Convenient route to trisubstituted oxazoles via a copper-catalysed tandem oxidative cyclisation by oxygen oxidation

Article abstract of DOI:10.3184/174751915X14192609116136

A novel copper-catalysed oxidative cyclisation has been developed for the synthesis of trisubstituted oxazoles, which is thought to proceed through cascade formation of C-N and C-O bonds by oxygen oxidation. The desired products can be obtained from readily available starting materials while avoiding hazardous materials. Therefore, a green synthetic method for the preparation of oxazoles has been found.

Full text of DOI:10.3184/174751915X14192609116136

JOURNAL OF CHEMICAL RESEARCH 2015
VOL. 39 JANUARY, 7–10
RESEARCH PAPER 7
Convenient route to trisubstituted oxazoles via a copper-catalysed tandem
oxidative cyclisation by oxygen oxidation
Chengqun Chen*, Wenfu Chen and Qianhong Bao
Department of Chemical Engineering, Fuzhou University Zhicheng College, Fuzhou 350002, P.R. China
A novel copper-catalysed oxidative cyclisation has been developed for the synthesis of trisubstituted oxazoles, which is thought
to proceed through cascade formation of C–N and C–O bonds by oxygen oxidation. The desired products can be obtained from
readily available starting materials while avoiding hazardous materials. Therefore, a green synthetic method for the preparation
of oxazoles has been found.
Keywords: copper, oxazoles, β‑diketones, benzylamine, oxygen oxidation
The oxazole moiety, which has attracted increasing attention in
both industrial and academic fields for decades, is a significant
structure in numerous bioactive natural products. Furthermore,
a large number of natural and synthetic compounds which
contain the moiety exhibit significant biological activities
such as anticancer, antifungal, antitumour, anti‑inflammatory,
and antiviral properties. For instance, leucamide A and its
analogues, which are important anticancer reagents,1 are
mainly composed of several oxazole moieties. Moreover,
oxazole derivatives can also be used as organic materials,
such as a corrosion inhibitor.2 Thus, various synthetic
methodologies of oxazole derivatives have been developed,
such as cyclodehydration of acyclic precursors3–11 or oxidative
functionalisation of alkynes.12–14 Iodine catalysed C–H bond
functionalisation has been extensively explored.15,16 However,
the development of simple and efficient methods for the
preparation of trisubstituted oxazoles is still desirable. Wang17
and Zhu18 have reported the synthesis of polysubstituted
oxazoles, but they used tert‑butyl hydroperoxide (TBHP),
which is a hazardous material. Narender has developed a
route to utilise the iodine promoted C–H functionalisation for
accessing poly‑substituted oxazoles under metal and peroxide
free conditions.19 Copper, among the transition metals, is
particularly attractive in organic synthesis because of its low
price, slight toxicity, and environmentally benign features.
There have been many reports on copper‑catalysed oxidative
C–H bond activation.13,18 We now describe a copper‑catalysed
oxidative tandem cyclisation of 1, 3‑dicarbonyl derivatives and
benzylamines to form oxazoles by O2 oxidation (Scheme 1).
Results and discussion
To initiate our study, the reaction of ethyl acetoacetate with
benzylamine was chosen as a model reaction in the presence of
a copper source in CH3CN at 40 °C. First, the model reaction
was carried out with copper catalysts; the desired product, ethyl
5‑methyl‑2‑phenyloxazole‑4‑carboxylate, was isolated in low
yields or there was no reaction (Table 1, entries 1–5). Then we
optimised the reaction conditions to increase the reaction yield.
Changing the reaction temperature had a positive influence
on this reaction, resulting in an increase in the yield to 63%
(Table 1, entry 6). When the model reaction was carried out in
the presence of O2, the desired product 1ab was isolated in 79%
yield (Table 1, entry 7). Encouraged by this result, we have tried
to raise the temperature to 80 °C or modulated the reaction
time, but they had no positive influence (Table 1, entries 8–9).
Following these results, the other solvents, such as toluene,
THF, CHCl3 and DMF, were used in place of CH3CN for this
reaction. CH3CN gave the best result (Table 1, entries 7, 10–13).
Table 1 Optimisation of the reaction conditionsa
O
O
[Cu(o-phen)2Cl]Cl,O2
CH CN,60°C 4A M.S., I
O
O
N
BnNH2
+
O
,
3
2
O
Ph
1a
1b
1ab
Entry
Catalyst
Solvent Temp./°C Atmosphere Yield/%b
1
2
3
4
5
6
7
8
Cu(OAc)2.H2O
CuCl2
Cu(OTf)2
CuSO4
[Cu(o-phen)2Cl]Cl CH3CN
[Cu(o-phen)2Cl]Cl CH3CN
[Cu(o-phen)2Cl]Cl CH3CN
[Cu(o-phen)2Cl]Cl CH3CN
CH3CN40
CH3CN
CH3CN
CH3CN
Air
40
40
40
40
60
60
80
60
60
60
60
60
60
13
Air
Air
Air
Air
Air
O2
O2
O2
O2
O2
O2
O2
O2
NRc
0
O
0
R2
42
63
79
78
79
0
64
69
73
77
[Cu(o-phen)2Cl]Cl,O2
O
O
N
R1
+
NH2
Ar
CH CN,60°C 4A M.S., I
R1
R2
,
3
2
O
Ar
a
b
9d [Cu(o-phen)2Cl]Cl CH3CN
10 [Cu(o-phen)2Cl]Cl Toluene
11 [Cu(o-phen)2Cl]Cl THF
12 [Cu(o-phen)2Cl]Cl CH3Cl
13 [Cu(o-phen)2Cl]Cl DMF
14e [Cu(o-phen)2Cl]Cl CH3CN
ab
N
N
N
[Cu(o-phen)2Cl]Cl =
Cl Cu
N
Cl
aReaction conditions: benzylamine (1.5 equiv., 1.5 mmol), ethyl
acetoacetate (1 equiv., 1 mmol), catalyst (0.1 mmol, 10 mmol%), 4 Å
M.S. (0.1 g), I2 (1.2 equiv., 1.2 mmol), solvent (5 mL). The reaction was
performed for 24 h.
Scheme 1 Synthesis of trisubstituted oxazoles.
bIsolated yield.
cNo reaction.
dThe reaction was performed for 48 h.
* Correspondent. E‑mail: ccq0591@yeah.net
eThe addition of benzylamine was reduced to 1.1 equiv.

8 JOURNAL OF CHEMICAL RESEARCH 2015
Table 2 Synthesis of oxazole derivativesa
To explore the reaction process, the reaction of ethyl 2‑iodo‑
3‑oxobutanoate with benzyl amine was performed. It was found
that the reaction can give the oxazole product with a yield of
65%, as shown in Scheme 2. When the reaction substrate was
changed to substrate c,20 no product was observed.
On the basis of the experimental results and the previous
reports,17 we proposed a mechanism, as shown in Scheme 3.
First, 2 is formed from 1a and 1b in the presence of iodine.
The oxidation of 2 and coordination of copper ions provides 3,
which undergoes an intramolecular cyclisation via an oxygen
atom attacking the double bond giving the intermediate 4 in a
tandem process.
Entry
Ar
R1
R2
Time/h
Yield /%b
1
2
3
4
5
6
7
8
9
10
11
12
13
14
C6H5
C6H5
C6H5
C6H5
C6H5
C6H5
C6H5
C6H5
C6H5
C6H5
CH3
CH3
CH3
n-Pr
i-Pr
t-Bu
C6H5
4-ClC6H4
C6H5
CH3
OEt
24
26
25
24
24
24
24
36
24
24
24
28
24
28
79
77
80
78
75
77
82
79
73
70
63
72
71
64
OMe
Ot-Bu
OEt
OEt
OEt
OEt
OEt
C6H5
CH3
OEt
4-CH3 C6H4
4-Cl C6H4
4-F C6H4
CH3
CH3
CH3
CH3
Conclusions
OEt
OEt
OEt
In summary, we developed a facile and efficient oxidative
synthesis of poly‑substituted oxazoles from readily available
starting materials. This transformation from benzylamine
and β‑diketone derivatives into oxazoles was achieved using
a tandem oxidative cyclisation. In contrast to the traditional
synthetic methods for oxazoles, this reaction avoided the use
of hazardous materials. Therefore, this method is an attractive
alternative to synthesise oxazoles. The mechanism and
synthetic applications of this reaction are being studied further
in our laboratory and the results will be reported in due course.
2-Cl C6H4
15
16
CH3
CH3
OEt
OEt
24
48
58
0
O
NH2
aReaction conditions: a (1.5 equiv., 1.5 mmol), b (1 equiv., 1 mmol), catalyst
(0.1 mmol, 10 mmol%), 4 Å M.S.(0.1 g), I2 (1.2 equiv., 1.2 mmol),solvent
(5 mL).
bIsolated yield.
Experimental
NMR spectra were performed on a Mercury 4N‑PEG‑300 (1H:
300 MHz; 13C: 75 MHz) spectrometer, using CDCl3 as a solvent and
TMS as the internal standard. IR spectra were recorded on Perkin‑
Elmer 2000 FTIR spectrometer. HRMS data were determined on a
Bruker Daltonics APEXII 47e FT‑ICR spectrometer. Melting points
are uncorrected.
When toluene was used there was no reaction. This may be due
to the catalyst having a low solubility in a non‑polar solvent.
If the addition of benzylamine was reduced to 1.1 equiv., it
increased the yield (Table 1, entry 14) since the benzylamine
can be oxidised by O2.
Using the optimised conditions, we investigated the scope of
this protocol between various 1,3‑dicarbonyl compounds and
different benzylamines. The experimental results are listed
in Table 2. First, we examined the reaction with a series of
1,3‑dicarbonyl compounds. When the ethyl acetoacetate was
switched to methyl or t‑butyl acetoacetate, a satisfactory yield
was obtained (entries 1–3). This meant that steric effect and
electronic effect had little influence on the reaction. Moreover,
β‑keto esters with different alkyl substituents can also react
with benzylamine smoothly, furnishing the desired products
in 75–82% yield (entries 4–8). When the reaction substrates
were switched from β‑keto esters to β‑diketones, the reactions
can be carried out smoothly to give the corresponding products
with good yields (entries 9 and 10). Subsequently, different
benzylamine derivatives were investigated as reaction
substrates.
Synthesis of [Cu(o-phen)2Cl]Cl 21
The synthesis of [Cu(o‑phen)2Cl]Cl complex was carried out by
addition of 1,10‑phenatroline ligand (2 equiv.) in ethanol (10.0 mL)
to CuCl2.2H2O (1 equiv.) in ethanol (10.0 mL). The mixture was
stirred for 1 h at 25 °C. The solvent was slowly evaporated at room
temperature and the complex was collected and dried，yield: 91% as
a green solid.
Typical procedure
Iodine (1.2 mmol), 1,3‑dicarbonyl compounds (1 mmol), [Cu
(o-phen)2Cl]Cl (0.2 mmol), 4 Å M.S.(0.1 g) were successively added
to a solution of benzylamine (1.5 mmol) derivatives in CH3CN (5 mL).
O
O
O
I2
The results demonstrated that both electron‑rich and
electron‑deficient benzylamines might be used. They react
readily with β‑keto esters to afford the oxazoles in 58–72%
yield (entries 11–15). However, iso‑butylamine failed to yield
the desired product (entries 16).
O
BnNH2
Ph
N
H
+
O
[O]
O
1b
1a
2
O
N
O
H
O
2+
Cu(o-phen)2
O
O
O
Ph
Ph
N
[Cu(o-phen)2Cl]Cl,O2
O
O
[O]
2+O
O
H
Cu(o-phen)2
N
BnNH2
+
4
CH CN,60°C 4A M.S.
,
O
3
O
3
Ph
I
1a
1ab(65%)
O
O
O
-H+
[O]
Bn
[Cu(o-phen)2Cl]Cl,O2
N
O
Ph
N
N.R
CH CN,60°C 4A M.S.,I
,
O
3
2
1ab
c
Scheme 2
Scheme 3 Possible mechanism for the formation of oxazoles.

JOURNAL OF CHEMICAL RESEARCH 2015 9
The resulting solution was stirred at 60 °C for 24 h (the progress of the
reaction was monitored by TLC). Then H2O (5 mL) and EtOAc (15 mL)
were added. The aqueous phase was extracted with EtOAc (3×15 mL).
The combined organic phases were washed with brine, dried (anhyd.
Na2SO4), filtered, and concentrated. The residue was purified by
column chromatography to give ethyl 5‑methyl‑2‑phenyloxazole‑4‑
carboxylate (1ab); yield 182 mg (79%).
8.23–8.16 (m, 3H), 8.15–8.04 (m, 2H), 8.02–7.94 (m, 1H),7.66–7.54
(m, 1H), 7.52–7.40 (m, 8H); 13C NMR (75 MHz, CDCl3): δ 187.7, 158.1,
153.5, 136.4, 132.2, 130.0, 129.5, 129.4, 128.6, 128.1, 127.3, 127.2,
126.8, 126.4, 125.7, 125.1；HRMS (ESI) calcd for C22H16NO2 (M+ + H ):
326.1176; found: 326.1176; error: 0 ppm.
1-(5-Methyl-2-phenyloxazol-4-yl) ethanone：Colourless solid; m.p.
77–78 °C (lit.17, 78–79 °C); IR (film)=2963, 1619, 1380, 1078, 693,
674 cm–1; 1H NMR (300 MHz, CDCl3): δ 8.05–7.99 (m, 2H), 7.49–7.43
(m, 3H),2.69 (s, 3H), 2.60 (s, 3H); 13C NMR (75 MHz, CDCl3): δ 196.0,
159.3, 155.0, 130.9, 129.4, 127.3, 126.9, 28.5, 12.9；HRMS (ESI) calcd
for C12H12NO2 (M+ +H): 202.0863; found: 202.0863; error: 0 ppm.
Ethyl 5-methyl-2-p-tolyloxazole-4-carboxylate：Yellow solid; m.p.
65–67 °C (lit.17 67–68 °C); IR (film)=2988, 1712, 1612, 1486, 1192,
Ethyl 5-methyl-2-phenyloxazole-4-carboxylate：Colourless solid;
m.p. 49–51 °C (lit.3 51–52 °C); IR (film)=2967, 1713, 1608, 1377,
1
1183, 1112, 712, 691 cm–1; H NMR (300 MHz, CDCl3):δ 8.10–8.05
(m, 2H), 7.48–7.43 (m, 3H), 4.45 (q, J=7.2 Hz, 2H), 2.71 (s, 3H), 1.42
(t, J=7.2 Hz, 3H); 13C NMR (75 MHz, CDCl3): δ 161.3, 158.5, 155.0,
129.6, 127.4, 127.2, 125.4, 125.3, 60.0, 14.2, 12.3；HRMS (ESI) calcd
for C13H14NO3 (M+ +H): 232.0968; found: 232.0969; error: 0.4 ppm.
Methyl 5-methyl-2-phenyloxazole-4-carboxylate: Light yellow solid;
m.p. 86–88 °C (lit.6 89–91 °C); IR (film)=2980, 1734, 1615, 1372, 1341,
1236, 1104, 783, 709, 691 cm–1; 1H NMR (300 MHz, CDCl3):δ 8.10–8.05
(m, 2H), 7.50–7.45 (m, 3H), 3.94 (s, 3H), 2.70 (s, 3H); 13C NMR
(75 MHz, CDCl3): δ 161.5, 160.1, 158.6, 131.5, 127.7, 127.3, 125.7, 125.6,
61.3, 28.2, 21.2, 14.2, 13.5；HRMS (ESI) calcd for C12H12NO3 (M+ + H ):
218.0812; found: 218.0812; error: 0 ppm.
tert-Butyl 5-methyl-2-phenyloxazole-4-carboxylate: Light yellow
solid; m.p. 67–69 °C (lit.17 72–73 °C); IR (film)=2977, 1730, 1710, 1369,
1166, 1111, 710, 497 cm–1; 1H NMR (300 MHz, CDCl3):δ 8.08–8.02 (m,
2 H),7.45–7.41 (m, 3 H), 2.66 (s, 3 H), 1.61 (s, 9 H); 13C NMR (75 MHz,
CDCl3): δ 160.5, 158.4, 154.9, 131.0, 129.1, 129.3, 125.5, 125.3, 28.7,
12.2；HRMS (ESI) calcd for C15H18NO3 (M+ +H): 260.1281; found:
260.1283; error: 0.7 ppm.
1
1007, 821, 719 cm–1; H NMR (300 MHz, CDCl3):δ 7.95 (d, J=7.5 Hz,
2H), 7.28–7.22 (m, 2H), 4.43 (q, J=7.2 Hz, 2H), 2.68 (s, 3H), 2.38 (s,
3H), 1.40 (t, J=7.2 Hz, 3H); 13C NMR (75 MHz, CDCl3): δ 162.0, 159.3,
155.3, 140.1, 128.9, 127.7, 127.6, 125.9, 60.4, 21.1, 13.9, 11.9；HRMS
(ESI) calcd for C14H16NO3 (M+ +H): 246.1125; found: 246.1126; error:
0.4 ppm.
Ethyl
2-(4-chlorophenyl)-5-methyloxazole-4-carboxylate：Light
yellow solid; m.p. 80–82 °C (lit.18 68–69 °C); IR (film)=2980, 1712,
1609, 1480, 1183, 1104, 836, 728 cm–1; H NMR (300 MHz, CDCl3):
1
δ 8.00 (d, J=7.2 Hz,2H), 7.38 (d, J=7.2 Hz, 2H), 4.42 (q, J=7.2 Hz,
2H), 2.71 (s, 3H), 1.43 (t, J=7.2 Hz, 3H); 13C NMR (75 MHz, CDCl3):
δ 161.4, 157.9, 155.5, 135.9, 128.7, 127.7, 126.9, 124.2, 60.2, 13.5,
12.3; HRMS (ESI) calcd for C13H13ClNO3 (M+ +H): 266.0578; found:
266.0578; error: 0 ppm.
Ethyl
2-(4-fluorophenyl)-5-methyloxazole-4-carboxylate：Yellow
solid; m.p. 70–72 °C (lit.17 72–73 °C); IR (film)=3012, 2986, 1709,
1622, 1500, 1199, 820, 733 cm–1; 1H NMR (300 MHz, CDCl3): δ
8.08–8.01 (m, 2H), 7.15–7.05 (m, 2H), 4.41 (q, J=7.2 Hz, 2H), 2.67 (s,
3H), 1.40 (t, J=7.2 Hz, 3H); 13C NMR (75 MHz, CDCl3): δ 165.8, 162.5,
162.0, 157.8, 155.3, 135.9, 128.8, 128.7, 127.9, 127.7, 115.1, 114.8, 60.3,
60.2, 14.2, 12.1；HRMS (ESI) calcd for C13H13FNO3 (M+ + H ): 250.0874;
found: 250.0874; error: 0 ppm.
Ethyl 2-phenyl-5-propyloxazole-4-carboxylate：Light yellow solid;
m.p. 51–53 °C (lit.17 53–54 °C); IR (film)=2980, 1715, 1611, 1363, 1332,
1229, 1099, 775, 701, 682 cm–1; 1H NMR (300 MHz, CDCl3):δ 8.12–8.01
(m, 2H), 7.48–7.42 (m, 3H), 4.41 (q, J=7.2 Hz, 2H), 3.08 (t, J=7.5 Hz,
2H), 1.84–1.72 (m, 2H), 1.41 (t, J=7.2 Hz, 3H), 1.2 (t, J=7.5 Hz,
3H).; 13C NMR (75 MHz, CDCl3): δ 160.5, 157.7, 157.5, 128.6, 126.4,
126.0, 124.2, 124.4, 59.1, 26.2, 19.9, 13.4, 12.7；HRMS (ESI) calcd for
C15H18NO3 (M+ +H): 260.1281; found: 260.1281; error: 0 ppm.
Ethyl
2-(2-chlorophenyl)-5-methyloxazole-4-carboxylate：Yellow
solid; m.p. 63–65 °C (lit.17 65–66 °C); IR (film)=2979, 1732, 1614,
Ethyl 5-iso-propyl-2-phenyloxazole-4-carboxylate：Light yellow
solid; m.p. 56–58 °C (lit.18 57–58 °C); IR (film)=2972, 1718, 1605, 1438,
1433, 1227, 1109, 1078, 1030, 758 cm–1; H NMR (300 MHz, CDCl3):δ
1
1
1363, 1239, 1053, 693 cm–1; H NMR (300 MHz, CDCl3):δ 8.10–8.02
8.02–7.96 (m, 1H), 7.50–7.45 (m, 1H), 7.43–7.32 (m, 2H), 4.42 (q,
J=7.2 Hz, 2H), 2.72 (s, 3H), 1.41 (t, J=7.2 Hz, 3H); 13C NMR (75 MHz,
CDCl3): δ 157.0, 155.9, 131.8, 130.6, 130.4, 130.0, 126.5, 125.8, 125.0,
60.0, 13.7, 12.1；HRMS (ESI) calcd for C13H13ClNO3 (M+ + H ):
266.0578; found: 266.0576; error: 0.8 ppm.
Ethyl 2-(furan-2-yl)-5-methyloxazole-4-carboxylate：Light yellow
solid; m.p. 76–78 °C (lit.17 75–76 °C); IR (film)=3111, 2923, 1709, 1607,
1372, 1106, 768 cm–1; 1H NMR (300 MHz, CDCl3): δ 7.57–7.53 (m, 1H),
7.11–7.06 (m, 1H), 6.56–6.50 (m, 1H), 4.40 (q, J=7.2 Hz, 2H), 2.68 (s,
3H), 1.40 (t, J=7.2 Hz, 3H); 13C NMR (75 MHz, CDCl3): δ 161.8, 155.0,
151.6, 143.9, 141.4, 127.9, 111.5, 111.1, 60.5, 13.7, 11.7；HRMS (ESI)
calcd for C11H12NO4 (M+ + H): 222.0761; found: 222.0761; error: 0 ppm.
(m, 2 H), 7.50–7.40 (m, 3 H), 4.32 (q, J=7.2 Hz, 3 H), 3.89–3.76 (m, 1
H), 1.38 (t, J=7.2 Hz, 3 H), 1.35 (d, J=7.2 Hz, 6H); 13C NMR (75 MHz,
CDCl3): δ 160.1, 158.3, 155.1, 126.2, 124.3, 122.1, 122.5, 122.1, 57.9,
24.2, 19.6, 12.3；HRMS (ESI) calcd for C15H18NO3 (M+ + H ): 260.1281;
found: 260.1281; error: 0 ppm.
Ethyl 5‑tert-butyl-2-phenyloxazole-4-carboxylate：Light yellow
solid; m.p. 70–73 °C (lit.17 75–76 °C); IR (film)=2964, 1707, 1606,
1
1374, 1333, 1229, 1092, 707, 681 cm–1; H NMR (300 MHz, CDCl3):δ
8.06–8.02 (m, 2H), 7.48–7.43 (m,3H), 4.40 (q, J=7.2 Hz, 2H), 1.51 (s,
9H), 1.45–1.39 (m, 3H); 13C NMR (75 MHz, CDCl3): δ 164.7, 160.4,
155.7, 128.9, 126.8, 126.3, 124.7, 124.5, 59.0, 32.4, 26.0；HRMS (ESI)
calcd for C16H20NO3 (M+ +H): 274.1438; found: 274.1441; error: 1.1 ppm.
Ethyl 2,5-diphenyloxazole-4-carboxylate：Light yellow solid; m.p.
84–86 °C (lit.5 86–87 °C); IR (film)=3060, 2979, 1718, 1489, 1317,
1214, 1092, 707, 386 cm–1; 1H NMR (300 MHz, CDCl3):δ 8.19–8.04 (m,
4H),7.55–7.45 (m, 6H), 4.45 (q, J=7.2 Hz, 2H), 1.43 (t, J=7.2 Hz, 3H);
13C NMR (75 MHz, CDCl3): δ 162.4, 160.1, 155.8, 132.9, 131.8, 131.9,
130.2, 129.1, 128.9, 127.8, 127.0, 125.9, 60.0, 15.8；HRMS (ESI) calcd
for C18H16NO3 (M+ +H): 294.1125; found: 294.1125; error: 0 ppm.
We gratefully acknowledge the Fuzhou University Zhicheng
College for financial support of this work.
Received 25 August 2014; accepted 26 November 2014
Paper 1402851 doi: 10.3184/174751915X14192609116136
Published online: 9 January 2015
Ethyl
5-(4-chlorophenyl)-2-phenyloxazole-4-carboxylate：White
References
solid; m.p. 86–88 °C (lit.12 110–111 °C); IR (film)=2983, 1718, 14869,
1214, 1092, 753, 712 cm–1; 1H NMR (300 MHz, CDCl3):δ 8.17–8.06 (m,
4H),7.53–7.41 (m, 5H), 4.46 (q, J=7.2 Hz, 2H), 1.43 (t, J=7.2 Hz, 3H);
13C NMR (75 MHz, CDCl3): δ 162.2, 159.9, 153.9, 136.3, 131.2, 129.8,
128.8, 128.7, 128.6, 126.9, 126.2, 125.6, 61.6, 14.3；HRMS (ESI) calcd
for C18H15ClNO3 (M+ +H): 328.0735; found: 328.0734; error: 0.3 ppm.
(2,5-Diphenyloxazol-4-yl)(phenyl) methanone：Light yellow solid;
m.p. 82–84 °C (lit.17 79–80 °C); IR (film)=3041, 2904, 1638, 1536,
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