Copper-catalyzed oxidative cyclization of chalcone and benzylic amine leading to 2,5-diaryl oxazoles via carbon-carbon double bond cleavage

Article abstract of DOI:10.1016/j.tet.2013.12.077

A copper-catalyzed oxidative cyclization of chalcone with benzylic amine is achieved, providing 2,5-diaryl oxazoles in moderate to good yields. The procedure employs O₂ as a clean oxidant and involves an oxidative cleavage of the CC bond as the key step.

Full text of DOI:10.1016/j.tet.2013.12.077

Tetrahedron 70 (2014) 1149e1153
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Tetrahedron
journal homepage: www.elsevier.com/locate/tet
Copper-catalyzed oxidative cyclization of chalcone and benzylic
amine leading to 2,5-diaryl oxazoles via carbonecarbon double
bond cleavage
,b,
Dongfang Liu a, Jintao Yu b, Jiang Cheng a
*
a College of Chemistry and Materials Engineering, Wenzhou University, Wenzhou 325035, PR China
b School of Petrochemical Engineering, Jiangsu Key Laboratory of Advanced Catalytic Materials and Technology, Jiangsu Province Key Laboratory of Fine
Petrochemical Engineering, Changzhou University, Changzhou 213164, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 10 October 2013
Received in revised form 19 December 2013
Accepted 26 December 2013
Available online 3 January 2014
A copper-catalyzed oxidative cyclization of chalcone with benzylic amine is achieved, providing 2,5-
diaryl oxazoles in moderate to good yields. The procedure employs O2 as a clean oxidant and involves
an oxidative cleavage of the C]C bond as the key step.
Ó 2014 Elsevier Ltd. All rights reserved.
Keywords:
Copper-catalyzed
Cyclization
Oxazoles benzylic amine
1. Introduction
Employing a-aminoketones, Wang reported an iodine-catalyzed
tandem oxidative cyclization of 2-amino-1-phenylethanone hy-
drochloride and aldehyde toward 2,5-disubstituted oxazole
(Scheme 1).26 The participation of carboxamides could also be
Oxazoles are widely found in natural occurring compounds,1e4
and pharmaceutical active small molecules.5,6 Moreover, this
framework is also a useful synthetic building block in organic
synthesis.7e14 Hence, many strategies have been developed for the
synthesis of oxazole derivatives. Among these strategies, un-
doubtedly, the direct intermolecular annulation of acyclic pre-
cursors to polysubstituted oxazoles from readily available materials
is more attractive for the diversity of target molecules in compar-
ison with the traditional intramolecular annulation, such as the
Robinson reaction.15,16 For example, the Van Leusen reaction17,18
and Fischer oxazole synthesis19,20 are powerful tools for the con-
struction of this framework. Despite the straightforward strategy,
the strong acidic or basic reaction conditions decrease the tolerance
of the functional groups. Recently, Zhang described a mild synthesis
of oxazole with this direct annulation strategy, which employs
gold-catalyzed [2þ2þ1] annulation of terminal alkyne, nitrile, and
an oxygen donor.21 This elegant strategy was further developed by
Saito, in which the oxazole annulation was mediated by iodi-
ne(III).22 An efficient construction of the oxazole framework using
the
a-diazocarbonyl compounds was developed by Lacour and
Moody, respectively.23e25
* Corresponding author. E-mail addresses: jiangcheng@cczu.edu.cn, shcheng-
Scheme 1. The selected recent reports on the intermolecular annulation leading to
jiang@yahoo.com.cn (J. Cheng).
oxazole.
0040-4020/$ e see front matter Ó 2014 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tet.2013.12.077

1150
D. Liu et al. / Tetrahedron 70 (2014) 1149e1153
utilized in the construction of oxazole motifs, and several in-
dependent reports from Zhan,27 Liu,28 Moses,29 Ray,30 and Buch-
wald are depicted in Scheme 1.31 Meanwhile, benzylamine was
wildly employed as commercially available material in the con-
struction of oxazole framework. For example, either the copper-
catalyzed or metal-free tandem oxidative cyclization from benzyl-
amine or 1,3-diketone has been reported by Wang and Zhu, re-
spectively (Scheme 1).32,33 In 2010, Jiang described a TBHP/I2-
mediated domino oxidative cyclization of aryl alkene and benzylic
amine toward a polysubstituted oxazole (Scheme 1).34 In 2012, Jiao
reported a copper-mediated aerobic oxidative annulation of alde-
hyde and benzylamine in the synthesis of oxazole (Scheme 1).35 In
2013, Jiang and Wu independently described an I2-promoted an-
nulation of acetophenone and benzylic amine to construct 2,5-
disubstituted oxazole (Scheme 1).36
Under the optimized reaction conditions, the scope of benzyl-
amine derivatives was explored, as shown in Table 2.. All substrates
including ortho, meta, and para substituted benzylamines ran
smoothly, affording the annulation products in moderate to good
yields. The reactions seemed not sensitive to the hindrance of or-
tho-substitution of benzylamine (entry 1). Notably, the chloro and
bromo groups survived under the reaction conditions, which pro-
vided handles for further functionalization.
Table 2
The scope of benzylic aminea
Although great progress has been made in the construction of
oxazole framework, the development of annulation of benzylamine
with readily available starting material involving new type of bond
cleavage remains urgent and necessary, one such example develops
an annulation of benzylamine with readily available starting ma-
terial involving new type of bond cleavage. (E)-Chalcone is a readily
available reagent either from commercially sources or through
laboratory condensation of acetophenone and aldehyde. Herein, we
report a copper-catalyzed oxidative cyclization of benzylamine and
chalcone via carbonecarbon double bond cleavage, which employs
O2 as a clean oxidant leading to 2,5-diaryl oxazole.
Entry
1
2
3
2
2. Result and discussion
3
4
5
We started the cyclization of chalcone and benzylamine in the
presence of CuBr, pyridine, and K2CO3 in toluene at 110 ꢀC under O2.
The desired oxazole 3aa was isolated in 5% yield (entry 1, Table 1).
To our delight, in the presence of 2.5 equiv of LiBr, the yield was
dramatically increased to 63% (entry 2, Table 1). Replacing CuBr
with CuCl, CuCl2 and Cu(OAc)2 decreased the yield (entries 3e5,
Table 1). Gratifyingly, 78% yield was obtained when CuBr2 was used
as catalyst (entry 6, Table 1). The yield dramatically decreased if O2
was replaced with air or N2. Conducting the reaction in xylene,
ClCH2CH2Cl or dioxane (entries 7e9, Table 1) either resulted to low
yield or no reaction at all. Further studies revealed that t-BuOK and
Na2CO3 were inferior to K2CO3 as the base (entries 10 and 11, Table
1). The employment of LiCl and KBr also resulted in low yields for
this transformation (entries 12 and 13, Table 1).
6
7
Table 1
Selected results of screening the optimal conditions
8
Entry
Copper
Additive
Base
Solvent
Yielda (%)
1
2
3
4
5
6
7
8
CuBr
CuBr
CuCl
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
t-BuOK
Na2CO3
K2CO3
K2CO3
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Xylene
ClCH2CH2Cl
Dioxane
Toluene
Toluene
Toluene
Toluene
5
63
35
40
LiBr
LiBr
LiBr
LiBr
LiBr
LiBr
LiBr
LiBr
LiBr
LiBr
LiCl
KBr
9
CuCl2
Cu(OAc)2
CuBr2
CuBr2
CuBr2
CuBr2
CuBr2
CuBr2
CuBr2
CuBr2
8
78 (63)b (13)c
a
Reaction conditions: 1a (0.2 mmol), benzylic amine
2
(0.4 mmol), CuBr2
52
0
0
21
48
33
18
(20 mol %), pyridine (0.4 mmol), K2CO3 (0.1 mmol), LiBr (0.5 mmol) in dry toluene
(2 mL), 110 ꢀC, 11 h, under O2.
9
10
11
12
13
Next, the substrate scope of chalcone derivatives was studied.
Once again, the cyclization ran smoothly with tolerance of func-
tional groups, such as bromo and trifluoromethyl. Notably, both
phenyl groups in chalcone could be substituted and still remain
good reactivity in the cyclization. (Table 3). Particularly, the chal-
cone derivatives with heteroaryl attached in the carbonyl group,
such as 1g and 1h, provided the oxazoles in 58% and 31% yields,
a
Reaction conditions: 1a (0.2 mmol), benzylamine 2a (0.4 mmol), CuBr2
(20 mol %), pyridine (0.4 mmol), K2CO3 (0.1 mmol), LiBr (0.5 mmol) in dry toluene
(2 mL), 110 ꢀC, 11 h, under O2.
b
Air.
N2.
c

D. Liu et al. / Tetrahedron 70 (2014) 1149e1153
1151
Table 3
The scope of chalcone derivativesa
Entry
1
Substrate 1
Product 3
Scheme 2. The mechanism study.
2
3
MeC6H4e) afforded no deuterium atom in the oxazole product
(Scheme 2, Eq. 4).
Based on these observations, a tentative mechanism is illus-
trated in Scheme 3. Firstly, the oxidation of Cu(II) by O2 via a single
electron transferring (SET) process produces a Cu(III) species A.41,42
Then, the intermediate B is formed by the addition of A to the
enone. Secondly, an intramolecular SET followed by the cyclization
takes place to form four-membered intermediate C, along with
a regeneration of Cu(II) species to re-enter the catalytic cycle.
4
5
Subsequently, the intermediate
C decomposes to 2-oxo-2-
phenylacetaldehyde and benzaldehyde.43,44 Condensation of 2-
oxo-2-phenylacetaldehyde with benzylamine produces in-
termediate D, which produces intermediate E via [1,5] s-H migra-
tion. It is consistent with the fact that no deuterium atom was
incorporated in the annulation of chalcone with 4-MeC6H4CD2NH2
(Scheme 2, Eq. 4). Finally, the cyclization of intermediate E forms
the intermediate F, which is oxidized by Cu(II) and/or O2 to oxazole.
However, the role of LiBr remains unclear at the current stage.
6
7
8
a
Reaction conditions:
1 (0.2 mmol), benzylic amine 2a (0.4 mmol), CuBr2
(20 mol %), pyridine (0.4 mmol), K2CO3 (0.1 mmol), LiBr (0.5 mmol) in dry toluene
(2 mL), 110 ꢀC, 11 h, under O2.
respectively. However, 1i bearing an a-methyl substitution did not
work under this procedure.
The radical scavengers, such as 2,2,6,6-tetramethylpiperidine-1-
oxyl (TEMPO) and 2,6-di-tert-butyl-p-methyl phenol (BHT),
inhibited the annulation reaction (Scheme 2, Eq. 1), indicating
a radical intermediate may be involved in this cyclization. During
the reaction, N-benzylbenzamide was detected as a by-product
(Scheme 2, Eq. 2). When 2-oxo-2-phenylacetaldehyde was sub-
jected to the procedure, 3aa was isolated in a comparable 74%
yield (Scheme 2, Eq. 3). These results revealed 2-oxo-2-
phenylacetaldehyde may act as the intermediate for this annula-
tion. Thus, this procedure may involve a C]C bond cleavage leading
to 2-oxo-2-phenylacetaldehyde and benzaldehyde. Benzaldehyde
and benzylamine further condensed to N-benzylbenzamide as by-
product.37e40 Replacing ArCH2NH2 with ArCD2NH2 (Ar¼4-
Scheme 3. The proposed mechanism.
3. Conclusions
In conclusion, we developed a new strategy leading to 2,5-diaryl
oxazoles from readily available chalcone and benzylic amine. This
procedure involves an oxidative C]C bond cleavage using O2 as
a clean oxidant and represents an attractive alternation for the
synthesis of oxazoles.

1152
D. Liu et al. / Tetrahedron 70 (2014) 1149e1153
4. Experimental section
130.0 (q, JCeF¼32.5 Hz), 128.9, 127.1, 126.5, 125.9 (q, JCeF¼3.8 Hz),
125.2, 124.2, 123.9 (q, JCeF¼272.5 Hz).
4.1. General information
4.3.6. 5-(Naphthalen-2-yl)-2-phenyloxazole (3fa, Table 3).34 White
Chemicals were either purchased or purified by standard tech-
niques. 1H NMR and 13C NMR spectra were measured on a 500 MHz
spectrometer (1H 500 MHz, 13C 125 MHz) with CDCl3 as the solvent
at room temperature. Chemical shifts (in ppm) were referenced to
solid; 1H NMR (CDCl3, 500 MHz):
d
¼8.18e8.16 (m, 3H), 7.91e7.87
(m, 2H), 7.85e7.74 (m, 2H), 7.55e7.48 (m, 6H); 13C NMR (CDCl3,
125 MHz):
d¼161.3, 151.3, 133.4, 133.1, 130.3, 128.8, 128.7, 128.2,
127.8, 127.5, 126.8, 126.5, 126.3, 125.3, 124.0, 122.9, 122.0.
tetramethylsilane (
were obtained by using the same NMR spectrometer and were
d
¼0 ppm) or CHCl3 (7.26 ppm). 13C NMR spectra
4.3.7. 2-Phenyl-5-(thiophen-2-yl)oxazole (3ga, Table 3).36 Greenish
calibrated with CDCl3
(
d
¼77.00 ppm). Chemical shifts are given in
solid;
7.35e7.34 (m, 1H), 7.31 (s, 1H), 7.11e7.09 (m, 1H); 13C NMR (CDCl3,
125 MHz):
d
¼8.10e8.08 (m, 2H), 7.50e7.46 (m, 3H), 7.38e7.37 (m, 1H),
d
relative to TMS, the coupling constants J are given in hertz (Hz).
Column chromatography was performed using EM Silica gel 60
(300e400 mesh).
d¼160.6, 146.8, 130.4, 129.9, 128.8, 127.8, 127.2, 126.3,
125.6, 124.3, 123.1.
4.3.8. 5-(Furan-2-yl)-2-phenyloxazole (3ha, Table 3).34 White solid;
4.2. General procedure to oxazoles from chalcone and ben-
zylic amine
1H NMR (CDCl3, 500 MHz):
d
¼8.09e8.08 (m, 2H), 7.47 (d, J¼1.5 Hz,
4H), 7.35 (s, 1H), 6.70 (d, J¼3.0 Hz, 1H), 6.53e6.52 (m, 1H); 13C NMR
(CDCl3, 125 MHz):
d
¼160.7, 143.8, 143.7, 142.9, 130.4, 128.8, 127.2,
Under O2, a mixture of chalcone (0.2 mmol), benzylamine
(0.4 mmol), CuBr2 (20 mol %), pyridine (0.4 mmol), K2CO3
(0.1 mmol), and LiBr (0.5 mmol) in dry toluene (2 mL) was refluxed
at 110 ꢀC for 11 h. After the completion of the reaction, as monitored
by TLC, the solvent was concentrated under vacuum and the resi-
due was purified by flash column chromatography through silica
gel (300e400 mesh) with petroleum ether/EtOAc as eluant to give
the desired product.
126.4, 123.4, 111.6, 107.3.
4.3.9. 2-(2-Chlorophenyl)-5-phenyloxazole (3ab, Table
2).26 Yellowish solid; 1H NMR (CDCl3, 500 MHz):
d
¼8.12e8.09
(m, 1H), 7.75e7.74 (m, 2H), 7.55e7.53 (m, 2H), 7.45 (t, J¼8.0 Hz, 2H),
7.39e7.34 (m, 3H); 13C NMR (CDCl3,125 MHz):
d¼159.0,151.7,132.3,
131.3, 131.0, 130.7, 129.0, 128.6, 127.8, 126.9, 126.2, 124.3, 123.2.
4.3.10. 2-(3-Bromophenyl)-5-phenyloxazole (3ac, Table
47
4.3. Synthesis of deuterated substrates 4eMeC6H4CD2NH2
2).35 Yellowish solid; 1H NMR (CDCl3, 500 MHz):
d
¼8.25e8.24
(m, 1H), 8.05e8.03 (m, 1H), 7.73e7.72 (m, 2H), 7.59e7.57 (m, 1H),
Compound 4eMeC6H4CD2NH2 was synthesized by reduction of
4-methylbenzonitrile (0.52 g, 5 mmol) with lithium aluminum
deuteride (0.5 g, 5 mmol) in ice cooled dry ether (50 ml). The
mixture was refluxed for 4 h before cooling to 0 ꢀC, EtOAc (0.4 mL)
was added to dilute the reaction. After filtration, the filter cake was
washed with ether, evaporated under vacuum (25 ꢀC) to afford p-
tolylmethanamine as a colorless oil. The isotopic purity determined
by 1H NMR was 95%.
7.47e7.44 (m, 3H), 7.37e7.34 (m, 2H); 13C NMR (CDCl3, 125 MHz):
d
¼159.6, 151.7, 133.2, 130.4, 129.2, 129.1, 129.0, 128.7, 127.7, 124.7,
124.3, 123.6, 122.9.
4.3.11. 2-(4-Chlorophenyl)-5-phenyloxazole (3ad, Table
2).26 Yellowish solid; 1H NMR (CDCl3, 500 MHz):
d
¼8.04 (d, J¼8.5
Hz, 2H), 7.71 (d, J¼8.0 Hz, 2H), 7.47e7.44 (m, 5H), 7.32 (t, J¼7.5 Hz,
1H); 13C NMR (CDCl3, 125 MHz):
d¼160.2, 151.5, 136.4, 129.1, 129.0,
128.6, 127.8, 127.5, 125.9, 124.2, 123.5.
4.3.1. 2,5-Diphenyloxazole (3aa).34 Yellow solid; 1H NMR (CDCl3,
500 MHz):
d
¼8.13e8.11 (m, 2H), 7.73e7.72 (m, 2H), 7.51e7.43 (m,
4.3.12. 2-(4-Fluorophenyl)-5-phenyloxazole (3ae, Table
6H), 7.36e7.33 (m, 1H); 13C NMR (CDCl3, 125 MHz):
d¼161.1, 151.2,
2).35 Yellowish solid; 1H NMR (CDCl3, 500 MHz):
d
¼8.10e8.07 (m,
2H), 7.71e7.69 (m, 2H), 7.45e7.42 (m, 3H), 7.35e7.32 (m, 1H),
7.18e7.14 (m, 2H); 13C NMR (CDCl3, 125 MHz):
130.2, 128.8, 128.7, 128.3, 128.0, 127.4, 126.2, 124.1, 123.4.
d
¼165.0 (d,
4.3.2. 5-(4-Bromophenyl)-2-phenyloxazole (3ba, Table
JCeF¼249.5 Hz), 160.3, 151.3, 128.9, 128.4 (d, JCeF¼11.5 Hz), 128.3,
3).34 Yellowish solid; 1H NMR (CDCl3, 500 MHz):
d
¼8.11e8.09 (m,
2H), 7.59e7.56 (m, 4H), 7.49e7.45 (m, 4H); 13C NMR (CDCl3,
125 MHz):
127.9, 124.2, 123.8 (d, JCeF¼3.3 Hz), 123.4, 116.0 (d, JCeF¼22.1 Hz).
d
¼161.4, 150.3, 132.1, 130.5, 128.8, 127.3, 127.0, 126.4,
4.3.13. 2-(3-Fluorophenyl)-5-phenyloxazole (3af, Table
2). Yellowish solid, mp: 68e70 ꢀC; 1H NMR (CDCl3, 500 MHz):
125.6, 123.9, 122.3.
d
¼7.90 (d, J¼8.0 Hz, 1H), 7.80 (d, J¼9.5 Hz, 1H), 7.72 (d, J¼7.5 Hz,
2H), 7.46 (t, J¼7.5 Hz, 4H), 7.36 (t, J¼7.5 Hz, 1H), 7.18e7.14 (m, 1H);
13C NMR (CDCl3, 125 MHz):
4.3.3. 2-Phenyl-5-(p-tolyl)oxazole (3ca, Table 3).35 White solid; 1H
NMR (CDCl3, 500 MHz):
7.50e7.45 (m, 3H), 7.40 (s, 1H), 7.26e7.24 (m, 2H), 2.39 (s, 3H); 13
NMR (CDCl3, 125 MHz): 160.8, 151.5, 138.5, 130.2, 129.6, 128.8,
127.5, 126.2, 125.3, 124.2, 122.8, 21.4.
d
¼8.11e8.09 (m, 2H), 7.61 (d, J¼8.5 Hz, 2H),
d¼162.9 (d, JCeF¼244.8 Hz), 159.4, 151.7,
C
130.5 (d, JCeF¼8.1 Hz), 129.0, 128.7, 127.7, 124.3, 123.6, 122.0 (d,
JCeF¼3.0 Hz), 117.2 (d, JCeF¼21.3 Hz), 113.2 (d, JCeF¼23.8 Hz); IR
(prism, cmꢁ1): 1593, 1543, 1481, 1267, 1191, 940, 791; HRMS calcd
for C15H11FNO ([MþH]þ): 240.0819; found: 240.0800.
d
4.3.4. 5-(4-Methoxyphenyl)-2-phenyloxazole (3da, Table
3).36 Yellow solid; 1H NMR (CDCl3, 500 MHz):
¼8.09 (d, J¼8.5
d
4.3.14. 5-Phenyl-2-(m-tolyl)oxazole (3ag, Table 2).26 Yellowish
Hz, 2H), 7.66 (d, J¼8.5 Hz, 2H), 7.49e7.45 (m, 3H), 7.33 (s, 1H), 6.97
solid; 1H NMR (CDCl3, 500 MHz):
d
¼7.94e7.91 (m, 2H), 7.73 (d,
(d, J¼9.0 Hz, 2H), 3.86 (s, 3H); 13C NMR (CDCl3, 125 MHz):
d
¼160.6,
J¼7.0 Hz, 2H), 7.47e7.44 (m, 3H), 7.39e7.33 (m, 2H), 7.27 (d,
159.8, 151.3, 130.1, 128.8, 127.6, 126.1, 125.7, 122.0, 120.9, 114.4, 55.4.
J¼7.5 Hz, 1H), 2.45 (s, 3H); 13C NMR (CDCl3, 125 MHz):
¼161.3,
d
151.2, 138.6,131.1, 128.9, 128.7, 128.4,128.1,127.3, 126.8,124.2, 123.4,
123.4, 21.4.
4.3.5. 2-Phenyl-5-(4-(trifluoromethyl)phenyl)oxazole (3ea, Table
3).45 White solid; 1H NMR (CDCl3, 500 MHz):
d
¼8.14e8.12(m, 2H)
7.82 (d, J¼8.0 Hz, 2H), 7.69 (d, J¼8.0 Hz, 2H), 7.56 (s, 1H), 7.51e7.49
(m, 3H); 13C NMR (CDCl3, 125 MHz):
¼162.0, 149.8, 131.2, 130.7,
4.3.15. 5-Phenyl-2-(p-tolyl)oxazole (3ah, Table 2).26 Yellowish
d
solid; 1H NMR (CDCl3, 500 MHz):
d¼7.99 (d, J¼8.0 Hz, 2H), 7.69 (d,

D. Liu et al. / Tetrahedron 70 (2014) 1149e1153
1153
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5445e5459.
J¼7.5 Hz, 2H), 7.42 (t, J¼7.5 Hz, 3H), 7.33e7.24 (m, 3H), 2.39 (s, 3H);
13C NMR (CDCl3,125 MHz):
128.1, 126.2, 124.7, 124.1, 123.3, 21.5.
d
¼161.4,150.9,140.6,129.5,128.9,128.3,
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16. Gabriel, S. Ber. 1910, 43, 134e138.
4.3.16. 2-(4-Methoxyphenyl)-5-phenyloxazole (3ai, Table
2).46 White solid; 1H NMR (CDCl3, 500 MHz):
d
¼8.07e8.04 (m,
2H), 7.72e7.70 (m, 2H), 7.46e7.41 (m, 3H), 7.33 (t, J¼8.0 Hz, 1H),
7.00e6.99 (m, 2H), 3.88 (s, 3H); 13C NMR (CDCl3, 125 MHz):
d
¼161.3, 161.2, 150.7, 128.9, 128.2, 128.2, 128.0, 124.0, 123.2, 120.3,
17. van Leusen, A. M.; Hoogenboom, B. E.; Siderius, H. Tetrahedron Lett. 1972, 13,
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4.3.17. 2-(3-Methoxyphenyl)-5-phenyloxazole (3aj, Table
2).26 White solid; 1H NMR (CDCl3, 500 MHz):
d
¼7.74e7.70 (m,
3H), 7.65e7.64 (m, 1H), 7.47e7.38 (m, 4H), 7.37e7.33 (m, 1H),
7.03e7.01 (m, 1H), 3.90 (s, 3H); 13C NMR (CDCl3, 125 MHz):
20. Wiley, R. H. Chem. Rev. 1945, 37, 401e442.
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d¼161.0,
159.9, 151.3, 130.0, 128.9, 128.6, 128.5, 127.9, 124.2, 123.4, 118.7,
116.8, 110.9, 55.4.
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We thank the National Natural Science Foundation of China
(nos. 21272028 and 21202013), ‘Innovation & Entrepreneurship
Talents’ Introduction Plan of Jiangsu Province, Jiangsu Key Labora-
tory of Advanced Catalytic Materials &Technology, Jiangsu Province
Key Laboratory of Fine Petrochemical Engineering, and the Priority
Academic Program Development of Jiangsu Higher Education In-
stitutions for financial support.
Supplementary data
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Supplementary data associated with this article can be found in
the online version, at http://dx.doi.org/10.1016/j.tet.2013.12.077.
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