Iron trichloride catalyzed one-pot synthesis of substituted oxazoles from propargylic acetates and amides under microwave irradiation

Article abstract of DOI:10.2174/157017809790442970

A rapid and efficient procedure under microwave irradiation, using silica gel-supported reagents for the synthesis of substitued oxazoles has been developed through iron trichloride catalyzed propargylation/ cycloisomerization tandem reaction of propargyl

Full text of DOI:10.2174/157017809790442970

664
Letters in Organic Chemistry, 2009, 6, 664-668
Iron Trichloride Catalyzed One-Pot Synthesis of Substituted Oxazoles
from Propargylic Acetates and Amides under Microwave Irradiation
Wen-Wen Xu, Min Lin*, Yang Huang, Li Chen and Zhuang-Ping Zhan
Department of Chemistry, College of Chemistry and Chemical Engineering, and State Key Laboratory for Physical
Chemistry of Solid Surfaces, Xiamen University, Xiamen 361005, P. R. China
Received August 17, 2009: Revised September 15, 2009: Accepted October 14, 2009
Abstract: A rapid and efficient procedure under microwave irradiation, using silica gel-supported reagents for the
synthesis of substitued oxazoles has been developed through iron trichloride catalyzed propargylation/cycloisomerization
tandem reaction of propargylic acetates and amides.
Keywords: Iron trichloride, oxazole, microwave irradiation.
A promising new area of research seeks to employ a
single catalyst to mediate two or more reactions that are
fundamentally different in nature [1]. This research area has
great potential, as there are many different reactions
susceptible to catalysis which might be linked in sequence.
Our work described in this paper adds an additional example
to demonstrate this idea.
reaction developed by our laboratory [13], we sought to
explore the synthesis of substituted oxazoles by employing
FeCl3, which works as a bifunctional catalyst and effectively
mediates two different transformations under the same
conditions (Scheme 1).
Recently, efficient propargylation/cycloisomerization
sequential reactions of terminal propargylic alcohols with
amides in the presence of [Cp*RuCl(μ2-SMe)2RuCp*Cl]/
AuCl3/NH4BF4 [14(a)], or Zn(OTf)2/TpRuPPh3(CH3CN)2
PF6 [14(b)], which lead to the synthesis of substituted
oxazoles, have been reported. However, these methods are
Oxazoles have attracted great interest due to their
appearance as subunit of various biologically active natural
products as well as their utilities as valuable precursors in
many useful synthetic transformations [2]. They are most
R1
OAc
O
N
+
R2
R1
R3
NH2
R3
O
R2
HOAc
O
propargylation
FeCl3
cycloisomerization
HN
R3
FeCl3
R1
R2
HOAc
Scheme 1. Propargylation/cycloisomerization tandem reaction of propargylic acetates and amides.
commonly obtained by the Hantzsch reaction [3] or the
cyclodehydration of ꢀ-ketoamides [4]. Strategies including
the dehydrogenation of oxazolines [5], cycloisomerization of
N-propargylamides [6], Aza-Wittig reactions [7], decom-
position of diazo ketones in nitriles [8], Schmidt
rearrangements [9], the use of isocyanides [10], TosMIC
[11], and ꢁ-acyloxy ketones [12], have also been developed.
To further extend the scope of propargylic substitution
only applicable to a relatively narrow range of substrates,
and two different catalysts were employed. Very recently, an
access to substituted oxazoles via PTSA-catalyzed
propargylation/cycloisomerization tandem reaction has been
released by our lab [15]. However, stoichiometric amount of
PTSA and toxic toluene was used in the protocol.
As microwave (MW) irradiation can provide unique
opportunities that are not easily accessible by conventional
thermal processes [16], and there are few examples of
oxazole formations via microwave irradiation [17], we wish
to report herein an efficient approach to substituted oxazoles
*Address correspondence to the author at the Department of Chemistry,
College of Chemistry and Chemical Engineering, Xiamen University,
Xiamen 361005, P. R. China; E-mail: Minlin@xmu.edu.cn
1570-1786/09 $55.00+.00
© 2009 Bentham Science Publishers Ltd.

Iron Trichloride Catalyzed One-Pot Synthesis
Letters in Organic Chemistry, 2009, Vol. 6, No. 8
665
Table 1. Synthesis of Substituted Oxazole 3aa from Propargylic Acetate 1a and Benzamide 2aa
Ph
OAc
FeCl3
O
N
TMS
+
Ph
Ph
Ph
NH2
O
TMS
2a
1a
3aa
Entry
Condition
Catalyst loading [mol%]
Time
Isolated yield [%]b
1
2
3
Reflux in MeCN
MWc
10
10
15
30 h
18 min
8 min
52
70
82
MWc
aReaction conditions: 1 (0.3 mmol), 2 (0.9 mmol). bIsolated yields of pure products based on propargylic acetate 1a. cAbsorbed on the silica gel.
from propargylic acetates and amides under microwave
irradiation.
yield (Table 2, entry 3). Moreover, substrates 1d and 1e
possessing bromo and ester functionalities at aryl ring were
also successfully employed in the reaction to give 3da and
3ea in good yields (Table 2, entries 4 and 5). It can be
concluded that electron-rich propargylic acetates provided
the expected products in higher yields than electron-poor
counterparts. Compared with propargylic acetate 1a (Table 2,
entry 1), reaction of propargylic acetate 1f (R2 = H) with
benzamide 2a gave 3fa much lower yields and needed even
longer time (Table 2, entry 6). To our surprise, reaction of
propargylic acetate 1g (R2 = n-Bu) with 2a provided 3ga as
well as a rearrangement product 4 in 39% and 51% yields
[18], respectively (Scheme 2; Table 2, entry 7).
Initially, treatment of propargylic acetate 1a with
benzamide 2a in the presence of 10 mol% FeCl3 in MeCN at
reflux for 30 h afforded 3aa in 52% yield (Table 1, entry 1).
Gratifyingly, when we carried out the reaction under MW
conditions by irradiating it for 18 minutes, we obtained the
oxazole 3aa in 70% yield (Table 1, entry 2). Increasing the
amount of FeCl3 to 15 mol% shortened the reaction time to 8
minutes and improved the yield to 82% (Table 1, entry 3).
Reactions of a variety of propargylic acetates with
various amides were carried out under MW conditions.
Typical results are shown in Table 2. Propargylic acetate 1b
with benzamide 2a gave desired product 3ba in 84% yield
(Table 2, entry 2). Propargylic acetate 1c possessing p-MeO
at aryl ring with benzamide afforded the oxazole 3ca in high
We next investigated the scope of amides 2. Both
aromatic and aliphatic amides were employed. The reaction
proceeded smoothly when the aryl groups of amides were
Table 2. Synthesis of Substituted Oxazoles 3 from Propargylic Acetates 1 and Amides 2a
MW
R1
OAc
O
Silica gel support
NH2
15 mol% FeCl3
N
+
R2
R1
R3
R3
O
R2
1
3
2
Entry
Propargylic acetate
1a: R1 = Ph; R2 = TMS
Amide
Time [min]
3/%b
1
2
2a: R3 = Ph
2a: R3 = Ph
2a: R3 = Ph
2a: R3 = Ph
2a: R3 = Ph
2a: R3 = Ph
2a: R3 = Ph
8
4
3aa/82
3ba/84
3ca/85
1b: R1 = 1-Naphthyl; R2 = TMS
1c: R1 = p-MeOC6H4; R2 = TMS
1d: R1 = p-BrC6H4; R2 = TMS
1e: R1 = p-MeOOCC6H4; R2 = TMS
1f: R1 = Ph; R2 = H
3
4
4
8
3da/79
3ea/63
5
11
17
14
8
6
3fa/34
7
1g: R1 = Ph; R2 = n-Bu
3ga/39 (51)c
3ab/87
3ac/84
8
1a: R1 = Ph; R2 = TMS
2b: R3 = p-MeOC6H4
2c: R3 = p-MeC6H4
2d: R3 = p-NO2C6H4
2e: R3 = p-ClC6H4
2f: R3 = Me
9
1a: R1 = Ph; R2 = TMS
4
10
11
12
1a: R1 = Ph; R2 = TMS
11
11
4
3ad/61
3ae/79
1a: R1 = Ph; R2 = TMS
1a: R1 = Ph; R2 = TMS
3af/80
aReaction conditions: 1 (0.3 mmol), 2 (0.9 mmol), and FeCl3 (0.045 mmol) were absorbed on 1 g silica gel and irradiated under MW. bIsolated yields of pure products based on
propargylic acetate 1. cIsolated yield of rearrangement product 4, see Scheme 2.

666 Letters in Organic Chemistry, 2009, Vol. 6, No. 8
Xu et al.
MW
Ph
O
OAc
O
N
Silica gel support
+
+
Ph
n-Bu
Ph
Ph
Ph
NH2
O
15 mol% FeCl3
4
n-Bu
4 51%
3ga 39%
[3,3]
hydrolysis
Ph
OAc
n-Bu
Ph
OH
n-Bu
Scheme 2.
substituted with both electron-donating (Table 2, entries 8
and 9) and electron-withdrawing groups (Table 2, entries 10
and 11). It should be noted that electron-rich amides
provided the desired products in higher yields and needed
shorter time than electron-poor ones. Additionally,
acetamide 2f readily participated in the reaction to afford the
substituted oxazole 3af in good yield with complete
regioselectivity (Table 2, entry 12).
13C NMR (100 MHz, CDCl3): ꢂ -1.1, 16.4, 125.9, 126.7,
126.9, 127.8, 128.5, 128.7, 129.7, 132.9, 134.3, 147.2, 158.6.
ESI-MS: m/z (%) = 308 (100) [M+H+], 330 (21)
[M+Na+].
4-(naphthalen-1-yl)-2-phenyl-5-((trimethylsilyl)methyl)
oxazole (3ba) [15]
a yellow oil. IR (film) ꢁmax: 3046, 1591, 1487 cm-1.
1H NMR (400 MHz, CDCl3): ꢂ 0.10 (s, 9H), 2.23 (s, 2H),
7.44-7.52 (m, 3H), 7.53-7.62 (m, 4H), 7.91-7.96 (m, 2H),
8.13-8.20 (m, 3H).
13C NMR (100 MHz, CDCl3): ꢂ -1.3, 15.4, 125.2, 125.8,
125.9, 126.07, 126.10, 126.13, 127.7, 127.8, 128.2, 128.5,
128.7, 129.7, 129.9, 132.1, 133.9, 148.8, 158.9.
In conclusion, we have developed a rapid and efficient
microwave-assisted procedure for the synthesis of
substituted oxazoles starting from propargylic acetates and
amides using iron trichloride as catalyst. This method
provides a rapid and efficient approach to substituted
oxazoles.
EXPERIMENTAL SECTION
General
ESI-MS: m/z (%) = 358 (29) [M+H+], 380 (100)
[M+Na+].
4-(4-methoxyphenyl)-2-phenyl-5-((trimethylsilyl)methyl)
oxaz-ole (3ca) [15]
Melting points were uncorrected. Column chromato-
graphy was performed on silica gel (99% SiO2, 300-400
mesh, surface area 300-400 m2/g). All reagents were
obtained commercially. MeCN was dried with MgSO4
overnight and then distilled. Silica gel was activated at 110
ꢀ for 16 h before use.
o
a yellow solid (mp 60-62 C). IR (film) ꢁmax: 3063, 1598,
1509 cm-1.
1H NMR (400 MHz, CDCl3): ꢂ 0.15 (s, 9H), 2.39 (s, 2H),
3.86 (s, 3H), 6.99 (d, 2H, J = 8.8 Hz), 7.40-7.49 (m, 3H),
7.67 (d, 2H, J = 8.8 Hz), 8.04-8.07 (m, 2H).
13C NMR (100 MHz, CDCl3): ꢂ -1.1, 16.3, 55.3, 113.9,
125.6, 125.9, 127.9, 128.0, 128.7, 129.6, 134.2, 146.2, 158.6,
158.7.
Typical Procedure for the Synthesis of 2,4-diphenyl-5-
((trimethylsilyl)methyl)oxazole
To a solution of FeCl3 (7.3 mg, 0.045 mmol) in MeCN (3
ml) was added complex 1a (74 mg, 0.3 mmol), 2a (109 mg,
0.9 mmol) and silica gel (1 g). Then the mixture was mixed
thoroughly and dried under reduced pressure. The contents
were transferred into a 5 ml conical flask and irradiated at
680 W in a microwave oven (cooking type, Galanz WP 700P
21-6). After completion indicated by TLC, the reaction
mixture was extracted with dichoromethane (3ꢀ10 ml). After
evaporation, the residue was puried by flash column
chromatography to afford pure oxazole 3aa (76 mg, 82%).
ESI-MS: m/z (%) = 338 (45) [M+H+], 360 (100)
[M+Na+].
4-(4-bromophenyl)-2-phenyl-5-((trimethylsilyl)methyl)
oxazole (3da) [15]
a yellow oil. IR (film) ꢁmax: 3064, 1602, 1584, 1487 cm-1.
1H NMR (400 MHz, CDCl3): ꢂ 0.13 (s, 9H), 2.39 (s, 2H),
7.40-7.49 (m, 3H), 7.54 (d, 2H, J = 8.4 Hz), 7.62 (d, 2H, J =
8.4 Hz), 8.02-8.04 (m, 2H).
13C NMR (100 MHz, CDCl3): ꢂ -1.1, 16.7, 120.8, 125.9,
127.6, 128.2, 128.8, 129.9, 131.6, 131.9, 133.3, 147.5, 158.8.
Characterization Data of All the Products
ESI-MS: m/z (%) = 386 (100) [M+H+].
2,4-diphenyl-5-((trimethylsilyl)methyl)oxazole (3aa) [15]
a yellow oil. IR (film) ꢁmax: 3061, 1595, 1494 cm-1.
methyl
4-(2-phenyl-5-((trimethylsilyl)methyl)oxazol-4-
yl)benz-oate (3ea) [15]
1H NMR (400 MHz, CDCl3): ꢂ 0.14 (s, 9H), 2.41 (s, 2H),
7.27-7.32 (m, 1H), 7.41-7.45 (m, 5H), 7.72-7.75 (m, 2H),
8.04-8.06 (m, 2H).
a white solid (mp 138-140 oC). IR (film) ꢁmax: 3059, 1720,
1612, 1489 cm-1.

Iron Trichloride Catalyzed One-Pot Synthesis
Letters in Organic Chemistry, 2009, Vol. 6, No. 8
667
1H NMR (400 MHz, CDCl3): ꢀ 0.14 (s, 9H), 2.47 (s, 2H),
3.93 (s, 3H), 7.41-7.49 (m, 3H), 7.84 (d, 2H, J = 8.4 Hz),
8.04-8.06 (m, 2H), 8.10 (d, 2H, J = 8.8 Hz).
13C NMR (100 MHz, CDCl3): ꢀ -1.1, 16.9, 52.1, 126.0,
126.2, 127.5, 128.3, 128.8, 129.8, 129.9, 133.4, 137.5, 148.8,
158.9, 166.9.
4-phenyl-2-p-tolyl-5-((trimethylsilyl)methyl)oxazole (3ac)
[15]
a pale yellow oil. IR (film) ꢁmax: 3031, 1613, 1503 cm-1.
1H NMR (400 MHz, CDCl3): ꢀ 0.13 (s, 9H), 2.39 (s, 3H),
2.40 (s, 2H), 7.24-7.30 (m, 3H), 7.39-7.44 (m, 2H), 7.71-
7.74 (m, 2H), 7.94 (d, 2H, J = 8.4 Hz).
13C NMR (100 MHz, CDCl3): ꢀ -1.2, 16.4, 21.5, 125.1,
125.9, 126.7, 126.9, 128.5, 129.4, 133.0, 134.1, 139.9, 146.8,
159.9.
ESI-MS: m/z (%) = 366 (100) [M+H+], 388 (43)
[M+Na+].
5-methyl-2,4-diphenyloxazole (3fa) [19]
ESI-MS: m/z (%) = 322 (100) [M+H+].
a yellow oil. IR (film) ꢁmax: 3062, 1600, 1495 cm-1.
1H NMR (400 MHz, CDCl3): ꢀ 2.62 (s, 3H), 7.30-7.35
(m, 1H), 7.42-7.48 (m, 5H), 7.72-7.75 (m, 2H), 8.06-8.09
(m, 2H).
13C NMR (100 MHz, CDCl3): ꢀ 12.0, 126.1, 126.8,
127.3, 127.7, 128.6, 128.7, 129.9, 132.4, 135.9, 143.9, 159.4.
2-(4-nitrophenyl)-4-phenyl-5-((trimethylsilyl)methyl)oxazole
(3ad) [15]
o
a yellow solid (mp 124-126 C). IR (film) ꢁmax: 3060,
1606, 1588, 1494 cm-1.
1H NMR (400 MHz, CDCl3): ꢀ 0.15 (s, 9H), 2.47 (s, 2H),
7.32-7.36 (m, 1H), 7.43-7.47 (m, 2H), 7.72-7.74 (m, 2H),
8.19 (d, 2H, J = 8.8 Hz), 8.32 (d, 2H, J = 8.8 Hz).
13C NMR (100 MHz, CDCl3): ꢀ -1.1, 16.8, 124.2, 126.4,
126.7, 127.5, 128.7, 132.2, 133.1, 135.5, 148.1, 149.3, 156.6.
ESI-MS: m/z (%) = 236 (100) [M+H+], 258 (73)
[M+Na+].
5-pentyl-2,4-diphenyloxazole (3ga) [15]
a yellow oil. IR (film) ꢁmax: 3061, 1599, 1580, 1495 cm-1.
ESI-MS: m/z (%) = 353 (100) [M+H+].
1H NMR (400 MHz, CDCl3): ꢀ 0.91 (t, 3H, J = 7.2 Hz),
1.34-1.44 (m, 4H), 1.75-1.84 (m, 2H), 2.92 (t, 2H, J = 7.6
Hz), 7.29-7.33 (m, 1H), 7.41-7.47 (m, 5H), 7.71-7.73 (m,
2H), 8.07-8.09 (m, 2H).
2-(4-chlorophenyl)-4-phenyl-5-((trimethylsilyl)methyl)
oxazole (3ae) [15]
a yellow oil. IR (film) ꢁmax: 3060, 1593, 1484 cm-1.
1H NMR (400 MHz, CDCl3): ꢀ 0.15 (s, 9H), 2.43 (s, 2H),
7.30-7.34 (m, 1H), 7.43-7.46 (m, 4H), 7.72-7.75 (m, 2H),
7.99 (d, 2H, J = 8.8 Hz).
13C NMR (100 MHz, CDCl3): ꢀ -1.1, 16.5, 126.3, 126.7,
127.1, 127.2, 128.6, 129.0, 132.7, 134.5, 135.7, 147.6, 157.8.
13C NMR (100 MHz, CDCl3): ꢀ 13.9, 22.4, 26.0, 27.9,
31.4, 126.1, 126.9, 127.3, 127.7, 128.5, 128.6, 129.9, 132.5,
135.7, 148.2, 159.4.
ESI-MS: m/z (%) = 292 (39) [M+H+], 314 (100)
[M+Na+].
ESI-MS: m/z (%) = 342 (100) [M+H+], 364 (93)
(E)-1-phenylhept-1-en-3-one (4) [20]
[M+Na+].
a yellow oil. IR (film) ꢁmax: 1686, 1620, 1500, 1181,
1069 cm-1.
2-methyl-4-phenyl-5-((trimethylsilyl)methyl)oxazole (3af)
[15]
a colorless oil. IR (film) ꢁmax: 3062, 1599, 1493 cm-1.
1H NMR (400 MHz, CDCl3): ꢀ 0.08 (s, 9H), 2.30 (s, 2H),
2.44 (s, 3H), 7.23-7.27 (m, 1H), 7.36-7.40 (m, 2H), 7.61-
7.63 (m, 2H).
1H NMR (400 MHz, CDCl3): ꢀ 0.95 (t, 3H, J = 7.2 Hz),
1.34-1.44 (m, 2H), 1.63-1.71 (m, 2H), 2.67 (t, J = 7.3 Hz,
2H), 6.75 (d, J = 16.4 Hz, 1H), 7.38-7.40 (m, 3H), 7.53-7.57
(m, 3H).
13C NMR (100 MHz, CDCl3): ꢀ 13.9, 22.4, 26.5, 40.7,
116.3, 128.2, 128.9, 130.4, 134.6, 142.3.
13C NMR (100 MHz, CDCl3): ꢀ -1.2, 13.8, 16.1, 126.4,
126.7, 128.4, 132.5, 132.9, 146.5, 158.4.
2-(4-methoxyphenyl)-4-phenyl-5-((trimethylsilyl)methyl)
oxaz-ole (3ab) [15]
ESI-MS: m/z (%) = 246 (77) [M+H+], 268 (100)
[M+Na+].
o
a pale yellow solid (mp 101-103 C). IR (film) ꢁmax: 3058,
1614, 1591, 1503 cm-1.
ACKNOWLEDGEMENTS
1H NMR (400 MHz, CDCl3): ꢀ 0.13 (s, 9H), 2.39 (s, 2H),
3.86 (s, 3H), 6.97 (d, 2H, J = 8.8 Hz), 7.26-7.31 (m, 1H),
7.39-7.44 (m, 2H), 7.71-7.73 (m, 2H), 7.98 (d, 2H, J = 8.8
Hz).
13C NMR (100 MHz, CDCl3): ꢀ -1.1, 16.3, 55.4, 114.1,
120.7, 126.7, 126.9, 127.5, 128.5, 133.1, 134.0, 146.5, 158.8,
160.9.
The research was financially supported by the National
Natural Science Foundation of China (No. 20772098), the
Program for New Century Excellent Talents in Fujian
Province University, and NFFTBS (No. J0630429).
REFERENCES
ESI-MS: m/z (%) = 338 (100) [M+H+], 360 (57)
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