Synthesis of 3-substitued 5-(2-thienyl)isoxazoles

Article abstract of DOI:10.1134/S1070428011060169

The reaction of 3,4,4-trichloro-1-(2-thienyl)but-3-en-1-one with hydroxylamine gave 3-hydroxyiminomethyl-5-(2-thienyl)isoxazole which was converted into 5-(2-thienyl)isoxazole-3-carbonitrile by the action of acetic anhydride in pyridine. 5-(2-Thienyl)isox

Full text of DOI:10.1134/S1070428011060169

ISSN 1070-4280, Russian Journal of Organic Chemistry, 2011, Vol. 47, No. 6, pp. 928–932. © Pleiades Publishing, Ltd., 2011.
Original Russian Text © V.I. Potkin, S.K. Petkevich, P.V. Kurman, 2011, published in Zhurnal Organicheskoi Khimii, 2011, Vol. 47, No. 6, pp. 911–914.
Synthesis of 3-Substitued 5-(2-Thienyl)isoxazoles
V. I. Potkin, S. K. Petkevich, and P. V. Kurman
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 October 27, 2010
Abstract—The reaction of 3,4,4-trichloro-1-(2-thienyl)but-3-en-1-one with hydroxylamine gave 3-hydroxy-
iminomethyl-5-(2-thienyl)isoxazole which was converted into 5-(2-thienyl)isoxazole-3-carbonitrile by the
action of acetic anhydride in pyridine. 5-(2-Thienyl)isoxazole-3-carbonitrile reacted with hydroxylamine to
produce the corresponding amide oxime. Heterocyclization of its O-acyl derivatives in acetic acid afforded
5-substituted 3-[5-(2-thienyl)isoxazol-3-yl]-1,2,4-oxadiazoles.
DOI: 10.1134/S1070428011060169
Functionally substituted isoxazoles and thiophenes
exhibit a broad spectrum of biological activity and are
used as medicines and chemical means for plant pro-
tection [1]. Combination of both isoxazole and thio-
phene rings in a single molecule makes such com-
pounds attractive from the viewpoint of their biologi-
cal properties, and the presence of reactive substituents
provides the possibility for subsequent functionaliza-
tion and purposeful synthesis of useful products.
excess compound I, the only product was oxime II
(Scheme 1).
1
Compound II was identified by IR, H NMR, and
mass spectra and elemental analysis. In the IR spec-
trum of II, stretching vibrations of the C=C and C=N
bonds gave rise to absorption bands at 1500, 1523, and
1599 cm–1, and a broad absorption band centered at
3230 cm–1 was assigned to H-bonded hydroxy group.
1
The H NMR spectrum of II contained singlets from
We recently reported on a convenient synthesis
of 3,4,4-trichloro-1-(2-thienyl)but-3-en-1-one (I,
2-thienyl trichloroallyl ketone) from accessible
3,4,4-trichlorobut-3-enoic acid [2] and showed that
compound I may be regarded as synthetic equivalent
of β-chlorovinyl ketones; reaction of the latter with
hydroxylamine underlies a classical procedure for the
synthesis of isoxazoles [3].
The goal of the present work was to synthesize
5-(2-thienyl)isoxazoles containing various functional
groups in position 3 of the isoxazole ring on the basis
of thienyl trichloroallyl ketone I. The reaction of
ketone I with hydroxylamine in boiling methanol
involved the carbonyl group and internal chlorine atom
and was accompanied by heterocyclization to produce
isoxazole ring. 3-Dichloromethyl-5-(2-thienyl)isoxa-
zole (IIa) thus formed reacted with the second hy-
droxylamine molecule at the dichloromethyl group to
give 3-hydroxyiminomethyl-5-(2-thienyl)isoxazole (II)
in 89% yield. Unlike the reaction of hydroxylamine
with aryl trichloroallyl ketones [4], we failed to isolate
intermediate dichloromethyl-substituted isoxazole IIa.
Even under mild conditions and in the presence of
the CH proton in the isoxazole ring (δ 6.88 ppm),
exocyclic =CH proton (δ 8.18 ppm), and hydroxy
proton (δ 11.19 ppm). Protons in the thiophene ring
resonated as a multiplet at δ 7.21 ppm and two dou-
blets at δ 7.68 and 7.72 ppm. Compound II displayed
in the 13C NMR spectrum signals from C4H in the
isoxazole ring (δC 96.08 ppm), CH carbon atoms in the
thiophene ring (δC 127.92, 128.48, and 129.01 ppm),
exocyclic CH=N carbon atom (δC 139.52 ppm), and
three quaternary carbon atoms (δC 135.75, 159.46, and
165.23 ppm). The mass spectrum of II contained the
molecular ion peak with m/z 194 and peaks belonging
to fragment ions, the most abundant of which being
that with m/z 111; it corresponds to elimination of thio-
phene residue from the molecular ion.
The hydroxyiminomethyl group in II was con-
verted into cyano by the action of acetic anhydride in
pyridine. The yield of nitrile III was 44%, and the
reaction was accompanied by considerable tarring. The
structure of nitrile III was confirmed by the presence
in its IR spectrum of a characteristic absorption band at
2255 cm–1 due to stretching vibrations of the C≡N
1
bond. The H and 13C NMR spectra of III lacked
928

SYNTHESIS OF 3-SUBSTITUED 5-(2-THIENYL)ISOXAZOLES
929
Scheme 1.
Cl
Cl
O
NH2OH
NH2OH
Cl
Cl
NOH
N
N
Cl
S
S
O
S
S
O
I
IIa
II
H2N
N
H2N
N
CN
NOH
N
O
Ac2O
NH2OH
RCOCl
R
N
S
O
S
O
O
O
III
IV
V–VII
R
O
N
AcOH, Δ
S
N
O
N
VIII–X
V, VIII, R = Me; VI, IX, R = Ph; VII, X, R = 4,5-dichloro-1,2-thiazol-3-yl.
signals assignable to the exocyclic CH group typical of
initial oxime II. Instead, a signal at δC 110.01 ppm
(CN) was observed in the 13C NMR spectrum, The
mass spectrum of III contained the molecular ion peak
with m/z 176.
thiazole-3-carbonyl chloride to obtain the correspond-
ing N-acyloxy derivatives V–VII (yield 79–92%,
Scheme 1). The reaction was selective, and only the
oxime group in IV was involved. This follows from
conservation of absorption bands due to stretching vi-
brations of amino group in the regions 3322–3326 and
3421–3437 cm–1 in the IR spectra of V–VII and disap-
pearance of OH absorption band (2767 cm–1). The
1H NMR spectra of V–VII contained no OH proton
signal at about δ 9.5 ppm, whereas protons in the NH2
group resonated as a broadened two-proton singlet in
the region δ 5.97–6.36 ppm. The carbonyl group in
O-acyl amide oximes V–VII gave rise to a strong IR
absorption band in the region 1722–1768 cm–1. The
carbonyl carbon signal appeared in the 13C NMR spec-
tra of V–VII at δC 164–168 ppm.
Due to the presence of a cyano group compound III
can be subjected to further functionalization. Nitriles
are widely used in the synthesis of compounds be-
longing to various classes, including heterocyclic com-
pounds [5]. In the present work the cyano group in III
was used to build up 1,2,4-oxadiazole ring according
to [6], through intermediate amide oxime IV and its
O-acyl derivatives.
Amide oxime IV was obtained in quantitative yield
by addition of hydroxylamine at the C≡N group of
nitrile III. No C≡N absorption band was observed in
the IR spectrum of IV, but two bands assignable to
N–H stretching vibrations (3339 and 3449 cm–1) and
a broad absorption band typical of stretching vibrations
of H-bonded hydroxy group (2767 cm–1) were present.
Heterocyclization of the O-acyl amide oxime
moiety in azoles V–VII to 1,2,4-oxadiazole ring was
effected by heating in boiling acetic acid. As a result,
the corresponding 5-substituted 3-[5-(2-thienyl)isoxa-
zol-3-yl]-1,2,4-oxadiazoles VIII–X were obtained in
73–85% yield. Unlike previously described heterocy-
clization of O-acyl amide oximes derived from 5-aryl-
isoxazoles, the reaction was accompanied by trans-
acylation of initial O-acyl amide oximes VI and VII
with acetic acid, and the reaction mixtures contained
5-methyl-3-[5-(2-thienyl)isoxazol-3-yl]-1,2,4-oxadi-
azole (VIII) as by-product (8–15%) together with the
corresponding oxadiazoles IX and X.
1
The H NMR spectrum of IV contained a broadened
two-proton singlet at δ 5.55 ppm from the NH2 group
and a one-proton singlet at δ 9.51 ppm from the
13
hydroxy proton. The C NMR spectrum of IV lacked
signal typical of cyano group (δC 110 ppm in the spec-
trum of III), while a signal appeared at δC 158.29 ppm
due to carbon atom in the amide oxime fragment.
Amide oxime IV was subjected to acylation with
acetyl chloride, benzoyl chloride, and 4,5-dichloroiso-
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 47 No. 6 2011

POTKIN et al.
930
Oxadiazoles VIII–X were identified by elemental
δC, ppm: 96.08 (C4), 127.92 (CH, thiophene), 128.48
(CH, thiophene), 129.10 (CH, thiophene), 139.52
(CH=N); 135.75, 159.46, 165.23 (Cquat). Found, %:
C 49.58; H 3.39; N 14.76; S 16.75. m/z 194 [M]+.
C8H6N2O2S. Calculated, %: C 49.47; H 3.11; N 14.42;
S 16.51. M 194.21.
1
13
analyses and IR and H and C NMR spectra. The IR
spectra of isoxazolyl-1,2,4-oxadiazoles VIII–X con-
tained no absorption bands assignable to carbonyl and
amino groups, which were present in the spectra of
initial O-acylated amide oximes V–VII. Stretching
vibrations of the C=C and C=N bonds were charac-
terized by absorption bands in the region 1493–
1646 cm–1. Like 3,5-substituted 1,2,4-oxadiazoles pos-
sessing a (2,5-dimethylphenyl)isoxazole fragment [7],
their analogs VIII–X having a thiophene ring instead
of the p-xylene moiety displayed in the IR spectra low-
frequency shift of the high-frequency absorption bands
as compared to initial O-acyl amide oximes V–VII
(from 1644–1653 to 1614–1646 cm–1); however, the
low-frequency shift was smaller than that observed
previously [7].
5-(2-Thienyl)isoxazole-3-carbonitrile (III). Acetic
anhydride, 7.66 g (75 mmol), was added under stirring
to a solution of 9.71 g (50 mmol) of oxime II in 20 ml
of pyridine, and the mixture was stirred for 6 h at 100–
105°C, poured into water, acidified to pH 5 with
hydrochloric acid, and extracted with chloroform. The
extract was dried over CaCl2, the solvent was removed
under reduced pressure, and the solid residue was re-
crystallized from hexane. Yield 3.88 g (44%), mp 77–
78°C. IR spectrum, ν, cm–1: 2255 (C≡N); 1597, 1585,
1
1510, 1497 (C=C, C=N). H NMR spectrum, δ, ppm:
1
In the H NMR spectra of 1,2,4-oxadiazoles VIII–
6.68 s (1H, 4-H), 7.18 d.d (1H, thiophene), 7.57 d (1H,
3
3
X, singlets from the 4-H proton in the isoxazole ring,
signals from protons in the thiophene ring, and signals
from methyl (VIII) or phenyl substituent (IX) were
present. The 13C NMR spectra of VIII–X were consist-
ent with the assumed structure.
thiophene, J = 5 Hz), 7.60 d (1H, thiophene, J =
4 Hz). 13C NMR spectrum, δC, ppm: 101.06 (C4),
110.01 (C≡N), 128.74 (CH, thiophene), 129.00 (CH,
thiophene), 130.23 (CH, thiophene); 126.95, 140.18,
167.80 (Cquat). Found, %: C 54.91; H 2.55; N 15.76;
S 18.17. m/z 176 [M]+. C8H4N2OS. Calculated, %:
C 54.53; H 2.29; N 15.90; S 18.20. M 176.20.
EXPERIMENTAL
N′-Hydroxy-5-(2-thienyl)isoxazole-3-carbox-
imidamide (IV). Hydroxylamine hydrochloride,
0.36 g (5.2 mmol), was added to a solution of 0.29 g
(5.2 mmol) of potassium hydroxide in 20 ml of metha-
nol, the mixture was stirred for 0.5 h at 30°C, 0.88 g
(5 mmol) of nitrile III was added in one portion, and
the mixture was stirred for an additional 1 h. The mix-
ture was then poured into water, and the precipitate
was filtered off, washed with water, dried under
reduced pressure, and recrystallized from chloroform–
hexane (6:1). Yield 1.03 g (99%), mp 169–171°C. IR
spectrum, ν, cm–1: 3449, 3339 (NH2); 2767 (OH);
The IR spectra were recorded in KBr on a Nicolet
Protégé-460 spectrometer with Fourier transform. The
13
1H and C NMR spectra were measured on a Bruker
Avance-500 spectrometer from solutions in CDCl3
(nitrile III), acetone-d6 (II, IV), and DMSO-d6 (V–X);
the chemical shifts were determined relative to tetra-
methylsilane (1H) and signals of the corresponding
deuterated solvents, δC, ppm: 77.2 ppm (CDCl3), 30.2
(acetone-d6), 40.1 (DMSO-d6). The mass spectra were
obtained on a Hewlett–Packard HP 5890/5972 GC–MS
system (electron impact, 70 eV; HP-5MS capillary col-
umn, 30 m×0.25 mm, film thickness 0.25 μm; injector
temperature 250°C).
1
1650, 1580, 1530 (C=C, C=N); 1609 (δNH). H NMR
spectrum, δ, ppm: 5.55 br.s (2H, NH2), 6.79 s (1H,
5-(2-Thienyl)isoxazole-3-carbaldehyde oxime
(II). A mixture of 6.39 g (25 mmol) of ketone I and
4.16 g (126 mmol) of hydroxylamine in 50 ml of meth-
anol was heated for 15 h under reflux and poured into
an ice–water mixture. The slightly colored precipitate
was filtered off, washed with water, and recrystallized
from chloroform. Yield 4.32 g (89%), mp 138–140°C.
IR spectrum, ν, cm–1: 3230 (O–H); 1599, 1523, 1500
4-H), 7.20 d.d (1H, thiophene), 7.66 d (1H, thiophene,
3
3J = 4 Hz), 7.71 d (1H, thiophene, J = 5 Hz), 9.51 s
(1H, OH). 13C NMR spectrum, δC, ppm: 96.68 (C4),
127.78 (CH, thiophene), 128.45 (CH, thiophene),
129.94 (CH, thiophene); 128.66, 144.30, 158.29,
165.05 (Cquat). Found, %: C 45.86; H 3.42; N 20.22;
S 15.51. C8H7N3O2S. Calculated, %: C 45.92; H 3.37;
N 20.08; S 15.33.
1
(C=C, C=N). H NMR spectrum, δ, ppm: 6.88 s (1H,
4-H), 7.21 m (1H, thiophene), 7.68 d (1H, thiophene,
N′-Acetoxy-5-(2-thienyl)isoxazole-3-carboximid-
amide (V). Acetyl chloride, 0.39 g (5 mmol), was
added in one portion to a suspension of 1.05 g
3
3J = 5 Hz), 7.72 d (1H, thiophene, J = 4 Hz), 8.17 s
(1H, N=CH), 11.19 s (1H, OH). 13C NMR spectrum,
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 47 No. 6 2011

SYNTHESIS OF 3-SUBSTITUED 5-(2-THIENYL)ISOXAZOLES
931
(5 mmol) of compound IV in 50 ml of anhydrous di-
ethyl ether, a solution of 0.41 g (5.2 mmol) of pyridine
in 5 ml of anhydrous diethyl ether was then added
dropwise under stirring, and the mixture was heated
for 3 h under reflux. The precipitate was filtered off,
washed with water, and dried under reduced pressure.
Yield 1.16 g (92%), mp 180–182°C. IR spectrum, ν,
cm–1: 3421, 3322 (NH2); 1754 (C=O); 1652, 1639,
5-Methyl-3-[5-(2-thienyl)isoxazol-3-yl]-1,2,4-oxa-
diazole (VIII). A solution of 1.26 g (5 mmol) of
compound V in 20 ml of glacial acetic acid was heated
for 6 h under reflux. The mixture was poured into
200 ml of water and neutralized to pH 7 with a solu-
tion of potassium hydroxide. The precipitate was
filtered off, washed with water, dried under reduced
pressure, and recrystallized from ethanol. Yield 0.95 g
(81%), mp 123–124°C. IR spectrum, ν, cm–1: 1614,
1
1593, 1529 (C=C, C=N). H NMR spectrum, δ, ppm:
1
2.12 s (3H, CH3), 6.36 br.s (2H, NH2), 7.09 s (1H,
4-H), 7.22 d.d (1H, thiophene), 7.79 d (1H, thiophene,
1584, 1566, 1499 (C=C, C=N). H NMR spectrum, δ,
ppm: 2.67 s (3H, CH3), 7.41 s (1H, 4-H), 7.24 d.d (1H,
3
3
3J = 4 Hz), 7.82 d (1H, thiophene, J = 5 Hz).
thiophene), 7.83 d (1H, thiophene, J = 4 Hz), 7.86 d
13C NMR spectrum, δC, ppm: 20.17 (CH3), 98.53 (C4),
129.29 (2C, CH, thiophene), 130.59 (CH, thiophene);
128.08, 149.38, 157.73, 165.96 (Cquat); 168.45 (C=O).
Found, %: C 48.05; H 4.07; N 16.63; S 12.81.
C10H9N3O3S. Calculated, %: C 47.80; H 3.61; N 16.72;
S 12.76.
(1H, thiophene, J = 5 Hz). 13C NMR spectrum, δC,
3
ppm: 12.59 (CH3), 99.37 (C4), 129.32 (CH, thiophene),
129.65 (CH, thiophene), 131.00 (CH, thiophene);
127.80, 153.63, 161.21, 166.86, 179.17 (Cquat). Found,
%: C 51.65; H 3.23; N 18.17; S 13.88. C10H7N3O2S.
Calculated, %: C 51.49; H 3.02; N 18.02; S 13.75.
Isoxazolyloxadiazoles VIII and IX were synthe-
sized in a similar way and were purified by recrystal-
lization from acetone.
N′-Acyloxy-5-(2-thienyl)isoxazole-3-carboximid-
amides VI and VII were synthesized in a similar way
by acylation of amide oxime IV with the correspond-
ing acid chlorides.
5-Phenyl-3-[5-(2-thienyl)isoxazol-3-yl]-1,2,4-oxa-
diazole (IX). Yield 85%, mp 177–179°C. IR spectrum,
ν, cm–1: 1638, 1605, 1561, 1493 (C=C, C=N). 1H NMR
spectrum, δ, ppm: 7.51 s (1H, 4-H), 7.26 d.d (1H,
N′-Benzoyloxy-5-(2-thienyl)isoxazole-3-carbox-
imidamide (VI). Yield 91%, mp 204–206°C. IR spec-
trum, ν, cm–1: 3425, 3325 (NH2); 1722 (C=O); 1653,
3
1
thiophene), 7.64 d (1H, thiophene, J = 4 Hz), 7.72 d
1638, 1597, 1523, 1492 (C=C, C=N). H NMR spec-
(1H, thiophene, 3J = 5 Hz), 7.87 m (3H, Harom), 8.15 m
trum, δ, ppm: 5.97 br.s (2H, NH2), 7.16 s (1H, 4-H),
13
(2H, Harom). C NMR spectrum, δC, ppm: 99.51 (C4),
3
7.24 d.d (1H, thiophene), 7.51 d (1H, thiophene, J =
3
128.68 (2C, CHarom), 130.24 (2C, CHarom); 129.38,
129.78, 131.09, 134.36 (CHarom, CH, thiophene);
123.43, 127.79, 153.60, 161.89, 166.99, 176.88 (Cquat).
Found, %: C 61.17; H 3.28; N 14.31; S 10.67.
C15H9N3O2S. Calculated, %: C 61.01; H 3.07; N 14.23;
S 10.86.
4 Hz), 7.64 d (1H, thiophene, J = 5 Hz), 7.82 m (3H,
Harom), 8.20 m (2H, Harom). 13C NMR spectrum, δC,
ppm: 98.68 (C4); 129.16, 129.31, 129.35, 130.28,
130.61, 133.89 (8C, CHarom and CH, thiophene);
128.11, 129.45, 149.68, 157.85, 163.76 (Cquat); 166.06
(C=O). Found, %: C 57.87; H 3.97; N 13.55; S 10.42.
C15H11N3O3S. Calculated, %: C 57.50; H 3.54;
N 13.41; S 10.23.
5-(3,4-Dichloroisothiazol-3-yl)-3-[5-(2-thienyl)-
isoxazol-3-yl]-1,2,4-oxadiazole (X). Yield 73%,
mp 137–139°C. IR spectrum, ν, cm–1: 1646, 1601,
N′-(4,5-Dichloroisothiazol-3-ylcarbonyloxy)-
5-(2-thienyl)isoxazole-3-carboximidamide (VII).
Yield 79%, mp 151–153°C. IR spectrum, ν, cm–1:
3437, 3326 (NH2); 1768 (C=O); 1644, 1600, 1587,
1
1585, 1564, 1513, 1494 (C=C, C=N). H NMR spec-
trum, δ, ppm: 7.51 s (1H, 4-H), 7.25 d.d (1H, thio-
13
phene), 7.86 m (2H, thiophene). C NMR spectrum,
δC, ppm: 99.51 (C4), 129.36 (CH, thiophene), 129.84
(CH, thiophene), 131.15 (CH, thiophene); 124.91,
148.36, 151.93, 153.19, 161.80, 167.15, 169.56, 172.55
(Cquat). Found, %: C 38.98; H 1.24; Cl 19.37; N 15.44;
S 17.36. C12H4Cl2N4O2S2. Calculated, %: C 38.83;
H 1.09; Cl 19.10; N 15.09; S 17.28.
1
1527, 1493 (C=C, C=N). H NMR spectrum, δ, ppm:
6.29 br.s (2H, NH2), 7.20 s (1H, 4-H), 7.24 d.d (1H,
3
thiophene), 7.81 d (1H, thiophene, J = 4 Hz), 7.84 d
3
(1H, thiophene, J = 5 Hz). 13C NMR spectrum, δC,
ppm: 98.72 (C4), 129.32 (CH, thiophene), 129.45 (CH,
thiophene), 130.75 (CH, thiophene); 124.78, 127.99,
150.40, 151.48, 154.44, 157.34, 165.96 (Cquat); 166.27
(C=O). Found, %: C 37.12; H 1.83; Cl 18.36; N 14.51;
S 16.62. C12H6Cl2N4O3S2. Calculated, %: C 37.03;
H 1.55; Cl 18.22; N 14.39; S 16.48.
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