New one-pot approach to regio-synthesis of substituted 2-aminothiazoles from the corresponding keto-aziridines

Article abstract of DOI:10.1007/s13738-013-0276-7

Ring expansion of keto-aziridines to the corresponding 2-aminothiazoles (54-67 %) using ammonium thiocyanate in the presence of RuCl₃ under refluxing acetonitrile is described. A plausible mechanism for the synthesis of substituted 2-aminothiazoles has been proposed.

Full text of DOI:10.1007/s13738-013-0276-7

J IRAN CHEM SOC
DOI 10.1007/s13738-013-0276-7
ORIGINAL PAPER
New one-pot approach to regio-synthesis of substituted
2-aminothiazoles from the corresponding keto-aziridines
•
Heshmat A. Samimi Somaye Mohammadi
Received: 6 February 2013 / Accepted: 16 May 2013
Ó Iranian Chemical Society 2013
Abstract Ring expansion of keto-aziridines to the cor-
responding 2-aminothiazoles (54–67 %) using ammonium
thiocyanate in the presence of RuCl3 under refluxing ace-
tonitrile is described. A plausible mechanism for the syn-
thesis of substituted 2-aminothiazoles has been proposed.
for the treatment of eating disorders such as obesity and
hyperphagia [24]. This class of compounds is well known as a
prodrug for the treatment of type 2 diabetes [25] and ami-
nothiazole-4-carboxylate derivatives as active compounds
against Mycobacterium tuberculosis H37Rv and the a-ke-
toacyl-ACP synthase mtFabH [26]. Besides, the aromatic
compounds containing sulfur are candidates for organic
semiconductors [27]. 2-Aminothiazoles are generally pre-
pared by the condensation of a-halo ketones/a-tosylketone
with thioureas [28–34] or a-thiocyanato carbonyl compounds
with aromatic or aliphatic amine hydrochlorides [35–38].
Despite work reported on the synthesis of 2-aminothia-
zole derivatives, novel and widely applicable methods for
the synthesis of them are still in demand.
Keywords Aziridine Á 2-Aminothiazoles Á
Ring expansion Á RuCl3
Introduction
Thiazole derivatives are the prominent players in the
pharmaceutical research as they possess several biological
properties [1–8].
In continuation of our efforts in the development and
synthesis of heterocycle molecules, especially from keto-
aziridines [39–43], we have discovered a new and simple
one-pot method for the synthesis of 2-aminothiazoles by
the reaction of 2-aroyl-3-arylaziridines with ammonium
thiocyanate in the presence of RuCl3. This method
describes a novel method which for the first time utilizes
keto-aziridine for the synthesis of 2-aminothiazoles in a
one-pot reaction. According to our knowledge, the syn-
thesis of -5-arolyl-4-aryl-2-aminothiazole derivatives has
not been reported in the literature, except, one report that
shows synthesis of 4-phenyl-2-aminothiazole from a-halo
ketones using ammonium thiocyanate in the presence of N-
methylimidazole [43].
Recently, thiazole analogues have been described as the
possible core skeletons of A3 receptor antagonists with
moderate affinity and selectivity [9–11]. In addition, thiazole
derivatives are reported to be cyclin dependent kinase (CDK)
inhibitors for the treatment of cancer and 11b-hydroxysteroid
dehydrogenasetype1(11b-HSD1)inhibitorsforthe treatment
of metabolic disorders [12]. Particularly, 2-aminothiazoles
have been reported to possess antitumor [13], antiviral [14,
15], antibacterial [16–18], anti-prion [19], psychotropic
activities [20], anti-allergies [21], anti-hypertension [22], and
anti-inflammation [23]. 2-Aminothiazole analogue MB06322
has also been used for the neuropeptide Y5 (NPY5) receptor
Electronic supplementary material The online version of this
article (doi:10.1007/s13738-013-0276-7) contains supplementary
material, which is available to authorized users.
Results and discussion
H. A. Samimi (&) Á S. Mohammadi
Trans-2-aroyl-3-arylaziridines (1) were prepared via a
Gabriel-Cromwell procedure, by bromination of the related
a,b-unsaturated carbonyl compounds, followed by the
Department of Chemistry, Faculty of Sciences, The University
of Shahrekord, P. O. Box 115, Shahrekord, Iran
e-mail: samimi-h@sci.sku.ac.ir
123

J IRAN CHEM SOC
reaction with ammonia solution in methanol at room
temperature [44].
Table 1 Reaction of trans-2-benzoyl-3-phenyl aziridines (1a) with
ammonium thiocyanate in the presence of some different catalysts
Our studies in this area began by choosing 2-benzoyl-3-
phenylaziridine 1a and NH4SCN as the model substrates to
test the possibility of reaction in acetonitrile but it failed. In
order to evaluate the effect of catalyst in these reactions,
we tried the reaction of 2-benzoyl-3-phenyl aziridine 1a
with NH4SCN in the presence of H2SO4 and AgNO3 or
some Lewis acids such as I2, BF3ÁOEt2, ZnCl2, RuCl3,
Fe(NO3)3, AlCl3 or NiCl2 in acetonitrile under refluxing
conditions (Scheme 1; Table 1). The results of this study
revealed that the ring expansion reaction in the presence of
AlCl3, Fe(NO3)3 and NiCl2 (entry 1–3) gave no products
but in the presence of H2SO4, BF3ÁOEt2, AgNO3 or ZnCl2
(entry 4–7) gave a mixture of products and Chalcone with
only a small amount (17–33 %) of the desired 2-amino-
thiazole (2a), while with I2 a moderate yield (45 %) of
2-aminothiazole (2a) was obtained (entry 8).
Entry
Catalystb
Time (h)
Yield (%)a
2a
Chalcone
1
AlCl3
Fe(NO3)3
NiCl2
H2SO4
BF3ÁOEt2
AgNO3
ZnCl2
I2
2
0
0
0
50
0
2
2
3
2
0
4
7
19
28
33
17
45
56
54
29
0
21
0
5
2
6
2
30
24
35
10
28
23
12
7
7
8
2
9
RuCl3
RuClc3
RuCld3
–
2.5
2.5
8
10
11
12
a
8
Isolated yield after purification
b
c
d
Catalyst (20 % mol)
Surprisingly only RuCl3 (20 % mol) was shown to be an
effective catalyst for the ring expansion of 2-benzoyl-3-
phenyl aziridine 1a to 2-amino-4-benzoyl-5-phenyl thia-
zole (2a) (entry 9–10). No reaction occurred at lower
temperatures even in the presence of RuCl3 (20 % mol). In
addition with KSCN in the presence of RuCl3 (20 % mol)
in refluxing acetonitrile, a small amount of the desired
thiazole (2a) was obtained (33 %).
RuCl3 (60 % mol)
In room temperature and in the presence of RuCl3 (20 % mol)
Bold values indicate the best conditions for transformation of keto-aziri-
dine to the corresponding 2-aminothiazole
We previously reported the mechanism of the iodide ion
catalyzed isomerization of 1-acylaziridines by attack of the
nucleophile on the C-2 1-acylaziridines, which subsequently
cyclizes to the oxazoline [39]. Now, the action of thiocyanate
ion like iodide on 2-aroyl-3-aryl aziridine would involve a
ring opening processes. No trace of another regio-isomer of
these aminothiazoles were detected.
These observations encouraged us to develop a direct
method for the conversion of keto-aziridines 1a–g to the
corresponding 2-aminothiazole 2a–g (Scheme 2). The results
of these ring expansion reactions are summarized in Table 2.
The structure of 2-aminothiazoles was deuced from IR,
1H, 13C NMR, as well as elemental analyses. The 1H NMR
spectrum showed a broad singlet at d 5.2–5.7 ppm corre-
sponding to the two hydrogens of amino group attached to
C-2. Whereas in the 13C NMR spectrum, the signal at d
165.2–166.3 ppm corresponds to C-2 of thiazole ring. The
FT-IR spectra of the compounds exhibited the broad peak at
3,400–3,450 and 1,650–1,660 cm-1 which corresponds to
the primary amino and carbonyl group, respectively. Based
on elemental analyses of products and the above spectral
data, the structure of the compounds was assigned as 5-aroyl-
4-aryl-2-aminothiazoles. These observations reveal that the
5-benzoyl-4-phenyl-2-aminothiazole can be obtained by the
reaction of 2-benzoyl-3-phenylaziridine with ammonium
thiocyanate inthe presence of RuCl3 in refluxingacetonitrile.
Although the exact mechanism of the reaction is not
very clear, one possible sequence of steps to explain the
conversion of keto-aziridines to corresponding 2-amino-
thiazoles involves activation of aziridine ring with RuCl3
by coordinating with the nitrogen and carbonyl group of
the aziridine, then the reaction of thiocyanate with aziridine
to give intermediate A. After that, amino group attacks the
thiocyanate group of the intermediate A, which leads to the
intermediate B and which is oxidized to 2-aminothiazole 2
in the presence of air (Scheme 3).
In conclusion, this work describes an efficient and novel
one-pot method for the synthesis of 2-aminothiazole
derivatives from the corresponding keto-aziridines with
ammonium thiocyanate in the presence of RuCl3 under
refluxing conditions. The synthesis of 2-aminothiazoles
Scheme 1 The reaction of
trans-2-benzoyl-3-phenyl
aziridines with ammonium
thiocyanate in the presence of
different catalysts
NH2
Ph
H
N
O
N
S
O
catalyst/Reflux
CH3CN
H
+
+
NH4SCN
Ph
Ph
Ph
Ph
H
Ph
2a
O
Chalcone
1a
123

J IRAN CHEM SOC
Scheme 2 Synthesis of trans-
2-aminothiazole from trans-2-
aroyl-3-arylaziridines and
ammonium thiocyanate in the
presence of RuCl3
NH2
NH2
H
N
O
C
S
C
N
i
N
S
H
Ar1
+
NH4SCN
Ar2
+
H
Ar2
Ar1
2a-g
Ar2
Ar1
3a-g
1a-g
O
O
Not formed
i) RuCl3/Reflux/CH3CN/Air
Infrared spectroscope JASCO, FT/IR-6300. All solvents used
were dried and distilled according to standard procedures.
Table 2 Synthesis of 5-aroyl-4-aryl-2-aminothiazoles (2) from trans-
2-aroyl-3-arylaziridines (1) with ammonium thiocyanate in the pres-
ence of RuCl3 (20 % mol)
Entry
Ar1
Ar2
Time (h)
Yield (%)a
Typical procedure for the synthesis of 2-aminothiazoles (2)
2a
2b
2c
2d
2e
2f
2g
a
C6H5
C6H5
7
56
55
60
62
54
55
67
RuCl3 (0.2 mmol) was added to a solution of aziridines (1)
(1.0 mmol) and ammonium thiocyanate (1.0 mmol) in
CH3CN (10 mL). The mixture was refluxed for the stipu-
lated time (Table 2). After the desired product formation
indicated by TLC/or GC–MS, the solvent was evaporated
in rotary vacuum evaporator to obtain the crude products.
The crude product was purified by silica gel column
chromatography (EtOAc/Hexane: 2/5) to provide the
desired 2-aminothiazole (54–67 %).
3-NO2C6H4
4-ClC6H4
2,4-Cl2C6H3
2,4-Cl2C6H3
3-NO2C6H4
C6H5
C6H5
7
C6H5
7
4-ClC6H4
C6H5
7
7
4-ClC6H4
4-BrC6H4
2.5
7
Isolated yield after purification
Cl3
Ru
5-Benzoyl-4-phenyl-2-aminothiazole (2a)
N
C
H
O
N
Yellow solid, Mp 94–98 °C; IR (KBr): 3,422, 3,081, 1,635,
S
H2N
H
Ar2
1
1,597, 1,527, 1,448, 758, 686 cm-1. H NMR (400 MHz,
Ar1
H
H
H
Ar1
CDCl3) (d, ppm): 7.70–7.68 (m, 2H), 7.40–7.11 (m, 8H), d
5.7 (br s, 2H). 13C NMR (100 MHz, CDCl3) (d, ppm): 195.5,
165.6, 151.9, 136.4, 129.9, 129.8, 129.6, 129.3, 129.0, 128.8,
128.3, 123.9. Anal. Calcd for C16H12N2OS: C, 68.55; H,
4.31; N, 9.99 %. Found: C, 68.43; H, 4.35; N, 9.97 %.
Ar2
-SCN
+
1
NH4
O
A
NH
NH2
S
NH2
S
HN
Ar1
S
[o]
Ar2
N
N
Ar2
Ar1
Ar2
Ar1
5-Benzoyl-4-(3-nitrophenyl)-2-aminothiazole (2b)
O
O
O
2
B
Yellow solid, Mp 122–125 °C; IR (KBr): 3,428, 3,062,
Scheme 3 Proposed mechanism for synthesis of 5-aroyl-4-aryl-2-
aminothiazoles (2) from trans-2-aroyl-3-arylaziridines (1) and ammo-
nium thiocyanate in the presence of RuCl3
1,638, 1,590, 1,527, 1,355, 1,440 cm-1
.
1H NMR
(400 MHz, CDCl3) (d, ppm): 8.02 (d, J = 2.0 Hz, 1H), 7.8
(d, J = 7.2 Hz, 2H), 7.6 (d, J = 7.6 Hz, 1H), 7.5 (d,
J = 3.2 Hz, 1H), 7.41–7.25 (m, 2H), 7.0 (t, J = 7.9 Hz,
1H), 5.5 (br s, 2H). 13C NMR (100 MHz, CDCl3) (d, ppm):
189.9, 165.3, 152.0, 138.2, 136.1, 133.1, 132.2, 132.1,
132.0, 130.6, 129.8, 129.2, 128.7, 124.0 ppm. Anal. Calcd
for C16H11N3O3S: C, 59.07; H, 3.41; N, 12.92 %. Found:
C, 58.98; H, 3.45; N, 12.87 %.
from the corresponding keto-aziridines has not been
reported in the literature.
Experimental
All yields refer to isolated products after purification by col-
umn chromatography or distillation in vacuum. Products were
characterized with IR and 1HNMR spectra, TLC, melting, and
boiling points. NMR spectra were recorded on a Bruker
AMX-400 spectrometer (1H at 400 MHz and 13C at
100 MHz) in CDCl3 with chemical shift values in ppm
downfield from TMS and IR spectra were recorded on FT
5-Benzoyl-4-(4-chlorophenyl)-2-aminothiazole (2c)
Yellow solid, Mp 129-132 °C; IR (KBr): 3,417, 3,077,
1
1,639, 1,597, 1,501, 755, 679 cm-1. H NMR (400 MHz,
CDCl3) (d, ppm): 7.8 (d, J = 8.2 Hz, 2H), 7.40–7.26 (m,
7H), 5.4 (br s, 2H). 13C NMR (100 MHz, CDCl3) (d, ppm):
123

J IRAN CHEM SOC
192.4, 165.2, 131.6, 130.5, 129.6, 129.3, 128.9, 128.7,
128.6, 128.5, 128.4, 128.2 ppm. Anal. Calcd for
C16H11ClN2OS: C, 61.05; H, 3.52; N, 8.90 %. Found: C,
61.13; H, 3.55; N, 8.92 %.
128.1 ppm. Anal. Calcd for C16H11BrN2OS: C, 53.49; H,
3.09; N, 7.80 %. Found: C, 53.45; H, 2.99; N, 7.69 %.
Acknowledgments We are thankful to Research Council of Shah-
rekord University for the supporting this work.
5-(4-Chlorobenzoyl)-4-(2,4-dichlorophenyl)-2-
aminothiazole (2d)
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