Gold-Catalyzed Synthesis of 2,5-Disubstituted Oxazoles from Carboxamides and Propynals

Article abstract of DOI:10.1002/adsc.201801386

2,5-Disubstituted oxazoles are synthesized by oxidative gold catalysis. In contrast to a reported procedure that delivers 2,4-disubstituted oxazoles starting from terminal alkynes, a switch in selectivity towards a 2,5-disubstitution is achieved by the use of propynals as starting materials. In the new reaction, the key intermediate is formed by the nucleophilic attack of the carboxamide onto a gold carbenoid, and then condensates with the more electrophilic aldehyde moiety already present in the substrate and not with the ketone that is derived from the oxygen donor. This new cyclization mode introduces a new carbonyl moiety as substituent at the 2,5-disubstituted oxazole, an attractive motive that can be found in bioactive compounds or be used for further derivatizations. (Figure presented.).

Full text of DOI:10.1002/adsc.201801386

Title: Gold-Catalyzed Synthesis of 2,5-Disubstituted Oxazoles from
Carboxamides and Propynals
Authors: Yun Xu, Qian Wang, Zhongyi Zeng, Matthias Rudolph, and A.
Stephen K. Hashmi
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10.1002/adsc.201801386
Advanced Synthesis & Catalysis
FULL PAPER
DOI: 10.1002/adsc.201801386
Gold-Catalyzed Synthesis of 2,5-Disubstituted Oxazoles from
Carboxamides and Propynals
Yun Xu,a, b Qian Wang,a Yufeng Wu,a Zhongyi Zeng,a Matthias Rudolph,a A. Stephen
K. Hashmia, c, *
a
Institute of Organic Chemistry, Heidelberg University, Im Neuenheimer Feld 270, 69120 Heidelberg, Germany
Fax: (+ 49)-6221-54-4205
E-mail: hashmi@hashmi.de
Homepage: http://www.hashmi.de
Department of Environmental Science, School of Engineering, China Pharmaceutical University, 211198 Nanjing,
b
China
c
Chemistry Department, Faculty of Science, King Abdulaziz University (KAU), 21589 Jeddah, Saudi Arabia
Received: ((will be filled in by the editorial staff))
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201######.((Please
delete if not appropriate))
Abstract. 2,5-Disubstituted oxazoles are synthesized by ketone that is derived from the oxygen donor. This new
oxidative gold catalysis. In contrast to a reported procedure cyclization mode introduces a new carbonyl moiety as
that delivers 2,4-disubstituted oxazoles starting from terminal substituent at the 2,5-disubstituted oxazole, an attractive
alkynes, a switch in selectivity towards a 2,5-disubstitution is motive that can be found in bioactive compounds or be used
achieved by the use of propynals as starting materials. In the for further derivatizations.
new reaction, the key intermediate is formed by the
nucleophilic attack of the carboxamide onto a gold carbenoid,
and then condensates with the more electrophilic aldehyde
moiety already present in the substrate and not by the
Keywords: gold catalysis; gold carbenoids; carboxamides;
N-oxides; oxazoles
alkyne (Scheme 1, eq. 3).[11] Based on these reports we
envisioned that if propynals were used as α-oxo gold
carbene precursors a different pathway might be
opened. As depicted in Scheme 1 (eq. 4), the additional,
more active carbonyl moiety should condensate with
the amide moiety, which instead of 2,4-oxazoles
should deliver 2,5-oxazoles. Related methods for gold-
catalyzed oxidative cyclization of yones were reported
in previous studies.[12]
Introduction
2,5-Disubstituted oxazoles exhibit extremely valuable
properties，and they are found as an important motif
in bioactive compounds[1] and pharmaceuticals.[2] As a
consequence, a large number of intramolecular
strategies towards these targets were established in the
last decades[3] (Scheme 1, eq. 1). In order to increase
the modularity of the synthetic protocols, efficient
intermolecular methods to access 2,5-substituted
oxazoles are desirable. For this purpose, many metal
catalysts were applied, such as copper,[4] palladium,[5]
ruthenium[6] and cobalt.[7] In the field of gold
catalysis[8] an elegant strategy was introduced by
Zhang’s group in which intermediary formed α-oxo
gold carbene intermediates, generated by N-oxides,[9]
were trapped by nitriles to form 2,5-disubstituted
oxazoles via an intermolecular process (Scheme 1, eq.
2).[10] The drawback of this protocol is the need for at
least 3 equivalents of the nitrile or its use as reaction
solvent. If carboxamides were used as nucleophiles in
combination with adjusted ligands at the gold center,
better ratios of the two reactants were possible. In this
case 2,4-disubstituted oxazoles are formed by
condensation of the amide unit with the carbonyl
moiety derived from the N-oxide and the terminal
Scheme 1. Gold-catalyzed strategies towards oxazoles
1
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10.1002/adsc.201801386
Advanced Synthesis & Catalysis
For this type of substrates, the oxygen transferred from
the N-oxide should remain in the obtained products.
This should enable a more efficient access to 2,5-
disubstituted oxazoles which in addition bear one
carbonyl unity as substituent. These are valuable
substructures which can be found in bioactive
compounds such as fura-2 acetoxymethyl ester[13] and
ACC1 inhibitors (Figure 1).[14] The results on this
project are summarized herein.
Entry
catalyst
N-
oxide
Solvent
Yieldb)
(%)
a)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
IPrAuCl
Mor-DalPhosAuCl
SPhosAuCl
(4-CF3C6H4)3PAuCl
CyJohnphosAuCl
Ph3PAuCl
1,2-DCE
1,2-DCE
1,2-DCE
1,2-DCE
1,2-DCE
1,2-DCE
1,2-DCE
1,2-DCE
1,2-DCE
1,2-DCE
1,2-DCE
TFE
44
50
43
42
53
61
n.r.
10
43
20
2
2d
2d
2d
2d
2d
2d
2d
2a
2b
2c
2d
2d
2d
2d
KB(C6F5)4
Ph3PAuCl
Ph3PAuCl
Ph3PAuCl
Ph3PAuCl
Ph3PAuCl
Ph3PAuCl
Ph3PAuCl
56
57
20
PhCF3
1,4-
dioxane
15
16
17
Ph3PAuCl
Ph3PAuCl
Ph3PAuCl
THF
toluene
PhCl
31
51
2d
2d
2d
71(64)c
Figure 1. Bioactive oxazoles
18
19
20
Ph3PAuCl
Ph3PAuCl
Ph3PAuCl
PhCl
PhCl
PhCl
7d
2d
2d
2d
20e
41f
Results and Discussion
Table 1. Optimization of the reaction conditions a,b
a) All reactions were carried out on a 0.1 mmol scale in 0.6
ml solvent in presence of 1.2 eq N-oxide, 10 mmol%
b)
KB(C6F5)4 at 80 °C for 24 h. Yield was determined by
c)
NMR analysis using CH2Br2 as the internal standard.
c)
d)
Isolated yield.
Isolated yield.
KBPh4 was used as
additive. e) AgSbF6 was used as additive. f) AgNTf2 was used
as additive.
Initially,
(4-methylphenyl)propynal
(1a)
and
benzamide were selected as model substrates for
reaction optimizations. In order to get the activated
gold species, KB(C6F5)4 which has been used as
excellent chloride scavenger[15] was chosen to obtain
-
the weakly coordinating anion B(C6F5)4 . To our
delight, a first test reaction with pyridine N-oxide (2a)
and IPr as ligand at 80 °C delivered 44% of the desired
product in a ratio 3a:3a’>30:1 (entry 1). A series of
phosphine ligands was tested, simple PPh3 delivered a
better result (entry 6) than Buchwald-type ligands
(entries 3,5), a more electron-deficient simple
phosphane ligand (entry 4), or basic Mor-Dal-Phos
(entry 2). The control experiment with KB(C6F5)4
alone showed no conversion of 1a (entry 7). After that,
a variety of N-oxides was tested. Halogen-substituted
N-oxides showed better yields than simple pyridine N-
oxide or 8-methylquinoline N-oxide (entry 8-11).
2
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10.1002/adsc.201801386
Advanced Synthesis & Catalysis
Ph3PAuCl/KB(C6F5)4 turned out to be the best catalyst
system together with 3,5-dichloropyridine N-oxide as
oxygen donor. A solvent screening including TFE,
PhCF3, 1,4-dioxane, THF, toluene and chlorobenzene
showed the best result of 71% (isolated yield 64%) for
chlorobenzene (entry 17). Under the optimized
conditions, KBPh4, AgSbF6 and AgNTf2 were used as
additive to check if they were better than KB(C6F5)4
(entry18-20). It was proved that Ph3PAuCl/KB(C6F5)4
was best catalytic system.
alkenyl-substituted propynals were also suitable
substrates leading to 3o and 3p in moderate yield. Next,
a series of carboxamides was subjected to the standard
reaction conditions (Table 3). In general, both
electron-rich and electron-deficient substituents at the
aromatic rings were tolerated, affording the desired
compounds 5a-5i in 37%-63% yield. Like in Table 2
case, substituent effects turned out to be only marginal
for most of the applied starting materials no matter if
electron-rich (5a-5c) or electron-deficient (5d-5h)
substrates were converted. Also in line with the results
from Table 2, the position of a substituent on the
aromatic moiety did not influence the yield (5e-5g) but
the installation of a nitro group significantly reduced
Table 2. Reaction scope with respect to the propynal a)
the yield (5i).
Trans-cinnamamide as alkenyl
precursor delivered the best yield (81%) in this series
(5j). Cyclohexyl amide was also suitable and alkyl
substituted oxazole 5k was obtained in 59% yield.
Heterocyclic and other carbocyclic substrates were
tested, too. The yields of thienyl (5l) or naphthyl (5m)
oxazole were 48% and 41% respectively. Efforts with
the pyridyl-substituted amide failed. An ester instead
of an aldehyde at the alkyne part was tested as well but
in this case only an unselective reaction was observed
which can be explained by the lower electrophilicity
of the ester carbonyl.
Table 3. Reaction scope with respect to the amide
compound a)
a) All reactions were carried out on a 0.2 mmol scale in 1.0
ml solvent in presence of 1.2 eq N-oxide, 10 mol%
KB(C6F5)4 at 80 °C for 24 h. All yields given in this table
refer to isolated products. b) Reaction time of 36 h.
Under the optimized conditions, the scope with respect
to the propynal was investigated first (Table 2).
Different substituted aromatic groups at the alkyne
terminus were all tolerated well. For electron-donating
groups like alkyl (3a, 3b) or methoxy (3c-3e) no strong
influence on yield was observed and all of the
corresponding oxazoles were obtained in yields up to
75%. The position of the substituent also plays a minor
role, which was tested in the series of ortho-, meta- and
para-substituted starting materials (1c-1e). For
electron-withdrawing group at the aromatic moiety
similar reactivities and yields were observed.
Substrates with varying halogen substituents including
fluorides, chlorides and bromides, afforded product 3f-
3h in 67-73% yield. Other important functional groups
such as trifluoromethyl (3i), esters (3j), or nitriles (3k)
also delivered good yields. A drop in yield was
observed for nitro substituted compound 1l. Even
under prolonged reaction time of 36 h only 52% of 3l
were obtained. A larger aromatic system (3m) and one
heterocyclic thiophen substituent (3n) delivered the
corresponding products in moderate yield. Alkyl- and
a) All reactions were carried out on a 0.2 mmol scale in
1.0 ml solvent in presence of 1.2 eq N-oxide, 10 mmol%
KB(C6F5)4 at 80 °C for 24 h;; yields are isolated yields. b)
Reaction time of 36 h.
Scheme 2. mmol-Scale synthesis of 3a.
3
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10.1002/adsc.201801386
Advanced Synthesis & Catalysis
157.2 (s), 149.2 (s), 137.3 (d), 134.2 (s), 131.9 (d), 129.0 (d,
2C), 129.0 (d, 2C), 127.5 (d, 2C), 126.3 (s), 125.8 (d, 2C),
35.2 (s), 31.1 (q) ppm; IR (ATR): ṽ 3124, 3067, 2968, 2958,
2904, 2866, 1925, 1635, 1607, 1552, 1522, 1470, 1446,
1405, 1359, 1318, 1302, 1268, 1245, 1161, 1108, 1070,
1024, 996, 946, 901, 891, 841, 781, 770, 720, 709, 682, 624;
HRMS (ESI) Calcd for [C20H19NO2] ([M+H]+) 306.1494,
found 306.1483.
The reaction of 1a with benzamide could be easily run on
1 mmol scale with a catalyst loading of 5.9 mol%, 3a was
still isolated in 64% yield (Scheme 2).
Conclusion
In summary, a gold-catalyzed pathway towards 2,5-
disubstituted oxazoles bearing one acyl substituent
was developed. Key for the selectivity switch from the
known synthesis of 2,4-disubstituted oxazoles from
terminal alkynes and carboxamides, towards the
synthesis of 2,5-oxazoles, is the installation of an
aldehyde moiety on the alkyne. Due to its higher
electrophilicity, compared to the ketone derived from
the oxidation of the alkyne, condensation towards the
target aromatic system occurs with maintenance of the
ketone moiety in the product. Due to the simplicity of
the starting materials, this protocol offers an attractive
alternative to the existing strategies towards the target
molecules.
(2-Methoxyphenyl)(2-phenyloxazol-5-yl)methanone (3c)
1
Yield 70%, yellow solid, m.p. 134-135 °C; H NMR (500
MHz, CDCl3)  8.18 - 8.16 (d, J = 8.0 Hz, 2H), 7.67 (s, 1H),
7.54-7.48 (dt, J = 11.3, 7.2 Hz, 5H), 7.08-7.03 (m, 2H), 3.83
(s, 3H) ppm. 13C NMR (125 MHz, CDCl3)  181.7 (s), 164.6
(s), 157.6 (s), 149.9 (s), 137.9 (d), 133.0 (d), 131.9 (d), 129.8
(d, 2C), 129.0 (d), 127.5 (d, 2C), 127.4 (s), 126.3 (s), 120.6
(d), 111.7 (s), 55.7(q) ppm. IR (ATR): ṽ 2962, 2925, 2853,
2836, 1654, 1599, 1580, 1563, 1527, 1491, 1449, 1361,
1284, 1255, 1159, 1096, 1047, 1022, 993, 946, 922, 900,
802, 783, 764, 714, 685, 665, 614 ; HRMS (ESI) Calcd for
[C17H13NO3] ([M+H]+), 280.0973, found 280.0968.
(3-Methoxyphenyl)(2-phenyloxazol-5-yl)methanone (3d)
Experimental Section
Yield 72%, white solid, m.p. 95-96 °C; 1H NMR (500 MHz,
CDCl3):  8.21-8.20 (m, J = 7.8 Hz, 2H), 7.88 (s, 1H), 7.60-
7.58 (d, J = 7.7 Hz, 1H), 7.55-7.50 (m, 4H), 7.47-7.44 (t,
1H), 7.20-7.18 (m, J = 10.2 Hz, 1H), 3.89 (s, 3H) ppm; 13C
NMR (125 MHz, CDCl3):  181.2 (s), 164.8 (s), 159.9 (s),
149.0 (s), 138.2 (s), 137.8 (d), 132.0 (d), 129.8 (d),129.0 (d,
2C), 127.5 (d, 2C ), 126.2 (s), 121.5 (d), 119.7 (d), 113.4 (d),
55.5 (q) ppm; IR (ATR): ṽ 3126, 3078, 3003, 2836, 1645,
1557, 1524, 1478, 1446, 1425, 1357, 1302, 1254, 1151,
1080, 1040, 1013, 995, 957, 918, 895, 860, 803, 793, 777,
748, 711, 684; HRMS (ESI) Calcd for [C17H13NO3]
([M+H]+), 280.0973, found 280.0968.
A round bottom flask equipped with a magnetic stirrer bar
was charged with (PPh3)PAuCl (5 mol%, 2.47 mg),
KB(C6F5)4 (10 mol%, 7.2 mg), 3,5-dichloropyridine N-
oxide (1.2 eq, 19.6 mg), 1a (0.1 mmol, 14.4 mg), benzamide
(0.12 mmol, 14.5 mg) and chlorobenzene (0.6 ml). The
mixture was stirred for 24 h at 80 °C. After cooling to room
temperature, the solvent was removed under vacuum and the
residue was purified by column chromatography on silica
gel (eluent: PE/EA=5:1 to 10:1) to afford the product 3a.
(2-Phenyloxazol-5-yl)(p-tolyl)methanone (3a)
1
(4-Methoxyphenyl)(2-phenyloxazol-5-yl)methanone (3e)
Yield 64%, light yellow solid, m.p.132-133 °C; H NMR
(500 MHz, CDCl3)  8.20-8.19 (d, J = 6.6 Hz, 2H), 7.93-
7.91 (d, J = 8.1 Hz, 2H), 7.86 (s, 1H) 7.56-7.49 (m, 3H),
7.36-7.34 (d, J = 7.9 Hz, 2H), 2.46 (s, 3H) ppm; 13C NMR
(125 MHz, CDCl3)  181.2 (s), 164.6 (s), 149.1 (s), 144.3
(d), 137.3 (s), 134.3 (s), 131.9 (d), 129.5 (d, 2C), 129.2 (d,
2C), 129.0 (d, 2C), 127.5 (d, 2C), 126.3 (d), 21.76 (q) ppm;
IR (ATR): ṽ 2925, 2853, 1649, 1605, 1555, 1523, 1473,
1447, 1355, 1301, 1243, 1153, 1109, 1071, 997, 945, 899,
868, 832, 786, 753, 719, 704, 691, 642, 628; HRMS (ESI)
Calcd for [C17H13NO2] ([M+H]+), 264.1024, found
264.1017.
1
Yield 75%, white solid, m.p. 135-136 °C; H NMR (500
MHz, CDCl3)  8.20-8.18 (d, J = 8.2 Hz, 2H), 8.06-8.04 (d,
J = 8.9 Hz, 2H), 7.87 (s, 1H), 7.56-7.50 (t, J = 7.7 Hz, 3H),
7.04-7.02 (d, J = 8.9 Hz, 2H), 3.91 (s, 3H) ppm. 13C NMR
(125 MHz, CDCl3)  180.0 (s), 164.4 (s), 163.8 (s), 149.2
(s), 136.8 (d), 131.8 (d, 2C ), 131.5 (d), 129.5 (s), 129.0 (d),
127.4 (d), 126.4 (s), 114.1 (d), 55.6 (q) ppm; IR (ATR): ṽ
3128, 3033, 2959, 2846, 1653, 1601, 1559, 1525, 1511,
1473, 1453, 1360, 1323, 1263, 1246, 1163, 1116, 1073,
1024, 992, 947, 901, 840, 795, 783, 761, 719, 707, 692, 636,
624; HRMS (ESI) Calcd for [C17H13NO3] ([M+H]+)
280.0973, found 280.0968.
(4-(tert-Butyl)phenyl)(2-phenyloxazol-5-yl)methanone (3b)
1
(4-Fluorophenyl)(2-phenyloxazol-5-yl)methanone (3f)
Yield 66 %, white solid, m.p. 130-131 °C; H NMR (500
MHz, CDCl3)  8.21-8.19 (d, J = 8.2 Hz, 2H), 7.98-7.96 (d,
J = 8.5 Hz, 2H), 7.89 (s, 1H), 7.57-7.50 (m, 5H), 1.38 (s, 9H)
ppm; 13C NMR (125 MHz, CDCl3)  181.1 (s), 164.6 (s),
4
This article is protected by copyright. All rights reserved.

10.1002/adsc.201801386
Advanced Synthesis & Catalysis
1
Yield 71 %, white solid, m.p. 153-154 °C; H NMR (500
MHz, CDCl3):  8.13-8.11 (m, 2H), 8.01-7.98 (m, 2H), 7.82
(s, 1H), 7.50-7.43 (dq, 3H), 7.19-7.15 (m, 2H) ppm; 13C
NMR (125 MHz, CDCl3)  179.8 (s), 166.9 (s), 164.8 (s),
164.8 (s), 148.9 (s), 137.6 (d), 133.1 (s), 132.0 (d), 131.7 (d),
131.6 (d), 129.0 (d, 2C), 127.5 (d, 2C), 126.2 (d), 116.1 (d),
116.0 (d) ppm; IR (ATR): ṽ 3129, 3077, 3055, 1925, 1746,
1651, 1600, 1558, 1526, 1507, 1478, 1449, 1409, 1359,
1319, 1290, 1233, 1175, 1159, 1103, 1073, 1016, 995, 947,
922, 899, 853, 813, 780, 758, 715, 702, 685, 639, 621;
HRMS (ESI) Calcd for [C16H10FNO2] ([M+H]+), 268.0774,
found 268.0764.
Interlaken, Switzerland, 1-4 July, 2010, pp. 581-584; c)
R. N. Misra, H.-y. Xiao, K. S. Kim, S. Lu, W.-C. Han,
S. A. Barbosa, J. T. Hunt, D. B. Rawlins, W. Shan, S. Z.
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2009, 331, 636-647.
[2] a) D. Kikelj, U. Urleb, Science of Synthesis 2:
Heteroarenes (Ed. E. Schaumann), Thieme, Stuttgart,
2002 ; b) P. Wipf, Chem. Rev. 1995, 95, 2115-2134; c)
D. S. Dalisay, E. W. Rogers, A. S. Edison, T. F. Molinski,
J. Nat. Prod. 2009, 72, 732-738; d) M. Suganuma, H.
Fujiki, H. Furuya-Suguri, S. Yoshizawa, S. Yasumoto,
Y. Kato, N. Fusetani, T. Sugimura, Cancer Res. 1990,
50, 3521-3525; e) D. R. Allington, M. P. Rivey, Clin.
Ther. 2001, 23, 24-44.
(3,5-Dichlorophenyl)(2-phenyloxazol-5-yl)methanone (3g)
Yield 73%, white solid, m.p. 80-81 °C; 1H NMR (500 MHz,
CDCl3):  8.19-8.18 (d, J = 6.9 Hz, 2H), 7.93 (s, 1H), 7.87
(d, J = 1.9 Hz, 2H), 7.63-7.62 (t, J = 1.9 Hz, 1H), 7.59-7.51
(dt, J = 14.4, 7.0 Hz, 3H) ppm; 13C NMR (125 MHz, CDCl3)
 178.4 (s), 165.4 (s), 148.3 (s), 139.1 (s), 138.5 (d), 135.8
(s), 132.9 (d), 132.3 (d), 129.1 (d, 2C), 127.6 (d, 2C), 127.4
(d, 2C), 125.9 (s) ppm; IR (ATR): ṽ 3133, 3076, 1647, 1605,
1556, 1523, 1473, 1448, 1418, 1393, 1356, 1293, 1238,
1186, 1144, 1101, 1071, 1017, 999, 953, 933, 870, 806, 780,
750, 712, 684, 619; HRMS (ESI) Calcd for [C16H9Cl2NO2]
([M+H]+) 318.0088, found 318.0078.
[3] a) W. Zhou, C. Xie, J. Han, Y. Pan, Org. Lett. 2012, 14,
4766-4769; b) I. Romero-Estudillo, V. R. Batchu, A.
Boto, Adv. Synth. Catal. 2014, 356, 3742-3748; c) A. S.
K. Hashmi, M. Rudolph, S. Schymura, J. Visus, W. Frey,
Eur. J. Org. Chem. 2006, 4905-4909; d) J. P. Weyrauch,
A. S. K. Hashmi, A. Schuster, T. Hengst, S. Schetter, A.
Littmann, M. Rudolph, M. Hamzic, J. Visus, F.
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4602.
(4-Bromophenyl)(2-phenyloxazol-5-yl)methanone (3h)
Yield 67%, white solid, m.p. 152-153 °C; 1H NMR (500
MHz, CDCl3)  8.18 -8.17 (d, J = 7.1 Hz, 2H), 7.89-7.88 (m,
3H), 7.70-7.68 (d, J = 8.4 Hz, 2H), 7.57-7.50 (dq, 3H) ppm;
13C NMR (125 MHz, CDCl3)  180.2 (s), 164.9 (s), 148.8
(s), 137.8 (d), 135.5 (s), 132.1 (d,)2C, 130.5 (d, 2C), 129.1
(d, 2C), 128.5 (s), 127.5 (d, 2C), 126.1 (s) ppm; IR (ATR):
ṽ 3135, 3062, 1921,1737, 1638, 1585, 1550, 1472, 1448,
1395, 1360, 1322, 1309, 1247, 1169, 1011, 995, 946, 927,
898, 836, 780, 753, 717, 688, 618; HRMS (ESI) Calcd for
[C16H10BrNO2] ([M+H]+), 327.9973, found 327.9965.
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Acknowledgements
Y.X is grateful to the National Natural Science Foundation of
China (no. 21808246) and Jiangsu Overseas Visiting Scholar
Program for University Prominent Young ＆ Middle-aged
Teachers and Presidents; Q. W is grateful to the CSC (China
Scholarship Council) for a Ph.D. fellowship, Z. Z. for a Ph.D.
fellowship from the Guangzhou Elites Scholarship Council.
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Gold-Catalyzed Synthesis of 2,5-Disubstituted
Oxazoles from Carboxamides and Propynals
Adv. Synth. Catal. Year, Volume, Page – Page
Yun Xu, Qian Wang, Yufeng Wu, Zhongyi Zeng,
Matthias Rudolph, A. Stephen K. Hashmi*
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