Direct synthesis of 2,5-disubstituted oxazoles through an iodine-catalyzed decarboxylative domino reaction

Article abstract of DOI:10.1021/jo400753n

An efficient iodine-catalyzed synthesis of highly substituted oxazoles is presented. Starting from readily available aryl methyl ketones, β-keto esters, or styrenes, in combination with α-amino acids as amine-containing coupling partners, the corresponding 2-alkyl-5-aryl- substituted oxazoles were obtained in up to 80% yield via a decarboxylative domino reaction.

Full text of DOI:10.1021/jo400753n

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Article
Direct synthesis of 2,5-disubstituted oxazoles through
an iodine-catalyzed decarboxylative domino reaction
Wei Xu, Ulrich Kloeckner, and Boris Johannes Nachtsheim
J. Org. Chem., Just Accepted Manuscript • DOI: 10.1021/jo400753n • Publication Date (Web): 27 May 2013
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Direct synthesis of 2,5-disubstituted oxazoles through an iodine-catalyzed
decarboxylative domino reaction
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Wei Xu, Ulrich Kloeckner and Boris J. Nachtsheim*
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Institute of Organic Chemistry, Eberhard Karls University Tuebingen, Auf der Morgenstelle 18, D-
72076 Tuebingen, Germany
Boris.nachtsheim@uni-tuebingen.de
ABSTRACT: An efficient iodine-catalyzed synthesis of highly substituted oxazoles is presented.
Starting from readily available aryl methyl ketones, β-keto esters or styrenes, in combination with
α-amino acids as amine-containing coupling partners, the corresponding 2-alkyl-5-aryl substituted
oxazoles were obtained in up to 80% yield via a decarboxylative domino reaction.
INTRODUCTION
The oxazole motif is a commonly found core structure in pharmaceuticals and natural products.
Representative examples are the PPARα/γ agonist aleglitazar,1 the non-steroidal anti-inflammatory
drug oxaprozin,2 the peptide alkaloide muscoride A3 or the antimycobacterial natural product
texaline (Figure 1).4 As a consequence of the extraordinary abundance of oxazoles in biological
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active compounds, a variety of synthetic procedures yielding highly substituted oxazoles have been
developed during the last decades.
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Figure 1. Representative examples of oxazoles in pharmaceuticals and other biological active
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compounds.
Classical routes to the oxazole motif include the cyclodehydration of (α-acylamino)ketones
(Robinson-Gabriel Synthesis), the cyclodehydration of (α-acyloxy)ketones in the presence of
ammonia, the condensation of cyanhydrines and aromatic aldehydes (Fischer Synthesis) and the
annulation of enamides.5-15 2-Alkyl substituted oxazoles are accessible by reactions of aryl alkyl
ketones and nitriles using hypervalent iodine reagents,16-18 mercury-19 and thallium-salts20, by [3+2]
cycloadditions21-23 by the cycloisomerization of propargylamides,24-30 or by a Ritter reaction of α-
oxotosylates.31 However, an underevaluated route to oxazoles, which primarily yields 2,4-
disubstituted derivatives, is the cyclocondensation of α-halogenated ketones and carboxamides as
initially described by Blümlein and Lewy in the 1880s (Scheme 1 – a).32,33 Major drawbacks of this
transformation are harsh reaction conditions and the necessity of hazardous α-halo ketones as
precursors. The in situ generation of α-halo ketones or synthetic equivalents thereof from α-
unsubstituted ketones by in situ halogenation or oxidation, subsequent substitution with an amine
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and cyclization in a domino process would be much more efficient (Scheme 1 – b).34 The utilization
of acetophenones for the de novo synthesis of heterocycles via oxidative domino reactions has been
studied intensively and resulted in the development of a variety of transformations including the
synthesis of 2-acylbenzo[d]thiazoles,35 isooxazoles,36 quinazolinones,37 imidazoles and
thiazoles,38,39 imidazo[1,2-a]pyridines,40 and 1,2,3-triaroylindolizines.41
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Scheme 1. Synthesis of oxazoles starting from α-halo ketones and carboxamides (a) or ketones and
amines (b)
However, the synthesis of oxazoles via oxidative domino reactions is less common.21,22,42,43 For
example, Wang and co-workers developed an efficient synthesis of 2,5-disubstituted oxazoles
starting from α-amino ketones and aromatic carboxaldehydes.44 Jiang and co-workers reported an
oxidative domino reaction yielding 2,5- or 2,4,5-substituted oxazoles starting from styrenes and
benzyl amines. Here, α-halo ketones, as key intermediates, are formed in situ from styrenes by
combining molecular iodine and tert-butyl hydroperoxide as co-oxidant.45 The same authors
described an efficient iodine-mediated synthesis of polysubstituted imidazoles.46 2-aryl-4-acyl-
substituted oxazoles can be generated by an iodine-catalyzed cascade sp3 C-H activation starting
from alkyl acetoacetates and benzyl amines.47
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Scheme 2. Mechanistic proposal for (a) the known oxidative domino synthesis of 2-aryl-5-aryl-
disubstituted oxazoles from aryl methyl ketones and benzyl amines and (b) a decarboxylative
domino reaction based on α-amino acids.
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Wu and co-workers presented an iodine-mediated synthesis of oxazoles based on benzoins, aryl
methyl ketones and ammonium acetate by the convergent integration of two self-labor domino
sequences.48
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Very recently, the same group developed an iodine-mediated and peroxide-free oxidative domino
synthesis of 2-aryl-5-aryl-disubstituted oxazoles utilizing aryl methyl ketones and benzyl amines as
substrates.49 However, all oxidative domino sequences described above give 2-aryl-substituted
oxazoles exclusively. The complementary 2-alkyl-substituted derivatives are not accessible via
these iodine-mediated oxidative domino sequences.21,25,50,51 An efficient synthetic approach towards
2-alkyl substituted oxazoles was reported by Martínez-Alvarez and co-workers through the reaction
of 1-(methylthio)acetone with nitriles in the presence of triflic acid.52 However, an explanation for
the preferred 2-aryl substituent in iodine-mediated oxidative domino sequences for the synthesis of
oxazoles is given in Scheme 2 (a). In either case, the α-iodinated ketone A is generated by in situ
iodination. A reacts with the corresponding amine to the aza enone C1 either by a
substitution/oxidation mechanism through the α-amino ketone B1 or, depending on the reaction
conditions, via a Kornblum oxidation through 1,2-dicarbonyl intermediate B2. However, for the key
cyclization step, C1 must isomerize via a [1,5]-H-shift to compound D1, which after cyclization to
E1 and subsequent oxidation gives the desired oxazole. Looking at this cascade reaction it seems
obvious that formation of D1 is highly favored for R3 being an (hetero)aromatic residue due to the
generation of a highly conjugated π-system explaining the high preference for benzyl amines in this
transformation. Inspired by the mechanism shown in Scheme 2 (a) we were intended to introduce
aliphatic residues in 2-position of the oxazole by using α-amino acids in a decarboxylative process
(Scheme 2 – b). The carboxylic acid would act as a directing group that has two positive effects on
this domino process: (1) intermediate C2 would be favored, even for an aliphatic residue at R3 due
to a conjugation with the carboxylic acid and (2) an iodine mediated oxidative decarboxylation can
be formulated (F) generating the oxazole directly through the oxidative loss of CO2. Our working
hypothesis was strongly supported by two recently published articles describing iodine mediated
decarboxylative domino reactions based on α-amino acids for the construction of highly substituted
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pyridines and quinazolines by Wang and co-workers.53,54 Based on this proposal and on recent
results of our group and others in iodine-mediated oxidative couplings, halogenations and domino
reactions,55-63 we herein report the first iodine-catalyzed synthesis of 2-alkyl-substituted oxazoles by
a decarboxylative domino reaction starting from aryl methyl ketones or styrenes and α-amino acids.
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RESULTS AND DISCUSSION
In initial experiments we investigated the reaction between acetophenone 1a and (DL)-valine 2a in
the presence of an iodine-source and a co-oxidant. Since it is well known, that the combination of I2
and the co-oxidant TBHP (tert-butyl hydroperoxide) gives undesired α-ketoamides from
acetophenones and amines,64 it was not surprising to us, that initial experiments using the common
co-oxidant TBHP gave the desired oxazole 3a in only low yields of 27% (Table 1, entry 1). Other
oxidants such as NaOCl or H2O2 did not yield 3a at all (Table 1, entries 2 and 3). To our great
delight, the co-oxidant Oxone (2KHSO5·KHSO4·K2SO4) gave 3a in a promising isolated yield of
53% (Table 1, entry 4). Switching the iodine-source from molecular iodine to tetrabutylammonium
iodide (TBAI) resulted in an almost complete loss of reactivity (Table 1, entry 5). For a mechanistic
discussion it is important to note that the reaction can be exclusively performed in DMSO. Other
polar aprotic solvents such as DMF and CH3CN gave the desired product only in trace amounts
(Table 1, entries 6 and 7). The reason for this solvent dependency is most likely a Kornblum
oxidation giving 1,2-diketone of type B2 as discussed in Scheme 2. Addition of base additives such
as K2CO3 or Na2CO3 as well as acid additives such as acetic acid did not improve yields
significantly (Table 1, entries 8-10). However, when the amount of co-oxidant was increased from
2.5 to 3 equivalents the isolated yield of 3a could be increased from 53 to 80% (Table 1, entry 11).
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Table 1. Optimization studies.a
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entry
cat
(mol%)
I2 (20)
I2 (20)
I2 (20)
I2 (20)
TBAI (20)
I2 (20)
I2 (20)
I2 (20)c
I2 (20)d
I2 (20)e
I2 (20)
I2 (20)
I2 (20)
I2 (20)
oxidant
(equiv.)
solvent
T
[°C]
95
95
95
95
95
95
95
95
95
95
95
95
75
115
t
[h]
2
2
2
2
2
2
2
2
2
2
2
2
2
2
yield 3a
[%]a,b
27
1
2
3
4
5
TBHP (2.5)c
NaOCl (2.5)d
H2O2 (2.5)e
Oxone (2.5)
Oxone (2.5)
Oxone (2.5)
Oxone (2.5)
Oxone (2.5)
Oxone (2.5)
Oxone (2.5)
Oxone (3.0)
Oxone (3.5)
Oxone (3.0)
Oxone (3.0)
DMSO
DMSO
DMSO
DMSO
DMSO
DMF
CH3CN
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
0
0
53
traces
traces
traces
53
53
57
80
74
60
67
6
7
8f
9g
10h
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14
aGeneral reaction conditions: 0.4 mmol (1 equiv) acetophenone 1a, 1.2 mmol (3 equiv) DL-valine
2a and catalyst in 3 mL of solvent Isolated yield after flash column chromatography. 70% aq.
solution. 30% aq. solution. 5% aq. solution. 2 equiv. of K2CO3 were added. 2 equiv. of Na2CO3
were added. h2 equiv. of AcOH were added.
b
c
d
e
f
g
When 3.5 equivalents of Oxone were used, yields dropped again (Table 1, entry 12). The initially
chosen 95 °C seemed to be the ideal reaction temperature since increasing or lowering the reaction
temperature resulted in a significant drop in yields (Table 1, entries 13 and 14). Having these
optimized reaction conditions in hand, we tested the reactivity of a variety of aryl methyl ketones
and α-amino acids in this novel domino reaction (Scheme 3). Besides valine, other aliphatic amino
acids, in particular leucine and isoleucine reacted smoothly to give the desired oxazoles 3b and 3c
in 70% and 52% yield respectively.
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Scheme 3. Reaction of various aryl methyl ketones with α-amino acids.a,b
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aGeneral reaction conditions: 0.4 mmol (1 equiv) 1, 1.2 mmol 2 (3 equiv), 0.08 mmol (20 mol%)
I2, 1.2 mmol (3 equiv) Oxone, 3 ml DMSO. bIsolated yield after flash column chromatography.
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To our surprise yields dropped significantly to 20% when unbranched aliphatic α-amino acids such
as alanine or norvaline were used as substrates (3d and 3e). Furthermore, it is worth mentioning that
enantiopure (2S,3S)-isoleucine gave the corresponding oxazole 3c as a racemic mixture. Electron
rich aryl methyl ketones such as 4-methoxy acetophenone or 2',5'-dimethylacetophenone gave
oxazoles 3f-3j in good yields of up to 63%. Here, α-aryl substituted amino acids such as phenyl
glycine could be used as substrates as well. A variety of ortho-, meta- and para-halogenated
acetophenones could be utilized giving 3k-3o in up to 70% yield. 4-Nitro and 4-CF3-substituted
acetophenones yielded oxazoles 3p and 3q in 41% and 60% yield respectively. 1- and 2-
naphthalenones resulted in formation of 5-(1-naphthyl) and 5-(2-naphthyl)-substituted oxazoles 3r-
3t in 53-74% yield. Finally, the 1,3-dicarbonyl compound ethyl benzoyl acetate was tested in this
domino process. Reaction with valine and isoleucine gave the 2,4,5-trisubstituted oxazoles 3u and
3v in 77% and 60% yield. However, it is worth mentioning that other, higher substituted, aryl alkyl
ketones such as propiophenone which would give an access to 2,4,5-trisubstuted oxazoles cannot be
used under our optimized reaction conditions. Aliphatic ketones such as cyclopentanone or butan-2-
one also showed no reactivity. To gain detailed insides into the mechanism of our newly developed
domino reaction, we were intended to isolate side products and reaction intermediates formed
during the reaction cascade (Scheme 4). First, we could isolate the α-hydroxylated 1-naphthalenone
4 (Scheme 4 – 1) which is formed by hydrolysis of the α-iodoketone. Subsequently, 4 was shown to
react with valine to the desired oxazole 3r in 75% yield under the same optimized reaction
conditions previously found in Table 1. Thus 4 can be seen as an intermediate which is formed in a
constructive side pathway coexistent to the Kornblum oxidation pathway. Since an excess of the
corresponding amino acid (relative to the ketone) is needed, an undesired decomposition of the
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amino acid was expected. As a control experiment, phenylglycine was treated with molecular iodine
in the presence of Oxone, without the addition of the corresponding ketone.
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Scheme 4. Decomposition pathways.
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In this experiment we observed benzaldehyde as the only side product in 30% yield, most likely due
to an iodine-mediated decarboxylation and subsequent hydrolysis of the emerging imine (Scheme 4
– 2). During our investigations towards the substrate scope we already recognized, that phenyl
alanine was a surprisingly poor substrate for our domino reaction. A remarkable side reaction was
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observed when phenyl alanine was reacted with Oxone in the presence of iodine. As the only
product, the unexpected 2-acyl oxazole 5 was isolated in 50% yield (Scheme 4 – 3). Here, a
benzylic oxidation of phenyl alanine to its β-oxo derivative A is proposed. A must be prone to two
convergent decomposition processes yielding 1,2-diketone C via an iodine-mediated oxidative
decarboxylation. Simultaneously, decarboxylation of the β-keto acid A gives α-amino acetophenone
D. Condensation of both substrates finally yields 2-acyl-5-aryl oxazole 5. Finally, we tried to
elucidate the poor reactivity of higher substituted aryl alkyl ketones. When propiophenone was
reacted under our optimized reaction conditions no oxazole formation was observed. Instead, we
could only isolate the corresponding α-hydroxlated derivative 6 in 50 % yield upon reaction with
valine. In contrast to the primary alcohol 4, this secondary hydroxyl group can obviously not be
further oxidized to the corresponding 1,2-diketone since further reaction of isolated 6 with valine
under the same reaction conditions did also not result in the formation of desired 2,4,5-trisubstituted
oxazole 3aa. Next, we investigated an even more ambitious domino reaction. The conversion of
styrenes into the corresponding α-iodo ketones with electrophilic iodine reagents is well
known.35,38,39,65-71 Thus, a direct conversion of styrenes to oxazoles with I2 and a co-oxidant should
be possible as well. One example for such a reaction was reported in 2010 by Jiang and co-
workers.45 However, stoichiometric amounts of molecular iodine were necessary and again only 2-
aryl substituted oxazoles are accessible via this method. Therefore, we wondered whether we could
extend the substrate scope of our decarboxylative domino reaction sequence to styrenes as well
(Scheme 5). Initial optimization studies for the reaction between styrene and valine revealed that
this reaction can be performed under similar reaction conditions as described in Table 1 (data not
shown). The only significant difference is a previously not observed positive effect of acetic acid as
an additive.
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Scheme 5. Reaction of styrene with an α- amino acid to give 2-alkyl-5-aryl oxazoles via a
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decarboxylative domino reaction.
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Finally, oxazole 3a could be isolated in only moderate yield of 36% (Scheme 6), which was
significantly lower in comparison with the same reaction based on acetophenone as substrate (80%).
However, we observed, that the reaction with styrenes are in general significantly slower leading to
a substantial negative influence of the amino acid decomposition pathways as described in Scheme
4, which explains the significant drop in yield. Furthermore, undesired side reaction of the styrene
such as epoxidations and/or iodohydroxylations cannot be ruled out.
Scheme 6. Reaction of styrenes with α-amino acids.a,b
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aGeneral reaction conditions: 0.3 mmol 1 0.6 mmol (2 equiv) styrene, 0.06 mmol I2 (20 mol%),
0.9 mmol AcOH (3 equiv) and 0.75 mmol (2.5 equiv) Oxone in 2 mL of DMSO. Isolated yield
after flash column chromatography.
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Even dropwise addition of the amino acid and/or styrene did not improve product yields. Electron
poor styrenes such as 2-chloro styrene reacted messy and the desired oxazole 3z could not be
isolated in significant amounts. However, moderately electron rich styrenes, such as p-methyl
styrene, yielded the desired oxazoles 3w-3y in up to 48% yield.
In summary we have developed an efficient synthesis of 2-alkyl-5-aryl oxazoles starting from
acetophenones and α-amino acids based on an iodine-catalyzed decarboxylative domino reaction.
The reaction must be conducted in DMSO which strongly supports an initial Kornblum oxidation
pathway. With only 20 mol% of molecular iodine and Oxone as cheap and readily available co-
oxidant, a variety of 2-alkyl-5-aryl-substituted oxazoles could be isolated in excellent yields of up to
80% using a variety of alkyl-substituted amino acids. Finally, this domino reaction could be
extended to styrenes as substrates as well, however with significantly lower yields.
EXPERIMENTAL SECTION
1H NMR spectra were recorded on a 400 MHz instrument. Chemical shifts for 1H NMR were
reported as δ (parts per million) relative to the signal of CHCl3 at 7.26(s) ppm. Chemical shifts for
13C NMR were reported as δ (parts per million) relative to the CDCl3 triplet at 77.0 ppm. The
following abbreviations were used to describe splitting patterns: br = broad, s = singlet, d = doublet,
t = triplet, q = quartet, sept. = septet, m = multiplet. Coupling constants J are given in Hz. Mass
spectra were recorded using EI ionization method with a quadrupole mass analyzer. High resolution
mass spectra were recorded using ESI ionization method with a FT-ICR mass analyzer.
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Unless otherwise stated, all chemicals were either used as received from their commercial supplier
or purified according to Purification of Common Laboratory Chemicals.72 Solvents for flash column
and thin layer chromatography including cyclohexane, ethyl acetate, toluene, and diethyl ether were
distilled prior to use. DMSO was 99.5% pure and used without further drying or purification. Thin
layer chromatography was performed on fluorescence indicator marked precoated silica gel 60
plates and visualized either by UV light (254 nm/366 nm). Flash column chromatography was
performed on silica gel (0.040 – 0.063 mm). Melting points are uncorrected.
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General experimental procedure 1 for preparation of 2-alkyl-5-aryl oxazoles from aryl
ketones. A solution of the corresponding acetophenone 1 (0.40 mmol, 1 equiv.), the α-amino acid
(1.2 mmol, 3 equiv.), Oxone (1.2 mmol, 3 equiv.) and iodine (0.08 mmol, 0.2 equiv.) in DMSO (3
mL) was heated to 95 °C. The resulting mixture was stirred at 95 °C until the starting material was
completely converted. The reaction mixture was allowed to cool to rt and then treated with saturated
aqueous solutions of Na2S2O3 and NaHCO3. The aqueous phase was extracted with EtOAc (3x) and
the combined organic layers were dried over anhydrous Na2SO4, filtered and concentrated. The
residue was purified by flash column chromatography on silica gel to afford 3a-v.
General experimental procedure 2 for preparation of 2-alkyl-5-aryl oxazoles from styrenes. A
solution of the corresponding styrene (0.30 mmol, 1 equiv.), the α-amino acid (0.90 mmol, 3
equiv.), Oxone (0.75 mmol, 2.5 equiv.), iodine (0.06 mmol, 0.2 equiv.) and acetic acid (0.90 mmol,
3 equiv.) in DMSO (2 mL) was heated to 95 °C. The resulting mixture was stirred at 95 °C until the
starting material was completely converted. The reaction mixture was allowed to cool to rt and then
treated with saturated aqueous solutions of Na2S2O3 and NaHCO3. The aqueous phase was extracted
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with EtOAc (3x) and the combined organic layers were dried over anhydrous Na2SO4, filtered and
concentrated. The residue was purified by flash column chromatography on silica gel to afford
3a,3w-3z.
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2-Isopropyl-5-phenyloxazole (3a): Prepared from acetophenone and valine following general
experimental procedure 1; Yield: 60 mg (80%). Prepared from styrene and valine following general
experimental procedure 2; Yield 20 mg (36 %) yellow liquid; eluent: petroleum ether/ethyl acetate
(7:1); 1H NMR (400 MHz, CDCl3) δ 7.54 (dt, J = 8.1, 1.6 Hz, 2H), 7.34 – 7.31 (m, 2H), 7.24 – 7.20
13
(m, 1H), 7.15 (s, 1H), 3.08 (hept, J = 7.0 Hz, 1H), 1.34 (d, J = 7.0 Hz, 6H); C NMR (101 MHz,
CDCl3) δ 168.5, 150.6, 128.7, 128.2, 128.0, 123.9, 121.5, 28.4, 20.4; IR (neat): 2974, 1691, 1552,
1489, 1363, 1273, 1203, 1139, 943, 823, 762, 712, 657, 613 (cm-1); MS (EI) m/z: 187.1 (100) [M]+ ,
172.1 (87) [M-CH3] +·, 105.1 (9) [M-C5H8N] +·, 82.2 (12) [M-C7H5O] +·, 77.1 (16) [M-C6H8NO] +·
The spectral data were in good agreement with the literature.17
2-Isobutyl-5-phenyloxazole (3b): Prepared from acetophenone and leucine following general
experimental procedure 1; Yield: 42 mg (70%), yellow liquid; eluent: petroleum ether/ethyl acetate
(8:1); 1H NMR (400 MHz, CDCl3) δ 7.78 (d, J = 7.2 Hz, 2H), 7.57 (t, J = 7.7 Hz, 2H), 7.49 – 7.44
13
(m, 1H), 7.40 (s, 1H), 2.88 (d, J = 7.1 Hz, 2H), 2.43 – 2.33 (m, 1H), 1.19 (d, J = 6.7 Hz, 6H); C
NMR (101 MHz, CDCl3) δ 164.0, 150.8, 128.8, 128.3, 128.0, 123.9 , 121.7, 37.1, 27.6, 22.3; IR
(neat): 2958, 1554, 1137, 1082, 940, 822, 759, 690, 671 (cm-1); HRMS (ESI) m/z calcd. for
C13H15NO [M+H]+: 202.1226, found: 202.1225.
2-(1-Methylpropyl)-5-phenyloxazole (3c): Prepared from acetophenone and isoleucine following
general experimental procedure 1; Yield: 42 mg (52%), yellow liquid; eluent: petroleum ether/ethyl
1
acetate (8:1); H NMR (400 MHz, CDCl3) δ 7.62 (dd, J = 5.2, 3.3 Hz, 2H), 7.42 – 7.38 (m, 2H),
7.32 – 7.27 (m, 1H), 7.22 (s, 1H), 2.97 (h, J = 7.0 Hz, 1H), 1.92 – 1.84 (m, 1H), 1.74 – 1.69 (m,
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1H), 1.38 (d, J = 7.0 Hz, 3H), 0.95 (t, J = 7.4 Hz, 3H); 13C NMR (101 MHz, CDCl3) δ 168.0, 150.6,
128.8, 128.3, 128.0, 123.9, 121.5, 35.3, 28.2, 18.0, 11.6; IR (neat): 2967, 1552, 1449, 1138, 1055,
955, 822, 761, 742, 689 (cm-1); HRMS (ESI) m/z calcd. for C13H15NO [M+H]+: 202.1226, found:
202.1226.
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2-Methyl-5-phenyloxazole (3d): Prepared from acetophenone and alanine following general
experimental procedure 1; Yield: 13 mg (20%), yellow solid; eluent: petroleum ether/ethyl acetate
(8:1); mp.: 56.0 – 58.0 °C; 1H NMR (400 MHz, CDCl3) δ 7.61 – 7.59 (m, 2H), 7.40 (t, J = 7.6 Hz,
13
2H), 7.30 (tt, J = 8.0 Hz, 2.0 Hz, 1H), 7.21 (s, 1H), 2.54 (s, 3H); C NMR (101 MHz, CDCl3) δ
161.0, 151.1, 128.8, 128.1, 128.1, 123.9, 121.6, 14.1; IR (neat): 3119, 1754, 1668, 1558, 1484,
1304, 1214, 1130, 1061, 942, 834, 760, 692 (cm-1); MS (EI) m/z: 159.1. (100) [M]+ , 130.1 (50) [M-
C2H5]+ , 104.2 (28) [M-C2H1NO]+ . The spectral data were in good agreement with the literature.21
2-Propyl-5-phenyloxazole (3e):Prepared from acetophenone and norvaline following general
experimental procedure 1; Yield: 15 mg (20%), pale yellow liquid; eluent: petroleum ether/ethyl
acetate (7:1); 1H NMR (400 MHz, CDCl3) δ 7.56 – 7.54 (m, 2H), 7.34 (t, J = 7.6 Hz, 2H), 7.24 (t, J
= 7.4 Hz, 1H), 7.17 (s, 1H), 2.77 (t, J = 7.5 Hz, 2H), 1.84 – 1.75 (m, 2H), 0.97 (t, J = 7.4 Hz, 3H);
13C NMR (101 MHz, CDCl3) δ 150.9, 130.84, 128.8, 128.2, 128.0, 123.9, 121.7, 30.1, 20.5, 13.7;
IR (neat): 2964, 1556, 1448, 1133, 1026, 941, 760, 709, 690, 665 (cm-1); MS (EI) m/z : 187.2 (36)
+·
+·
[M]+ , 172.1 (13) [M-CH3] , 159.1 (100) [M-C2H6]+·, 105.1 (6) [M- C5H8N] , 77.1 (7) [M-
C6H8NO]+·, The spectral data were in good agreement with the literature.17
2-Isopropyl-5-(4-methoxyphenyl)oxazole (3f): Prepared from 4-methoxyacetophenone and valine
following general experimental procedure 1; Yield: 52 mg (60%), yellow liquid; eluent: petroleum
1
ether/ethyl acetate (8:1); H NMR (400 MHz, CDCl3) δ 7.55 – 7.51 (m, 2H), 7.07 (s, 1H), 6.94 –
6.90 (m, 2H), 3.82 (s, 3H), 3.12 (dt, J = 13.9, 7.0 Hz, 1H), 1.38 (d, J = 7.0 Hz, 6H); 13C NMR (101
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MHz, CDCl3) δ 167.9, 159.4, 150.6, 125.4, 121.2, 120.0, 114.2, 55.3, 28.4, 20.4; IR (neat): 2970,
1557, 1503, 1304, 1248, 115, 1137, 1029, 960, 831, 737, 685, 604 (cm-1); HRMS (ESI) m/z calcd..
for C13H15NO2 [M+H]+: 218.1176, found: 218.1175.
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2- Isobutyl -5-(4-methoxyphenyl)oxazole (3g): Prepared from 4-methoxyacetophenone and leucine
following general experimental procedure 1; Yield: 50 mg (54%), yellow liquid; eluent: petroleum
1
ether/ethyl acetate (8:1); H NMR (400 MHz, CDCl3) δ 7.48 – 7.45 (m, 2H), 7.03 (s, 1H), 6.87 –
6.84 (m, 2H), 3.76 (s, 3H), 2.62 (d, J = 7.1 Hz, 2H), 2.13 (dp, J = 13.6, 6.8 Hz, 1H), 0.95 (d, J = 6.7
Hz, 6H); 13C NMR (101 MHz, CDCl3) δ 163.3, 159.4, 150.8, 125.4, 121.2, 120.1, 114.2, 55.3, 37.1,
27.5, 22.3; IR (neat): 2957, 1620, 1558, 1503, 1463, 1295, 1250, 1174, 1028, 831, 796, 681, 608
(cm-1); HRMS (ESI) m/z calcd.. for C14H17NO2 [M+H]+: 232.1332, found: 232.1332.
2-(1-Methylpropyl)-5-(4-methoxyphenyl)oxazole (3h): Prepared from 4-methoxyacetophenone and
isoleucine following general experimental procedure 1; Yield: 47 mg (51%), yellow liquid; eluent:
petroleum ether/ethyl acetate (8:1); 1H NMR (400 MHz, CDCl3) δ 7.54 – 7.51 (m, 2H), 7.08 (s, 1H),
6.93 – 6.90 (m, 2H), 3.81 (s, 3H), 2.93 (h, J = 7.0 Hz, 1H), 1.89 – 1.82 (m, 1H), 1.71 – 1.66 (m,
1H), 1.36 (d, J = 7.0 Hz, 3H), 0.94 (t, J = 7.4 Hz, 3H); 13C NMR (101 MHz, CDCl3) δ 167.3, 159.4,
150.5, 125.4, 121.2, 119.9, 114.2, 55.2, 35.2, 28.2, 17.9, 11.5; IR (neat): 2967, 1556, 1503, 1460,
1291, 1246, 1175, 1028, 831, 799, 742, 610 (cm-1); HRMS (ESI) m/z calcd. for C14H17NO2
[M+H]+: 232.1332, found: 232.1333.
2-Phenyl-5-(4-methoxyl)oxazole (3i): Prepared from 4-methoxyacetophenone and phenylglycine
following general experimental procedure 1; Yield: 58 mg (58%), pale yellow solid; eluent:
petroleum ether/ethyl acetate (8:1); mp.: 77.0 – 78.0 °C; 1H NMR (400 MHz, CDCl3) δ 8.09 (dt, J =
8.3, 2.2 Hz, 2H), 7.65 – 7.63 (m, 2H), 7.48 – 7.46 (m, 3H), 7.32 (s, 1H), 6.98 – 6.95 (m, 2H), 3.84
(s, 3H); 13C NMR (101 MHz, CDCl3) δ 160.5, 159.7, 151.2, 130.0, 128.7, 127.5, 126.1, 125.7,
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121.8, 120.8, 114.3, 55.3; IR (neat): 2973, 1498, 1300, 1250, 1021, 951, 823, 772, 705, 614 (cm-1);
MS (ESI) m/z [M]+: 251.1. The spectral data were in good agreement with the literature.73
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2-Phenyl-5-(2,5-dimethylphenyl)oxazole (3j): Prepared from 2,5-dimethylacetophenone and
phenylglycine following general experimental procedure 1; Yield: 62 mg (63%), white solid; eluent:
petroleum ether/ethyl acetate (15:1); mp.: 79.0 – 80.0 °C; 1H NMR (400 MHz, CDCl3) δ 8.14 (dd, J
= 8.0, 1.7 Hz, 2H), 7.59 (s, 1H), 7.52 – 7.46 (m, 3H), 7.34 (s, 1H), 7.18 (d, J = 7.7 Hz, 1H), 7.09
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(dd, J = 7.7, 1.3 Hz, 1H), 2.50 (s, 3H), 2.41 (s, 3H); C NMR (101 MHz, CDCl3) δ 160.7, 150.9,
135.7, 131.8, 131.2, 130.2, 129.2 128.8 , 127.4, 127.2, 127.0, 126.2, 126.0, 21.4, 21.0; IR (neat):
2921, 1538, 1494, 1445, 1066, 963, 811, 772, 704, 687 (cm-1); HRMS (ESI) m/z calcd. for
C17H15NO [M+H]+: 250.1226, found: 250.1225.
2-Isopropyl-5-(2-bromophenyl)oxazole (3k): Prepared from 2-bromoacetophenone and valine
following general experimental procedure 1; Yield: 74 mg (70%), pale yellow liquid; eluent:
petroleum ether/ethyl acetate (10:1); 1H NMR (400 MHz, CDCl3) δ 7.72 – 7.70 (m, 2H), 7.63 (dd, J
= 8.0, 1.2 Hz, 1H), 7.35 (td, J = 7.7, 1.2 Hz, 1H), 7.15 – 7.11 (m, 1H), 3.15 (hept, J = 7.0 Hz, 1H),
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1.40 (d, J = 7.0 Hz, 6H); C NMR (101 MHz, CDCl3) δ 168.5, 148.1, 134.0, 128.9, 128,9, 128.3,
127.4, 126.3 119.7, 28.4, 20.4; IR (neat): 2972, 1562, 1469, 1146, 1021, 939, 833, 756, 710, 638
(cm-1); HRMS (ESI) m/z calcd. for C12H12BrNO [M+H]+: 266.0175, found: 266.0175.
2-(1-Methylpropyl)-5-(2-bromophenyl)oxazole (3l): Prepared from 2-bromoacetophenone and
isoleucine following general experimental procedure 1; Yield: 71 mg (64%), yellow liquid; eluent:
petroleum ether/ethyl acetate (6:1); 1H NMR (400 MHz, CDCl3) δ 7.72 – 7.70 (m, 2H), 7.64 (dd, J
= 8.0, 1.0 Hz, 1H), 7.38 – 7.34 (m, 1H), 7.14 (td, J = 7.8, 1.6 Hz, 1H), 2.97 (h, J = 7.0 Hz, 1H), 1.91
– 1.84 (m, 1H), 1.69 (dt, J = 14.0, 7.3 Hz, 1H), 1.38 (d, J = 7.0 Hz, 3H), 0.95 (t, J = 7.4 Hz, 3H);
13C NMR (101 MHz, CDCl3) δ 168.0, 148.0, 134.0, 129.0, 128.9, 128.3, 127.4, 126.3, 119.7, 35.3,
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28.2, 17.9, 11.6; IR (neat): 2967, 1562, 1548, 1469, 1146, 1021, 939, 833, 756, 711, 640 (cm-1);
HRMS (ESI) m/z calcd. for C13H14BrNO [M+H]+: 280.0332, found: 280.0334.
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2-Isopropyl-5-(3-bromophenyl)oxazole (3m): Prepared from 3-bromoacetophenone and valine
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following general experimental procedure 1; Yield: 59 mg (56%), yellow liquid; eluent: petroleum
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ether/ethyl acetate (8:1); H NMR (400 MHz, CDCl3) δ 7.74 (t, J = 1.7 Hz, 1H), 7.53 – 7.50 (m,
1H), 7.41 (m, 1H), 7.27 – 7.25 (m, 1H), 7.23 (d, J = 1.9 Hz, 1H), 3.14 (hept, J = 7.0 Hz, 1H), 1.40
(d, J = 7.0 Hz, 6H); 13C NMR (101 MHz, CDCl3) δ 169.1, 149.2, 130.9, 130.3, 130.2, 126.8, 122.9,
122.6, 122.4, 28.5, 20.4; IR (neat): 2974, 1582, 1549, 1472, 1281, 1139, 1074, 963, 825, 781, 684,
612 (cm-1);. HRMS (ESI) m/z calcd. for for C12H12BrNO [M+H]+: 266.0175, found: 266.0174.
2-(1-Methylpropyl)-5-(3-bromophenyl)oxazole (3n): Prepared from 3-bromoacetophenone and
isoleucine following general experimental procedure 1; Yield: 57 mg (51%), colorless liquid;
eluent: petroleum ether/ethyl acetate (7:1); 1H NMR (400 MHz, CDCl3) δ 7.75 (t, J = 1.7 Hz, 1H),
7.54 – 7.51 (m, 1H), 7.42 – 7.39 (m, 1H), 7.27 – 7.23 (m, 2H), 2.96 (h, J = 7.0 Hz, 1H), 1.93 – 1.80
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(m, 1H), 1.73 – 1.68 (m, 1H), 1.37 (d, J = 7.0 Hz, 3H), 0.94 (t, J = 7.4 Hz, 3H); C NMR (101
MHz, CDCl3) δ 168.5, 149.1, 130.8, 130.3, 130.2, 126.8, 122.9, 122.5, 122.4, 35.3, 28.2, 17.9, 11.6;
IR (neat): 2967, 1697, 1547, 1472, 1211, 1138, 1075, 958, 782, 737, 684 (cm-1); HRMS (ESI) m/z
calcd. for C13H14BrNO [M+H]+: 280.0332, found: 280.0332.
2-(1-Methylpropyl)-5-(4-chlorophenyl)oxazole (3o): Prepared from 4-chloroacetophenone
isoleucine following general experimental procedure 1; Yield: 66 mg (70%), pale yellow liquid;
eluent: petroleum ether/ethyl acetate (7:1)1H NMR (400 MHz, CDCl3) δ 7.52 (d, J = 8.6 Hz, 2H),
7.35 (d, J = 8.6 Hz, 2H), 7.20 (s, 1H), 2.99 – 2.90 (m, 1H), 1.91 – 1.80 (m, 1H), 1.74 – 1.64 (m,
1H), 1.36 (d, J = 7.0 Hz, 3H), 0.94 (t, J = 7.4 Hz, 3H); 13C NMR (400 MHz, CDCl3) δ 168.2, 149.6,
133.7, 129.0, 126.8, 125.1, 121.9, 35.3, 28.1, 17.9, 11.5; IR (neat): 2967, 1549, 1485, 1139, 1092,
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1012, 820, 737 (cm-1); HRMS (ESI) m/z calcd. for C13H14ClNO [M+H]+: 236.0837, found:
236.0836.
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2-(1-Methylpropyl)-5-(4-nitrylphenyl)oxazole (3p): Prepared from 4-nitroacetophenone and
isoleucine following general experimental procedure 1; Yield: 40 mg (41%), yellow liquid; eluent:
petroleum ether/ethyl acetate (6:1); 1H NMR (400 MHz, CDCl3) δ 8.27 – 8.24 (m, 2H), 7.75 – 7.73
(m, 2H), 7.43 (s, 1H), 2.99 (h, J = 7.0 Hz, 1H), 1.91 – 1.84 (m, 1H), 1.75 – 1.70 (m, 1H), 1.38 (d, J
= 7.0 Hz, 3H), 0.95 (t, J = 7.4 Hz, 3H); 13C NMR (101 MHz, CDCl3) δ 169.9, 148.6, 146.8, 134.0,
125.1, 124.4, 124.2, 35.4, 28.1, 17.8, 11.5; IR (neat): 2968, 1607, 1548, 1514, 1457, 1332, 1108,
1073, 956, 851, 752, 691 (cm-1); HRMS (ESI) m/z calcd. for C13H14N2O3 [M+H]+: 247.1077, found:
247.1078.
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2-(1-Methylpropyl)-5-(4-trifluoromethyl)oxazole
(3q):
Prepared
from
4-
(trifluoromethyl)acetophenone and isoleucine following general experimental procedure 1; Yield:
1
64 mg (60%),pale yellow liquid; eluent: petroleum ether/ethyl acetate (8:1); H NMR (400 MHz,
CDCl3) δ 7.83 (s, 1H), 7.77 (d, J = 7.1 Hz, 1H), 7.54 – 7.49 (m, 2H), 7.31 (s, 1H), 2.97 (h, J = 7.0
Hz, 1H), 1.92 – 1.85 (m, 1H), 1.74 – 1.69 (m, 1H), 1.38 (d, J = 7.0 Hz, 3H), 0.95 (t, J = 7.4 Hz,
3H); 13C NMR (101 MHz, CDCl3) δ 168.8, 149.3, 131.4 (q, J = 32.3 Hz), 129.3, 129.1, 126.9, 145.5
(q, J = 4.0 Hz), 123.8 (q, J = 272.7 Hz), 122.8, 120.6 (q, J = 4.0 Hz), 35.4, 28.2, 17.9, 11.5; IR
(neat): 2971, 1553, 1454, 1334, 1266, 1166, 1123, 1096, 960, 897, 829, 799, 745, 696, 651 (cm-1);
HRMS (ESI) m/z calcd. for C14H14F3NO [M+H]+: 270.1100, found: 270.1099.
2-Isopropyl-5-(2-naphthyl)oxazole (3r): Prepared from 2-acetylnaphthalene and valine following
general experimental procedure 1; Yield: 50 mg (53%), pale yellow solid; eluent: petroleum
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ether/ethyl acetate (10:1); mp.: 59.0 – 61.0 °C; H NMR (400 MHz, CDCl3) δ 7.99 (s, 1H), 7.79 –
7.72 (m, 3H), 7.60 (dd, J = 7.2, 1.1 Hz, 1H), 7.43 – 7.38 (m, 2H), 7.24 (s, 1H), 3.11 (hept, J = 6.9
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Hz, 1H), 1.36 (d, J = 7.0 Hz, 6H); 13C NMR (101 MHz, CDCl3) δ 168.8, 150.8, 133.4, 132.9, 128.6,
128.1, 127.8, 126.7, 126.3, 125.6, 122.5, 122.1, 122.0, 28.6, 20.5; IR (neat): 2967, 1570, 1508,
1129,1104, 1049, 897, 865, 818, 752, 737, 675 (cm-1); HRMS (ESI) m/z calcd. for C16H15NO
[M+H]+: 238.1226, found: 238.1227.
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2-Phenyl-5-(2-naphthyl)oxazole (3s): Prepared from 2-acetylnaphthalene and phenylglycine
following general experimental procedure 1; Yield: 80 mg (74%), pale yellow solid; eluent:
1
petroleum ether/ethyl acetate (8:1); mp.: 100.0 – 102.0 °C; H NMR (400 MHz, CDCl3) δ 8.18 –
13
8.16 (m, 3H), 7.91 – 7.83 (m, 3H), 7.76 (dd, J = 8.5, 1.7 Hz, 1H), 7.55 – 7.47 (m, 6H); C NMR
(101 MHz, CDCl3) δ 161.2, 151.3, 133.3, 133.0, 130.3, 128.8, 128.7, 128.2, 127.8, 127.4, 126.7,
126.4, 126.3, 125.2, 123.9, 122.8, 122.0; IR (neat): 3091, 1562, 1485, 1137, 973, 891, 857, 819, 744
706, 618 (cm-1); MS (EI) m/z: 271.1 (100) [M]+, 243.2 (22) [M-CH2N] +·, 127.1 (12) [M-C9H6NO] +·
The spectral data were in good agreement with the literature.45
2-Phenyl-5-(1-naphthyl)oxazole (3t): Prepared from 1-acetylnaphthalene and phenylglycine
following general experimental procedure 1; Yield: 54 mg (50%); pale yellow solid; eluent:
petroleum ether/ethyl acetate (15:1); mp.: 113.0 – 114.0 °C; 1H NMR (400 MHz, CDCl3) δ 8.39 (d,
J = 8.4 Hz, 1H), 8.20 – 8.17 (m, 2H), 7.92 (t, J = 8.3 Hz, 2H), 7.84 (dd, J = 7.2, 1.1 Hz, 1H), 7.63 –
7.49 (m, 7H); 13C NMR (101 MHz, CDCl3) δ 161.5, 150.5, 133.8, 130.4, 130.0, 129.5, 128.8, 128.7,
127.4, 127.1, 126.7, 126.4, 126.3, 126.2, 125.3, 125.3, 124.8; IR (neat): 3053, 1589, 1485, 1397,
1233, 1119, 990, 925, 839, 767, 703, 684, 655, 623, 601 (cm-1); MS (EI) m/z: 271.1 (100) [M]+,
243.2 (25) [M-CH2N]+· The spectral data were in good agreement with the literature.45
.
2-Isopropyl-4-(carboxylic acid ethyl ester)-5-phenyloxazole (3u): Prepared from ethyl
benzoylacetate and valine following general experimental procedure 1; Yield: 80 mg (77%), pale
1
yellow solid; eluent: petroleum ether/ethyl acetate (5:1); mp.: 41.0 – 42.0 °C; H NMR (400 MHz,
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CDCl3) δ 8.01 – 7.98 (m, 2H), 7.42 (m, 3H), 4.38 (d, J = 21.4 Hz, 2H), 3.18 (hept, J = 7.0 Hz, 1H),
1.40 – 1.34 (m, 9H); 13C NMR (101 MHz, CDCl3) δ 167.1, 162.2, 154.7, 129.9, 128.2, 128.1, 127.2,
126.6, 61.1, 28.4, 20.2, 14.2; IR (neat): 2980, 1710, 1585, 1450, 1371, 1240, 1205, 1157, 1025, 835,
768, 693 (cm-1); HRMS (ESI) m/z calcd. for C15H17NO3 [M+H]+: 260.1281, found: 260.1271
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2-(1-Methylpropyl)-4-(carboxylic acid ethyl ester)-5-phenyloxazole (3v): Prepared from ethyl
benzoylacetate and isoleucine following general experimental procedure 1; Yield: 65 mg (60%),
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colorless liquid; eluent: petroleum ether/ethyl acetate (10:1); H NMR (400 MHz, CDCl3) δ 8.01
(dt, J = 8.5, 2.3 Hz, 2H), 7.47 – 7.40 (m, 3H), 4.40 (q, J = 7.1 Hz, 2H), 3.01 (h, J = 7.0 Hz, 1H),
1.92 – 1.84 (m, 1H), 1.74 – 1.68 (m, 1H), 1.38 (dt, J = 7.1, 3.6 Hz, 6H), 0.94 (t, J = 7.4 Hz, 3H); 13C
NMR (101 MHz, CDCl3) δ 166.6, 162.3, 154.8, 129.9, 128.3, 128.2 , 127.3, 126.6 , 61.2, 35.3, 28.0,
17.8, 14.3, 11.7; IR (neat): 2971, 2360, 1716, 1492, 1372, 1230, 1185, 1089, 1038, 837, 766, 690
(cm-1); HRMS (ESI) m/z calcd. for C16H19NO3 [M+Na]+: 296.1257, found: 296.1258.
2-isopropyl-5-(4-methylphenyl)oxazole (3w): Prepared from 4-methylstyrene and valine following
general experimental procedure 2; Yield: 29 mg (48%), yellow liquid; eluent: petroleum
1
ether/diethyl ether (10:1); H NMR (400 MHz, CDCl3) δ 7.50 (d, J = 8.2 Hz, 2H), 7.20 (d, J = 8.3
Hz, 2H), 7.15 (s, 1H), 3.14 (hept, J = 7.0 Hz, 2H), 2.37 (m, 3H), 1.40 (d, J = 7.0 Hz, 6H); 13C NMR
(101 MHz, CDCl3) δ 168.3, 150.8, 138.0, 129.5, 125.6, 123.9, 120.9, 28.5, 21.3, 20.5; IR (neat):
2974, 1556, 1504, 1138, 1106, 1065, 1053, 940, 812, 738(cm-1); HRMS (ESI/ TOF) m/z calcd. for
C13H15NO [M+H]+: 202.12264, found: 202.12269.
2-(1-Methylpropyl)-5-(4-methylphenyl)oxazole (3x): Prepared from 4-methylstyrene and isoleucine
following general experimental procedure 2; Yield: 26 mg (40%), yellow liquid; eluent: petroleum
1
ether/diethyl ether (10:1); H NMR (400 MHz, CDCl3) δ 7.50 (d, J = 8.2 Hz, 2H), 7.21 (d, J = 8.0
Hz, 2H), 7.17 (s, 1H), 2.96 (hext, J = 7.0 Hz, 1H), 2.37 (s, 3H), 1.89 – 1.84 (m, 1H), 1.72 – 1.70 (m,
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1H), 1.38 (d, J = 7.0 Hz, 3H), 0.95 (t, J = 7.4 Hz, 3H) ; 13C NMR (101 MHz, CDCl3) δ 167.7, 150.9,
138.1, 129.5, 125.5, 124.0, 120.6, 35.3, 28.2, 21.3, 18.0, 11.6; IR (neat): 2966, 1554, 1504, 1455,
1137, 1111, 1053, 955, 812; HRMS (ESI/ TOF) m/z calcd. for C14H17NO [M+H]+: 216.13829,
found: 216.13813.
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2-phenyl-5-(4-methylphenyl)oxazole (3y): Prepared from 4-methylstyrene and phenylglycine
following general experimental procedure 2; Yield: 23 mg (33%), yellow liquid; eluent: petroleum
1
ether/diethyl ether (10:1); H NMR (400 MHz, CDCl3) δ 8.12 – 8.10 (m, 2H), 7.62 (d, J = 8.2 Hz,
2H), 7.50 – 7.40 (m, 3H), 7.26 (d, J = 8.0 Hz, 2H), 7.25 (s, 1H), 2.40 (s, 3H)) ; 13C NMR (101 MHz,
CDCl3) δ 160.8, 151.5, 138.5, 130.2, 129.6, 128.8, 127.5, 126.2, 125.3, 124.2, 122.7, 21.4; IR
(neat): 2965, 2923, 1675, 1504, 1068, 1018, 814; HRMS (ESI /TOF) m/z calcd. for C16H13NO
[M+H]+: 236.10699, found: 236.10705.
2-Hydroxy-1-naphthalen-2-yl-ethanone (4): A solution 2-acetylnaphthalene (0.4 mmol, 1 equiv.),
valine (1.2 mmol, 3 equiv.) iodine (0.08 mmol, 0.2 equiv.) and Oxone (1.2 mmol, 3 equiv.) in
DMSO (3 mL) was heated to 95 °C for 10 minutes. The reaction mixture was allowed to cool to rt
and then treated with saturated aqueous solutions of Na2S2O3 and NaHCO3. The aqueous phase was
extracted with EtOAc (3x) and the combined organic layers were dried over anhydrous Na2SO4,
filtered and concentrated. The residue was purified by flash column chromatography on silica gel to
afford 4. Yield: 21 mg (28%), white solid; eluent: petroleum ether/ethyl acetate (7:1); mp.: 117.3
1
°C; H NMR (400 MHz, CDCl3) δ 8.42 (s, 1H), 7.99 – 7.88 (m, 4H), 7.66 – 7.56 (m, 2H), 5.02 (s,
13
2H), 3.62 (s, 1H). C NMR (101 MHz, CDCl3) δ 198.3, 136.1, 132.4, 130.6, 129.6, 129.5, 129.0,
128.9, 127.9, 127.1, 123.0, 65.5; IR (neat): 3422, 1678, 1406, 1245, 1185, 1100, 938, 821, 749, 603
(cm-1); MS (EI) m/z: 186.1 (12) [M]+, 155.1 (100) [M-CH3O]+·, 127.1 (80) [M-C2H3O2]+·. The
spectral data were in good agreement with the literature.74
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phenyl(5-phenyloxazol-2-yl)methanone (5): A solution phenylalanine (0.6 mmol, 1 equiv.) and
iodine (0.06 mmol, 0.10 equiv.), Oxone® (0.75 mmol, 3 equiv.) in DMSO (3 mL) was heated to 95
°C. The resulting mixture was stirred at 95 °C until the starting material was completely converted.
The reaction mixture was allowed to cool to rt and then treated with saturated aqueous solutions of
Na2S2O3 and NaHCO3. The aqueous phase was extracted with EtOAc (3x) and the combined
organic layers were dried over anhydrous Na2SO4, filtered and concentrated. The residue was
purified by flash column chromatography on silica gel to afford 5. Yield: 75 mg (50%), yellow
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solid; eluent: petroleum ether/diethyl ether (10:1); H NMR (400 MHz, CDCl3) δ 8.50 – 8.48 (m,
2H), 7.84 – 7.81 (m, 2H), 7.67 – 7.63 (m, 1H), 7.61 (s, 1H), 7.56 - 7.52 (m, 2H), 7.50 – 7.42 (m,
3H) ; 13C NMR (101 MHz, CDCl3) δ 178.7, 157.0, 154.2, 135.3, 133.7, 130.7, 130.0, 129.1, 128.4,
126.6, 125.4, 123.9; IR (neat): 3068, 1650, 1473, 1446, 1361, 1256, 1171, 947, 904, 764, 685, 638
cm-1; MS (EI) m/z: 249.1 (36) [M]+ , 221.1 (24) [M-CH2N] +·, 105.1 (100) [M-C9H6NO]+·, 77.1 (46)
[M-C10H6NO2] +·. The spectral data were in good agreement with the literature.75
2-Hydroxy-1-phenyl-1-propanone (6)
Prepared from propiophenone and valine following general experimental procedure 1; Yield 50%
(30 mg), pale yellow liquid; eluent: petroleum ether/ethyl acetate (7:1); 1H NMR (400 MHz, CDCl3)
δ 7.92 (dt, J = 8.5, 1.6 Hz, 2H), 7.64 – 7.58 (m, 1H), 7.54 – 7.46 (m, 2H), 5.16 (q, J = 7.0 Hz, 1H),
4.17 – 3.17 (m, 1H), 1.44 (d, J = 7.0 Hz, 3H); 13C NMR (101 MHz, CDCl3) δ 202.3, 133.9, 133.3,
128.8, 128.6, 69.3, 22.2. MS (EI) m/z [M]+: 151.1. The spectral data were in good agreement with
the following literature.76
ACKNOWLEDGMENT
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We thank the DFG (NA 955/1-1) and the Fonds der Chemischen Industrie (Sachkostenzuschuss) for
financial support.
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SUPPORTING INFORMATION
1H and 13C NMR spectra of oxazoles 3a-y and compounds 4, 5 and 6. This material is available
free of charge via the Internet at http://pubs.acs.org.
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