An efficient synthesis of 2-substituted benzoxazoles via RuCl 3·3H2O catalyzed tandem reactions in ionic liquid

Article abstract of DOI:10.1002/cjoc.201190156

An efficient synthesis of 2-substituted benzoxazoles through RuCl ₃·3H₂O catalyzed, air oxidized tandem reactions of 2-aminophenols and aldehydes in [bmim]BF₄ was developed. This synthetic strategy has such advantages as mild reaction conditions, cost-free oxidant, readily available starting materials, and recyclable catalyst and solvent. As an application, it was successfully used in the synthesis of the unreported 5-(benzoxazol-2-yl)-2′-deoxyuridines with potential biological activities. A novel and efficient synthesis of 2-substituted benzoxazoles through RuCl₃ catalyzed, air oxidized tandem reactions of 2-aminophenols and aldehydes in [bmim]BF₄ was developed.

Full text of DOI:10.1002/cjoc.201190156

FULL PAPER
An Efficient Synthesis of 2-Substituted Benzoxazoles via
RuCl3·3H2O Catalyzed Tandem Reactions in Ionic Liquid
Fan, Xuesen*(范学森)
He, Yan(何艳)
Wang, Yangyang(王洋洋)
Zhang, Xinying*(张新迎)
Wang, Jianji(王键吉)
School of Chemistry and Environmental Science, Key Laboratory of Green Chemical Media and Reactions,
Ministry of Education, Henan Key Laboratory for Environmental Pollution Control,
Henan Normal University, Xinxiang, Henan 453007, China
An efficient synthesis of 2-substituted benzoxazoles through RuCl3•3H2O catalyzed, air oxidized tandem reac-
tions of 2-aminophenols and aldehydes in [bmim]BF4 was developed. This synthetic strategy has such advantages
as mild reaction conditions, cost-free oxidant, readily available starting materials, and recyclable catalyst and sol-
vent. As an application, it was successfully used in the synthesis of the unreported 5-(benzoxazol-2-yl)-2'-deoxy-
uridines with potential biological activities.
Keywords 2-Substituted benzoxazoles, ruthenium chloride, tandem reaction, ionic liquids, nucleoside
Introduction
chemoselectivity. The use of catalytic amount of ruthe-
nium(III) together with various oxidants have been used
in an array of oxidative transformations.16-23 At the
same time, ionic liquids (ILs) have been frequently used
as green alternatives to conventional solvents due to
their nonvolatility, nonflammability and thermal stabil-
ity. Their high polarity and the ability to solubilize both
inorganic and organic compounds can result in en-
hanced rate of chemical processes and provide higher
selectivity compared to commonly used volatile sol-
vents.24-25 In addition, the ability of ILs to immobilize
and recycle catalysts makes them an ideal medium for
the transition-metal catalyzed oxidations.26
As a continuation of our research program in
RuCl3•3H2O catalyzed oxidations in IL,27 we were in-
terested in the development of a practical and sustain-
able synthesis of 2-substitued benzoxazoles from the
readily available 2-aminophenols and aldehydes by
taking the advantages offered by RuCl3•3H2O as cata-
lyst and IL as reaction medium.
Substituted benzoxazoles have drawn much attention
due to their broad biological activities1 and their in-
creasing applications in material science.2 To date, three
commonly used strategies for the synthesis of
2-substituted benzoxazoles have been developed. The
first approach is through condensation of 2-amino-
phenols with carboxylic acids in the presence of a
strong acid at high temperature. The drastic reaction
conditions employed therein often cause low yields and
side reactions.3 The second method is via the oxidative
cyclization of phenolic Schiff bases promoted by
various oxidants such as 2,3-dichloro-5,6-dicyanoben-
zoquinone,4 PhI(OAc)2,5 ThClO4,6 PCC,7 I2,8 K2S2O8,9
Pd(OAc)2/O2,10 HAuCl4•4H2O/O2 and activated car-
11
bon/O2.12 Unfortunately, many of these oxidants are
neither economically nor environmentally sustainable.
The third approach to 2-substituted benzoxazoles is the
intramolecular cross-coupling of 2-bromoanilides pro-
moted by copper(I)/(II),13 iron(III) complex,14 or cop-
per(II) oxide nanoparticles.15 These coupling reactions
usually proceed under mild conditions with wide sub-
strate scope. However, the utility and applicability of
this protocol are often compromised by specially made
starting materials, expensive and/or toxic catalyst, and
adverse impact on the environment due to the use of
volatile organic solvents.
Experimental
General
Analytical Thin Layer Chromatography (TLC) was
carried out by using silica gel 60 F254 pre-coated plates.
Visualization was accomplished with UV lamp. NMR
spectra were recorded on Bruker AC-400 NMR spec-
trometer, using TMS as an internal standard and peak
multiplicities are designed as follows: s, singlet; d, dou-
blet; t, triplet; m, multiplet; dd, doublet of doublets; td,
Meanwhile, organic transformations catalyzed by
ruthenium species have emerged as a focus of attention
in recent years due to their high efficiency and excellent
*
E-mail: xuesen.fan@henannu.edu.cn; Tel.: 0086-0373-3329261; Fax: 0086-0373-3326336
Received November 7, 2010; revised December 14, 2010; accepted December 27, 2010.
Project supported by the National Natural Science Foundation of China (Nos. 20772025, 20972042), Innovation Scientists and Technicians Troop
Construction Projects of Henan Province (No. 104100510019), and the Natural Science Foundation of Henan Province (No. 092300410237).
Chin. J. Chem. 2011, 29, 773— 777
© 2011 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
773

Fan et al.
FULL PAPER
triplet of doublets. Chemical shifts (δ) were expressed
downfield from the internal standard tetramethylsilane
and coupling constants J were given in Hz. Mass spectra
were obtained in API mode using a Waters Acquity SQ
HPLC-mass spectrometer. Elemental analyses were
performed on an EA-1110 instrument.
the formation of 3a. To our knowledge, this is the first
example in which air acts as an oxidant for the synthesis
of 2-substituted benzoxazole via the oxidative cycli-
zation of the in situ formed phenolic Schiff base from
2-aminophenol and aldehyde. In literature proce-
dures,4-12 stoichiometric or excessive oxidants other
than air are indispensable. In another aspect, raising the
temperature to 80 ℃ increased the yield to 68% (Entry
8). However, higher temperature did not enhance the
yield obviously (Entry 9). After the reaction time was
optimized, the yield of 3a was improved to 71% (Entry
10). The reaction of 1a and 2a was also run in several
commonly used organic solvents (Entries 12—15). It
followed that compared with ethanol, CH3CN, toluene
or THF, [bmim]BF4 exhibited enhanced reactivity by
reducing the reaction time and improving the yields sig-
nificantly. Therefore, the optimal conditions were ob-
tained as: the mixture of 1a and 2a in [bmim]BF4 was
heated at 80 ℃ for 10 h with 5 mol% of RuCl3 as cata-
lyst and air as oxidant.
Typical procedure for the preparation of 3a
A mixture containing 2-aminophenol (1a, 1 mmol),
benzaldehyde (2a, 1 mmol) and RuCl3•3H2O (0.05
mmol) in [bmim]BF4 (1 mL) was stirred at 80 ℃ for
10 h. Upon completion, the mixture was extracted with
diethyl ether (5 mL×3). The combined organic phases
were concentrated under vacuum. The crude product
was purified by column chromatography eluting with
ethyl acetate/hexane (5%—20%, volume ratio) to give
3a. Other 2-substituted benzoxazoles were obtained in a
similar manner.
Typical procedure for the preparation of 6a
A mixture containing 2-amino-4-methylphenol (1b,
1 mmol), 3',5'-di-O-acetyl-5-formyl-2'-deoxyuridine (5a,
1 mmol) and RuCl3•3H2O (0.05 mmol) in [bmim]BF4 (1
mL) was stirred at 80 ℃ for 10 h. Upon completion,
the crude product was purified by column chromatog-
raphy eluting with ethyl acetate/hexane (20%—50%,
volume ratio) to give 6a. Other 5-(benzoxazol-2-yl)-
2'-deoxyuridines were obtained in a similar manner.
With the optimized reaction conditions, the scope of
both aldehydes and 2-aminophenols was explored
(Scheme 2) and the results are listed in Table 2.
Scheme 2
Results and discussion
As showed in Table 2, various aromatic aldehydes
were investigated by reacting with 2-aminophenol (1a).
The results demonstrated that aryl aldehydes with
electron-donating or electron-withdrawing groups on
the aryl ring underwent the reactions successfully (En-
tries 1—7). Then, we further validated the generality
and efficiency of our synthetic methodology by using
2-amino-4-methylphenol (1b) and 2-amino-4-chloro-
phenol (1c) as the substrates and they reacted smoothly
with aromatic aldehydes (Entries 8—18). On the other
hand, with aliphatic aldehyde, the corresponding prod-
uct was obtained in lower yield (Entry 19). Interestingly,
when phenyl acetaldehyde was treated with 1c under the
standard conditions, 2-benzoyl-5-chlorobenzoxazole (4)
was obtained together with the expected 2-benzyl-5-
chlorobenzoxazole (3t) (Entry 20). Though further
study of the generality of this procedure toward
2-benzoylbenzoxazoles is still under way, this mecha-
nistically interesting and synthetically promising proc-
ess is expected to be developed as a simple and practical
route to 2-benzoylbenzoxazole derivatives.
Initially, 2-aminophenol (1a) and benzaldehyde (2a)
were chosen as model reactants to study the feasibility
of our envisioned route (Scheme 1).
Scheme 1
Thus, a mixture of 1a and 2a was treated with 0.05
equiv. of RuCl3•3H2O and 3 equiv. of tert-butyl hy-
droperoxide (TBHP) in [bmim]BF4 (1-butyl-3-methyl
imidazolium tetrafluoroborate). It turned out that 1a and
2a were completely consumed in 3 h upon stirring at 60
℃ and 2-phenylbenzoxazole (3a) was isolated in a
yield of 57% (Table 1, Entry 1). In a further screening,
it was found that similar yields of 3a could be obtained
with less amount of TBHP, or even without TBHP (En-
tries 2—5) if longer reaction period was allowed. On the
other hand, in the absence of either RuCl3 (Entry 6) or
air (Entry 7), only trace amount of 3a were formed as
indicated by TLC, indicating the combination of RuCl3
(cat.) and air serves as an efficient oxidant system for
Based on the above observation, the mechanism for
the formation of 2-substituted benzoxazoles through
RuCl3•3H2O/air promoted tandem process is proposed
in Scheme 3. Firstly, condensation of 2-aminophenol (1)
with aldehyde (2) gives an imine A, which undergoes an
intramolecular cyclization to afford intermediate B.
Dehydrogenation of B promoted by Ru(III)•3H2O/air
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© 2011 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Chin. J. Chem. 2011, 29, 773— 777

Synthesis of 2-Substituted Benzoxazoles in Ionic Liquid
Table 1 Optimization of the synthesis of 3aa
acidity of the hydrogen on the 2-position of the imida-
zolium cation and its ability to act as a hydrogen bond
donor. The fo＋rmation of the O—H hydrogen bond be-
tween [bmim] and the carbonyl oxygen of 2a induces
electrophilic activation of aldehyde, which benefits the
initial condensation of 1 with 2 to give an imine A.
Moreover, it may further form an N—H hydrogen bond
with the in situ formed imine A and this would acceler-
ate the subsequent intramolecular nucleophilic cycliza-
tion to afford B. When it comes to 2-benzyl-5-chloro-
benzoxazole (3t), it can be further oxidized through
benzylic oxidation to give 2-benzoyl-5-chlorobenzox-
azole (4). Meanwhile, the in situ formed Ru(II) could be
oxidized back to Ru(III) by air to complete the catalytic
cycle.28 To confirm the mechnism, an independent study
was conducted in which 3t was treated with
Ru(III)•3H2O/air in [bmim]BF4. From this reaction,
compound 4 was obtained in 85% yield.
Oxidant
(equiv.)
Time/ Temp./ Yieldb/
Entry
Solvent
h
2
℃
%
57
58
55
56
55
1
2
[bmim]BF4
[bmim]BF4
[bmim]BF4
[bmim]BF4
[bmim]BF4
[bmim]BF4
[bmim]BF4
[bmim]BF4
[bmim]BF4
[bmim]BF4
[bmim]BF4
Ethanol
TBHP (3)
60
TBHP (2)
3
60
3
TBHP (1)
4
60
4
TBHP (0.5)
6
60
5
air
air
—
air
air
air
air
air
air
air
air
8
60
c
6
8
60
—
d
7
8
60
—
68
69
71
70
25
32
36
31
8
8
80
9
8
100
80
10
11
12
13
14
15
10
12
10
10
10
10
80
reflux
reflux
reflux
80
THF
CH3CN
Toluene
Scheme 3
a
Reaction conditions: 1 mmol of 1a and 2a, 0.05 mmol of
RuCl3•3H2O, 1 mL of solvent for Entries 1—11, 10 mL of sol-
vent for Entries 12—15. b Isolated yields. c Without RuCl3•3H2O.
d Under N2.
Table 2 Synthesis of 2-substituted benzoxazolesa
Entry R1 (1)
R2
Product
3a
Yieldb/%
1
2
3
4
5
6
7
8
9
H
C6H5
71
H
2-FC6H4
2-ClC6H4
3-CH3C6H4
3-BrC6H4
4-BrC6H4
4-NO2C6H4
C6H5
3b
3c
66
H
65
H
3d
3e
72
H
72
H
3f
78
H
3g
85
4-CH3
4-CH3
3h
3i
71
It is well known that many 5-heteroaryl-2'-deoxyuri-
dine derivatives, such as thiazole,29 thiophen,29,30 imi-
dazole,29c benzothiazole,29a oxazole,29b isoxazole,31 tri-
azole,32 furan,30,33 and pyridine29c substituted 2'-deoxy-
uridines have attracted much attention by acting as
highly efficient therapeutic or fluorescent agents.29-33 To
our knowledge, however, there is still no report of the
synthesis and biological study of 5-(benzoxazol-2-yl)-
2'-deoxyuridines. With the synthetic procedure devel-
oped above, we envisioned that 5-(benzoxazol-2-yl)-2'-
deoxy-uridines could be obtained through the reaction
of 5-formyl-2'-deoxyuridine (5) and 2-aminophenol (1)
with the assistance of RuCl3/air (Scheme 4).
2-FC6H4
2-ClC6H4
3-CH3C6H4
3-FC6H4
4-CH3C6H4
4-BrC6H4
C6H5
63
10 4-CH3
11 4-CH3
12 4-CH3
13 4-CH3
14 4-CH3
15 4-Cl
16 4-Cl
17 4-Cl
18 4-Cl
19 4-Cl
20 4-Cl
3j
65
3k
3l
76
72
3m
3n
3o
78
80
73
2-FC6H4
3-CH3C6H4
4-BrC6H4
n-Propyl
C6H5CH2
3p
3q
3r
62
71
76
3s
38
Thus, a mixture of 5 and 1 was treated with
RuCl3•3H2O in [bmim]BF4. It turned out that the reac-
tions underwent smoothly and afforded the pyrimidine
nucleoside-benzoxazole hybrids (6) in moderate yields
(Table 3). It should be noted that literature procedures
for the preparation of 5-heteroaryl substituted
pyrimidine nucleosides usually involve palladium and
cupric salt catalyzed coupling of the pre-protected
3t (4)
15 (52)
a
Reaction conditions: 1 mmol of 1 and 2, 0.05 mmol of
RuCl3•3H2O, 1 mL of [bmim]BF4, 80 ℃, 10 h. b Isolated yields.
affords 2-substituted benzoxazoles (3). The accelerating
effect showed by [bmim]BF4 compared with conven-
tional organic solvents is most probably attributed to the
Chin. J. Chem. 2011, 29, 773— 777
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775

Fan et al.
FULL PAPER
5-iodo-2'-deoxyuridine with the pre-made heteroaryltin
derivatives. Considering the tedious procedure, expen-
sive catalyst, and specially made starting materials in-
volved in literature methods, it is reasonable to suggest
our method toward 5-heteroaryl-2'-deoxyuridine is not
only economically, but also environmentally much more
practical and sustainable.
Compared with literature methods, advantages of this
procedure include readily available starting materials,
efficient and recyclable catalyst, cost-free oxidant, and
environmentally benign reaction medium. Further
studies to search for more applications of this novel
oxidative system and to evaluate the biological activities
of the novel pyrimidine nucleoside-benzoxazole hybrids
are currently underway and the results will be reported
in due course.
Scheme 4
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