2-Aryl-5-arylsulfanyl-1,3-oxazole-4-carboxylic acids and their derivatives

Article abstract of DOI:10.1134/S1070363210070212

A convenient preparative procedure has been developed for the synthesis of previously unknown 2- aryl-5-arylsulfanyl-1,3-oxazole-4-carboxylic acids and their functional derivatives from accessible multicenter substrates of the general formula Cl₂C=C(NHCOR)C(O)OMe. The products turned out to be suitable for various subsequent transformations. Some oxazole-4-carboxylic acid hydrazide derivatives containing a substituted oxazol-5-yl fragment at the N² atom in the hydrazine moiety underwent recyclization on heating in acetic acid; as a result, one oxazole ring was converted into 1,3,4-oxadiazole. Pleiades Publishing, Ltd., 2010.

Full text of DOI:10.1134/S1070363210070212

ISSN 1070-3632, Russian Journal of General Chemistry, 2010, Vol. 80, No. 7, pp. 1345–1350. © Pleiades Publishing, Ltd., 2010.
Original Russian Text © S.G. Pil’o, V.M. Prokopenko, V.S. Brovarets, B.S. Drach, 2010, published in Zhurnal Obshchei Khimii, 2010, Vol. 80, No. 7,
pp. 1179–1184.
2-Aryl-5-arylsulfanyl-1,3-oxazole-4-carboxylic Acids
and Their Derivatives
S. G. Pil’o, V. M. Prokopenko, V. S. Brovarets, and B. S. Drach†
Institute of Bioorganic and Petroleum Chemistry, National Academy of Sciences of Ukraine,
ul. Murmanskaya 1, Kiev, 02660 Ukraine
e-mail: brovarets@bpci.kiev.ua
Received June 18, 2009
Abstract—A convenient preparative procedure has been developed for the synthesis of previously unknown 2-
aryl-5-arylsulfanyl-1,3-oxazole-4-carboxylic acids and their functional derivatives from accessible multicenter
substrates of the general formula Cl2C=C(NHCOR)C(O)OMe. The products turned out to be suitable for
various subsequent transformations. Some oxazole-4-carboxylic acid hydrazide derivatives containing a
substituted oxazol-5-yl fragment at the N2 atom in the hydrazine moiety underwent recyclization on heating in
acetic acid; as a result, one oxazole ring was converted into 1,3,4-oxadiazole.
DOI: 10.1134/S1070363210070212
Extensive development of the chemistry of func-
tional 1,3-oxazole derivatives over the past two
decades was undoubtedly related to search for effect-
tive bioregulators. The role of 1,3-oxazole derivatives
in vital processes turned out to be more significant
than it was believed previously; this followed from the
recent isolation of numerous bio-active oxazole
compounds from natural sources [1–3]. Among these,
virginiamycin, madumycin II, griseoviridin, dendro-
amide A, and other antibiotics are derivatives of
oxazole-4-carboxylic acid, which exhibit not only
strong bactericidal but also antiblastic effect [4–7].
The key transformation sequence II → III → V
afforded reactive compounds, 2-aryl-5-arylsulfanyl-
1,3-oxazole-4-carbonyl chlorides which were brought
into further transformations to obtain the correspond-
ing hydrazides IV and N-arylamides IX (Table 1). It is
more convenient to synthesize compounds IV from
substrates II, while acid chlorides V are indispensable
for the synthesis of IX and X.
More profound modification of the substituent on
C4 in the oxazole ring can be achieved following the
transformation sequences IV → VII and V → X →
XIII. The first process is a particular case of
cyclization of carboxylic acid hydrazides with triethyl
orthoformate [14]. On the other hand, the second
process is much more complex, for it is likely to
involve regioselective reaction of compounds V and
VIII, probable prototropic transformation X ↔ XI,
and subsequent recyclization XI → XII → XIII.
Analogous recyclization was recently examined in
detail for simpler acylation products of methyl 2-aryl-
5-hydrazino-1,3-oxazole-4-carboxylates [13].
It is quite obvious that studies performed in 1950–
60 [1, 8–10] on the development of methods for the
preparation of oxazole-4-carboxylic acid derivatives
should be continued with account taken of con-
siderable improvement of synthetic tools in the
chemistry of oxazole, which was attained in the recent
years.
In the present work we used accessible methyl 2-
acylamino-3,3-dichloroacrylates І and their cyclization
products ІІ to develop convenient synthetic approaches
to numerous new 5-sulfanyl-1,3-oxazole-4-carboxylic
acid derivatives III–XIII (Scheme 1). The first reac-
tion in Scheme 1 was reported previously [11–13], while
all other reactions were performed for the first time.
The structure of all new oxazole derivatives shown
in Scheme 1 directly followed from their synthesis and
1
was confirmed by IR and H NMR spectroscopy
(Table 2). The products obtained by alkaline hydrolysis
of ester II lacked signal at δ 3.87 ppm (MeO) in the 1H
NMR spectra, but displayed a downfield signal at δ
13.21 ppm due to proton in the carboxyl group of acid
† Deceased.
1345

1346
PIL’O et al.
Scheme 1.
H
N
O
O
O
(1) Ar1SH;
(2) Ag2CO3
R1
OMe
N
OMe
(1) NaOH;
(2) HCl
N
OH
R1
R1
O
Cl
SAr1
SAr1
Cl
Iа, Ib
O
O
IIIа−IIIc
IIа−IIc
H2NNH2·H2O
SOCl2
H2O2,
MeC(O)OH
O
O
O
H2NNH2·H2O
N
OH
N
NHNH2
SAr1
N
Cl
R1
R1
R1
SO2Ar1
SAr1
O
O
O
VIа−VIc
IVа−IVc
Vа−Vc
Ar2NH2, Et3N
HC(OEt)3, Δ
+
H
O
N
N
O
N
N
NHAr2
SAr1
N
OMe
O
H2N
R1
R1
O
N
SAr1
O
O
R2
VIIIа, VIIIb
IXа−IXd
VIIа−VIIc
O
O
H
N
O
O
N
N
N
OMe
OMe
N
N
R1
R1
H
H
O
O
N
N
O
SAr1
O
SAr1
R2
R2
Xа−Xd
XIа−XId
MeC(O)OH, Δ
OMe
O
O
O
N
O
N
O
H
MeC(O)OH, Δ
OMe
R2
N
N
H
R2
N
N
N
O
N
R1
R1
O
SAr1
SAr1
O
H
O
O
Me
XIIа−XIId
XIIIа−XIIId
I, R1 = Ph (a), 4-MeC6H4 (b); II–VII, R1 = Ph, Ar1 = 4-MeC6H4 (a), R1 = Ar1 = 4-MeC6H4 (b), R1 = 4-MeC6H4, Ar1 = 4-
ClC6H4 (c); VIII, R2 = Ph (a), 4-MeC6H4 (b); IX, R1 = Ar1 = Ar2 = 4-MeC6H4 (a), R1 = Ar1 = 4-MeC6H4, Ar2 = 4-ClC6H4 (b),
R1 = Ar2 = 4-MeC6H4, Ar1 = 4-ClC6H4 (c), R1 = 4-MeC6H4, Ar1 = Ar2 = 4-ClC6H4 (d); X–XIII, R1 = R2 = Ph, Ar1 = 4-MeC6H4
(a), R1 = Ph, Ar1 = R2 = 4-MeC6H4 (b), R1 = Ar1 = 4-MeC6H4, R2 = Ph (c), R1 = 4-MeC6H4, Ar1 = 4-ClC6H4, R2 = Ph (d).
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 80 No. 7 2010

2-ARYL-5-ARYLSULFANYL-1,3-OXAZOLE-4-CARBOXYLIC ACIDS
1347
Table 1. Yields, melting points, and elemental analyses of compounds II–VII and IX–XIII
Found, %
Calculated, %
N (Cl) S
Comp.
no.
Yield, %
mp, °C (solvent)
Formula
N (Cl)
S
64
65
69
70
75
75
84b
82
85
80
80
85
76
80
80
60
62
65
80
80
78
80
82
82
85
85
76
78
75
75
93–95 (EtOH)
4.22
3.95
9.79
9.35
8.85
10.27
9.80
9.29
9.80
9.44
8.83
9.58
9.12
8.68
9.21
8.99
8.55
9.50
9.03
8.52
7.68
7.22
7.25
6.88
6.01
5.89
5.90
5.75
6.12
5.98
5.91
5.75
C18H15NO3S
4.30
4.13
9.85
9.45
8.91
10.30
9.85
9.27
9.85
9.45
8.91
9.72
9.33
8.80
9.34
8.97
8.49
9.56
9.18
8.67
7.74
7.37
7.37
7.04
6.09
5.93
5.93
5.72
6.09
5.93
5.93
5.72
IIa
98–100 (EtOH)
C19H17NO3S
IIb
126–129а (EtOH)
188–190 (EtOH)
191–194 (EtOH)
205–208 (EtOH)
162–163 (EtOH)
125–127 (EtOH)
176–178 (EtOH)
118–120c
3.78 (9.80)
4.52
C18H14ClNO3S
C17H13NO3S
3.89 (9.85)
4.50
IIc
IIIa
IIIb
IIIc
IVа
IVb
IVc
Vа
4.21
C18H15NO3S
4.30
3.95 (10.31)
12.85
C17H12ClNO3S
C17H15N3O2S
C18H17N3O2S
C17H14ClN3O2S
C17H12ClNO2S
C18H14ClNO2S
C17H11Cl2NO2S
C17H13NO5S
4.05 (10.25)
12.91
12.34
12.38
11.61 (9.92)
4.12 (10.83)
3.85 (10.43)
3.70 (19.56)
3.89
11.68 (9.85)
4.25 (10.75)
4.07 (10.31)
3.85 (19.47)
4.08
153–155c
140–142c
Vb
Vc
190–192 (EtOH)
178–180 (EtOH)
192–196 (EtOH)
112–115 (EtOH)
128–132 (EtOH)
135–138 (EtOH)
146–148 (МеCN)
170–173 (МеCN)
168–172 (МеCN)
180–184 (МеCN)
212–216 (МеCN–DMF, 3:1)
222–224 (МеCN–DMF, 3:1)
222–224 (МеCN–DMF, 3:1)
220–224 (МеCN–DMF, 3:1)
179–182 (EtOH)
172–175 (EtOH)
190–192 (AcOH)
215–217 (AcOH)
VIа
VIb
VIc
VIIа
VIIb
VIIc
IXа
IXb
IXc
IXd
Xа
3.90
C18H15NO5S
3.92
3.65 (9.40)
12.39
C17H12ClNO5S
C18H13N3O2S
C19H15N3O2S
C18H12ClN3O2S
C25H22N2O2S
C24H19ClN2O2S
C24H19ClN2O2S
3.71 (9.38)
12.53
11.85
12.03
9.40
9.59
6.71
6.76
6.28 (8.19)
6.30 (8.20)
6.03 (15.63)
10.69
6.44 (8.15)
6.44 (8.15)
C23H16Cl2N2O2S 6.15 (15.57)
C28H22N4O5S
C29H24N4O5S
C29H24N4O5S
C28H21ClN4O5S
C28H22N4O5S
C29H24N4O5S
C29H24N4O5S
C28H21ClN4O5S
10.64
10.36
10.25
Xb
10.45
10.36
Xc
9.85 (6.40)
10.55
9.99 (6.32)
10.64
Xd
XIIIа
XIIIb
XIIIc
XIIId
10.30
10.36
10.40
10.36
9.88 (6.30)
9.99 (6.32)
The melting point corresponds to published data [16]. b Yield according to method a. c After washing with hexane.
a
1
III. The transformation IV → VII leading to closure of
oxadiazole ring is accompanied by disappearance from
the IR spectra of absorption bands typical of carbonyl
and hydrazine groups in the regions 1645–1800 and
3050–3600 cm–1, respectively. Recyclization X →
XIII characteristically gives rise to CH–NH proton
signals (3JHH = 7.5 Hz) in the H NMR spectrum of
XIII, which is consistent with published data [13].
To conclude, it should be noted that acids III
smoothly undergo oxidation with hydrogen peroxide in
acetic acid to produce previously unknown 2-aryl-5-
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 80 No. 7 2010

1348
PIL’O et al.
Table 2. IR and 1H NMR spectra of compounds II–VII and IX–XIII
Comp.
no.
IR spectrum (KBr), ν, cm–1
1H NMR spectrum, δ, ppm (DMSO-d6)
1730 (C=O)
1728 (C=O)
2.37 s (3H, CH3), 3.87 s (3H, OCH3), 7.30–7.79 m (9H, Harom)
IIa
2.37 s (6H, 2CH3), 3.87 s (3H, OCH3), 7.28–7.68 m (8H, Harom
)
IIb
1687 (C=O), 3667 (OH, assoc.)
1693 (C=O), 3640 (OH, assoc.)
2.37 s (3H, CH3), 7.30–7.72 m (9H, Harom), 13.21 br.s (1H, COOH)
IIIa
IIIb
2.31 s (3H, CH3), 2.35 s (3H, CH3), 7.24–7.72 m (8H, Harom), 13.21 br.s
(1H, COOH)
1690 (C=O), 3590 (OH, assoc.)
1654 (C=O)a, 3208 (NH, NH2, assoc.)
2.38 s (3H, CH3), 7.30–7.74 m (8H, Harom), 13.21 br.s (1H, COOH)
IIIc
IVa
2.33 s (3H, CH3), 4.48 br.s (2H, NH2), 7.24–7.86 m (9H, Harom), 9.47 s
(1H, NH)
1657 (C=O)a, 3252 (NH, NH2, assoc.)
1660 (C=O)a, 3210 (NH, NH2, assoc.)
2.33 s (3H, CH3), 2.38 s (3H, CH3), 4.46 br.s (2H, NH2), 7.30–7.80 m (8H,
Harom), 9.40 s (1H, NH)
IVb
IVc
VIa
VIb
VIc
2.38 s (3H, CH3), 4.55 br.s (2H, NH2), 7.30–7.80 m (8H, Harom), 9.50 s
(1H, NH)
1149, 1303 (SO2), 1723 (C=O)a, 3250 (NH, assoc.) 2.44 s (3H, CH3), 7.44–8.01 m (9H, Harom
1122, 1305 (SO2), 1735 (C=O)a, 3232 (NH, assoc.) 2.42 s (6H, 2CH3), 7.43–8.03 m (8H, Harom
1118, 1301 (SO2), 1729 (C=O)a, 3228 (NH, assoc.) 2.41 s (3H, CH3), 7.41–8.15 m (8H, Harom
)
)
)
1645–1800, 3050–3600 (no absorption)
1645–1800, 3050–3600 (no absorption)
2.32 s (3H, CH3), 7.24–7.96 m (9H, Harom), 9.40 s (1H, 5′-H)
VIIa
VIIb
2.29 s (3H, CH3), 2.39 s (3H, CH3), 7.24-7.87 m (8H, Harom), 9.47 s (1H,
5′-H)
1645–1800, 3050–3600 (no absorption)
1672 (C=O), 3390 (NH, assoc.)
2.39 s (3H, CH3), 7.40–7.94 m (8H, Harom), 9.44 s (1H, 5′-H)
VIIc
IXa
2.28 s (3H, CH3), 2.32 s (3H, CH3), 2.37 s (3H, CH3), 7.24–7.88 m (12H,
Harom), 10.07 s (1H, NH)
1679 (C=O), 3392 (NH, assoc.)
1670 (C=O), 3380 (NH, assoc.)
2.33 s (3H, CH3), 2.37 s (3H, CH3), 7.30–7.86 m (12H, Harom), 10.30 s
(1H, NH)
IXb
IXc
2.28 s (3H, CH3), 2.38 s (3H, CH3), 7.20–7.84 m (12H, Harom), 10.13 s
(1H, NH)
1675 (C=O), 3375 (NH, assoc.)
2.38 s (3H, CH3), 7.40–7.94 m (12H, Harom), 10.37 s (1H, NH)
IXd
Xa
1634 (C=O), 1677 (C=O), 3284 (NH, assoc.)
2.34 s (3H, CH3), 3.82 s (3H, OCH3), 7.32–7.94 m (14H, Harom), 9.32 s
(1H, NH), 10.75 s (1H, NH)
1641 (C=O), 1681 (C=O), 3265 (NH, assoc.)
1638 (C=O), 1685 (C=O), 3310 (NH, assoc.)
1643 (C=O), 1679 (C=O), 3292 (NH, assoc.)
1642 (C=O), 1762 (C=O), 3264 (NH, assoc.)
2.34 s (6H, 2CH3), 3.81 s (3H, OCH3), 7.26–7.92 m (13H, Harom), 9.27 s
(1H, NH), 10.72 s (1H, NH)
Xb
2.34 s (3H, CH3), 2.40 s (3H, CH3), 3.82 s (3H, OCH3), 7.30–7.87 m
(13H, Harom), 9.28 s (1H, NH), 10.69 s (1H, NH)
Xc
2.40 s (3H, CH3), 3.81 s (3H, OCH3), 7.40–7.84 m (13H, Harom), 9.35 s
(1H, NH), 10.80 s (1H, NH)
Xg
2.31 s (3H, CH3), 3.82 s (3H, OCH3), 6.24 d (1H, CH, 3JHH = 7.5 Hz),
XIIIa
7.26–7.97 m (14H, Harom), 9.73 d (1H, NH, 3JHH = 7.5 Hz)
1646 (C=O), 1755 (C=O), 3198 (NH, assoc.)
1645 (C=O), 1758 (C=O), 3205 (NH, assoc.)
1642 (C=O), 1763 (C=O), 3244 (NH, assoc.)
2.31 s (3H, CH3), 2.39 s (3H, CH3), 3.82 s (3H, OCH3), 6.25 d (1H, CH,
XIIIb
XIIIc
XIIId
3JHH = 7.5 Hz), 7.30–8.03 m (13H, Harom), 9.63 d (1H, NH, 3JHH = 7.5 Hz)
2.31 s (3H, CH3), 2.40 s (3H, CH3), 3.83 s (3H, OCH3), 6.24 d (1H, CH,
3JHH = 7.5 Hz), 7.22–7.96 m (13H, Harom), 9.70 d (1H, NH, 3JHH = 7.5 Hz)
2.41 s (3H, CH3), 3.82 s (3H, OCH3), 6.27 d (1H, CH, 3JHH = 7.5 Hz),
7.36–7.98 m (13H, Harom), 9.71 d (1H, NH, 3JHH = 7.5 Hz)
a
Band with a shoulder.
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2-ARYL-5-ARYLSULFANYL-1,3-OXAZOLE-4-CARBOXYLIC ACIDS
1349
arylsulfonyl-1,3-oxazole-4-carboxylic acids VI. The
properties of acids VI and their analogs III having a
bivalent sulfur atom will be compared elsewhere.
removed under reduced pressure, the residue was
treated with anhydrous hexane, and the precipitate was
filtered off, dried under reduced pressure, and used in
further syntheses without additional purification.
EXPERIMENTAL
2-Aryl-5-arylsulfonyl-1,3-oxazole-4-carboxylic
acids VIa–VIc (general procedure). A solution of
0.005 mol of compound IIIa–IIIc in 10 ml of glacial
acetic acid was heated to the boiling point, a 30%
solution of hydrogen peroxide was added to the hot
mixture in three 1-ml portions over a period of 1 h, the
mixture was kept for 8 h at room temperature, the
solvent was removed under reduced pressure, the
residue was treated with water, and the precipitate was
filtered off and purified by recrystallization.
The IR spectra were recorded in KBr on a Vertex 70
1
spectrometer. The H NMR spectra were measured on
a Varian VXR-300 spectrometer from solutions in
DMSO-d6 using tetramethylsilane as internal reference.
Methyl 2-acylamino-3,3-dichloroacrylates Ia and
Ib were synthesized according to the procedure
reported in [11]. Methyl 2-aryl-5-arylsulfanyl-1,3-
oxazole-4-carboxylates (IIa–IIc) were prepared as
described in [15].
2-(2-Aryl-5-arylsulfanyl-1,3-oxazol-4-yl)-1,3,4-
oxadiazoles VIIa–VIIc (general procedure). A mix-
ture of 0.001 mol of compound IVa–IVc and 0.005 mol
of triethyl orthoformate was heated for 12 h under
reflux and was then left to stand for 12 h at room
temperature. The resulting oily material was treated
with water, and the precipitate was filtered off and
purified by recrystallization.
2-Aryl-5-arylsulfanyl-1,3-oxazole-4-carboxylic
acid IIIa–IIIc (general procedure). A solution of
0.015 mol of sodium hydroxide in 5 ml of water was
added to a solution of 0.01 mol of compound IIa–IIc
in 30 ml of ethanol, and the mixture was stirred for 1 h
on heating. After cooling, the mixture was neutralized
to pH ~7–6 with hydrochloric acid, the solvent was
removed under reduced pressure, the residue was
treated with water, and the precipitate was filtered off
and purified by recrystallization.
2-Aryl-5-arylsulfantl-1,3-oxazole-4-carbox-
amides IXa–IXd (general procedure). The corres-
ponding amine, 0.001 mol, was dissolved in 20 ml of
anhydrous acetonitrile, 0.001 mol of triethylamine and
0.001 mol of compound Va–Vc were added, and the
mixture was heated to the boiling point and left to
stand for 12 h at room temperature. The precipitate
was treated with water, filtered off, and purified by
recrystallization.
2-Aryl-5-arylsulfanyl-1,3-oxazole-4-carboxylic
acid hydrazides IVa–IVc. a. Hydrazine hydrate,
0.015 mol, was added to a solution of 0.005 mol of
compound IIa–IIc in 20 ml of ethanol. The mixture
was heated for 3 h under reflux and was then left to
stand for 12 h at room temperature. The precipitate
was filtered off, washed with water, and pyrified by
recrystallization.
Methyl 2-aryl-5-[2-(2-aryl-5-arylsulfanyl-1,3-oxa-
zole-4-carbonyl)hydrazino]-1,3-oxazole-4-carboxy-
lates Xa–Xd (general procedure). Compound VIIIa or
VIIIb, 0.005 mol, was dissolved in 30 ml of anhyd-
rous acetonitrile, 0.005 mol of dimethylaniline and
0.005 mol of compound Va–Vc was added, and the
mixture was heated to the boiling point and left to
stand for 12 h at room temperature. The precipitate
was treated with water, filtered off, and purified by
recrystallization.
b. A solution of 0.03 mol of hydrazine hydrate in
50 ml of water was cooled with ice water, a solution of
0.01 mol of compound Va in 15 ml of anhydrous
acetonitrile was added dropwise under stirring, and the
mixture was stirred for 1 h. The precipitate was filtered
off and purified by recrystallization. Yield of com-
pound IVa 88%. Samples of IVa obtained by the two
methods (a and b) showed no depression of the
1
melting point on mixing, and their IR and H NMR
spectra were identical.
Methyl acylamino[5-(2-aryl-5-arylsulfanyl-1,3-
oxazol-4-yl)-1,3,4-oxadiazol-2-yl]acetates XIIIa–XIIId
(general procedure). A solution of 0.005 mol of
compound Xa–Xd in 20 ml of glacial acetic acid was
heated for 8 h under reflux and was then left to stand
for 12 h at room temperature. The solvent was
removed under reduced pressure, the residue was
2-Aryl-5-arylsulfanyl-1,3-oxazole-4-carbonyl
chlorides Va–Vc (general procedure). Thionyl chlo-
ride, 0.006 mol, was added to a solution of 0.005 mol
of compound IIIa–IIIc in 20 ml of anhydrous ben-
zene. The solution was heated under reflux until
gaseous products no longer evolved, the solvent was
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PIL’O et al.
8. Cornforth, J.W. and Cornforth, R.H., J. Chem. Soc.,
treated with water, and the precipitate was filtered off
and purified by recrystallization.
1947, p. 96.
9. Cornforth, J.W. and Huang, H.T., J. Chem. Soc., 1948,
p. 1964.
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