are
1
given: 5b: H NMR (CDCl3); d 2.51 (s, 3H, –CH3), 7.19–7.23 (m, 2H), 7.34–
7.37 (m, 2H), 7.55–7.59 (m,1H), 7.74–7.77 (m, 1H), 7.93–7.95 (m, 1H); 13C NMR

Med Chem Res (2010) 19:541–550
545
(CDCl3); d 15.63, 110.28, 119.60, 124.58, 126.74, 130.28, 142.23, 145.78, 152.24;
C12H9NOS; Found (C, 66.83; H, 4.14; N, 6.48%); Requires (C, 66.97; H, 4.18; N,
6.51%). m/z M? 215.
1
6a: H NMR (CDCl3); d 7.21–7.24 (dd, 1H, J1 = 6.9 Hz, J2 = 1.2 Hz), 7.31–
7.34 (dd, 1H, J1 = 6.9 Hz, J2 = 1.2 Hz), 7.43–7.50 (m, 1H), 7.60–7.62 (m, 1H),
7.93–7.95 (m, 1H); 13C NMR (CDCl3); d 111.10, 119.74, 125.30, 128.36, 129.10,
130.49, 143.20, 149.04, 160.32; C11H6NOSCl Found (C, 56.02; H, 2.44; N, 5.88%);
requires (C, 56.05; H, 2.54; N, 5.94%). m/z M? 235, 237.
6b: 1H NMR (CDCl3); d 2.51(s, 3H, –CH3), 7.02–7.03 (d, 1H, J = 5.1 Hz), 7.28–
7.38 (m, 1H), 7.46–7.49 (m, 3H), 7.73–7.74 (m, 1H); 13C NMR (CDCl3); d 15.68,
110.42, 119.82, 124.72, 125.07, 129.68, 130.23, 142.02, 150.45; C12H8NOSCl
Found (C, 57.62; H, 3.16; N, 5.58%); Requires (C, 57.71; H, .20; N, 5.61%). m/z
M? 249, 251.
1
6c: H NMR (CDCl3); d 2.52(s, 3H, –CH3), 6.85–6.86 (m, 1H), 7.26–7.29 (m,
2H), 7.42–7.45 (m, 1H), 7.71–7.73 (m, 1H); 13C NMR (CDCl3); d 15.70, 110.96,
119.52, 124.96, 126.90, 130.88, 143.23, 146.28, 150.24; C12H8NOSCl Found (C,
57.62; H, 3.16; N, 5.58%); Requires (C, 57.71; H, 3.20; N, 5.61; O, 6.41%). m/z M?
249, 251.
1
6d: H NMR (CDCl3); d 7.22–7.34 (m, 2H), 7.46–7.49 (m, 2H), 7.60–7.94 (m,
2H), 8.89–8.99 (m, 1H); 13C NMR (CDCl3); d 111.10, 119.74, 125.30, 128.37,
130.86, 149.04; C12H7N2OCl Found (C, 62.58; H, 2.99; N, 12.14%); Requires (C,
62.60; H, 3.04; N, 12.17%). m/z M? 230, 232.
Biological investigation
In vitro antifungal assay
Potato dextrose media (PDA media) was prepared in the flask and sterilized. Stock
solutions of 500 lg/ml of each compound were prepared in dimethylsulfoxide
(DMSO) and were appropriately diluted to get final concentration of 10, 50, and
100 lg/ml. A volume of 100 ll of each sample was poured in the well in PDA
media containing sterilized Petri plates. Mycelial discs taken from the standard
cultures (Aspergillus flavus and Aspergillus niger) of fungi were grown on PDA
medium for 5–7 days. These cultures were used for the purpose of inoculation in the
center sterilized Petri dish, aseptically. Standard cultures inoculated at 28 1°C
were used as the control. The efficacy of each sample was determined by measuring
radial mycelial growth. The radial growth of the colony was measured in two
directions at right angle to each other, and the average of two replicates was
recorded in each case. Data were expressed as percent inhibition over control from
the size of colonies. The percentage inhibition given in the table was calculated
using the formula:
% inhibition ¼ ðC ꢁ TÞ ꢂ 100=C
where C = diameter of fungus colony in the control plate after 96-hour incubation,
T = diameter of fungus colony in the tested plate after 96-hour incubation.

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Med Chem Res (2010) 19:541–550
In vitro antibacterial assay
Primary screening
The antibacterial activities of the newly synthesized compounds were evaluated by
Agar Well Diffusion Assay technique (NCCLS, 2000; McFarland, 1907; Green-
wood et al., 1997) against two gram-positive bacteria, i.e., Bacillus subtilis (MTCC
121) and Bacillus stearothermophilus (MTCC 8508), and two gram-negative
bacteria, i.e., E. coli (MTCC 51) and Pseudomonas putida. The bacterial cultures
were maintained on the nutrient agar media by subculturing them on the fresh slants
after every 4–6 weeks and incubating them at the appropriate temperature for 24
hours. All stock cultures were stored at 48C. For the evaluation of antimicrobial
activity of the synthetic compounds, suspension of each test microorganism was
prepared. Turbidity of each suspension was adjusted to 0.5 McFarland units by
suspending the cultures in sterile, distilled water. The size of final a volume of
20 ml of agar media was poured into each Petri plate and plates were swabbed with
broth cultures of the respective microorganisms and kept for 15 minutes for
adsorption to take place. Using a punch, &8-mm diameter well was bored in the
seeded agar plates and a 100-ll volume of each test compound reconstituted in
DMSO was added into the wells. DMSO was used as control for all the test
compounds. After holding the plates at room temperature for 2 hours to allow
diffusion of the compounds in to the agar, the palates were incubated at 378C for 24
hours. Antibacterial activity was determined by measuring the inhibition zone
diameter. The entire tests were made in triplicates and mean of the diameter of
inhibition was calculated. The antimicrobial activities of the compounds were
compared against the standard drugs. Inoculum was adjusted to 5x107 CFU/ml.
A volume of 20 ml of agar media was poured into each Petri plate and plates
were swabbed with broth cultures of the respective microorganisms and kept for 15
minutes for adsorption to take place. Using a punch, &8-mm diameter well was
bored in the seeded agar plates and a 100-ll volume of each test compound
reconstituted in DMSO was added into the wells. DMSO was used as control for all
the test compounds. After holding the plates at room temperature for 2 hours to
allow diffusion of the compounds in to the agar, the palates were incubated at 378C
for 24 hours. Antibacterial activity was determined by measuring the inhibition zone
diameter. The entire tests were made in triplicates and mean of the diameter of
inhibition was calculated. The antimicrobial activities of the compounds were
compared against the standard drugs.
Conclusions
All ten chemically synthesized compounds, 5a, b, 6a–h, were tested in vitro for
their antifungal activities against two fungi, A. flavus and A. niger, and antibacterial
activity against two gram-positive bacteria (B. subtilis, B. stearothermophilus) and
two gram-negative bacteria (E. coli, P. putida).

Med Chem Res (2010) 19:541–550
547
Most of these compounds possessed excellent in vitro antifungal activity against
A. flavus. Of these synthesized compounds, 5a, 6b, 6c, 6d, and 6g were active
against both fungi: A. flavus and A. niger. The activities of the compounds 5a–b,
6a–h also were compared with commercially available antibiotic, cycloheximide,
(Table 2; Fig. 1). It was found that 5a, 5b, 6a, 6b, 6e, and 6h were more potent than
cycloheximide against A. flavus, whereas 5a, 6b, 6c, 6d, and 6g were more active
than cycloheximide against A. niger. It also was found that compounds having
Table 2 Physical data of 2-aryl/hetarylbenzoxazoles (5a, 5b, 6a–h)
Compounds
M.pt (°C)
Lit. M.pt (°C)
Yield (%)
5a
5b
6a
6b
6c
6d
6e
6f
105–106
107–108
120–121
134–135
99–100
105–106 (Chang, et al., 2002)
70
65
75
76
60
63
62
60
65
55
–
–
–
–
194–195
101–102
135–136
234–238
185–186
–
101–102 (Prakash et al., 1997, 2006)
138 (Prakash et al., 1997, 2006)
240 (Prakash et al., 1997, 2006)
186 (Prakash et al., 1997, 2006)
6g
6h
100
90
80
70
60
50
40
30
20
10
0
A. flavus
A. niger
compounds
Fig. 1 In vitro antifungal assay of test compounds

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Med Chem Res (2010) 19:541–550
Table 3 In vitro antibacterial
activity of newly synthesized
compounds by using well
diffusion method
Compound
Concentration Diameter of zone of growth
inhibitiona (%)
(lg/ml)
BꢀS
B.St
E.C
P.put
5a
10
50
12.66 10.33 11.66 09.66
21.26 20.43 26.28 24.34
31.72 31.33 36.66 33.33
100
10
5b
–
–
12.45 13.47
50
11.33 12.66 21.74 22.33
17.33 19.66 34.29 35.45
19.66 21.33 17.74 19.66
30.53 32.66 31.77 33.66
41.66 39.66 43.33 44.77
10.34 12.66 10.33 12.12
21.67 23.44 24.56 25.67
29.33 31.84 33.17 35.06
100
10
6a
50
100
10
6b
50
100
10
6c
–
–
–
–
50
10.33 12.66 17.44 18.33
16.66 18.47 31.74 32.66
13.33 11.33 12.66 09.66
22.16 21.43 27.33 25.33
33.33 32.33 36.74 35.33
10.21 10.66 19.92 15.21
26.47 27.21 33.87 57.66
36.74 36.44 45.33 76.33
17.22 17.92 18.62 14.22
31.37 30.87 32.77 54.56
42.13 43.33 43.33 73.47
100
10
6d
50
100
10
6e
50
100
10
6f
50
100
10
6g
–
–
–
–
50
10.13 12.26 17.14 18.13
17.22 19.37 30.24 31.16
09.33 10.66 09.33 12.66
20.43 21.33 21.43 26.33
31.33 31.74 31.33 37.33
26.33 25.33 20.33 19.33
47.20 53.20 50.40 43.40
64.20 77.20 65.40 71.24
100
10
6h
B.S = Bacillus subtilis;
B.St = Bacillus
stearothermophilus;
50
100
10
Chloramphenicol
E.C = Escherichia coli;
P.put = Pseudomonas putida
50
a
Mean of three replicates—no
100
activity
thienyl or substituted thienyl (5a, 6a–6c) or nitro group 6g were found to possess
more antifungal activity than other benzoxazoles synthesized.
These benzoxazoles possessed good antibacterial activity against both gram-
positive (B. subtilis, B. stearothermophilus) and gram-negative bacteria (E. coli,
P. putida). Their activities were compared with commercial antibiotics (chloram-
phenicol). The compounds 6e, 6f were found to be more potent than chloramphenicol

Med Chem Res (2010) 19:541–550
549
B.s
B.st
E.coli
P.put
90
80
70
60
50
40
30
20
10
0
compounds
Fig. 2 In vitro antibacterial assay of test compounds
against P. putida (Table 3; Fig. 2). The five chlorobenzoxazoles showed activity
comparable with five unsubstituted derivatives.
Acknowledgments The authors thank DST (SR/FTP/CF-125/2006) for financial assistance and
Kurukshetra University, Kurukshetra, for the award of University Research Scholarship to Reshmi
R. Nair.
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