KMnO4/HOAc system promoted one-pot synthesis of benzoxazoles from o-aminophenols or oxidative cyclization of o-hydroxyarylidene anilines at room temperature

Article abstract of DOI:10.1007/s13738-015-0795-5

1,3-Benzoxazoles via oxidative cyclization of corresponding o-hydroxyarylidene anilines was synthesized in the presence of KMnO₄/HOAc system. This system also was applied for the one-pot synthesis of 1,3-benzoxazoles from o-amino phenols and aldehydes. The both protocols were processed at room temperature under solvent-free conditions with good to excellent yields.

Full text of DOI:10.1007/s13738-015-0795-5

J IRAN CHEM SOC
DOI 10.1007/s13738-015-0795-5
ORIGINAL PAPER
KMnO4/HOAc system promoted one‑pot synthesis
of benzoxazoles from o‑aminophenols or oxidative cyclization
of o‑hydroxyarylidene anilines at room temperature
B. F. Mirjalili1 · A. Bamoniri2 · E. Bagheri1
Received: 25 July 2015 / Accepted: 7 December 2015
© Iranian Chemical Society 2015
Abstract 1,3-Benzoxazoles via oxidative cyclization of
corresponding o-hydroxyarylidene anilines was synthe-
sized in the presence of KMnO4/HOAc system. This sys-
tem also was applied for the one-pot synthesis of 1,3-ben-
zoxazoles from o-amino phenols and aldehydes. The both
protocols were processed at room temperature under sol-
vent-free conditions with good to excellent yields.
heat-resistant polybenzoxazole nanofibers [15], 5-lipooxy-
genase inhibitor [16], (HIF)-1 transcriptional inhibitor and
[17] mPGES-1 inhibitors [18]. Some of biological active
benzoxazoles are shown in Scheme 1.
Classical protocols for the preparation of benzoxazoles
involved (1) one-pot reaction of carboxylic acids [19, 20],
aldehydes [20–22], orthoesters [23] or acid chlorides [24, 25]
with o-aminophenols (2) cyclization of o-haloanilides [26,
27], o-nitro phenyl esters [28], o-hydroxyarylidene anilines
[29–33] and o-hydroxyanilides [34], (3) flash vacuum pyroly-
sis of 2-methoxy-N-(arenylidene)anilines [35] and (4) copper-
catalyzed reaction of o-aryl dihalides with nitriles [36]. Oxi-
dative cyclization of o-hydroxyarylidene anilines or phenolic
Schiff’s bases was done using various oxidants such as hyper-
valent iodine [29], 2,3-dichloro-5,6-dicyanobenzo-1,4-qui-
none [30], barium manganite [32], silver (I) oxide [33], PCC/
silica [37]. One-pot synthesis of benzoxazoles from alde-
hydes and o-aminophenols was achieved in the presence of
acidic catalysts such as H5[PMo10V2O40] [20], iodine [21],
nanoceria [22], 4-methoxy-TEMPO/O2 [31], activated car-
bon/O2 [38], CN−/O2 [39], K2S2O8/CuSO4 [40], ABMs/O2
[41], Zn(OTf)2 [42] and Cu-np, K2CO3/O2 [43].
Keywords 1,3-Benzoxazoles · o-Hydroxyarylidene
anilines · KMnO4 · o-Amino phenol · Oxidative cyclization
Introduction
Benzoxazoles are one of the important structural motifs
in many biologically active compounds such as antibiot-
ics [1], antimicrobials [2], anticancers, anti-HIV-1s [3, 4],
antituberculous [5], antimycobacterials [6], anti-inflam-
matories, and analgesic and kinase (CDK-1, CDK-5 and
GSK-3) inhibitors [7]. This class of compounds allowed
an access to many demonstrated bio-active agents such as
5-HT3 receptor partial agonist for treatment of diarrhea-
predominant irritable bowel syndrome [8], melatonin
receptor agonists [9], 5-HT3 receptor partial agonists in
the gut [10], cyclooxygenase inhibitors [11], hypoglycemic
agents [12], precursors for in vivo imaging of β-amyloid
plaques [13], membranes for CO2 separation [14],
Experimental
General
Chemicals were purchased from Sigma–Aldrich and Merck
chemical companies and were used without any purifi-
cation. All products were characterized by their FT-IR,
1H-NMR and comparison of their physical properties
with those reported in the literature. FT-IR spectra were
recorded on a Bruker, Eqinox 55 spectrometer. In all cases,
* B. F. Mirjalili
fmirjalili@yazd.ac.ir
1
Department of Chemistry, College of Science, Yazd
University, P.O.Box 89195-741, Yazd, Iran
2
Department of Organic Chemistry, Faculty of Chemistry,
1
the H-NMR spectra were recorded on a Bruker DRX-400
University of Kashan, Kashan, Iran
1 3

J IRAN CHEM SOC
One‑pot preparation of 2‑aryl‑1,3‑benzoxazoles
In a mortar, aldehyde (1 mmol), 2-aminophenol (1 mmol),
KMnO4 (3.4 mmol) and HOAc (0.08 mL) were charged
and ground for 10 min. The progress of reaction was moni-
tored by TLC (ethyl acetate:n-hexane, 20:80). Then, ace-
tone (5 mL) was added to the mortar and filtered the mix-
ture. By adding water to filtrate, 2-aryl-1,3-benzoxazole
was appeared as a pure solid.
Some spectroscopic data
2‑(4‑Nitrophenyl)‑1,3‑benzoxazole
Scheme 1 The structure of some benzoxazole containing drugs
FT-IR (ATR) = ῡ, cm−1: 3112 (C–H), 1598 (C=N), 1555,
1348 (N=O), 1519, 1451 (C=C).
instrument. The elemental analyses was done by Costech
ECS 4010 CHNS-O analyzer. Melting points were deter-
mined by a Buchi melting point B-540 B.V.CHI apparatus.
1H NMR (400 MHz, CDCl3): δ, ppm (J, Hz): 8.44 (sbr,
4H), 7.83 (sbr, 1H), 7.65 (sbr, 1H), 7.44 (sbr, 2H).
2‑(3‑Nitrophenyl)‑1,3‑benzoxazole
Preparation of 2‑aryl‑1,3‑benzoxazoles
from o‑hydroxyarylidene anilines
FT-IR (ATR) = ῡ, cm−1: 3097 (C–H), 1613 (C=N), 1525,
1351 (N=O), 1474, 1453 (C=C), 1241 (C–O).
In a mortar, o-hydroxyarylidene aniline (1 mmol), KMnO4
(1.7 mmol) and HOAc (0.08 mL) were charged and ground
for 5 min. The progress of reaction was monitored by TLC
(ethyl acetate:n-hexane, 20:80). Then, acetone (5 mL) was
added to the mortar and filtered the mixture. By adding
water to filtrate, 2-aryl-1,3-benzoxazole was appeared as a
pure solid.
1H NMR (400 MHz, acetone): δ, ppm (J, Hz): 8.97 (sbr, 1H),
8.77 (sbr, 1H), 8.64 (sbr, 1H), 7.7-8.0 (m, 3H), 7.47 (sbr, 2H).
2‑(2‑Nitrophenyl)‑1,3‑benzoxazole
FT-IR (ATR) = ῡ, cm−1: 1620 (C=N), 1534, 1349 (N=O),
1452 (C=C), 1240 (C–O).
Table 1 Synthesis of 2-(aryl)-benzoxazole from o-hydroxyarylidene anilines (1 mmol) under various conditions
Entry
X
Y
Oxidant
Reactant/oxidant
Acid (mL)
Solvent
Time (h)
Yield (%)* References
1
NO2
NO2
NO2
NO2
NO2
NO2
NO2
NO2
NO2
H
H
H
H
H
H
H
H
H
H
H
H
H
Me
–
1/−
1/1.5
1/1.5
1/1.7
1/2
HOAc (0.08)
–
0.5
0.5
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0.5
14
5
2
KMnO4
KMnO4
KMnO4
KMnO4
KMnO4
KMnO4
KMnO4
PhI (OAc)2
DDQ
–
–
20
3
HOAc (0.08)
–
83
4
HOAc (0.08)
–
91
5
HOAc (0.08)
–
91
6
1/1.7
1/1.7
1/1.7
1/1.1
–
HOAc (0.06)
–
76
7
HOAc (0.07)
–
85
8
HOAc (0.09)
–
91
9
–
–
–
–
–
CH3CN
CH2Cl2
CHCl3
CH2Cl2
CH2Cl2
93 [29]
93 [30]
72 [32]
76 [33]
89 [37]
10
11
12
13
NO2
H
BaMnO4
Ag2O
1/10
–
2–5
0.18
NO2
PCC/Silica
1/1.1
* Isolated yield
1 3

J IRAN CHEM SOC
Table 2 Synthesis of benzoxazoles via oxidative cyclization of o-hydroxyarylidene anilines
Entry
X
Y
Yield (%)*
m.p (°C)
Observed
References
Reported
1
H
H
4-NO2
3-NO2
2-NO2
4-Cl
92
87
90
85
88
0
259–261
196–198
204–205
143–146
119–120
–
257–263
[44]
[47]
2
207
3
H
–
4
H
143–145
[46]
[45]
–
5
H
2,4-Cl2
2,6-Cl2
4-Br
118–119
6
H
–
7
H
91
68
83
79
81
78
144–146
189–191
220–222
123–125
177–179
148–150
142–144
[46]
–
8
H
4-COOMe
4-NO2
3-NO2
4-Cl
–
9
Cl
Cl
Cl
Cl
–
–
10
11
12
184–186
[46]
–
–
–
2,4-Cl2
–
* 1 mmol phenolic schiff’s bases, 1.7 mmol KMnO4, 0.08 ml HOAc, solvent free, room temperature, 5 min
** Isolated yields
Table 3 One-pot synthesis of 2-(aryl)-benzoxazole under various conditions
Entry
X
Y
Oxidant
Reactant/catalyst
(mmol)
Acid (mL)
Solvent
T/°C
Time (h)
Yield (%)
1
NO2
NO2
NO2
NO2
NO2
NO2
NO2
NO2
NO2
NO2
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
–
1/−
HOAc (0.08)
–
R.T.
R.T.
R.T.
R.T.
R.T.
R.T.
R.T.
R.T.
R.T.
R.T.
Reflux
M.W.
R.T.
120
120
80
0.5
0.5
0.15
0.15
0.15
0.15
0.15
0.15
0.15
0.15
6.5
0.05
0.3
6
10
2
KMnO4
1/1.7
1/1.7
1/2.5
1/3
–
–
22
3
KMnO4
HOAc (0.08)
–
65
4
KMnO4
HOAc (0.08)
–
71
5
KMnO4
HOAc (0.08)
–
79
6
KMnO4
1/3.4
1/3.8
1/3.4
1/3.4
1/3.4
1/0.5
1/0.5
1/0.05
1/0.05
1/0.125
1/1
HOAc (0.08)
–
82
7
KMnO4
HOAc (0.08)
–
82
8
KMnO4
HOAc (0.06)
–
74
9
KMnO4
HOAc (0.07)
–
78
10
11
12
13
14
15
16
17
18
19
20
KMnO4
HOAc (0.09)
–
82
H5[PMO10V2O40]
Iodine
–
–
–
–
–
–
–
–
–
–
THF
–
88 [20]
95 [21]
98 [22]
91 [31]
78 [38]
78 [39]
90 [40]
92 [41]
91 [42]
75 [43]
Cl
NO2
NO2
H
Nanoceria
4-Methoxy-TEMPO/O2
Activated carbon/O2
CN−/O2
H2O
Xylene
Xylene
DMF
SDS/water
Toluene
Ethanol
MeOH
4
H
4
H
K2S2O8/CuSO4
ZnBr2/ABM/air
Zn(OTf)2
1/0.01
1/100 mg
1/0.01
1/0.01
60
0.83
0.3
5
NO2
H
110
80
NO2
Cu–np, K2CO3/O2
80
4.6
* o-Aminophenol (1 mmol) and aldehyde (1 mmol) were used
1 3

J IRAN CHEM SOC
1H NMR (400 MHz, CDCl3): δ, ppm (J, Hz): 8.43–8.45
(m, 4H), 8.15 (d, J = 7.5 Hz, 1H), 7.90 (d, J = 7.5 Hz, 1H),
7.82 (sbr, 1H), 7.75–7.71 (m, 2H), 7.58 (sbr, 1H), 7.4 (sbr,
1H).C13H8N2O3, calculated: C, 65.00; H, 3.36; N, 11.66.
Found: C, 64.6; H, 3.39; N, 11.67 %.
1H NMR (400 MHz, acetone): δ, ppm (J, Hz): 8.25 (sbr,
1H), 7.8 (m, 3H), 8.64 (sbr, 1H), 7.7 (sbr, 1H), 7.45 (sbr,
2H).
2‑(4‑Bromophenyl)‑1,3‑benzoxazole
2‑(4‑Chlorophenyl)‑1,3‑benzoxazole
FT-IR (ATR) = ῡ, cm−1: 3057(C–H), 1616 (C=N), 1592,
1483 (C=C), 1294 (C–N), 1243 (C–O), 1068 (C–Br).
1H NMR (400 MHz, DMSO-d6): δ, ppm (J, Hz): 8.62
(sbr, 2H), 8.24 (sbr, 4H), 7.88 (sbr, 2H).
FT-IR (ATR) = ῡ, cm−1: 3060 (C–H), 1617 (C=N), 1574,
1483 (C=C), 1344 (C–N), 1244 (C–O), 1091 (C–Cl).
1H NMR (400 MHz, acetone): δ, ppm (J, Hz): 8.26 (sbr,
2H), 7.69–7.77 (m, 3H), 8.64 (sbr, 1H), 7.43–7.5 (m, 3H).
2‑(4‑Metoxycarbonyl phenyl) 1,3‑benzoxazole
2‑(2,4‑Dichlorophenyl)‑1,3‑benzoxazole
FT-IR (ATR) = ῡ, cm−1: 3163 (C–H), 2955 (C–H), 1726
(C=O), 1605 (C=N), 1557, 1454 (C=C), 1276 (C–O).
1H NMR (400 MHz, DMSO-d6): δ, ppm (J, Hz): 8.71
(sbr, 4H), 8.23 (sbr, 2H), 7.92 (sbr, 2H), 4.35 (s, 3H).
C15H11NO3, calculated: C, 71.14; H, 4.38; N, 5.53. Found:
C, 70.64; H, 4.42; N, 5.89 %.
FT-IR (ATR) = ῡ, cm−1: 3066 (C–H), 1590 (C=N), 1560,
1471 (C=C), 1375 (C–N), 1191 (C–O), 1027 (C–Cl).
Table 4 One-pot synthesis of 2-(aryl)-benzoxazoles in the presence
of KMnO4/HOAc system at room temperature
5‑Chloro, 2‑(4‑nitrophenyl)‑1,3‑benzoxazole
FT-IR (ATR) = ῡ, cm−1: 3093 (C–H), 1602 (C=N), 1552,
1348 (N=O), 1519, 1453 (C=C), 1285 (C–O), 1063
(C–Cl).
Entry
X
Y
Yield (%)
References
1H NMR (400 MHz, acetone): δ, ppm (J, Hz): 8.49 (sbr,
4H), 7.8–8.0 (m, 2H), 7.5 (sbr, 1H).
1
2
3
4
5
6
7
H
H
H
H
H
Cl
Cl
4-NO2
3-NO2
4-Cl
84
76
73
68
80
79
71
[44]
[47]
[46]
[45]
[46]
–
C13H7ClN2O3, calculated: C, 56.85; H, 2.57; N, 10.20.
Found: C, 56.75; H, 2.62; N, 10.25 %.
2,4-Cl2
4-Br
4-NO2
3-NO2
5‑Chloro, 2‑(3‑nitrophenyl)‑1,3‑benzoxazole
[46]
FT-IR (ATR) = ῡ, cm−1: 3093 (C–H), 1621 (C=N), 1527,
1353 (N=O), 1449 (C=C), 1195 (C–O), 1101 (C–Cl).
* The ratio of o-aminophenol (mmol):aldehyde (mmol):KMnO4
(mmol):HOAc (mL) is equal to 1:1:3.4:0.08
Scheme 2 A proposed mecha-
nism for synthesis of benzo-
xazoles using KMnO4/HOAc
system
1 3

J IRAN CHEM SOC
1H NMR (400 MHz, acetone): δ, ppm (J, Hz): 9.0 (sbr,
1H), 8.77 (sbr, 1H), 8.5 (sbr, 1H), 7.8 (sbr, 1H), 7.9 (sbr,
1H), 8.0 (sbr, 2H).
Our proposed mechanism for 2-(aryl)-benzoxazoles syn-
thesis is shown in Scheme 2. By addition oxygen of alde-
hyde carbonyl group to KMnO4, addition of 2-aminophenol
to aldehyde is accelerated and finally dihydrobenzoxazole
is formed. The resulted HMnO4 oxidize dihydrobenzoxa-
zole to benzoxazole.
5‑Chloro, 2‑(4‑chlorophenyl)‑1,3‑benzoxazole
FT-IR (ATR) = ῡ, cm−1: 3066 (C–H), 1611 (C=N), 1550,
1481 (C=C), 1332 (C–N), 1261 (C–O), 1089 (C–Cl).
1H NMR (400 MHz, acetone): δ, ppm (J, Hz): 8.3
(sbr, 2H), 7.6–7.8 (m, 4H), 8.64 (sbr, 1H), 7.5 (sbr, 1H).
C13H7Cl2NO, calculated: C, 59.12; H, 2.67; N, 5.30.
Found: C, 58.92; H, 2.45; N, 5.72 %.
In conclusion, KMnO4/HOAc system promotes synthe-
sis of 2-(aryl)-benzoxazoles at room temperature under sol-
vent-free condition. High to excellent yields, short reaction
times, easy workup and low cost are some advantages of
this protocol.
Acknowledgments The Research Council of Yazd University is
gratefully acknowledged for financial support of this work.
5‑Chloro, 2‑(2,4‑dichlorophenyl)‑1,3‑benzoxazole
FT-IR (ATR) = ῡ, cm−1: 3092 (C–H), 1586 (C=N), 1561,
1448 (C=C), 1388 (C–N), 1257 (C–O), 1092 (C–Cl).
1H NMR (400 MHz, CDCl3): δ, ppm (J, Hz): 8.25 (sbr,
1H), 7.81 (sbr, 1H), 7.4–7.5 (m, 2H), 7.01–7.12 (m, 2H),
ppm. C13H6Cl3NO, calculated: C, 52.30; H, 2.03; N, 4.69.
Found: C, 52.15; H, 2.39; N, 4.48 %.
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