Synthesis of functionally substituted isoxazoles from 5-(2,5- dimethylphenyl)-1,2-oxazole-3-carbonitrile

Article abstract of DOI:10.1134/S107042800906013X

Successive transformations of 5-(2,5-dimethylphenyl)-1,2-oxazole-3- carbonitrile, including Curtius rearrangement, led to the formation of 5-(2,5-dimethylphenyl)-1,2-oxazol-3-amine. Sulfonamides and urea derivative containing 5-(2,5-dimethylphenyl)isoxazo

Full text of DOI:10.1134/S107042800906013X

ISSN 1070-4280, Russian Journal of Organic Chemistry, 2009, Vol. 45, No. 6, pp. 879–883. © Pleiades Publishing, Ltd., 2009.
Original Russian Text © V.I. Potkin, S.K. Petkevich, E.G. Zalesskaya, 2009, published in Zhurnal Organicheskoi Khimii, 2009, Vol. 45, No. 6, pp. 892–896.
Synthesis of Functionally Substituted Isoxazoles
from 5-(2,5-Dimethylphenyl)-1,2-oxazole-3-carbonitrile
V. I. Potkin, S. K. Petkevich, and E. G. Zalesskaya
Institute of Physical Organic Chemistry, National Academy of Sciences of Belarus,
ul. Surganova 13, Minsk, 220072 Belarus
e-mail: potkin@ifoch.bas-net.by
Received July 28, 2008
Abstract—Successive transformations of 5-(2,5-dimethylphenyl)-1,2-oxazole-3-carbonitrile, including Curtius
rearrangement, led to the formation of 5-(2,5-dimethylphenyl)-1,2-oxazol-3-amine. Sulfonamides and urea
derivative containing 5-(2,5-dimethylphenyl)isoxazole fragments were synthesized.
DOI: 10.1134/S107042800906013X
The chemistry of isoxazole derivatives extensively
develops due to their wide application in the synthesis
of various organic compounds [1–3] and biological
activity of numerous representatives of this class of
heterocycles [4–6]. Functionalization of isoxazole ring
is one of the most practical ways of obtaining new
compounds that may be promising as potential biolog-
ically active substances. Among functionally substitut-
ed isoxazoles, their amino derivatives attract strong
interest as starting compounds for the synthesis of vari-
ous fused heterocyclic systems, as well as of cytostatic
agents, anticonvulsants, fungicides, herbicides, and
other bioactive products [7–9]. Development of new
synthetic approaches to substituted aminoisoxazoles
and search for convenient starting compounds for their
synthesis constitute important problems.
1,2-oxazole-3-carboxylic acid (III) was synthesized in
90% yield by alkaline hydrolysis of the cyano group in
compound I on heating in aqueous–alcoholic alkali.
Carboxylic acid III was converted into isoxazole-
3-carbonyl chloride IV by treatment with thionyl
chloride (yield 85%; Scheme 1). The structure of acid
III and acid chloride IV was determined on the basis
1
of their elemental analyses and IR, H and 13C NMR,
and mass spectra. The IR spectra of III and IV lacked
absorption band typical of stretching vibrations of the
cyano group in initial nitrile I (2253 cm–1), and
a strong absorption band due to stretching vibrations of
the carbonyl group appeared at 1708 (III) or 1766 cm–1
(IV). The electron impact mass spectra of compounds
III and IV contained the molecular ion peaks and
fragment ion peaks which were consistent with the
assumed structures.
We recently developed a procedure for the syn-
thesis of 5-(2,5-dimethylphenyl)-1,2-oxazole-3-carbo-
nitrile (I) [10] from 5-(2,5-dimethylphenyl)-1,2-oxa-
zole-3-carbaldehyde oxime which was prepared in turn
by condensation of accessible 1-(2,5-dimethylphenyl)-
3,4,4-trichlorobut-3-en-1-one with hydroxylamine
[11]. Isoxazole I molecule possesses a reactive cyano
group whose transformations could lead to various
derivatives [12].
Acid chloride IV was treated with sodium azide to
obtain 81% of 5-(2,5-dimethylphenyl)-1,2-oxazole-3-
carbonyl azide (V). The presence of an azido group in
compound V was confirmed by the IR spectrum which
characteristically contained an absorption band at
2171 cm–1; carbonyl stretching vibrations gave rise to
a strong band at 1690 cm–1. No molecular ion was ob-
served in the mass spectrum of V; instead, an ion peak
with m/z 214 [M – N2]+ (base peak) was present to-
gether with ion peaks corresponding to its fragmenta-
tion products.
The goal of the present work was to synthesize
5-(2,5-dimethylphenyl)-1,2-oxazol-3-amine (II), its
precursors, and some derivatives starting from com-
pound I. We planned to obtain aminoisoxazole II via
Curtius rearrangement of the corresponding carboxylic
acid azide, which is accompanied by loss of one car-
bon atom [13]. The required 5-(2,5-dimethylphenyl)-
Azide V was converted into target aminoisoxazole
II through carbamate VI which was obtained by heat-
ing compound V for 4 h in boiling anhydrous ethanol.
Ethyl 5-(2,5-dimethylphenyl)-1,2-oxazol-3-ylcarba-
879

POTKIN et al.
880
Scheme 1.
Me
Me
Me
CN
COOH
COCl
KOH, H2O–EtOH
SOCl2
N
N
N
O
O
O
Me
Me
Me
I
III
IV
Me
Me
Me
H
N
OEt
CON3
NH2
NaN3
Δ, EtOH
Δ, H2O–EtOH
O
N
N
N
O
O
O
Me
Me
Me
V
VI
II
Me
Me
H
H
4-MeCONHC6H4SO2Cl
pyridine
N
N
SO2C6H4NHCOMe-4
SO2C6H4NH2
N
N
O
O
Me
Me
VIII
IX
Me
Me
H
N
H
N
Δ, H2O
V
N
O
O
N
O
Me
Me
VII
mate (VI) was isolated in 79% yield. The use of
anhydrous ethanol was crucial; heating of azide V in
water for 1 h at 70°C resulted in the formation of 89%
of N,N′-bis[5-(2,5-dimethylphenyl)-1,2-oxazol-3-yl]-
urea (VII) (Scheme 1). Treatment of carbamate VI
with alkali in aqueous alcohol at 75°C afforded 92% of
5-(2,5-dimethylphenyl)-1,2-oxazol-3-amine (II).
in aromatic compounds, and a band at 1629 cm–1 due
to bending vibrations of the NH2 group. The latter gave
rise to a broadened singlet at δ 3.76 ppm in the
1H NMR spectrum. Compound II showed in the
13C NMR spectrum two signals from methyl groups,
six signals from carbon atoms in the benzene ring, and
three signals at δC 95.27, 163.52, and 169.96 ppm,
which were assigned to the C4, C3, and C5 atoms in the
isoxazole ring, by analogy with the spectrum of struc-
turally related aminoisoxazole [14]. The mass spec-
trum of II contained the molecular ion peak.
The structure of aminooxazole II, carbamate VI,
and substituted urea VII was confirmed by elemental
1
analyses and IR, H and 13C NMR, and mass spectra.
Compound VI displayed in the IR spectrum absorption
bands due to stretching vibrations of the N–H bond
(3256 cm–1) and C=O group (1737 cm–1, strong band).
Urea derivative VII was characterized by IR ab-
sorption bands at 3253 (N–H) and 1701 cm–1 (C=O).
Two isoxazole fragments in compound VII gave only
1
In the H NMR spectrum of VI, protons in the ethoxy
1
group resonated as a triplet at δ 1.33 ppm (CH3) and
a quartet at δ 4.29 ppm (CH2), and the NH proton
appeared as a broadened singlet at δ 8.02 ppm. The
13C NMR spectrum of VI contained signals from car-
bon atoms in the ethoxy group (δC 14.57, 62.29 ppm)
and carbonyl carbon atom (δC 152.08 ppm). The mo-
lecular ion peak (m/z 260) in the mass spectrum of VI
had the maximal intensity.
one set of proton signals in the H NMR spectrum,
indicating symmetric structure of its molecule.
Compound II was subjected to acylation with
sulfanilic acid chloride in which the amino group was
preliminarily protected by acetylation. The reaction
was carried out in pyridine at 80°C, and the product
was the corresponding sulfonamide, N-{4-[5-(2,5-di-
methylphenyl)-1,2-oxazol-3-ylsulfamoyl]phenyl}acet-
amide (VIII, yield 85%). The acetyl protection was
removed by treatment of compound VIII with aqueous
potassium hydroxide, and 4-amino-N-[5-(2,5-dimeth-
In the IR spectrum of aminoisoxazole II we ob-
served absorption bands at 3398 and 3316 cm–1, typ-
ical of stretching vibrations of a primary amino group
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 45 No. 6 2009

SYNTHESIS OF FUNCTIONALLY SUBSTITUTED ISOXAZOLES
881
1
ylphenyl)-1,2-oxazol-3-yl]benzenesulfonamide (IX)
(C=O); 1573 (C=N); 1613, 1503 (C=C). H NMR
was isolated in 90% yield (Scheme 1).
spectrum, δ, ppm: 2.37 s (3H, Me), 2.48 s (3H, Me),
3
6.99 s (1H, 4-H), 7.25 d (1H, Harom, J = 8 Hz), 7.27 d
The formation of sulfonamides VIII and IX was
confirmed by the presence in their IR spectra of ab-
sorption bands in the regions 1154–1165 and 1320–
1325 cm–1, corresponding to stretching vibrations of
the sulfonyl group. Stretching vibrations of the N–H
bonds gave rise to broad absorption bands in the region
3323–3486 cm–1. In addition, the IR spectrum of
N-acetyl derivative VIII contained carbonyl absorption
band at 1677 cm–1; no analogous band was present in
the IR spectrum of IX. In the 1H and 13C NMR spectra
of VIII and IX we observed signals from protons and
carbon atoms in the isoxazole and sulfanilic acid frag-
ments, as well as from the acetyl group in the spectra
of compound VIII.
3
(1H, Harom, J = 8 Hz), 7.60 s (1H, Harom). 13C NMR
spectrum, δC, ppm: 21.13 (Me), 21.55 (Me), 104.29
(C4 ), 129.98, 132.52, 132.67 (CHarom), 127.27, 134.45,
.
137.22, 158.22, 161.68 (Cquat), 172.80 (C=O). Found,
%: C 66.62; H 5.37; N 6.18. m/z 173 [M – CO2]+.
C12H11NO3. Calculated, %: C 66.35; H 5.10; N 6.45.
M 217.22.
5-(2,5-Dimethylphenyl)-1,2-oxazole-3-carbonyl
chloride (IV). Thionyl chloride, 8.94 g (75.12 mmol),
was added to a suspension of 3.23 g (14.87 mmol) of
acid III in 30 ml of anhydrous carbon tetrachloride,
and the mixture was heated for 6 h under reflux. The
solvent and excess thionyl chloride were removed
under reduced pressure, and the solid residue was re-
crystallized from hexane. Yield 2.98 g (85%), mp 46–
47°C. IR spectrum, ν, cm–1: 1766 (C=O); 1569 (C=N);
Sulfonamides VIII and IX are structural analogs of
bactericidal agents like Sulfamethoxazole [15], and
they attract interest from the viewpoint of their bio-
logical activity.
1
1613, 1506 (C=C). H NMR spectrum, δ, ppm: 2.40 s
(3H, Me), 2.48 s (3H, Me), 6.87 s (1H, 4-H), 7.23 br.s
(2H, Harom), 7.55 br.s (1H, Harom). 13C NMR spectrum,
δC, ppm: 21.51 (Me), 21.55 (Me), 102.86 (C4), 129.58,
132.24, 132.57 (CHarom), 125.92, 134.03, 136.85,
159.78, 161.52 (Cquat), 174.28 (C=O). Found, %:
C 61.05; H 4.22; Cl 15.35; N 5.88. m/z 235 [M]+.
C12H10ClNO2. Calculated, %: C 61.16; H 4.28;
Cl 15.04; N 5.95. M 235.66.
EXPERIMENTAL
The IR spectra were recorded in KBr on a Nicolet
Protégé-460 spectrometer with Fourier transform. The
1H and 13C NMR spectra were measured on a Bruker
Avance-500 instrument from solutions in CDCl3 (com-
pounds II and IV–VI), acetone-d6 (III), or DMSO-d6
1
(VII–IX). The H chemical shifts were determined
5-(2,5-Dimethylphenyl)-1,2-oxazole-3-carbonyl
azide (V). A solution of 2.36 g (10 mmol) of acid
chloride IV in 25 ml of anhydrous acetone was cooled
to 0°C, a solution of 0.78 g (12 mmol) of sodium azide
in 3 ml of water was added dropwise under stirring, the
cooling bath was removed, the mixture was stirred for
20 min and diluted with 100 ml of water, and the pre-
cipitate was filtered off and dried under reduced pres-
sure. Yield 1.96 g (81%), mp 77–78°C. IR spectrum, ν,
cm–1: 2171 (N3); 1690 (C=O); 1590, 1573, 1557, 1500
relative to tetramethylsilane, and the 13C chemical
shifts were determined relative to the corresponding
deuterated solvent signals. The mass spectra (electron
impact, 70 eV) were obtained on a Hewlett–Packard
5890/5972 GC–MS system (HP-5MS capillary col-
umn, 30 m×0.25 mm, stationary phase 5% of phenyl-
methylsilicone, film thickness 0.25 μm; injector tem-
perature 250°C).
5-(2,5-Dimethylphenyl)-1,2-oxazole-3-carbonitrile
(I) was synthesized according to the procedure de-
scribed in [5].
1
(C=C, C=N). H NMR spectrum, δ, ppm: 2.39 s (3H,
Me), 2.48 s (3H, Me), 6.87 s (1H, 4-H), 7.22 br.s
(2H, Harom), 7.57 br.s (1H, Harom). 13C NMR spectrum,
δC, ppm: 21.01 and 21.12 (Me), 102.57 (C4), 129.08,
131.64, 131.79 (CHarom), 125.68, 133.44, 136.25,
157.12, 166.14 (Cquat), 172.79 (C=O). Found, %:
C 59.14; H 4.39; N 23.42. m/z 214 [M – N2]+.
C12H10N4O2. Calculated, %: C 59.49; H 4.16; N 23.13.
M 242.24.
5-(2,5-Dimethylphenyl)-1,2-oxazole-3-carboxylic
acid (III). A solution of 1.3 g of potassium hydroxide
in 20 ml of water was added to a solution of 2.15 g
(10.85 mmol) of cyanoisoxazole I in 30 ml of ethanol,
and the mixture was heated to the boiling point, stirred
on heating under reflux until ammonia no longer
evolved (~10 h), poured into water, and acidified with
hydrochloric acid to pH 3. The precipitate was filtered
off, washed with water, dried under reduced pressure,
and recrystallized from chloroform. Yield 2.13 g
(90%), mp 151–153°C. IR spectrum, ν, cm–1: 1708
Ethyl 5-(2,5-dimethylphenyl)-1,2-oxazole-3-yl-
carbamate (VI). A solution of 3.63 g (15 mmol) of
azide V in 30 ml of anhydrous ethanol was heated for
4 h under reflux. The solvent was removed, and the
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 45 No. 6 2009

POTKIN et al.
882
residue was recrystallized from hexane. Yield 3.08 g
(79%), mp 103–104°C. IR spectrum, ν, cm–1: 3256
(NH); 1737 (C=O); 1632, 1613, 1547, 1491 (C=C,
C=N). 1H NMR spectrum, δ, ppm: 1.37 t (3H, Me, 3J =
7 Hz), 2.38 s (3H, Me), 2.50 s (3H, Me), 4.30 q (2H,
a solution of 1.88 g (10 mmol) of aminoisoxazole II in
10 ml of pyridine, and the mixture was stirred for 8 h
at 80°C. The mixture was cooled, poured into 100 ml
of water, and acidified with 15 ml of 10% hydrochloric
acid. The precipitate was filtered off, washed with
water, and dried under reduced pressure. Yield 3.27 g
(85%), mp 124–126°C. IR spectrum, ν, cm–1: 3413,
3323 (NH); 1677 (C=O); 1588, 1541, 1514, 1494
(C=C, C=N); 1325, 1165 (S=O). 1H NMR spectrum, δ,
ppm: 2.04 s (3H, Me), 2.27 s (3H, Me), 2.32 s (3H,
3
CH2O, J = 7 Hz), 7.01 s (1H, 4-H), 7.18 m (2H,
H
arom), 7.55 s (1H, Harom). 8.02 br.s (1H, NH).
13C NMR spectrum, δC, ppm: 14.57 (Me), 21.01 (Me),
21.10 (Me), 62.29 (CH2), 96.17 (C4), 129.01, 131.09,
131.46 (CHarom), 126.80, 132.38, 135.90, 153.13,
158.61 (Cquat), 170.55 (C=O). Found, %: C 64.55;
H 6.58; N 10.64. m/z 260 [M]+. C14H16N2O3. Calculat-
ed, %: C 64.60; H 6.20; N 10.76. M 260.29.
3
Me), 6.56 s (1H, 4-H), 7.18 d (1H, Harom, J = 8 Hz),
7.21 d (1H, Harom, 3J = 8 Hz), 7.45 s (1H, Harom), 7.74 d
3
3
(2H, Harom, J = 8.5 Hz), 7.82 d (2H, Harom, J =
8.5 Hz), 10.37 br.s and 11.51 br.s (1H each, NH).
13C NMR spectrum, δC, ppm: 20.89 (Me), 21.00 (Me),
24.69 (Me), 96.76 (C4), 119.22 (2C, CHarom), 128.80
(2C, CHarom), 128.97, 131.73, 131.99 (CHarom), 126.35,
133.23, 133.28, 136.19, 144.20, 158.36, 169.74 (Cquat),
170.13 (C=O). Found, %: C 59.44; H 5.23; N 10.76;
S 8.48. C19H19N3O4S. Calculated, %: C 59.20; H 4.97;
N 10.90; S 8.32.
5-(2,5-Dimethylphenyl)-1,2-oxazol-3-amine (II).
Potassium hydroxide, 10 g, was added to a solution of
1.3 g (5 mmol) of compound VI in 20 ml of 80%
aqueous alcohol, and the mixture was stirred for 8 h at
75°C. The mixture was then poured into water, and the
precipitate was filtered off, washed with water, and
dried under reduced pressure. Yield 0.86 g (92%),
mp 153–155°C. IR spectrum, ν, cm–1: 3398, 3316,
1629 (NH); 1599, 1515, 1500 (C=C, C=N). 1H NMR
spectrum, δ, ppm: 2.36 s (3H, Me), 2.45 s (3H, Me),
3.76 br.s (2H, NH2), 6.00 s (1H, 4-H), 7.15 d (1H,
4-Amino-N-[5-(2,5-dimethylphenyl)-1,2-oxazol-
3-yl]benzenesulfonamide (IX). Sulfonamide VIII,
3.85 g (10 mmol), was added to a solution of 1.2 g
(21 mmol) of potassium hydroxide in 20 ml of water,
and the mixture was kept for 2 days at 25°C, diluted
with water to a volume of 100 ml, and neutralized with
20% hydrochloric acid. The precipitate was filtered
off, washed with water, and dried under reduced pres-
sure. Yield 3.10 g (90%), mp 181–182°C. IR spectrum,
ν, cm–1: 3486, 3384, 1626 (NH); 1596, 1573, 1506,
3
3
H
arom, J = 8.5 Hz), 7.17 d (1H, Harom, J = 8.5 Hz),
7.50 s (1H, Harom). 13C NMR spectrum, δC, ppm: 21.02
(Me), 21.09 (Me), 95.27 (C4), 128.93, 130.83, 131.34
(CHarom), 127.12, 133.15, 135.83 (Cquat), 163.52 (C3),
169.96 (C5). Found, %: C 70.39; H 6.58; N 14.79.
m/z 188 [M]+. C11H12N2O. Calculated, %: C 70.19;
H 6.43; N 14.89. M 188.23.
1
1494 (C=C, C=N); 1320, 1154 (S=O). H NMR spec-
N,N′-Bis[5-(2,5-dimethylphenyl)-1,2-oxazol-3-yl]-
urea (VII). A suspension of 1.21 g (5 mmol) of azide
V in 20 ml of water was stirred for 1 h at 70°C. The
precipitate was filtered off, washed with water, and
dried under reduced pressure. Yield 0.89 g (89%),
mp 217–220°C. IR spectrum, ν, cm–1: 3253 (NH);
trum, δ, ppm: 2.28 s (3H, Me), 2.32 s (3H, Me), 6.52 s
(1H, 4-H), 6.57 d (2H, Harom, 3J = 8.5 Hz), 7.18 d (1H,
3
3
H
arom, J = 8 Hz), 7.22 d (1H, Harom, J = 8 Hz), 7.45 s
3
(1H, Harom), 7.50 d (2H, Harom, J = 8.5 Hz), 11.12 br.s
(1H, NH). 13C NMR spectrum, δC, ppm: 20.90 (Me),
21.190 (Me), 96.72 (C4), 113.24 (2C, CHarom), 129.58
(2C, CHarom), 128.90, 131.66, 131.99 (3C, CHarom),
124.48, 126.44, 133.20, 136.20, 153.84, 158.73, 169.76
(Cquat). Found, %: C 59.76; H 5.37; N 12.21; S 9.55.
C17H17N3O3S. Calculated, %: C 59.46; H 4.99;
N 12.24; S 9.34.
1
1701 (C=O); 1624, 1606, 1561 (C=C, C=N). H NMR
spectrum, δ, ppm: 2.30 s (6H, Me), 2.40 s (6H, Me),
3
7.03 s (2H, 4-H), 7.19 d (2H, Harom, J = 8 Hz), 7.24 d
3
(2H, Harom, J = 8 Hz), 7.52 s (2H, Harom), 9.79 br.s
(2H, NH). 13C NMR spectrum, δC, ppm: 20.93 (2C,
Me), 21.13 (2C, Me), 96.93 (2C, C4), 128.86, 131.57,
132.01 (6C, CHarom), 126.68, 133.20, 136.21, 151.07,
158.99 (10C, Cquat), 169.57 (1C, C=O). Found, %:
C 68.53; H 5.77; N 13.85. C23H22N4O3. Calculated, %:
C 68.64; H 5.51; N 13.92.
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