Palladium-catalyzed ortho-acylation of 2-arylbenzoxazoles

Article abstract of DOI:10.1016/j.tet.2012.10.033

An efficient and facile catalytic protocol for benzoxazole-directed ortho-acylation of 2-arylbenzoxazoles has been described using aldehydes as the easily accessible acyl source. This catalytic system was performed in the presence of tert-butyl hydroperoxide (TBHP) as the oxidant in chlorobenzene, affording the acylated 2-arylbenzoxazoles in moderate to good yields. The acylation exhibited high regioselectivity for the substrates containing a meta-substituent in the benzene ring and typically occurred at the less sterically hindered ortho-C-H bond of the directing group.

Full text of DOI:10.1016/j.tet.2012.10.033

Tetrahedron 69 (2013) 320e326
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Tetrahedron
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Palladium-catalyzed ortho-acylation of 2-arylbenzoxazoles
*
*
Qian Zhang, Chao Li, Fan Yang , Jingya Li, Yangjie Wu
The College of Chemistry and Molecular Engineering, Henan Key Laboratory of Chemical Biology and Organic Chemistry, Key Laboratory of Applied Chemistry of Henan Universities,
Zhengzhou University, Zhengzhou 450052, People’s Republic of China
a r t i c l e i n f o
a b s t r a c t
Article history:
An efficient and facile catalytic protocol for benzoxazole-directed ortho-acylation of 2-arylbenzoxazoles
has been described using aldehydes as the easily accessible acyl source. This catalytic system was per-
formed in the presence of tert-butyl hydroperoxide (TBHP) as the oxidant in chlorobenzene, affording the
acylated 2-arylbenzoxazoles in moderate to good yields. The acylation exhibited high regioselectivity for
the substrates containing a meta-substituent in the benzene ring and typically occurred at the less
sterically hindered ortho-CeH bond of the directing group.
Received 12 July 2012
Received in revised form
30 September 2012
Accepted 9 October 2012
Available online 26 October 2012
Ó 2012 Elsevier Ltd. All rights reserved.
Keywords:
Acylation
Palladium catalysis
2-Arylbenzoxazole
CeH activation
1. Introduction
workers developed an efficient catalytic system for amide-
directed ortho-acylation to generate acylated amides.5d
Aryl ketones as a family of organic intermediates could be easily
functionalized further, and therefore played an irreplaceable role in
the synthesis of numerous natural products and bioactive com-
pounds.1 The classical FriedeCrafts acylation reactions have served
as the main access to ketones for quite a long time, and do have
their own drawbacks and limitation, such as harsh reaction con-
ditions and poor regioselectivity.2
During the past few years, transition metal-catalyzed ligand-
directed ortho-functionalization of CeH-bond has emerged as
a valuable tool in organic synthesis.3 Many arenas containing
a directing group, such as pyridyl, imino, alkoxycarbonyl, carbonyl,
oxazolino, amido and cyano group can result in the cleavage of
ortho-CeH bond under palladium catalysis.3d,4 Particularly, the
ligand-directed ortho-acylation of CeH-bond has attracted con-
siderable attention within the recent few years.5 In 2009, Cheng
and co-workers described a facile route to aromatic ketones from 2-
arylpyridines and benzaldehyde via palladium-catalyzed ortho-
acylation using aldehydes and pyridyl ring as acylating reagents
and directed group, respectively.5a In 2010, Li and co-workers also
reported palladium-catalyzed ligand-directed oxidative sp2 CeH
bond acylation with aromatic or alkyl aldehydes.5b Meanwhile, Yu’s
group also reported oxime-directed ortho-CeH acylation of O-
methyl oximes with aldehydes.5c Just recently, Kwong and co-
Recent years, our research interest has been focused on the func-
tionalization of benzoxazoles due to their low toxicities and lots of
biological activities.6 Further extension and branching functional
substituents on benzoxazoles would have great importance for the
synthesis of pharmaceutical intermediates. In 2008, we realized the
catalytic the benzoxazole-directed ortho-arylation of 2-
arylbenzoxazoles in trifluoroacetic acid, and subsequently found
weak base could remarkably enhance the reaction process.7 Very
recently, we have described halogenations, the benzoxazole-directed
ortho-acetoxylation, and one-pot brominationeborylation of 2-
arylbenzoxazoles.8 It should be noted that the halogenation of 2-
arylbenzoxazoles is not the benzoxazole-directed ortho-CeH activa-
tion process, but an electrophilic substitution at the para-position to
the nitrogen atom in the benzo ring of the benzoxazole moiety. In-
spired by the aforementioned and our own reports, herein we would
like to report an efficient protocol for the benzoxazole-directed ortho-
acylation of 2-arylbenzoxazoles using aldehydes as the acylating
reagents.
2. Results and discussion
Initially, in the absence of the ligand, we performed the opti-
mization process using other oxidants other than TBHP (e.g., O2, BQ,
DDQ, Ag2O, Cu(OAc)2 and CuCl2) or other solvents other than
chlorobenzene (e.g., bromobenzene, 1,4-dioxane, DMSO and DMF),
and the reaction yields of 2-(3-methyphenyl)benzoxazole with
benzaldehyde were very low. And gratifyingly, with TBHP as the
* Corresponding authors. Fax: þ86 371 6797 9408; e-mail addresses: yangf@
zzu.edu.cn (F. Yang), wyj@zzu.edu.cn (Y. Wu).
0040-4020/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tet.2012.10.033

Q. Zhang et al. / Tetrahedron 69 (2013) 320e326
321
oxidant and chlorobenzene as solvent, the acylation could afford
the desired product in a yield of 44% (Table 1, entry 1). Then, effects
of several phosphine ligands (e.g., dppf, DavePhos, X-Phos and
PPh3) were investigated, and the desired product was generated in
yields of 50%, 49%, 60% and 68%, respectively (Table 1, entries 2e5).
It should be noted that the isolated yield could be up to 77% when
10 mol % of PPh3 was utilized, while higher ligand loading
(15 mol %) did not show the much better result (Table 1, entries 6
and 7). Other palladium sources, such as Pd2(dba)3 and PdCl2 were
also checked, and both of them did not exhibit higher catalytic
activity (Table 1, entries 8 and 9). When triphenylphosphine oxide
as the ligand, which was the oxidative product of triphenylphos-
phine was also evaluated, and the similar yield was obtained as that
of triphenylphosphine (Table 1, entry 10). Finally, using triphenyl-
phosphine oxide instead of TBHP, the reaction did not occur (Table
1, entry 11). These results indicate that triphenylphosphine oxide
behaves as not the oxidant but the real ligand in this catalytic
system (Table 1, entries 10 and 11). The molecular structure of the
acylated product (3a) was unambiguously determined by the single
crystal X-ray diffraction study (Fig. 1).9
Table 1
Effects of ligands and palladium source on the acylation of 2-(3-methylphenyl)
benzoxazolea
Fig. 1. Molecular structure of 3a.
O
O
Pd source (5 mol%)
Table 2
The palladium-catalyzed acylation of 2-arylbenzoxazolesa
O
N
N
O
TBHP (6 equiv), Ligand
H
+
Chlorobenzene, 8 h
1a
2a
3a
Entry
Palladium source
Ligand (mol %)
Yieldb (%)
1
2
3
4
5
6
7
8
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd2(dba)3
PdCl2
d
44
50
49
60
68
77
78
54
46
76
0
dppf (5)
DavePhos (5)
X-Phos (5)
PPh3 (5)
PPh3 (10)
PPh3 (15)
PPh3 (10)
PPh3 (10)
Ph3PO (10)
Ph3PO (6 equiv)
Entry R1
R2
Product
Yieldb (%)
1
m-Me (1a)
m-OMe (2b)
62 (3b)
9
10
Pd(OAc)2
Pd(OAc)2
11c
Pr
PPh
PCy
P
Fe
N
2
3
m-Me (1a)
m-Me (1a)
m,p-diOMe (2c)
65 (3c)
65 (3d)
Cy P Pr
Ph P
1,1'-Bis(diphenyl
phosphino)ferrocene
(dppf)
2-Dicyclohexylphosphino-
2'-(N,N-dimethylamino)biphenyl 2',4',6'-triisopropyl-1,1'-biphenyl
(DavePhos) (X-Phos)
2-(Dicyclohexylphosphino)-
Bold represents the optimal conditions.
a
Reaction conditions: 1a (0.4 mmol), benzaldehyde (2.4 mmol), catalyst (5 mol %
of Pd), phosphine ligand and TBHP (6 equiv) in chlorobenzene (2.0 mL) at reflux for
p-Me (2d)
8 h.
b
Isolated yield.
In the absence of TBHP.
c
4
5
6
m-Me (1a)
m-Me (1a)
m-Me (1a)
p-Cl (2e)
p-Br (2f)
p-F (2g)
85 (3e)
42 (3f)
71 (3g)
Under the optimized conditions, the scope of palladium-
catalyzed acylation of an array of 2-arylbenzoxazoles with various
substituted benzaldehydes was also investigated (Table 2). Gener-
ally, the acylation could tolerate various functional groups, such as
MeO, Cl, Br and F, affording the desired acylated products in mostly
moderate to good yields. Notably, when the benzene ring of the 2-
arylbenzoxazoles possessed
a meta-substituent, the acylation
would show high regioselectivity and occurred at the less sterically
hindered ortho-CeH bond of the directing group (Table 2, entries
1e10, 13e15). For the reaction of 2-(3-methylphenyl)benzoxazole
(1a) with substituted benzaldehydes, the substituents on the ben-
zene ring of aldehydes did not have evident electronic effect on this
acylation (Table 2, entries 1e6). In the case of this acylation of 2-(3-
(continued on next page)

322
Q. Zhang et al. / Tetrahedron 69 (2013) 320e326
Table 2 (continued )
acylated products would be generated in a relatively lower yields,
Entry R1
R2
Product
Yieldb (%)
although the reaction of 2-(3-methylphenyl)benzoxazole (1a) with
o-chlorobenzaldehyde (2k) gave the product in a moderate yield of
61% (Table 2, entries 7e10). The electronic effect of the substituents
on the benzene rings of 2-arylbenzoxazoles was also investigated,
and the isolated yields of the substrates bearing an electron-
donating or electron-neutral group in the benzene ring would be
higher than those of the substrates bearing an electron-
withdrawing group in the benzene ring (Table 2, entries 11e17).
Subsequently, we explored the acylation of 2-arylbenzoxazoles
bearing the substituents in the benzo ring of the benzoxazole
moiety (1i and 1j), and the results were summarized as Table 3.
When there is a methyl group in the benzo ring of the benzoxazole
moiety (1i), the substituent has little effect on the reaction and the
reactions proceeded similarly as those of 2-(3-methylphenyl)
7
8
9
m-Me (1a)
m-Me (1a)
m-Me (1a)
o-Me (2h)
44 (3h)
o-Cl (2i)
61 (3i)
42 (3j)
o,p-diCl (2j)
Table 3
Effect of the substituents in the benzo ring of the benzoxazole moiety on the acyl-
ation of 2-arylbenzoxazolesa
R
10
m-Me (1a)
o-Br (2k)
39 (3k)
O
Pd(OAc) (5 mol%)
O
O
N
TBHP (6 eqiv)
H
N
O
R
+
R
R
PPh (10 mol%)
chlorobenzene, reflux, 8 h
1
2
3
Entry
1
R1
R2
Product
Yieldb (%)
11
12
13
o-Me (1b)
p-Me (1c)
p-Cl (2l)
p-Cl (2l)
61 (3l)
O
7-CH3 (1i)
7-CH3 (1i)
7-CH3 (1i)
5-Cl (1j)
p-OMe (2m)
42 (3s)
N
O
76 (3m)
71 (3n)
OMe
O
2
3
4
5
H (2a)
70 (3t)
81 (3u)
40 (3v)
55 (3w)
N
O
m-OMe (1d) p-Cl (2l)
O
p-Cl (2l)
N
O
14
15
m-Cl (1e)
m-F (1f)
p-Cl (2l)
p-Cl (2l)
41 (3o)
62 (3p)
Cl
O
p-OMe (2m)
N
O
Cl
OMe
O
5-Cl (1j)
H (2a)
N
O
Cl
16
17
p-F (1g)
H (1h)
p-Cl (2l)
p-Cl (2l)
50 (3q)
76 (3r)
O
6
7
5-Cl (1j)
5-Cl (1j)
p-Cl (2l)
p-Br (2f)
42 (3x)
37 (3y)
N
O
Cl
Cl
a
Reaction conditions: 1aeh (0.4 mmol), aldehyde (2.4 mmol), Pd(OAc)2 (5 mol %),
PPh3 (10 mol %) and TBHP (2.4 mmol) in chlorobenzene (2.0 mL) at reflux for 8 h.
O
b
Isolated yield.
N
O
Cl
Br
methylphenyl)benzoxazole (1a) with p-chlorobenzaldehyde (2l),
the product could be obtained in a yield of 85% (Table 2, entry 4). It
should be noted that the ortho-steric effect on the benzene ring of
aldehydes could have influence on the reaction efficiency and the
a
Reaction conditions: 1i or 1j (0.4 mmol), aldehyde (2.4 mmol), Pd(OAc)2
(5 mol %), PPh3 (10 mol %) and TBHP (2.4 mmol) in chlorobenzene (2.0 mL) at reflux
for 8 h.
b
Isolated yield.

Q. Zhang et al. / Tetrahedron 69 (2013) 320e326
323
benzoxazole (1a), generating the products in moderate to good
3. Conclusions
yields (Table 3, entries 1e3). However, the benzo ring of the ben-
zoxazole moiety bearing a weakly electron-withdrawing chlorine
atom (1j) would result in a decrease of the reactivity of the sub-
strates, affording the acylated products in relatively low to mod-
erate yields (Table 3, entries 4e7).
Finally, the acylation using other heterocycles (e.g., benzothia-
zole, benzimidazole and pyridine) as the directing group was also
investigated (Scheme 1). The reaction of 2-(3-methylphenyl)ben-
zothiazole (1k) with p-chlorobenzaldehyde (2l) was carried out,
and the isolated yield of the desired product (4a) could be up to
83%. However, using benzimidazole as the directing group, the
acylation did not occur at all. Gratifyingly, the acylation of benzo[h]
quinoline (1m) with p-chlorobenzaldehyde (2l) could gave the
desired product (4c) in a yield of up to 96%.
A convenient and atom-economic synthesis of aromatic ketones
bearing benzoxazole moieties by palladium-catalyzed ortho-CeH
acylation of 2-arylbenzoxazoles has been developed. Remarkable
features of this acylation are high catalytic efficiency using
Pd(OAc)2/PPh3 as the catalyst, the high regioselectivity of the less
sterically hindered ortho-CeH bond, and tolerance of functional
groups, such as MeO and halogen atoms. This protocol provides
a new synthetic strategy for the acylated benzoxazoles, which have
shown low toxicities and lots of biological activities and thereby
would have wide applications in pharmaceutical chemistry. Further
application of this synthetic methodology is currently underway in
our laboratory.
4. Experimental section
4.1. General details
1H NMR, 13C NMR spectra were recorded on a Bruker DPX-400
spectrometer with CDCl3 as the solvent and TMS as an internal
standard. Melting points were measured by using a WC-1 micro-
scopic apparatus. GC analysis was performed on Agilent 4890D gas
chromatograph. Mass spectra were measured on an LC-MSD-Trap-
XCT instrument. High resolution mass spectra were measured on
a UHPLC Q-TOF HRMS. The single crystal X-ray diffraction study
was measured on
a Xcalibur, Eos, Gemini diffractometer.
Dichloromethane, ethyl acetate and hexane were used for column
chromatography without further purification, and other solvents
were purified according to the standard methods. 2-
Arylbenzoxazoles were prepared according to the reported liter-
ature.7a,8a Other chemicals were obtained from commercial sources
and used as-received unless otherwise noted.
4.2. General procedure for the palladium-catalyzed acylation
of 2-arylbenzoxazoles
Scheme 1. Other three kinds of heterocycle-directed acylation.
To a solution of 2-arylbenzoxazole (0.4 mmol) in chlorobenzene
(2 mL), aldehyde (2.4 mmol), Pd(OAc)2 (5 mol %), PPh3 (10 mol %)
and TBHP (2.4 mmol) were added. The resulting mixture was
heated at refluxing for 8 h. After the reaction was complete, the
mixture was added into H2O (25 mL) and extracted with ethyl ac-
etate (10 mL) for three times. The combined organic layer was dried
over anhydrous Na2SO4 and filtered. After removal of the solvent in
vacuo, the residue was purified by column chromatography (ethyl
acetate/hexane) to afford the pure product.
Based on the previous reports5,10 and our own results, the ten-
tative mechanism for palladium-catalyzed ortho-CeH acylation of
2-arylbenzoxazole is depicted in Scheme 2. The active palladium
reacted with 2-arylbenzoxazole to give the intermediate of palla-
dacycle A via ortho-CeH activation. The intermediate A would react
with the acyl radicals, which were generated in situ by the radical
reaction of TBHP with the aldehydes, affording the oxidative ad-
dition product as intermediate B. The reductive elimination of in-
termediate B would result in the desired product 3a and the
regeneration of active palladium species.
4.2.1. 2-(2-Benzoyl-5-methylphenyl)benzoxazole (3a). Yellow solid,
mp 149e150 ꢀC; 1H NMR (400 MHz, CDCl3):
d
2.53 (s, 3H), 7.21e7.27
(m, 2H), 7.29e7.35 (m, 3H), 7.39e7.46 (m, 3H), 7.59e7.63 (m, 1H),
7.76e7.81 (m, 2H), 8.11 (s, 1H); 13C NMR (100 MHz, CDCl3):
21.3,
d
110.4, 120.2, 124.5, 125.1, 125.5, 128.3, 128.8, 129.4, 129.8, 131.8,
132.9, 137.3, 137.6, 140.6, 141.6, 150.6, 161.7, 197.2; HRMS-ESI (m/z):
calcd for C21H16NO2 (MþH): 314.1176, found 314.1184.
4.2.2. 2-[2-(3-Methoxybenzoyl)-5-methylphenyl]benzoxazole
(3b). Yellow solid, mp 110e112 ꢀC; 1H NMR (400 MHz, CDCl3):
d
2.53 (s, 3H), 3.80 (s, 3H), 6.95e6.70 (m, 1H), 7.16e7.27 (m, 4H),
7.32e7.36 (m, 1H), 7.43e7.45 (m, 3H), 7.60e7.65 (m, 1H), 8.10 (s,
1H); 13C NMR (100 MHz, CDCl3):
21.3, 55.4, 110.5, 113.2, 119.6,
d
120.2, 122.6, 124.5, 125.2, 125.5, 128.8, 129.3, 129.8, 131.7, 137.3,
138.9, 140.6, 141.6, 150.6, 159.6, 161.7, 197.0; HRMS-ESI (m/z): calcd
for C22H18NO3 (MþH): 344.1281, found 344.1287.
4.2.3. 2-[2-(3,4-Dimethoxybenzoyl)-5-methylphenyl]benzoxazole
Scheme 2. Proposed mechanism for acylation of 2-arylbenzoxazoles.
(3c). White solid, mp 191e193 ꢀC; 1H NMR (400 MHz, CDCl3):

324
Q. Zhang et al. / Tetrahedron 69 (2013) 320e326
d
2.56 (s, 3H), 3.82 (s, 3H), 3.88 (s, 3H), 6.84 (d, J¼8.48 Hz, 1H),
7.43e7.49 (m, 2H), 7.55 (d, J¼7.84 Hz,1H), 7.64e7.68 (m,1H), 7.96 (s,
1H); 13C NMR (100 MHz, CDCl3):
21.4, 110.7, 120.2, 124.7, 125.4,
126.1, 126.5, 129.9, 130.4, 130.7,131.9, 132.0, 134.3, 135.8, 137.0, 137.7,
141.5, 142.2, 150.7, 161.5, 194.2; HRMS-ESI (m/z): calcd for
C21H14Cl2NO2 (MþH): 382.0402, found 382.0397.
7.14e7.18 (m, 1H), 7.28e7.32 (m, 2H), 7.43e7.46 (m, 2H), 7.52e7.56
d
(m, 2H), 7.58e7.62 (m, 1H), 8.10 (s, 1H); 13C NMR (100 MHz, CDCl3):
d
21.3, 56.0, 56.1, 110.8, 111.1, 111.6, 120.6, 125.1, 125.4, 125.9, 126.0,
129.3, 130.2, 131.3, 132.3, 138.4, 140.9, 142.3, 149.8, 151.2, 154.1,
162.3, 195.6; HRMS-ESI (m/z): calcd for C23H20NO4 (MþH):
374.1387, found 374.1390.
4.2.11. 2-[2-(2-Bromobenzoyl)-5-methylphenyl]benzoxazole
(3k). Yellow solid, mp 87e90 ꢀC; 1H NMR (400 MHz, CDCl3):
d 2.52
4.2.4. 2-[5-Methyl-2-(4-methylbenzoyl)phenyl]benzoxazole
(s, 3H), 7.06e7.16 (m, 2H), 7.25e7.34 (m, 3H), 7.43e7.53 (m, 3H),
(3d). Light yellow solid, mp 126e129 ꢀC; 1H NMR (400 MHz,
7.60 (d, J¼7.76 Hz, 1H), 7.65e7.68 (m, 1H), 7.95 (s, 1H); 13C NMR
CDCl3):
(m, 2H), 7.44e7.52 (m, 2H), 7.57e7.61 (m, 2H), 7.66 (d, J¼8.12 Hz,
2H), 8.10 (s, 1H); 13C NMR (100 MHz, CDCl3):
21.2, 21.5, 111.4,
d
2.31 (s, 3H), 2.56 (s, 3H), 7.22 (d, J¼8.04 Hz, 2H), 7.30e7.35
(100 MHz, CDCl3): d 21.4, 110.8, 120.2, 121.5, 124.5, 125.2, 126.7,
130.5, 130.7, 131.1, 131.8, 132.1, 132.5, 134.2, 136.8, 139.0, 141.5, 142.3,
150.8, 162.0, 195.5; HRMS-ESI (m/z): calcd for C21H15BrNO2 (MþH):
392.0286, found 392.0284.
d
120.8, 125.5, 126.1, 126.3, 129.6, 129.9, 130.2, 130.5, 132.8, 136.2,
138.8, 141.4, 142.6, 144.5, 151.4, 162.5, 196.6; HRMS-ESI (m/z): calcd
for C22H18NO2 (MþH): 328.1332, found 328.1337.
4.2.12. 2-[2-(4-Chlorobenzoyl)-6-methylphenyl]benzoxazole
(3l). Yellow solid, mp 97e99 ꢀC; 1H NMR (400 MHz, CDCl3):
d 2.60
4.2.5. 2-[2-(4-Chlorobenzoyl)-5-methylphenyl]benzoxazole
(s, 3H), 7.21e7.30 (m, 4H), 7.37e7.41 (m, 1H), 7.42e7.47 (m, 1H),
(3e). White solid, mp 160e163 ꢀC; 1H NMR (400 MHz, CDCl3):
7.52e7.55 (m, 1H), 7.61e7.66 (m, 3H); 13C NMR (100 MHz, CDCl3):
d
2.54 (s, 3H), 7.23e7.28 (m, 2H), 7.30 (d, J¼8.52 Hz, 2H), 7.34e7.38
(m, 1H), 7.39e7.47 (m, 2H), 7.59e7.64 (m, 1H), 7.73 (d, J¼8.48 Hz,
2H), 8.11 (s, 1H); 13C NMR (100 MHz, CDCl3):
21.3, 110.5, 120.3,
d 20.7, 110.5, 120.2, 124.5, 125.3, 126.4, 126.8, 128.4, 130.3, 130.8,
133.3, 135.4, 139.1, 139.5, 140.6, 141.3, 150.3, 161.0, 195.7; HRMS-ESI
(m/z): calcd for C21H15ClNO2 (MþH): 348.0791, found 348.0794.
d
124.6, 125.3, 125.4, 128.6, 128.7, 129.8, 130.7, 131.9, 136.0, 136.8,
139.3, 140.8, 141.6, 150.5, 161.3, 196.0; HRMS-ESI (m/z): calcd for
C21H15ClNO2 (MþH): 348.0791, found 348.0791.
4.2.13. 2-[2-(4-Chlorobenzoyl)-4-methylphenyl]benzoxazole
(3m). Light yellow solid, mp 190e194 ꢀC; 1H NMR (400 MHz,
CDCl3):
d 2.49 (s, 3H), 7.22e7.27 (m, 2H), 7.29e7.31 (m, 2H),
4.2.6. 2-[2-(4-Bromobenzoyl)-5-methylphenyl]benzoxazole
7.32e7.38 (m, 2H), 7.48 (d, J¼8.00 Hz, 1H), 7.56e7.61 (m, 1H), 7.73
(3f). Light yellow solid, mp 114e117 ꢀC; 1H NMR (400 MHz, CDCl3):
(d, J¼8.64 Hz, 2H), 8.19 (d, J¼8.00 Hz, 1H); 13C NMR (100 MHz,
d
2.57 (s, 3H), 7.29e7.38 (m, 2H), 7.49e7.55 (m, 2H), 7.56e7.63 (m,
4H), 7.66e7.70 (m, 2H), 8.12 (s, 1H); 13C NMR (100 MHz, CDCl3):
21.3, 111.4, 120.8, 125.7, 126.1, 126.5, 128.1, 129.7, 130.4, 131.7, 132.5,
CDCl3): d 21.5, 110.4, 120.2, 122.4, 124.5, 125.2, 128.7, 128.9, 129.2,
130.8, 131.0, 135.9, 139.4, 139.6, 141.7, 142.0, 150.5, 161.2, 196.2;
HRMS-ESI (m/z): calcd for C21H15ClNO2 (MþH): 348.0791, found
348.0793.
d
133.0, 137.8, 138.0, 141.9, 142.4, 151.4, 162.1, 196.1; HRMS-ESI (m/z):
calcd for C21H15BrNO2 (MþH): 392.0286, found 392.0284.
4.2.14. 2-[2-(4-Chlorobenzoyl)-5-methoxyphenyl]benzoxazole
4.2.7. 2-[2-(4-Fluorobenzoyl)-5-methylphenyl]benzoxazole
(3n). Light yellow solid, mp 183e186 ꢀC; 1H NMR (400 MHz,
(3g). Light yellow solid, mp 126e129 ꢀC; 1H NMR (400 MHz,
CDCl3): d 3.98 (s, 3H), 7.13e7.18 (m, 1H), 7.25e7.29 (m, 3H),
CDCl3):
7.28e7.38 (m, 1H), 7.41e7.46 (m, 2H), 7.55e7.65 (m, 1H), 7.78e7.84
(m, 2H), 8.11 (s, 1H); 13C NMR (100 MHz, CDCl3):
21.3, 110.4, 115.5
d
2.53 (s, 3H), 6.98 (t, J¼8.60 Hz, 2H), 7.23e7.27 (m, 2H),
7.30e7.31 (m, 1H), 7.34e7.38 (m, 1H), 7.49 (d, J¼8.48 Hz, 1H),
7.61e7.65 (m, 1H), 7.69e7.75 (m, 3H); 13C NMR (100 MHz, CDCl3):
d
d 55.8, 110.5, 113.9, 117.3, 120.3, 124.7, 125.5, 127.4, 128.6, 130.7,
(d, J¼21.9 Hz), 120.3, 124.6, 125.3, 125.4, 128.7, 129.8, 131.9, 132.0 (d,
J¼8.3 Hz), 134.1 (d, J¼2.9 Hz), 137.0, 140.7, 141.6, 150.6, 161.5, 165.5
(d, J¼253.3 Hz), 195.7; HRMS-ESI (m/z): calcd for C21H15FNO2
(MþH): 332.1087, found 332.1086.
130.8, 131.9, 136.3, 139.1, 141.5, 150.5, 161.1, 161.2, 195.5; HRMS-ESI
(m/z): calcd for C21H15ClNO3 (MþH): 364.0740, found 364.0738.
4.2.15. 2-[5-Chloro-2-(4-chlorobenzoyl)phenyl]benzoxazole
(3o). Light yellow solid, mp 164e166 ꢀC; 1H NMR (400 MHz,
4.2.8. 2-[2-(2-Methoxybenzoyl)-5-methylphenyl]benzoxazole
CDCl3):
d
7.25e7.29 (m, 2H), 7.33e7.39 (m, 3H), 7.47 (d, J¼1.60 Hz,
(3h). Yellow solid, mp 133e136 ꢀC; 1H NMR (400 MHz, CDCl3):
1H), 7.57e7.61 (m, 1H), 7.65 (dd, J¼1.48 Hz, 8.42 Hz, 1H), 7.74 (d,
d
2.49 (s, 3H), 3.64 (s, 3H), 6.74 (d, J¼8.40 Hz, 1H), 6.82e6.88 (m,
1H), 7.20e7.29 (m, 3H), 7.33e7.41 (m, 2H), 7.49 (d, J¼7.80 Hz, 1H),
7.58e7.65 (m, 2H), 7.94 (s, 1H); 13C NMR (100 MHz, CDCl3):
21.3,
J¼8.40 Hz, 1H), 8.25 (d, J¼8.48 Hz, 1H); 13C NMR (100 MHz,
CDCl3):
d 110.5, 120.5, 123.6, 124.8, 125.7, 128.4, 128.9, 130.4,
d
130.5, 130.8, 135.3, 137.7, 139.9, 141.0, 141.6, 150.5, 160.0, 194.2;
HRMS-ESI (m/z): calcd for C20H12Cl2NO2 (MþH): 368.0245, found
368.0243.
55.7, 110.5, 111.7, 120.0, 120.1, 124.3, 124.9, 125.7, 127.7, 128.9, 130.0,
131.5, 133.5, 139.6, 140.6, 141.7, 150.7, 158.7, 162.5, 195.8; HRMS-ESI
(m/z): calcd for C22H18NO3 (MþH): 344.1281, found 344.1292.
4.2.16. 2-[2-(4-Chlorobenzoyl)-5-fluorophenyl]benzoxazole
(3p). White solid, mp 153e155 ꢀC; 1H NMR (400 MHz, CDCl3):
4.2.9. 2-[2-(2-Chlorobenzoyl)-5-methylphenyl]benzoxazole
(3i). Light yellow solid, mp 154e158 ꢀC; 1H NMR (400 MHz, CDCl3):
d
7.25e7.31 (m, 2H), 7.32e7.35 (m, 1H), 7.36e7.37 (m, 2H),
7.38e7.40 (m, 1H), 7.48e7.54 (m, 1H), 7.59e7.63 (m, 1H), 7.70e7.74
(m, 2H), 7.97e8.01 (m, 1H); 13C NMR (100 MHz, CDCl3):
110.2,
d
2.52 (s, 3H), 7.09 (t, J¼7.40 Hz, 1H), 7.18 (td, J¼1.18 Hz, 7.53 Hz, 1H),
7.26e7.30 (m, 3H), 7.34e7.38 (m, 1H), 7.43e7.48 (m, 2H), 7.59 (d,
J¼7.84 Hz, 1H), 7.63e7.67 (m, 1H), 7.96 (s, 1H); 13C NMR (100 MHz,
d
116.2 (d, J¼24.4 Hz),118.3 (d, J¼21.6 Hz),120.6,124.9,125.9,127.6 (d,
J¼8.7 Hz), 128.8, 130.7, 130.8, 135.6 (d, J¼3.6 Hz), 135.8, 139.6, 141.5,
150.5, 159.8, 163.2 (d, J¼249.9 Hz), 194.9; HRMS-ESI (m/z): calcd for
C20H12ClFNO2 (MþH): 352.0541, found 352.0539.
CDCl3):
d 21.4, 110.7, 120.2, 124.5, 125.2, 126.1, 126.5, 130.2, 130.6,
130.8, 131.0, 131.8, 132.0, 133.2, 137.3, 137.4, 141.6, 142.1, 150.8, 161.9,
195.0; HRMS-ESI (m/z): calcd for C21H15ClNO2 (MþH): 348.0791,
found 348.0788.
4.2.17. 2-[2-(4-Chlorobenzoyl)-4-fluorophenyl]benzoxazole
(3q). Yellow solid, mp 129e130 ꢀC; 1H NMR (400 MHz, CDCl3):
4.2.10. 2-[2-(2,4-Dichlorobenzoyl)-5-methylphenyl]benzoxazole
(3j). Yellow solid, mp 155e158 ꢀC; 1H NMR (400 MHz, CDCl3):
d 7.19e7.24 (m, 1H), 7.25e7.29 (m, 2H), 7.33e7.39 (m, 4H), 7.57e7.61
d
2.52 (s, 3H), 7.07 (dd, J¼1.88 Hz, 8.32 Hz, 1H), 7.27e7.32 (m, 4H),
(m, 1H), 7.72e7.77 (m, 2H), 8.32 (dd, J¼5.28 Hz, 8.74 Hz, 1H); 13C

Q. Zhang et al. / Tetrahedron 69 (2013) 320e326
325
NMR (100 MHz, CDCl3):
d
110.5, 115.8 (d, J¼23.6 Hz), 117.5 (d,
(100 MHz, CDCl3):
d
21.3, 111.2, 120.2, 124.8, 125.7, 128.7, 128.8,
J¼21.8 Hz), 120.4, 121.5, 124.7, 125.5, 128.9, 130.7, 131.6 (d, J¼8.6 Hz),
135.2, 139.9, 141.6, 142.0, 150.5, 160.1, 164.0 (d, J¼254.8 Hz), 194.2;
HRMS-ESI (m/z): calcd for C20H12ClFNO2 (MþH): 352.0541, found
352.0538.
129.9, 130.1,130.8, 132.2, 135.9,137.0,139.5,140.9,142.7,149.1,162.7,
195.8; HRMS-ESI (m/z): calcd for C21H14Cl2NO2 (MþH): 382.0402,
found 382.0400.
4.2.25. 2-[2-(4-Bromobenzoyl)-5-methylphenyl]-5-chlorobenzoxazole
4.2.18. 2-[2-(4-Chlorobenzoyl)phenyl]benzoxazole (3r). Light yellow
(3y). Light yellow solid, mp 182e184 ꢀC; 1H NMR (400 MHz, CDCl3):
solid, mp 139e141 ꢀC; 1H NMR (400 MHz, CDCl3):
d
7.24e7.28 (m,
d
7.99 (s, 1H), 7.56 (d, J¼8.44 Hz, 2H), 7.50 (d, J¼1.68 Hz, 1H),
7.36e7.43 (m, 3H), 7.32 (d, J¼7.76 Hz, 1H), 7.17e7.22 (m, 1H),
7.12e7.16 (m, 1H), 2.44 (s, 3H); 13C NMR (100 MHz, CDCl3):
21.3,
2H), 7.32 (d, J¼8.48 Hz, 2H), 7.35e7.40 (m, 1H), 7.48e7.52 (m, 1H),
7.58e7.69 (m, 3H), 7.73 (d, J¼8.36 Hz, 2H), 8.28e8.32 (m, 1H); 13C
d
NMR (100 MHz, CDCl3):
d
110.5, 120.4, 124.6, 125.2, 125.4, 128.4,
111.2, 120.2, 124.2, 125.7, 128.3, 128.7, 129.9, 130.1, 130.9, 131.8, 132.2,
136.3, 136.9, 140.9, 142.7, 162.7, 196.0; HRMS-ESI (m/z): calcd for
C21H14BrClNO2 (MþH): 425.9896, found 425.9895.
128.8, 129.2, 130.3, 130.8, 131.2, 135.8, 139.5, 139.6, 141.6, 150.5,
161.0, 195.9; HRMS-ESI (m/z): calcd for C20H13ClNO2 (MþH):
334.0635, found 334.0636.
4.2.26. 2-[2-(Chlorobenzoyl)-5-methylphenyl]benzothiazole
4.2.19. 2-[2-(4-Methoxybenzoyl)-5-methylphenyl]-7-methylbenzoxa-
(4a). Light yellow solid, mp 130e132 ꢀC; 1H NMR (400 MHz,
zole (3s). Light yellow solid, mp 157e159 ꢀC; 1H NMR (400 MHz,
CDCl3): d 2.50 (s, 3H), 7.21e7.31 (m, 3H), 7.33e7.43 (m, 3H),
CDCl3):
d
8.17 (d, J¼0.64 Hz, 1H), 7.77e7.81 (m, 2H), 7.38e7.59 (m,
7.66e7.72 (m, 3H), 7.77 (d, J¼7.96 Hz, 2H); 13C NMR (100 MHz,
3H), 7.15 (t, J¼7.70 Hz, 1H), 7.01 (d, J¼7.44 Hz, 1H), 6.82 (d,
CDCl3): d 21.2, 121.2, 123.1, 125.2, 126.0, 128.3, 128.7, 130.0, 130.3,
J¼8.96 Hz,1H), 2.53 (s, 3H), 2.26 (s, 3H); 13C NMR (100 MHz, CDCl3):
130.9, 131.9, 135.0, 136.3, 136.4, 138.6, 140.6, 153.2, 165.1, 196.2;
HRMS-ESI (m/z): calcd for C21H15ClNOS (MþH): 364.0557, found
364.0580.
d
14.8, 21.2, 55.4, 109.6, 113.6, 117.3, 121.2, 124.4, 125.0, 126.0, 128.3,
129.6, 130.6, 131.8, 137.5, 140.0, 141.1, 149.9, 161.5, 163.4, 196.0;
HRMS-ESI (m/z): calcd for C23H20NO3 (MþH): 358.1438, found
358.1441.
4.2.27. 10-(4-Chlorobenzoyl)benzo[h]quinoline (4c). Light yellow
solid, mp 81e84 ꢀC; 1H NMR (400 MHz, CDCl3):
d 7.24e7.27 (m, 2H),
4.2.20. 2-[2-Benzoyl-5-methylphenyl]-7-methylbenzoxazole
7.33e7.37 (m, 1H), 7.59e7.62 (m, 1H), 7.65e7.67 (m, 2H), 7.73e7.81
(m, 2H), 7.90 (d, J¼8.84 Hz, 1H), 8.05 (dd, J¼0.88 Hz, 8.021 Hz, 1H),
8.11 (dd, J¼1.44 Hz, 8.04 Hz, 1H), 8.49 (dd, J¼1.84 Hz, 4.36 Hz, 1H);
(3t). Light yellow solid, mp 134e136 ꢀC; 1H NMR (400 MHz, CDCl3):
d
8.19 (s, 1H), 7.84 (d, J¼7.40 Hz, 2H), 7.40e7.48 (m, 4H), 7.31e7.36
(m, 2H), 7.13 (t, J¼7.68 Hz, 1H), 6.99 (d, J¼7.40 Hz, 1H), 2.53 (s, 3H),
2.25 (s, 3H); 13C NMR (100 MHz, CDCl3):
14.9, 21.3, 117.4, 121.1,
13C NMR (100 MHz, CDCl3):
d 121.8, 126.2, 126.4, 127.1, 127.8, 128.4,
d
129.1, 129.2, 130.0, 133.8, 135.4,137.8, 137.9,138.4, 144.5, 147.1,197.3;
HRMS-ESI (m/z): calcd for C20H13ClNO (MþH): 318.0686, found
318.0687.
124.4, 125.3, 126.1, 128.3, 128.6, 129.4, 129.6, 131.9, 133.0, 137.2,
137.5, 140.5, 141.1, 149.9, 161.4, 197.2; HRMS-ESI (m/z): calcd for
C22H18NO2 (MþH): 328.1332, found 328.1338.
Acknowledgements
4.2.21. 2-[2-(4-Chlorobenzoyl)-5-methylphenyl]-7-methylbenzoxazole
(3u). Light yellow solid, mp 163e165 ꢀC; 1H NMR (400 MHz, CDCl3):
We are grateful to the National Natural Science Foundation of
China (Nos. 21172200, 21102134) and the Innovation Fund for
Outstanding Scholar of Henan Province (No. 621001100) for fi-
nancial support to this research.
d
8.10 (s, 1H), 7.68 (d, J¼8.20 Hz, 2H), 7.31e7.40 (m, 3H), 7.24 (d,
J¼8.20 Hz, 2H), 7.08 (t, J¼7.68 Hz, 1H), 6.96 (d, J¼7.36 Hz, 1H), 2.47 (s,
3H), 2.21 (s, 3H); 13C NMR (100 MHz, CDCl3):
14.9, 21.3, 117.5, 121.1,
d
124.6, 125.2, 126.3, 128.4, 128.7, 129.7, 130.7, 131.9, 136.0, 136.7, 139.4,
140.7, 141.1, 149.8, 161.1, 196.1; HRMS-ESI (m/z): calcd for
C22H17ClNO2 (MþH): 362.0948, found 362.0947.
Supplementary data
4.2.22. 5-Chloro-2-[2-(4-methoxybenzoyl)-5-methylphenyl]benzox-
Supplementary data related to this article can be found at http://
dx.doi.org/10.1016/j.tet.2012.10.033.
azole (3v). White solid, mp 140e142 ꢀC; 1H NMR (400 MHz, CDCl3):
d
8.08 (s, 1H), 7.74e7.79 (m, 2H), 7.61 (d, J¼2.00 Hz, 1H), 7.39e7.47
(m, 2H), 7.25e7.29 (m, 1H), 7.19e7.23 (m, 1H), 6.80e6.85 (m, 2H),
3.81 (s, 3H), 2.53 (s, 3H); 13C NMR (100 MHz, CDCl3):
20.3, 54.4,
References and notes
d
108.6, 110.2, 112.6, 119.1, 123.7, 124.4, 127.6, 128.9, 129.4, 130.8, 131.0,
136.7, 139.3, 141.7, 148.2, 162.1, 162.5, 194.7; HRMS-ESI (m/z): calcd
for C22H17ClNO3 (MþH): 378.0897, found 378.0894.
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1502.
4.2.23. 2-[2-Benzoyl-5-methylphenyl]-5-chlorobenzoxazole
(3w). Light yellow solid, mp 144e146 ꢀC; 1H NMR (400 MHz,
CDCl3):
7.30e7.35 (m, 3H), 7.19e7.25 (m, 2H), 7.04e7.17 (m, 2H), 2.41 (s,
3H); 13C NMR (100 MHz, CDCl3):
21.3, 100.0, 111.2, 120.1, 125.0,
d
7.95 (s, 1H), 7.67 (d, J¼7.56 Hz, 2H), 7.46 (d, J¼1.32 Hz, 1H),
d
125.5, 126.6, 128.4, 128.9, 129.4, 129.9, 132.1, 133.0, 137.4, 140.7,
142.7, 149.1, 163.0, 196.9; HRMS-ESI (m/z): calcd for C21H15ClNO2
(MþH): 348.0791, found 348.0791.
4.2.24. 5-Chloro-2-[2-(4-chlorobenzoyl)-5-methylphenyl]benzox-
azole (3x). Light yellow solid, mp 155e157 ꢀC; 1H NMR (400 MHz,
4. (a) Zhang, Y.-H.; Feng, J.-Q.; Li, C.-J. J. Am. Chem. Soc. 2008, 130, 2900; (b) Wen, J.;
Qin, S.; Ma, L.-F.; Dong, L.; Zhang, J.; Liu, S.-S.; Duan, Y.-S.; Chen, S.-Y.; Hu, C.-W.;
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Soc. 2010, 132, 8569; (e) Nishikata, T.; Abela, A. R.; Huang, S.-L.; Lipshutz, B. H. J.
CDCl3):
d
8.11 (s, 1H), 7.75 (d, J¼8.48 Hz, 2H), 7.61 (d, J¼1.76 Hz, 1H),
7.49 (d, J¼7.80 Hz, 1H), 7.43 (d, J¼7.76 Hz, 1H), 7.29e7.37 (m, 2H),
7.28e7.31 (m, 1H), 7.23e7.27 (m, 1H), 2.56 (s, 3H); 13C NMR

326
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Am. Chem. Soc. 2010, 132, 4978; (f) Wang, G.-W.; Yuan, T.-T.; Wu, X.-L. J. Org.
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(b) Leng, Y.-T.; Yang, F.; Zhu, W.-G.; Zou, D.-P.; Wu, Y.-J.; Cai, R.-R. Tetrahedron2011,
67, 6191.
9. CCDC 902677 contains the supplementary crystallographic data for 3a. These
data can be obtained free of charge from the Cambridge Crystallographic Data
5. (a) Jia, X.-F.; Zhang, S.-H.; Wang, W.-H.; Luo, F.; Cheng, J. Org. Lett. 2009, 11,
3120; (b) Basle, O.; Bidange, J.; Shuai, Q.; Li, C.-J. Adv. Synth. Catal. 2010, 352,
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3258.
Centre via https://www.ccdc.cam.ac.uk/datarequest/cif. Crystal, data for com-
ꢁ
pound 3a: C21H15NO2, M¼313.34, triclinic, a¼8.1890(7) A,
a
¼78.067(5)ꢀ, b¼9.
3
ꢁ
¼70.628(7)ꢀ, c¼11.5319(9) A,
g
¼68.915(6)ꢀ, V¼823.10(10) A ,
ꢁ
ꢁ
6. (a) Sondhi, S. M.; Singh, N.; Kumar, A.; Lozach, O.; Meijer, L. Bioorg. Med. Chem.
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K.; Sherry, B. D.; Yin, J.-J.; Fleitz, F. J. Org. Lett. 2012, 14, 3458; (d) Rodríguez, A.
9514(5) A,
b
T¼291.15 K, space group¼Pꢁ1, Z¼2, number of reflections¼6609, independent
reflections¼2929[R(int)¼0.0192], final R indices [I>2
1230, R indices (all data) R1¼0.0524, wR2¼0.1313.
s
(I)] R1¼0.0439, wR2¼0.
ꢀ
D.; Ramírez, C.; Rodríguez, I. I.; Gonzalez, E. Org. Lett. 1999, 1, 527.
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914.