A sequential one-pot tandem approach for the synthesis of 4-tosyl-5-aryloxazoles from carboxylic acids

Article abstract of DOI:10.1007/s12039-018-1540-2

Abstract: An efficient method for the synthesis of 4-tosyl-5-aryloxazoles directly from aromatic carboxylic acids has been reported. The method involves the conversion of aromatic carboxylic acids to tosyl carboxylates by treating with tosyl chloride in t

Full text of DOI:10.1007/s12039-018-1540-2

J. Chem. Sci. (2018) 130:150
© Indian Academy of Sciences
https://doi.org/10.1007/s12039-018-1540-2
REGULAR ARTICLE
A sequential one-pot tandem approach for the synthesis
of 4-tosyl-5-aryloxazoles from carboxylic acids
NARASIMHAMURTHY RAJEEVa, TORESHETTAHALLY R SWAROOPb,
SEEGEHALLI M ANILa, KUPPALLI R KIRANa, KANCHUGARAKOPPAL S RANGAPPAa,∗
and MARALINGANADODDI P SADASHIVAa,∗
aDepartment of Studies in Chemistry, University of Mysore, Manasagangothri, Mysuru, Karnataka 570 006,
India
bDepartment of Studies in Organic Chemistry, University of Mysore, Manasagangothri, Mysuru,
Karnataka 570 006, India
E-mail: rangappaks@gmail.com; mpsadashiva@gmail.com
MS received 29 March 2018; revised 21 July 2018; accepted 16 August 2018
Abstract. An efficient method for the synthesis of 4-tosyl-5-aryloxazoles directly from aromatic carboxylic
acids has been reported. The method involves the conversion of aromatic carboxylic acids to tosyl carboxylates by
treating with tosyl chloride in the presence of potassium carbonate and its subsequent reaction with tosylmethyl
isocyanide in the presence of sodium hydride to get 4-tosyl-5-aryloxazoles.
Keywords. Oxazole; tosylmethylisocyanide; tandem; cyclization; one-pot.
1. Introduction
methylene isocyanides with phenyl carboxylates,17
multicomponent reaction of amines, aldehydes, ketones
18
Oxazole is a commonly encountered moiety in
antimicrobial,1 antifungal,2 anticancer3 and anti-
tuberculosis agents.4 Also they are present in lipid
peroxidation inhibitors,5 neurotrophic factor inducers,6
LSD 1 inhibitors,7 T-type calcium and sodium channel
blockers,8 phosphodiesterase9 and GSK-3 inhibitors.10
As a result, there is a continuing interest in the develop-
ment of new methods for the synthesis of oxazoles.
Vast numbers of synthetic methods for oxazoles are
reported in the literature. Thus, we focussed our inter-
est on the synthesis of oxazoles from isocyanides.
The most widely used and popular method is van
Leusen strategy which makes use of aldehydes and
tosylmethyl isocyanide (TosMIC).11 The modification
of this strategy makes use of solid phase equiva-
lent TosMIC,12 quaternary ammonium hydroxide ion
exchange resin as a catalyst,13 ultrasound as promot-
ers14 and ionic liquid [bmim]Br15 as reaction media.
Katritzky and co-workers have replaced TosMIC by
benzotriazolyl methyl isocyanide in original van Leusen
method.16 Alternative approaches are reaction of active
α
and -acidic isocyanides, Schöllkopf condensation
of lithiated methyl isocyanides with carboxylic acid
19
α
chlorides, oxidative decarboxylation-cyclization of -
oxo carboxylates and isocyanides,20 cycloaddition of
isocyanides with methyl ketones,21 cycloaddition of
active methylene isocyanides with acid chlorides in flow
reactor.22 Cuprous iodide catalysed cyclization of iso-
cyanide enones,23 Ugi/Robinson–Gabriel synthesis24
and conversion of carboxylic acid to acid chlorides fol-
lowed by reaction with ethyl isocyanoacetate.25
The above-reported methods suffer from limitations
such as the use of costly transition metal catalysts,20
unstable substrates22,25 and elevated temperatures.20,21
Recently, we reported the synthesis of oxazoles directly
from arylmethyl alcohols and benzyl bromides, which
is a remarkable change in van Leusen approach.26
Based on literature survey, van Leusen’s strategy has
majorly employed unstable/highly reactive precursors
(acid anhydrides/acid chlorides) for the synthesis of
oxazoles.27 The handling and storage of these reactive
starting materials is one of the major shortcomings,
*
For correspondence
Electronic supplementary material: The online version of this article (https://doi.org/10.1007/s12039-018-1540-2) contains
supplementary material, which is available to authorized users.
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150 Page 2 of 5
J. Chem. Sci. (2018) 130:150
whereas synthesis directly from simple carboxylic 2.2b Characterization data of 5-(4-fluorophenyl)
-4-tosyloxazole (3b): White solid, Yield 93%, 0.59g,
acids is not yet reported. Thus, we report herein
a sequential one-pot tandem synthesis of 4-tosyl-5-
aryloxazolesfromcarboxylicacidsviatosylcarboxylate
intermediates (13 examples).
M.p.: 96–98 ◦C; IR (KBr, cm−1): 685, 1025, 1048, 1255,
υ¯
1575, 2931; 1H NMR (CDCl3, 400 MHz): δ 2.41 (s, 3H, Me),
7.17–7.24 (m, 2H, Ar-H), 7.31 (d, J= 7.6 Hz, 2H, Ar-H),
7.82 (s, 1H, Ar-H), 7.87 (d, J= 8.4 Hz, 2H, Ar-H), 7.97–8.00
(m, 2H, Ar-H); 13C NMR (CDCl3, 100 MHz): δ 21.6, 115.8,
116.0, 121.7, 128.2, 129.8, 129.9, 131.2, 131.3, 136.9, 145.1,
151.7, 165.4; HRMS (ESI): Anal. Calcd for C16H12FNO3S:
317.0522; Found: 318.0608 [M+1]+.
2. Experimental
2.1 Materials and methods
The starting materials were purchased from commercial
sources. The solvents were of analytical grade and were used
without further purification. Reactions were monitored by
thin-layer chromatography (TLC) using pre-coated sheets of
silica gel 60 (Merck 60F254, 0.25 mm thickness) and visu-
alization under UV light. Melting points were determined
on SELACO melting point apparatus and are uncorrected.
1H NMR (400 MHz) and 13C NMR (100 MHz) spectra
were obtained using an Agilent NMR spectrometer. Chemical
shifts (δ) are given in parts per million (ppm) using the residue
solvent (CDCl3 and DMSO-d6) peaks as a reference relative
to TMS. Coupling constant (J) values are given in Hz. The
mass spectral analysis was performed using Waters-Synapt
G2 mass spectrometer. Infrared spectra were recorded on
Shimadzu FT-IR model 8300 spectrophotometer.
2.2c Characterization data of 5-(4-chlorophenyl)
-4-tosyloxazole (3c): White solid, Yield 88%, 0.58g,
M.p.: 110–112 ◦C; IR (KBr, cm−1): 692, 1018, 1038, 1255,
υ¯
1581, 2914; 1H NMR (CDCl3, 400 MHz): δ 2.41 (s, 3H, Me),
7.31 (d, J= 7.6 Hz, 2H, Ar-H) 7.46–7.50 (m, 2H, Ar-H) 7.84–
7.94 (m, 5H, Ar-H); 13C NMR (CDCl3, 100 MHz): δ 16.90,
119.2, 123.5, 124.2, 125.5, 126.2, 132.5, 137.0, 140.4, 144.5,
146.7, 147.3; Anal. Calcd for C16H12ClNO3S: 333.0226;
Found: 334.0309 [M+1]+.
2.2d Characterization data of 5-(4-bromophenyl)
-4-tosyloxazole (3d): White solid, Yield 86%, 0.65g,
M.p.: 116–118 ◦C; IR (KBr, cm−1):
675, 1025, 1050,
υ¯
1
1265, 1569, 2931; H NMR (CDCl3, 400 MHz) δ 2.40 (s,
3H, Me), 7.31 (d, J= 7.6 Hz, 2H, Ar-H) 7.62 (d, J= 10.8
Hz, 2H, Ar-H), 7.84–7.87 (m, 5H, Ar-H); 13C NMR (CDCl3,
100 MHz): δ 21.7, 125.0, 126.3, 128.9, 130.5, 131.1, 132.2,
132.6, 137.5, 145.9, 150.1, 152.3; HRMS (ESI): Anal. Calcd
for C16H12BrNO3S: 376.9721; Found: 377.9806 [M+1]+ and
379.978 [M+3]+.
2.2 General procedure for the synthesis of
4-tosyl-5aryl-oxazoles 3a–3m
To a solution of aromatic carboxylic acid 1 (2 mmol) in THF
(5 mL), potassium carbonate (2 mmol) was added and stirred
for 5–10 min. Later, tosyl chloride (2 mmol) was added and
continued stirring. The disappearance of carboxylic acid was
monitored by TLC. The reaction mass was cooled to 0 ◦C and
tosylmethylisocyanide (2 mmol) was added followed by the
addition of suspension of sodium hydride (2 mmol) in THF
(2 mL). The reaction mixture was stirred for 1 h. After the
completion of reaction monitored by TLC, water (25 mL) was
added to the reaction mixture and extracted with ethyl acetate
(25mL×2). Thecombinedorganiclayerwaswashedinwater
(25mL), brine(25mL), driedoveranhydroussodiumsulphate
andconcentratedunderreducedpressuretogetcrudeproducts
which were purified by column chromatography using 10%
ethyl acetate in hexane.
2.2e Characterization data of 5-(2-bromophenyl)
-4-tosyloxazole (3e): White solid, Yield 81%, 0.61g,
M.p.: 112–114 ◦C; IR (KBr, cm−1): 685, 1032, 1068, 1282,
υ¯
1
1585, 2932; H NMR (CDCl3, 400 MHz): δ 2.40 (s, 3H,
Me), 7.25–7.30 (m, 2H, Ar-H), 7.39–7.47 (m, 2H, Ar-H) 7.54
(d, J= 7.6 Hz, 1H, Ar-H) 7.68 (d, J= 7.6 Hz, 1H, Ar-H),
7.83 (d, J= 7.6 Hz, 2H, Ar-H), 7.92 (d, J= 1.2 Hz, 1H,
Ar-H); 13C NMR (CDCl3, 100 MHz): δ 21.6, 123.7, 127.1,
127.3, 128.3, 129.7, 132.4, 132.8, 133.4, 136.7, 145.0, 150.4,
151.9, 153.4; HRMS (ESI): Anal. Calcd for C16H12BrNO3S:
376.972; Found: 377.9804 [M+1]+ and 379.9786 [M+3]+.
2.2f Characterizationdataof5-(p-tolyl)-4-tosyloxazole
(3f): White solid, Yield 85%, 0.53g, M.p.: 120–122 ◦C; IR
2.2a Characterizationdataof5-phenyl-4-tosyloxazole
1
(KBr, cm−1): 689, 1028, 1048, 1278, 1589, 2941; H NMR
υ¯
(3a): White solid, Yield 91%, 0.54 g, M.p.: 142–144 ◦C; IR
1
(KBr, cm−1): 683, 1022, 1043, 1267, 1567, 2920; H NMR
υ¯
(CDCl3, 400 MHz): δ 2.39 (s, 6H, Me2), 7.29 (d, J= 8.0
Hz, 4H, Ar-H) 7.81–7.88 (m, 5H, Ar-H); 13C NMR (CDCl3,
100 MHz): δ 21.6, 123.3, 128.9, 129.6, 129.9, 130.4, 134.0,
137.9, 142.1, 145.6, 149.7, 153.8; HRMS (ESI): Anal. Calcd
for C17H15NO3S: 313.0773; Found: 314.0856 [M+1]+.
(CDCl3, 400 MHz): δ 2.40 (s, 3H, Me), 7.30 (d, J = 8.4 Hz,
2H, Ar-H), 7.46–7.50 (m, 3H, Ar-H), 7.83 (s, 1H, Ar-H), 7.88
(d, J =8.4Hz, 2H, Ar-H), 7.95–7.97(m, 2H, Ar-H); 13CNMR
(CDCl3, 100 MHz): δ 21.6, 125.5, 128.5, 128.8, 129.0, 129.4,
129.8, 130.9, 137.2, 144.9, 149.0, 152.7; HRMS (ESI): Anal.
Calcd for C16H13NO3S: 299.0616; Found: 300.0699 [M+1]+ 2.2gCharacterizationdataof5-(m-tolyl)-4-tosyloxazole
◦
Author /Editor difference in calcd. and found (1 unit, here and (3g): White solid, Yield 84%, 0.52g, M.p.: 96–98 C; IR
+.
1
for others) is due to comparison between M and M Data (KBr, cm−1): 692, 1038, 1053, 1275, 1575, 2939; H NMR
υ¯
given in SI are comparable.
(CDCl3, 400 MHz): δ 2.41 (s, 6H, Me2), 7.24–7.43 (m, 4H,

J. Chem. Sci. (2018) 130:150
Page 3 of 5 150
Ar-H), 7.23–7.77 (m, 2H, Ar-H), 7.84–7.88 (m, 3H, Ar-H); δ 21.6, 55.1, 55.9, 110.9, 111.9, 115.9, 118.0, 122.2, 128.1,
13C NMR (CDCl3, 100 MHz): δ 21.6, 21.7, 126.2, 128.2, 129.7, 137.4, 144.8, 148.4, 148.8, 151.2, 152.6; HRMS (ESI):
128.4, 128.8, 129.0, 129.8, 131.8, 137.1, 138.3, 145.0, 149.2, Anal. Calcd for C18H17NO5S: 359.0827; Found: 360.0909
152.9, 160.3; HRMS (ESI): Anal. Calcd for C17H15NO3S: [M+1]+.
313.0773; Found: 314.0858 [M+1]+.
2.2m Characterization data of 5-([1,1ꢀ-biphenyl]-3-
2.2hCharacterization data of 5-(o-tolyl)-4-tosyloxazole
(3h): White solid, Yield 78%, 0.48g, M.p.: 62–64 C; IR
yl)-4-tosyloxazole (3m): White solid, Yield 82%, 0.61g,
◦
M.p.: 128–130 ◦C; IR (KBr, cm−1): 689, 1035, 1043, 1267,
υ¯
1
(KBr, cm−1): 695, 1035, 1074, 1289, 1591, 2935; H NMR
1569, 2925; 1H NMR (CDCl3, 400 MHz): δ 2.41 (s, 3H, Me),
7.24–7.49 (m, 5H, Ar-H), 7.69–7.66 (m, 2H, Ar-H), 7.73 (d,
J= 8.4 Hz, 2H, Ar-H), 7.86–7.93 (m, 3H, Ar-H), 8.06–8.08
(m, 2H, Ar-H); 13C NMR (CDCl3, 100M Hz): δ 21.6, 124.3,
127.1, 128.0, 128.3, 128.5, 128.9, 129.7, 131.7, 135.6, 137.0,
139.8, 143.6, 144.9, 149.1, 152.5; HRMS (ESI): Anal. Calcd
for C22H17NO3S: 375.0929; Found: 376.1013 [M+1]+.
υ¯
(CDCl3, 400MHz):δ 2.17(s, 3H, Me), 2.41(s, 3H, Me), 7.26–
7.31 (m, 4H, Ar-H), 7.41 (d, J= 7.6 Hz, 2H, Ar-H), 7.78 (d, J=
8.0 Hz, 2H, Ar-H), 7.90 (s, 1H, Ar-H); 13C NMR (CDCl3, 100
MHz): δ 20.0, 21.6, 125.5, 128.1, 128.8, 129.7, 129.9, 130.3,
131.1, 131.5, 137.1, 138.2, 144.9, 150.1, 153.4; HRMS (ESI):
Anal. Calcd for C17H15NO3S: 313.0773; Found: 314.08588
[M+1]+.
2.3 X-ray crystallographic study of compound (3m)
2.2i Characterization data of 5-(3,4-dimethylphenyl)-
4-tosyloxazole (3i): White solid, Yield 80%, 0.52g, M.p.:
The single crystal X-ray diffraction data of the compound 3m
was generated on a Rigoku SMART Lab model, Japan; it uses
Cu source and works at room temperature in monochrome
beam method. The structure was established by direct meth-
ods and refined on F2 by full matrix least square methods
using the SHELXS program.
76–78 ◦C; IR (KBr, cm−1): 666, 1039, 1055, 1283, 1588,
υ¯
1
2915; H NMR (CDCl3, 400 MHz): δ 2.31 (s, 6H, (Me)2),
2.41 (s, 3H, Me), 7.24 (t, J= 4.4 Hz, 1H, Ar-H), 7.29 (d, J= 8.0
Hz, 2H, Ar-H),7.69–7.72 (m, 2H, Ar-H), 7.81 (s, 1H, Ar-H),
7.88 (d, J= 8.4 Hz, 2H, Ar-H); 13C NMR (CDCl3, 100 MHz):
δ 19.8, 19.8, 21.6, 122.9, 126.6, 128.2, 129.7, 129.8, 129.8,
134.7, 136.4, 137.2, 140.2, 144.8, 148.9, 153.2; HRMS (ESI):
Anal. Calcd for C18H17NO3S: 327.0929; Found: 328.1015
[M+1]+.
3. Results and Discussion
Initially, to optimize the reaction condition, we took a
synthesis of tosyl benzoate 2a by the reaction of benzoic
acid 1a with tosyl chloride in the presence of base and
solvent, and subsequent reaction of 2a with tosylmethyl
isocyanide in the presence of a base in the same solvent
to get 4-tosyl-5-phenyl oxazole 3a. Thus, we conducted
reactions in the presence of bases such as potassium
carbonate, sodium hydride and triethylamine and in sol-
vents such as DMF, THF, acetonitrile and toluene. The
desired product 3a was obtained in 91% yield in the
presence of potassium carbonate in the first step and
sodium hydride in the second step with THF as solvent
(Scheme 1).
2.2j Characterization data of 5-(4-methoxyphenyl)-4-
tosyloxazole (3j): White solid, Yield 78%, 0.51g, M.p.:
84–86 ◦C; IR (KBr, cm−1): 689, 1042, 1056, 1276, 1589,
υ¯
2945; 1H NMR (CDCl3, 400 MHz): δ 2.40 (s, 3H, Me), 3.87
(s, 3H, OMe), 7.00 (d, J= 9.2 Hz, 2H, Ar-H), 7.24–7.31 (m,
2H, Ar-H), 7.78–7.95 (m, 5H, Ar-H); 13C NMR (CDCl3, 100
MHz): δ 21.6, 55.4, 114.0, 117.9, 128.1, 128.3, 129.7, 129.9,
130.6, 137.3, 144.8, 148.6, 161.6; HRMS (ESI): Anal. Calcd
for C17H15NO4S: 329.0722; Found: 330.0806 [M+1]+.
2.2k Characterizationdataof5-(3-methoxyphenyl)-4-
tosyloxazole (3k): White solid, Yield 81%, 0.53g, M.p.:
60–62 ◦C; IR (KBr, cm−1): 691, 1038, 1085, 1284, 1575,
υ¯
2941; 1H NMR (CDCl3, 400 MHz): δ 2.40 (s, 3H, Me), 3.87
(s, 3H, OMe), 7.02–7.05 (m, 1H, Ar-H), 7.24–7.41 (m, 3H,
Ar-H), 7.52–7.59 (m, 2H, Ar-H), 7.83–7.89 (m, 3H, Ar-H);
13C NMR (CDCl3, 100 MHz): δ 21.6, 55.4, 113.9, 117.3,
121.2, 126.5, 128.2, 129.6, 129.8, 135.7, 137.0, 145.0, 149.0,
152.4, 159.4; HRMS (ESI): Anal. Calcd for C17H15NO4S:
329.0722; Found: 330.0804 [M+1]+.
Scheme 1. Synthesis of 4-tosyl-5-phenyloxazole 3a from
benzoic acid 1a.
2.2l Characterizationdataof5-(3,4-dimethoxyphenyl)
-4-tosyloxazole (3l: White solid, Yield 75%, 0.54g, M.p.:
134–136 ◦C; IR (KBr, cm−1): 686, 1028, 1056, 1278, 1575,
υ¯
2957; 1H NMR (CDCl3, 400 MHz): δ 2.40 (s, 3H, Me), 3.97
(s, 6H, (OMe)2), 6.97 (d, J= 8.4 Hz, 1H, Ar-H), 7.25–7.31 (m,
2H, Ar-H), 7.58 (d, J= 8.0 Hz, 1H, Ar-H), 7.79 (s, 1H, Ar-H),
7.87 (d, J= 8.0 Hz, 2H, Ar-H); 13C NMR (CDCl3, 100 MHz):
Scheme 2. Synthesis of 4-tosyl-5-aryloxazole 3.

150 Page 4 of 5
J. Chem. Sci. (2018) 130:150
R (1,3)
Table 1. Substrate scope for the synthesis of 4-tosyl-5-aryloxazole 3.
Entry
R (1,3)
% Yield (3)
Entry
% Yield (3)
1
2
3
4
5
6
7
C6H5
91 (3a)
93 (3b)
88 (3c)
86 (3d)
81 (3e)
85 (3f)
84 (3g)
8
9
10
11
12
13
2-Me-C6H4
3,4-Me2-C6H3
4-MeO-C6H4
78 (3h)
80 (3i)
78 (3j)
81 (3k)
75 (3l)
82 (3m)
4-F-C6H4
4-Cl-C6H4
4-Br-C6H4
2-Br-C6H4
4-Me-C6H4
3-Me-C6H4
3-MeO-C6H4
3,4-(MeO)2-C6H3
[1,1ꢀ-biphenyl]-4-yl
to our earlier reported research.28 The structure of one
of the oxazoles 3m was confirmed by single crys-
tal X-ray diffraction studies (CCDC reference number
1833176)29 and its ORTEP diagram is given in Figure 1.
4. Conclusion
Figure 1. ORTEP diagram of 4-tosyl-5-(1,1ꢀ-biphenyl-4-
yl)-oxazole (3m).
In summary, we report an efficient method for the
synthesis of 4-tosyl-5-aryloxazoles directly from aro-
matic carboxylic acids. The method involves the con-
version of aromatic carboxylic acids into tosyl esters
and their in situ reaction with tosylmethyl isocyanide
to access 4-tosyl-5-aryloxazoles. One-pot, simple, effi-
cient and rapid reaction time are the noteworthy features
of this protocol. Further investigations on isocyanide
cyclization reactions are underway in our laboratory.
With the optimized reaction conditions in hand, we
examinedthegeneralapplicabilityoftheprotocolforthe
synthesis of 4-tosyl-5-aryloxazole 3 (Scheme 2). Thus,
variouspara-substitutedaromaticcarboxylicacidsbear-
ing inductively electron withdrawing halogens (F, Cl
and Br) gave 4-tosyl-5-(4-fluro/chloro/bromophenyl)-
oxazoles 3b–d in 86–93% yield (entries 2–4, Table 1).
Notably, yield gradually decreases with a decrease
in electronegativity of substituted halogen. Further,
2-bromobenzoic acid furnished 4-tosyl-5-(2-bromophe-
nyl)oxazole 3e in 81% yield (entry 5, Table 1), which is
less when compared with 4-tosyl-5-(4-bromophenyl)-
oxazole 3d which is obtained in 86% yield (entry 4,
Table 1) probably due to steric factors. On the other
hand, substituted aromatic carboxylic acids bearing
inductively electron releasing methyl group at various
positions afforded corresponding 4-tosyl-5-(4/3/2-
methylphenyl)oxazoles 3f–h in 78–85% yield (entries
6–8, Table 1). Similarly, 3,4-dimethylbenzoic acid gave
Supplementary Information (SI)
Crystallographic data for the compound 3m reported in this
paper had been deposited in the Cambridge Crystallographic
DataCentrewithccdcreferencenumber:1833176. Thesedata
can be obtained free of charge from Cambridge Crystallo-
graphic Data Centre via www.ccdc.cam.ac.uk/getstructures.
Supplementary Information containing general information,
experimental details and characterization data of all the syn-
thesized compounds is available at www.ias.ac.in/chemsci.
Acknowledgements
MPS thanks UGC-SAP DRS III (Grant No. UGC F-540/10/
4-tosyl-5-(3,4-dimethylphenyl)oxazole 3i in 80% yield DRS-III (SAP-I) Dated 14-09-2016). Authors are grateful
to Institution of Excellence, University of Mysore, Mysuru
for spectral analysis. N. Rajeev thanks UGC, New Delhi for
providing RGNF.
(entry 9, Table 1). In addition, substituted aromatic
carboxylic acids bearing electron releasing methoxy
groups at various positions furnished 4-tosyl-5-(3/4-
methoxyphenyl)oxazoles 3j and 3k in 78 and 81%
yield respectively (entries 10 and 11, Table 1). Sim-
ilarly, 3,4-dimethoxybenzoic acid gave 4-tosyl-5-(3,4-
dimethoxyphenyl)oxazole 3l in 75% yield (entry 12,
Table 1). Finally, [1,1ꢀ-biphenyl]-4-carboxylic acid
afforded corresponding oxazole 3m in 82% yield (entry
13, Table 1). The mechanism of the reaction is similar
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of charge from The Cambridge Crystallographic Data
Centre via www.ccdc.cam.ac.uk/getstructures