Synthesis of 2-styrylbenzoxazole derivatives by the reaction of styrylphenolic schiff bases with thianthrene cation radical

Article abstract of DOI:10.1002/jhet.1707

A simple and fast preparative method of 2-styrylbenzoxazoles by oxidative intramolecular cyclization of styrylphenolic Schiff bases with thianthrene cation (Th^+.ClO₄^-) is described. The oxidative cyclization of Schiff bases in the presence of 2,6-di-tert-butyl-4- methylpyridine (DTBMP) gives 2-styrylbenzoxazole derivatives in better yields than those in the absence of DTBMP.

Full text of DOI:10.1002/jhet.1707

May 2013
Synthesis of 2-Styrylbenzoxazole Derivatives by the Reaction of
Styrylphenolic Schiff Bases with Thianthrene Cation Radical
663
a
b
c
c
*
Hee Jung Park, Myeong Soon Park, Tae Hee Lee, and Koon Ha Park
aKorea Basic Science Institute, Ochang, Chungbuk, 363-883, Korea
bHanchem Company, Yuseong-gu, Daejeon, 305-510, Korea
cDepartment of Chemistry, Chungnam National University, Yuseong-gu, Daejeon, 305-764, Korea
*
E-mail: khpark@cnu.ac.kr
Received February 16, 2012
DOI 10.1002/jhet.1707
Published online in Wiley Online Library (wileyonlinelibrary.com).
A simple and fast preparative method of 2-styrylbenzoxazoles by oxidative intramolecular cyclization of
styrylphenolic Schiff bases with thianthrene cation (Th+.ClO4À) is described. The oxidative cyclization of Schiff
bases in the presence of 2,6-di-tert-butyl-4-methylpyridine (DTBMP) gives 2-styrylbenzoxazole derivatives in
better yields than those in the absence of DTBMP.
J. Hetercyclic Chem., 50, 663 (2013).
INTRODUCTION
with 2,6-di-tert-butyl-4-methylpyridine (DTBMP) afforded
2-styrylbenzoxazole 2, corresponding cinnamaldehyde,
thianthrene (Th), and its 5-oxide (ThO) as shown in
Scheme 1.
2-Styrylbenzoxazoles have attracted interests for their
fluorescent and biological significance. For instance, 2-
styrylbenzoxazoles have been utilized as invisible markings
and brightening agents for polymeric organic materials
such as synthetic fibres and resin masses [1], and as
potent drugs [2]. Because of their diverse applications in
industry, many methods are documented in the literature
for the synthesis of 2-styrylbenzoxazoles, including the
condensation of 2-aminophenol and carboxylic acid [3],
cyclization of corresponding Schiff base with an oxidant [4],
Beckmann rearrangement of oximes [5], condensation of 2-
methylbenzoxazole with aldehydes [6], and o-dihalobenzene
and primary amides [7]. However, many of these methods
suffer from drawbacks due to long reaction times and high
temperatures. Herein, we have employed thianthrene
cation radical perchlorate (Th+.ClO4À) as an oxidant [8]
by which 2-styrylbenzoxazole is efficiently obtained from
styrylphenolic Schiff bases 1 under mild conditions.
Although the reactions were complete generally within
5–10min, the reaction was done after further stirring over-
night by extracting the neutralized aqueous solution with
dichloromethane. Each product was analysed quantitatively
by gas chromatography (GC). Considering the general
reaction conditions reported in the literature, that is, several
hours of heating (80–100ꢀC), the very fast reactions of 1 and
Th+.ClOÀ4 at room temperature in good yields (52–99%) are
noteworthy. The other products observed in these reactions
were Th as a redox partner and ThO. In all of the reactions,
formation of ThO is not related to the main reaction but
stems from the hydrolysis of Th+.ClO4À due to either adven-
titious water or water added during the work-up procedure.
The results from 12 reactions are shown in Table 1.
The addition of DTBMP was required to prevent aldehyde
formation in all cases and accordingly to make higher yields
of 2-styrylbenzoxazole products.
RESULTS AND DISCUSSION
In the absence of DTBMP, the reactions were sluggish
with much lower yields (16–47%) as shown in Table 2. In
these reactions, formation of aldehydes in appreciable
The reaction under argon of styrylphenolic Schiff base 1
and Th+.ClOÀ4 (2 equiv) at room temperature in acetonitrile
© 2013 HeteroCorporation

664
H. J. Park, M. S. Park, T. H. Lee, and K. H. Park
Vol 50
Scheme 1
more electron rich alkenyl groups gave lower yields, that
is, when R2 was electron-donating group.
According to our previous studies [8], we assumed that
phenoxyl radical might be the first formed intermediate by
the reaction of 1 with Th+.ClOÀ4 . The intermediate phenoxyl
radical with an electron-donating group might react
intramolecularly with adjacent double bond, C═N group
and a-styryl carbon.
CONCLUSIONS
A mild and convenient method for the synthesis of
2-styrylbenzoxazole derivatives has been described. The
reaction proceeded in good yields by the oxidative
intramolecular cyclization of styrylphenolic Schiff bases
1 with thianthrene cation radical perchlorate (Th+.ClO4À)
in the presence of DTBMP in acetonitrile.
amounts is obviously caused by the hydrolysis of substrates
by liberated acid (HClO4).
EXPERIMENTAL
The oxidative intramolecular cyclization reaction of
2-styrylphenolic Schiff bases has shown that the use of
Acetonitrile was purified by distillation over phosphorus
pentoxide under argon prior to use. Thianthrene was recrystallized
Table 1
Yielda (%) of products in the reactions of thianthrene cation radical perchlorate (Th+.ClOÀ4 ) with 1 in the presence of DTBMP.
Compound
R1
R2
2
Th
DTBMP
ThO
a
b
c
d
e
f
g
h
i
H
H
H
H
H
NO2
NO2
NO2
NO2
OCH3
OCH3
OCH3
OCH3
85
69
52
95
99
90
70
84
61
58
54
66
97
97
98
99
97
97
98
96
98
97
96
96
90
89
88
85
90
84
86
85
86
83
80
82
3
3
2
1
3
3
2
4
2
3
4
4
CH3
t-Butyl
Cl
H
CH3
t-Butyl
Cl
H
j
k
l
CH3
t-Butyl
Cl
aThe yield was quantitatively determined by GC.
Table 2
Yielda (%) of products in the reactions of thianthrene cation radical perchlorate (Th+.ClOÀ4 ) with 1 in the absence of DTBMP.
Compound
R1
R2
1
2
Th
Aldehyde
ThO
a
b
e
i
j
k
l
H
CH3
H
H
H
NO2
OCH3
OCH3
OCH3
OCH3
20
20
12b
0
0
0
34
22
47
26
16
18
24
81
84
92
77
85
84
83
24
20
24
24
16
24
16
1
2
0
0
1
1
2
H
CH3
t-Butyl
Cl
0
aThe yield was quantitatively determined by GC.
b2-Aminophenol.
Journal of Heterocyclic Chemistry
DOI 10.1002/jhet

May 2013
2-Styrylbenzoxazoles from Styrylphenolic Schiff Bases and Thianthrene Cation Radical Perchlorate 665
twice from acetone. Aldehydes and aminophenols were used
without further purification.
4-t-Butyl-2-[[3-(2-nitrophenyl)-2-propenylidene]amino]phenol
(1g). Yield; 72%, yellow crystal with mp 147À149ꢀC. 1H NMR
(300MHz, DMSO-d6): d 8.89 (s, 1H, OH), 8.63 (d, J = 8.8 Hz, 1H,
N═CH), 8.02 (d, J = 8.1 Hz, 1H, Ar-H), 7.99 (d, J = 7.7 Hz, 1H,
Ar-H), 7.77 (dd, J = 7.7 Hz, 7.4 Hz, 1H, Ar-H), 7.62
(dd, J = 8.1 Hz, 7.4 Hz, 1H, Ar-H), 7.60 (d, J = 15.8 Hz, 1H,
CH), 7.21À7.14 (dd, J = 15.8 Hz, 8.8 Hz, 1H, CH), 7.13 (s, 1H,
Ar-H), 7.09 (d, J = 8.4 Hz, 1H, Ar-H), 6.80 (d, J =8.4 Hz, 1H,
Ar-H), 1.26 (s, 9H, CH3).
4-Chloro-2-[[3-(2-nitrophenyl)-2-propenylidene]amino]phenol
(1h). Yield; 87%, mp 184À185ꢀC. 1H NMR (300 MHz, DMSO-d6): d
9.44 (s, 1H, OH), 8.55 (d, J = 8.8 Hz, 1H, N═CH), 8.04À7.99
(dd, J = 8.1Hz, 7.7 Hz, 2H, Ar-H), 7.77 (dd, J = 7.6 Hz, 7.5 Hz,
1H, Ar-H), 7.62 (dd, J = 8.1 Hz, 7.5 Hz, 1H, Ar-H), 7.65À7.59
(d, J = 15.8 Hz, 1H, CH), 7.20À7.11 (m, 2H, Ar-H, CH), 7.09
(d, J = 8.6 Hz, 1H, Ar-H), 6.87 (d, J = 8.6 Hz, 1H, Ar-H). Anal.
Calcd for C15H11ClN2O3: C, 59.60; H, 3.64; N, 9.27. Found:
C, 59.34; H, 3.62; N, 9.06.
Thianthrene cation radical perchlorate (Th+.ClO4). The
known procedure [9] was adopted as follows. To a solution of
perchloric acid (70%, 0.60 mL) in acetic anhydride (33 mL)
was added a solution of thianthrene (1.0 g, 4.6 mmol) in carbon
tetrachloride (66 mL). The reaction mixture was then allowed to
stand for 24 h in the dark at room temperature. Dark purple coloured
crystals were formed and were collected by filtration and washed
with carbon tetrachloride until the filtrate was colourless. The crystals
were then dried in a flask under vacuum. The product (1.2 g,
3.9 mol, 84%) was dried under vacuum for short period before use.
General procedure for the preparation of styrylphenolic
Schiff bases (1). To a stirred solution of 2-aminophenol
(20mmol) in ethanol (20mL) was added dropwise a solution of
trans-cinnamaldehyde (20mmol) in ethanol (20mL) at room
temperature. After stirring for 2 h, the compound was obtained as
yellow crystals (ethanol).
2-[(3-Phenyl-2-propenylidene)amino]phenol (1a). Yield; 76%,
2-[[3-(2-Methoxyphenyl)-2-propenylidene]amino]phenol (1i).
Yield; 81%, mp 76À78ꢀC. 1H NMR (300 MHz, DMSO-d6): d 8.92
(s, 1H, OH), 8.45 (d, J=8.9Hz, 1H, N═CH), 7.66 (d, J=7.7Hz,
1H, Ar-H), 7.51 (d, J= 16.1 Hz, 1H, CH), 7.36 (t, J=7.7Hz, 1H,
Ar-H), 7.19 (dd, J= 16.1 Hz, 8.9 Hz, 1H, CH), 7.11À6.99
(m, 4H, Ar-H), 6.85À6.78 (m, 2H, Ar-H), 3.86 (s, 3H, OCH3).
Anal. Calcd for C16H15NO2: C, 75.89; H, 5.93; N, 5.53. Found:
C, 75.60; H, 5.99; N, 5.41.
1
mp 90–92ꢀC (lit. mp 92ꢀC [10]). H NMR (300MHz, CDCl3): d
8.49 (d, J = 7.8 Hz, 1H, N═CH), 7.55 (d, J = 8.1 Hz, 1H, Ar-H),
7.42À7.38 (m, 5H, Ar-H), 7.26À7.13 (m, 4H, Ar-H, OH), 7.02
(d, J = 8.1Hz, 1H, CH), 6.90 (t, J = 7.7 Hz. 1H, CH).
4-Methyl-2-[(3-phenyl-2-propenylidene)amino]phenol (1b).
1
Yield; 74%, mp 76À77ꢀC. H NMR (300MHz, CDCl3): d 8.47
(d, J = 7.7Hz, 1H, N═CH), 7.57À7.35 (m, 5H, Ar-H, CH),
7.27À6.90 (m, 6H, Ar-H, CH, OH), 2.32 (s, 3H, CH3). Anal.
Calcd for C16H15NO: C, 81.01; H, 6.33; N, 5.91. Found:
C, 80.88; H, 6.46; N, 5.89.
4-Methyl-2-[[3-(2-methoxyphenyl)-2-propenylidene]amino]
1
phenol (1j). Yield; 96%, mp 109À110ꢀC. H NMR (300MHz,
DMSO-d6): d 8.67 (s, 1H, OH), 8.45 (d, J = 8.9 Hz, 1H, N═CH),
7.65 (d, J = 7.7 Hz, 1H, Ar-H), 7.50 (d, J = 16.1Hz, 1H, CH), 7.35
(dd, J = 7.7 Hz, 7.8Hz, 1H, Ar-H), 7.15 (dd, J = 16.1 Hz, 8.9 Hz,
1H, CH), 7.18À7.06 (m, 1H, Ar-H), 6.99 (t, J = 7.8 Hz, 1H,
Ar-H), 6.92 (s, 1H, Ar-H), 6.83 (d, J = 8.1 Hz, 1H, Ar-H), 6.72
(d, J = 8.1 Hz, 1H, Ar-H), 3.86 (s, 3H, OCH3), 2.19 (s, 3H,
CH3). Anal. Calcd for C17H17NO2: C, 76.40; H, 6.37; N, 5.24.
Found: C, 76.27; H, 6.51; N, 5.13.
4-tert-Butyl-2-[(3-phenyl-2-propenylidene)amino]phenol (1c).
Yield; 76%, mp 107À108ꢀC. 1H NMR (300 MHz, CDCl3): d 8.51
(d, J = 8.2Hz, 1H, N═CH), 7.58À7.38 (m, 5H, Ar-H, CH),
7.27À7.09 (m, 5H, Ar-H, CH), 6.95 (d, J = 8.2 Hz, 1H, Ar-H),
1.36 (s, 9H, CH3). Anal. Calcd for C19H21NO: C, 81.72; H, 7.53;
N, 5.02. Found: C, 81.23; H, 7.66; N, 4.83.
4-Chloro-2-[(3-phenyl-2-propenylidene)amino]phenol (1d).
1
Yield; 63%, mp 105À107ꢀC. H NMR (300 MHz, DMSO-d6):
4-tert-Butyl-2-[[3-(2-methoxyphenyl)-2-propenylidene]amino]
phenol (1k). Yield; 55%, mp 93À94ꢀC. 1H NMR (300 MHz,
DMSO-d6): d 8.71 (s, 1H, OH), 8.53 (d, J=8.9Hz, 1H, N═CH),
7.66 (d, J= 7.7 Hz, 1H, Ar-H), 7.54 (d, J= 16.1 Hz, 1H, CH), 7.36
(dd, J= 7.7 Hz, 7.9 Hz, 1H, Ar-H), 7.16 (dd, J= 16.1 Hz, 8.9 Hz,
1H, CH), 7.19À6.97 (m, 4H, Ar-H), 6.76 (d, J= 8.3 Hz, 1H, Ar-H),
3.86 (s, 3H, OCH3), 1.25 (s, 9H, CH3). Anal. Calcd for
C20H23NO2: C, 77.63; H, 7.44; N, 4.53. Found: C, 77.62; H, 7.61;
N, 4.52.
4-Chloro-2-[[3-(2-methoxyphenyl)-2-propenylidene]amino]
phenol (1l). Yield; 45%, mp 79À80ꢀC. 1H NMR (300 MHz,
DMSO-d6): d 9.26 (s, 1H, OH), 8.45 (d, J = 8.9 Hz, 1H,
N═CH), 7.66 (d, J = 7.7 Hz, 1H, Ar-H), 7.55 (d, J = 16.1 Hz,
1H, CH), 7.37 (dd, J = 7.7 Hz, 7.8 Hz, 1H, Ar-H), 7.18À6.97
(m, 5H, Ar-H, CH), 6.84 (d, J = 8.6 Hz, 1H, Ar-H), 3.86
(s, 3H, OCH3). Anal. Calcd for C16H14ClNO2: C, 66.90;
H, 4.88; N, 4.88. Found: C, 66.43; H, 4.84; N, 4.74.
General procedure for the synthesis of 2-styrylbenzoxazole
by thianthrene cation radical. Modified procedure [9] was
adopted as follows. The reaction was carried out with thianthrene
cation radical perchlorate (1.0 mmol), 1 (0.5mmol), and DTBMP
(1.50 mmol) in acetonitrile under argon at room temperature.
Generally, the colour of the thianthrene cation radical disappeared
in 5–10 min. The reaction mixture was neutralized using aqueous
sodium bicarbonate solution, followed by extraction of products
using dichloromethane.
d 9.35 (s, 1H, OH), 8.43 (d, J = 8.8 Hz, 1H, N═CH), 7.64
(m, 2H, Ar-H, CH), 7.45À7.33 (m, 4H, Ar-H, CH), 7.15
(d, J = 8.8 Hz, 1H, Ar-H), 7.12À7.09 (m, 1H, Ar-H), 7.06
(d, J = 8.5 Hz, 1H, Ar-H), 6.87 (d, J = 8.5 Hz, 1H, Ar-H). Anal.
Calcd for C15H12ClNO: C, 70.04; H, 4.67; N, 5.45. Found:
C, 69.64; H, 4.64; N, 5.28.
2-[[3-(2-Nitrophenyl)-2-propenylidene]amino]phenol (1e).
Yield; 90%, mp 126À128ꢀC. 1H NMR (300MHz, DMSO-d6):
d 9.10 (s, 1H, OH), 8.54 (d, J = 8.8 Hz, 1H, N═CH), 8.03
(d, J = 8.1 Hz, 1H, Ar-H), 7.99 (d, J = 7.9 Hz, 1H, Ar-H), 7.76
(dd, J =8.1 Hz, 7.8Hz, 1H, Ar-H), 7.63 (dd, J = 7.9 Hz, 7.8 Hz,
1H, Ar-H), 7.60 (d, J = 15.8Hz, 1H, CH), 7.15 (dd, J = 15.8Hz,
8.8 Hz, 1H, CH), 7.11 (d, J =7.7 Hz, 1H, Ar-H), 7.03
(dd, J = 8.1Hz, 7.6 Hz, 1H, Ar-H), 6.88 (d, J = 8.1 Hz, 1H, Ar-H),
6.80 (dd, J = 7.7 Hz, 7.6 Hz, 1H, Ar-H).
4-Methyl-2-[[3-(2-nitrophenyl)-2-propenylidene]amino]phenol
1
(1f). Yield; 93%, mp 151À153ꢀC. H NMR (300 MHz, DMSO-d6):
d 8.85 (s, 1H, OH), 8.54 (d, J = 8.8 Hz, 1H, N═CH), 8.03
(d, J = 8.1 Hz, 1H, Ar-H), 8.00 (d, J = 7.9 Hz, 1H, Ar-H), 7.76
(dd, J = 8.1 Hz, 7.8 Hz, 1H, Ar-H), 7.63 (dd, J = 7.9 Hz, 7.8 Hz,
1H, Ar-H), 7.56 (d, J = 15.8 Hz, 1H, CH), 7.20À7.12
(dd, J = 15.8 Hz, 8.8 Hz, 1H, CH), 6.96 (s, 1H, Ar-H), 6.86
(d, J = 8.1 Hz, 1H, Ar-H), 6.76 (d, J = 8.1 Hz, 1H, Ar-H), 2.20
(s, 3H, CH3). Anal. Calcd for C16H14N2O3: C, 68.03; H, 4.96;
N, 9.93. Found: C, 67.94; H, 4.96; N, 9.82.
Journal of Heterocyclic Chemistry
DOI 10.1002/jhet

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H. J. Park, M. S. Park, T. H. Lee, and K. H. Park
Vol 50
2-(2-Phenylethenyl)benzoxazole (2a). Colourless crystal with mp
7.5 Hz, 1H, Ar-H), 7.25 (d, J = 16.5Hz, 1H, CH), 7.17
(d, J = 8.3Hz, 1H, Ar-H), 7.10 (d, J = 8.1 Hz, 1H, Ar-H), 7.00
(dd, J = 7.7Hz, 7.5Hz, 1H, Ar-H), 3.90 (s, 3H, OCH3), 2.40
(s, 3H, CH3). Anal. Calcd for C17H15NO2: C, 76.98; H, 5.66;
N, 5.28. Found: C, 76.39; H, 5.66; N, 5.32.
5-tert-Butyl-2-[2-(2-methoxyphenyl)ethenyl]benzoxazole (2k).
Crystal with mp 84À85ꢀC. 1H NMR (300 MHz, DMSO-d6): d 7.99
(d, J = 16.5 Hz, 1H, CH), 7.82 (d, J= 7.7 Hz, 1H, Ar-H), 7.67 (s, 1H,
Ar-H), 7.60 (d, J = 8.6 Hz, 1H, Ar-H), 7.44 (d, J= 8.6 Hz, 1H, Ar-H),
7.40 (dd, J= 7.8 Hz, 7.5 Hz, 1H, Ar-H), 7.28 (d, J = 16.5 Hz, 1H,
CH), 7.10 (d, J= 7.8 Hz, 1H, Ar-H), 7.01 (dd, J = 7.7 Hz, 7.5 Hz,
1H, Ar-H), 3.90 (s, 3H, OCH3), 1.33 (s, 3H, CH3). Anal. Calcd for
C20H21NO2: C, 78.18; H, 6.84; N, 4.56. Found: C, 78.10; H, 6.86;
N, 4.63.
5-Chloro-2-[2-(2-methoxyphenyl)ethenyl]benzoxazole (2l).
Yellow crystal with mp 148À149ꢀC. 1H NMR (500 MHz,
CDCl3): d 8.04 (d, J = 16.6 Hz, 1H, CH), 7.83 (d, J = 7.7 Hz,
1H, Ar-H), 7.80 (s, 1H, Ar-H), 7.75 (d, J = 8.7 Hz, 1H, Ar-H),
7.43À7.40 (d, J = 8.7 Hz, 1H, Ar-H), 7.43À7.40 (m, 1H, Ar-
H), 7.29 (d, J = 16.6 Hz, 1H, CH), 7.11 (d, J = 8.40 Hz, 1H,
Ar-H), 7.02 (dd, J = 7.7 Hz, 7.3 Hz, 1H, Ar-H), 3.90 (s, 3H,
OCH3). Anal. Calcd for C16H12ClNO2: C, 67.37; H, 4.21; N,
4.91. Found: C, 66.41; H, 4.16; N, 4.94.
80À81ꢀC (lit. mp 84ꢀC [10]). 1H NMR (300MHz, DMSO-d6):
d 7.84À7.69 (m, 5H, Ar-H, CH), 7.47À7.29 (m, 6H, Ar-H, CH).
5-Methyl-2-(2-Phenylethenyl)benzoxazole (2b). Colourless
crystal with mp 89À91ꢀC (lit. mp 92À93ꢀC [11]). 1H NMR
(300 MHz, DMSO-d6): d 7.79À7.74 (m, 2H, Ar-H), 7.79À7.74
(d, J = 16.4 Hz, 1H, CH), 7.55 (d, J = 8.3 Hz, 1H, Ar-H), 7.51
(s, 1H, Ar-H), 7.45À7.39 (m, 3H, Ar-H), 7.27 (d, J = 16.4 Hz,
1H, CH), 7.18 (d, J = 8.3 Hz, 1H, Ar-H), 2.40 (s, 3H, CH3).
5-tert-Butyl-2-(2-Phenylethenyl)benzoxazole (2c). Colourless
crystal with mp 96À98ꢀC. 1H NMR (300 MHz, DMSO-d6):
d 7.80À7.74 (m, 2H, Ar-H), 7.80À7.74 (d, J= 16.4 Hz, 1H, CH),
7.68 (s, 1H, Ar-H), 7.58 (d, J= 8.6 Hz, 1H, Ar-H), 7.45À7.39
(m, 4H, Ar-H), 7.30 (d, J= 16.4 Hz, 1H, CH) 1.32 (s, 9H, CH3).
5-Chloro-2-(2-Phenylethenyl)benzoxazole (2d). Brown crystal
1
with mp 112À113ꢀC (lit. mp 104ꢀC [12]). H NMR (300 MHz,
DMSO-d6): d 7.87À7.81 (d, J = 16.6 Hz, 1H, CH), 7.84À7.79 (m,
3H, Ar-H), 7.75 (d, J = 8.7Hz, 1H, Ar-H), 7.45À7.42 (m, 3H, Ar-
H), 7.43 (d, J = 8.7 Hz, 1H, Ar-H), 7.33 (d, J = 16.6 Hz, 1H, CH).
2-[2-(2-Nitrophenyl)ethenyl]benzoxazole (2e). Yellow crystal
with mp 138À139ꢀC. 1H NMR (300MHz, DMSO-d6):
d 8.12À8.04 (m, 3H, Ar-H), 7.80À7.65 (m, 4H, Ar-H, CH),
7.43À7.36 (m, 3H, Ar-H, CH). Anal. Calcd for C15H10N2O3:
C, 67.67; H, 3.76; N, 10.53. Found: C, 66.96; H, 3.72; N, 10.53.
5-Methyl-2-[2-(2-nitrophenyl)ethenyl]benzoxazole (2f). Yellow
crystal with mp 131À133ꢀC. 1H NMR (300 MHz, DMSO-d6):
d 8.12À8.06 (m, 2H, Ar-H), 8.03 (d, J = 16.2Hz, 1H, CH),
7.82À7.77 (dd, J = 7.7Hz, 7.5 Hz, 1H, Ar-H), 7.68À7.61 (m, 1H,
Ar-H), 7.63À7.61 (d, J = 8.3Hz, 1H, Ar-H), 7.55 (s, 1H, Ar-H),
7.35 (d, J = 16.2Hz, 1H, CH), 7.23 (d, J = 8.3 Hz, 1H, Ar-H), 2.41
(s, 3H, CH3). Anal. Calcd for C16H12N2O3: C, 68.57; H, 4.29;
N, 10.00. Found: C, 68.07; H, 4.23; N, 9.99.
Acknowledgment. This study was financially supported by a
research fund from Chungnam National University in 2009.
REFERENCES AND NOTES
5-tert-Butyl-2-[2-(2-nitrophenyl)ethenyl]benzoxazole (2g).
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Yellow crystal with mp 130À132ꢀC. 1H NMR (300 MHz,
DMSO-d6):
d 8.12 (d, J = 7.9 Hz, 1H, Ar-H), 8.09 (d,
J = 7.7 Hz, 1H, Ar-H), 8.05À8.00 (d, J = 16.1 Hz, 1H, N═CH),
7.76 (dd, J = 7.7 Hz, 7.6 Hz, 1H, Ar-H), 7.72 (s, 1H, Ar-H),
7.67À7.61 (d, J = 8.7 Hz, 1H, Ar-H), 7.67À7.61 (dd, J = 7.9 Hz,
7.6 Hz, 1H, Ar-H), 7.50À7.46 (dd, J = 8.7 Hz, 1.94 Hz, 1H, Ar-
H), 7.38 (d, 16.1 Hz, 1H, CH), 1.33 (s, 9H, CH3). Anal. Calcd
for C19H18N2O3: C, 70.81; H, 5.59; N, 8.70. Found: C, 70.65;
H, 5.68; N, 8.85.
5-Chloro-2-[2-(2-nitrophenyl)ethenyl]benzoxazole (2h). Yellow
crystal with mp 153À154ꢀC. 1H NMR (500 MHz, CDCl3):
d 8.13À8.07 (m, 2H, Ar-H), 8.13À8.07 (d, J =16.1 Hz, 1H, CH),
7.89 (s, 1H, Ar-H), 7.83À7.79 (m, 1H, Ar-H), 7.81 (d, J = 8.7Hz,
1H, Ar-H), 7.67 (t, J = 7.95 Hz, 1H, Ar-H), 7.47 (d, J = 8.7Hz,
1H, Ar-H), 7.39 (d, 16.1 Hz, 1H, CH). Anal. Calcd for
C15H9ClN2O3: C, 59.90; H, 3.00; N, 9.32. Found: C, 59.36;
H, 2.92; N, 9.20.
2-[2-(2-Methoxyphenyl)ethenyl]benzoxazole (2i). Colourless
1
crystal with mp 70À71ꢀC. H NMR (500MHz, CDCl3): d 8.02
(d, J =16.6 Hz, 1H, CH), 7.82 (d, J = 7.7Hz, 1H, Ar-H),
7.74À7.69 (m, 2H, Ar-H), 7.41À7.32 (m, 3H, Ar-H), 7.29
(d, J = 16.6 Hz, 1H, CH), 7.10 (d, J = 8.1 Hz, 1H, Ar-H), 7.01
(dd, 7.7 Hz, 7.5Hz, 1H, Ar-H), 3.90(s, 3H, OCH3). Anal. Calcd
for C16H13ClNO2: C, 76.49; H, 5.18; N, 5.58. Found: C, 76.35;
H, 5.22; N, 5.75.
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5-Methyl-2-[2-(2-methoxyphenyl)ethenyl]benzoxazole (2j).
Colourless crystal with mp 79À81ꢀC. 1H NMR (300 MHz, DMSO-
d6): d 7.98 (d, J = 16.5Hz, 1H, CH), 7.80 (d, J = 7.7 Hz, 1H, Ar-H),
7.57 (d, J = 8.3Hz, 1H, Ar-H), 7.49 (s, 1H, Ar-H), 7.39 (dd, 8.1Hz,
Journal of Heterocyclic Chemistry
DOI 10.1002/jhet

May 2013
2-Styrylbenzoxazoles from Styrylphenolic Schiff Bases and Thianthrene Cation Radical Perchlorate 667
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Journal of Heterocyclic Chemistry
DOI 10.1002/jhet