Novel one-pot process for the synthesis of 1,3-thiazoles via organocatalysed epoxidation of nitro-olefins

Article abstract of DOI:10.1039/c1ob05260h

A facile one-pot two-step process for the synthesis of 1,3-thiazole heterocycles via organocatalytic epoxidation of nitro-olefins with the t-BuOOH/DBU system, and subsequent reaction of α-nitro-epoxides with thioamides under mild conditions has been devel

Full text of DOI:10.1039/c1ob05260h

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PAPER
Novel one-pot process for the synthesis of 1,3-thiazoles via organocatalysed
epoxidation of nitro-olefins†
Katharina M. Weiß, Shengwei Wei and Svetlana B. Tsogoeva*
Received 2nd January 2011, Accepted 23rd February 2011
DOI: 10.1039/c1ob05260h
A facile one-pot two-step process for the synthesis of 1,3-thiazole heterocycles via organocatalytic
epoxidation of nitro-olefins with the t-BuOOH/DBU system, and subsequent reaction of
a-nitro-epoxides with thioamides under mild conditions has been developed.
chlorooxiranes, a-nitro-epoxides) and thioamides or thioureas.11
This is surprising, since epoxides are particularly attractive
intermediates and building blocks for the synthesis of complex
organic molecules, e.g. of natural products and biologically
Introduction
Thiazoles are of eminent importance because of their potential
as bioactive compounds1,2 and versatile building blocks for
natural products and pharmaceuticals.3,4 Thiazole heterocycles
are important subunits in many complex natural compounds
and drugs, e.g. Vitamine B1, Epothilones, Thiostrepton, Niza-
tidine (ulcer therapeutic), Ritonavir (a potent inhibitor of HIV
protease) (Fig. 1) and thiamine pyrophosphate (TPP, a coen-
zyme that is part of the Krebs cycle in the process of cellular
respiration).3–6 Further, there are many other applications of
thiazole derivatives, for example in liquid crystals7 or cosmetics
(sunscreens).8
active compounds.12 Diverse approaches to catalytic epoxidations
have been developed in recent years.13 Notably, the thiazole
synthesis from the corresponding a-nitro-epoxides (generated
from nitro-olefins with H2O2/NaOH system) was first reported
by Newman.11a,b However, as far as we are aware, this has been the
only example of preparation and application of a-nitro-epoxides
for the thiazole synthesis (heating a mixture of a-nitro-epoxide
and thiobenzamide under reflux in MeOH for 16 h). Therefore,
the development of new efficient procedures for the synthesis of
a-nitro-epoxides and their use for the preparation of thiazole
heterocycles still remains a challenging task.
As a continuation of our interest in organocatalytic epoxidation
reactions,14 we decided to develop an organocatalytic epoxidation
of nitro-olefins with an intention to further apply the correspond-
ing products, a-nitro-epoxides, without workup and purification
for the synthesis of 1,3-thiazoles. Herein, we disclose a novel
and practical approach to thiazole derivatives: one-pot two-step
synthesis, which involves an organocatalytic epoxidation of nitro-
olefins with t-BuOOH/DBU (1,8-diazabicyclo[5.4.0]undec-7-ene)
system, followed by reaction with different thiobenzamides at
room temperature.
Fig. 1 Thiazoles as part of natural compounds and drugs.
Results and discussion
The most prominent method to synthesize thiazoles is the
Hantzsch thiazole synthesis where a-haloketones are reacted with
thioamides.9 Several other multi-step5 and also some multicom-
ponent syntheses10 are known. One of the less explored methods
is to prepare thiazoles from the corresponding epoxides (e.g. 2-
Notably, one-pot processes are very attractive and sustainable
methods in modern synthetic chemistry through the reduction of
work-up and purification steps, reduction of costs, materials and
labor input.15 In view of our idea to develop a one-pot procedure
towards thiazole synthesis, we first studied the epoxidation of b-
methyl-b-nitrostyrene 1. In 1995, Yadav reported the epoxidation
of a,b-unsaturated-d-lactones and other enones using anhydrous
TBHP (t-BuO◦OH) as an oxidant and DBU as a base (at 120 mol%
loading) at 0 C.16 We decided to extend the application of this
readily available system to the epoxidation of more challenging
Department of Chemistry and Pharmacy, Institute of Organic Chemistry
1, University of Erlangen-Nuremberg, Henkestrasse 42, 91054, Erlangen,
Germany. E-mail: tsogoeva@chemie.uni-erlangen.de; Fax: +49-9131-85-
26865; Tel: +49-9131-85-22541
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c1ob05260h
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Table 1 Catalytic epoxidation of trans-b-methyl-b-nitrostyrene 1 with
TBHP/DBU system
Table 3 Optimization of the one-pot process
Entry DBU (mol%)
Temp
Solvent
t (min)
Conv. (%)a
1
2
3
4
5
6
7
20
20
5
0 ◦
rt
rt
rt
rt
rt
rt
C
n-Hexane
n-Hexane
n-Hexane
Toluene
CHCl3
EtOH
MeOH
135
45
50
85
85
60
60
90
90
90
78
55
38
25b
Step 1:
Step 2:
5
5
5
5
Entry
TBHP (equiv.)
3 (equiv.)
Add. (equiv.)
Yielda (%)
1
2
3
4
5
6
7
8
2.0
1.0
2.0
1.0
1.0
1.0
1.0
1.0
2.0
1.0
1.0
2.0
2.0
2.0
2.0
2.0
—
—
—
—
34
22
25
56
56
60
56
63
a Conversions were determined by GC analysis based on the formation of
epoxide. The reaction was monitored until consumption of the alkene.
b Conversion was determined by 1H NMR analysis using an internal
standard.
MeOH (100)
AcOH (0.1)
AcOH (0.5)
MeOH (100)
AcOH (0.1)
TFA (0.1)
MeOH (100)
TFA (0.1)
nitro-olefins. Following our aim to develop a catalytic epoxidation
of b-methyl-b-nitrostyrene 1, we used catalytic amounts of DBU
(at 5–20 mol% loading) for optimization of reaction conditions
(Table 1).
9
10
1.0
1.0
2.0
2.0
70
51
In a typical procedure, TBHP was added to a mixture containing
1 in n-hexane, DBU and an internal standard (hexadecane).
The mixture was stirred and the progress of the reaction was
monitored by GC. To our delight, we found that DBU is indeed an
efficient catalyst for the epoxidation of 1 with TBHP as oxidant.
Intriguingly, while 90% conv. was observed after 135 min at
a Yield of the isolated product after column chromatography.
using n-hexane (entry 3) and MeOH (entry 4) as solvents. In both
cases, the yield was improved to 66% and 80%, respectively.
The next aim was to combine the optimized conditions of
the two single reaction steps (for the epoxidation reaction and
the formation of thiazole) in one-pot (Table 3). We started our
investigation of the one-pot process by simply adding 2.0 equiv. of
thiobenzamide 3 after 1 h reaction time for the epoxidation that
was conducted in n-hexane (entry 1). Surprisingly, the thiazole
product 4 was isolated in only 34% yield. Reduction of the amount
of 3 from 2.0 to 1.0 equiv. resulted in decreased yields (entries 2 and
3). To our delight, by perfoming the epoxidation with 1.0 equiv. of
TBHP followed by the addition of 2.0 equiv. of 3, we were able to
isolate 4 in 56% yield (entry 4).
Motivated by this result, we hoped to improve the yield further
by using additives in the second step of the one-pot process. MeOH
came to our mind, because as shown in Table 2, the formation of
4 was very efficient in this solvent (entry 4, Table 2). Thus, MeOH
was added to the second step of the one-pot process in 100 equiv.
with no improvement in the yield (entry 5, Table 3). Addition of
0.1 equiv. of acetic acid led to 60% yield (entry 6) and the use
of both additives, MeOH and acetic acid gave 63% yield (entry
8). Finally, trifluoroacetic acid (TFA) was tested to provide the
isolated product in 70% yield after two sequential steps (entry 9).
However, the combination of TFA with MeOH gave a lower yield
(entry 10).
◦
0 C, the same conversion was observed in only 45 min at room
temperature, using DBU at 20 mol% loading in both reactions
(entries 1 and 2). We further reduced the catalyst loading to 5
mol% without loss in activity with a slightly extended reaction time
(50 min, entry 3). Among various solvents screened, the originally
chosen n-hexane proved to be the most optimal solvent (entry 3
vs. entries 4–7).
Next, the optimal conditions for the thiazole formation from
epoxide 2 and thiobenzamide 3 were investigated (Table 2). First,
the thiazole synthesis was conducted under the conditions found
for the epoxidation step: in n-hexane as a solvent and at room
temperature (entry 1).
In this case, 4 was isolated only in 34% yield. Applying the same
solvent (MeOH) as reported in the literature,11a,b but carrying
out the reaction at room temperature instead of under reflux
conditions, the desired product was isolated in 62% yield (entry 2).
To improve the yield, the amount of 3 was increased to 2.0 equiv.
Table 2 Optimization of conditions for the thiazole formation
We next examined the scope of the one-pot two-step process
for different nitroalkenes and thiobenzamides using the optimized
reaction conditions. The results are summarized in Table 4.
To our delight, all reactions investigated can be effected in 40–
70% yield, applying DBU at only 5 mol% loading. Generally,
higher yields (65–70%) were obtained using 1 as alkene and
thiobenzamides with R2 = H, electron withdrawing or neutral
substituents (entries 1–3). Interestingly, the reaction conditions
Entry
Solvent
3 (equiv.)
Yielda (%)
1
2
3
4
n-Hexane
MeOH
n-Hexane
MeOH
1.0
1.0
2.0
2.0
34
62
66
80
a Yield of the isolated product after column chromatography.
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Table 4 Scope of substrates
The use of commercially available DBU as an organocatalyst
for the epoxidation of nitro-olefins with TBHP, followed by
the reaction with thiobenzamides, and the very mild conditions
used for both transformations, make this process a simple and
convenient approach to obtain thiazole heterocycles.
Experimental
General information
Chemicals were used as received from common commercial
sources. All solvents were distilled before use. n-Hexane (HPLC
grade) was obtained from Fisher Scientific and used as received.
NMR spectra were recorded on Jeol (400 and 400) or Bruker
Avance (300 or 400). NMR spectra were referenced to the residual
solvent signal (1H: CDCl3, 7.24 ppm; 13C: CDCl3, 77.0 ppm)
and recorded at ambient probe temperature. IR spectra were
recorded as thin films on a Varian IR-660 spectrometer. FAB
mass spectra were measured with a Micromass ZabSpec, EI mass
spectra were recorded with a Finnigan MAT 95 XP spectrom-
eter. Gas chromatography (GC) was conducted on a Thermo
instrument Trace GC Ultra with a 7 m TR5 column. Thin-Layer
Entrya
1
R1
H
R2
H
Product
Yield (%)b
70
2
3
H
H
p-Cl
65
65
p-tBu
R
chromatography (TLC) was carried out on ALUGRAMꢀ SIL
4
5
6
H
p-OMe
H
41
50
48
G/UV254 (Macherey-Nagel) and visualized by UV. Flash column
chromatography was performed using silica gel 60 M (Macherey-
Nagel).
p-Cl
p-Cl
General procedure for the one-pot two-step synthesis of thiazoles
A round bottom flask was charged with trans-b-methyl-b-
nitrostyrene (0.028 g, 0.170 mmol), DBU (1.3 mL, 0.008 mmol)
and n-hexane (0.56 mL). Then TBHP (34.1 ml, 0.170 mmol) was
added in one portion. The reaction mixture was stirred at room
temperature for 1 h. Then thiobenzamide (0.047 g, 0.340 mmol)
and TFA (1.3 ml, 0.017 mmol) were added and the reaction mixture
was stirred for 24 h. The crude product was extracted with Et2O
and purified by column chromatography (SiO2, PE/EtOAc 97 : 3)
to yield a white solid.
p-tBu
7
8
p-Br
H
52
40
o-OMe
p-Cl
4-Methyl-2,5-diphenylthiazole (4)17
9
p-Br
p-Cl
p-OMe
p-Cl
51
40
White solid. 1H NMR (400 MHz, CDCl3): d = 7.94–7.92 (m, 2H),
7.49–7.32 (m, 8H), 2.55 (s, 3H); 13C NMR (100 MHz, CDCl3): d =
165.0, 148.8, 133.7, 132.2, 129.8, 129.1, 128.9, 128.7, 127.7, 126.3,
16.4. MS (FAB) m/z: 252 (100%, M + H+).
10
2-(4-Chlorophenyl)-4-methyl-5-phenylthiazole (5)17
a One-pot conditions from Table 3, entry 9 were used for entries 1–4. One-
pot conditions from Table 3, entry 10 were used for entries 5–10. b Isolated
yield after column chromatography.
White solid. Mp 114 ◦C. 1H NMR (300 MHz, CDCl3): d = 7.88–
7.84 (m, 2H), 7.48–7.35 (m, 7H), 2.54 (s, 3H); 13C NMR (75 MHz,
CDCl3): d = 163.6, 149.0, 135.7, 132.6, 132.2, 131.9, 129.1, 129.1,
128.7, 127.9, 127.4, 16.3. IR (thin film): 2922, 2360, 1653, 1600,
1572, 1534 cm-1. MS (EI) m/z: 285 (100%, M+); HRMS (EI):
Calc. for C16H12ClNS: 285.0379; Found: 285.0372. Anal. Calc. for
C16H12ClNS: C, 67.24; H, 4.23; N, 4.90; S, 11.22. Found: C, 66.78;
H, 4.55; N, 4.67; S, 12.41.
from Table 3, entry 10 (MeOH/TFA as additives) proved to be
better suitable for olefines with R1 = electron withdrawing and/or
donating group (entries 5–10, Table 4).
Conclusion
2-(4-(tert-Butyl)phenyl)-4-methyl-5-phenylthiazole (6)
◦
1
In summary, a novel one-pot process providing a practical route for
the formation of 1,3-thiazoles from nitro-olefins was developed.
White solid. Mp 109 C. H NMR (300 MHz, CDCl3): d = 7.86
(d, J = 8.4 Hz, 2H), 7.50–7.39 (m, 6H), 7.36–7.31 (m, 1H), 2.55
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(s, 3H), 1.34 (s, 9H); 13C NMR (100 MHz, CDCl3): d = 165.3,
153.3, 148.7, 132.4, 131.8, 131.1, 129.2, 128.7, 127.7, 126.1, 125.9,
34.8, 31.1, 16.3. IR (thin film): 3058, 2961, 2868, 1773, 1736, cm-1.
MS (EI) m/z: 307 (85%, M+), 292 (100, M – CH3); HRMS (EI):
Calc. for C20H21NS: 307.1395; Found: 307.1408. Anal. Calc. for
C20H21NS: C, 78.13; H, 6.88; N, 4.56; S, 10.43. Found: C, 78.32;
H, 6.98; N, 4.55; S, 10.20.
5-(4-Bromophenyl)-2-(4-methoxyphenyl)-4-methylthiazole (12)
White solid. 1H NMR (300 MHz, CDCl3): d = 7.84 (d, J = 9.0 Hz,
2H), 7.54 (d, J = 9.0 Hz, 2H), 7.32 (d, J = 9.0 Hz, 2H), 6.93 (d,
J = 9.0 Hz, 2H), 3.84 (s, 3H), 2.50 (s, 3H); 13C NMR (75 MHz,
CDCl3): d = 164.9, 160.6, 148.4, 131.4, 130.8, 130.1, 129.4, 127.3,
126.0, 121.3, 113.8, 55.0, 15.9. MS (EI) m/z: 358 (100%, M+);
HRMS (EI) Calc. for C17H14ONBrS: 358.9979, Found: 358.9993.
Anal. Calc. for C17H14ONBrS: C, 56.67; H, 3.92; N, 3.89; S, 8.90,
Found: C, 56.32; H, 3.92; N, 3.87; S, 9.00.
2-(4-Methoxyphenyl)-4-methyl-5-phenylthiazole (7)
◦
1
White solid. Mp 63 C. H NMR (400 MHz, CDCl3): d = 7.87
(d, J = 9.2 Hz, 2H), 7.48–7.24 (m, 5H), 6.94 (d, J = 9.2 Hz, 1H),
3.84 (s, 3H), 2.53 (s, 3H); 13C NMR (100 MHz, CDCl3): d = 165.0,
161.0, 148.4, 132.4, 131.2, 129.1, 128.7, 127.7, 127.6, 126.7, 114.2,
55.4, 16.4. IR (thin film): 3001, 2960, 2931, 2853, 1604, 1574,
1559, 1515 cm-1. MS (EI) m/z: 281 (100%, M+); HRMS (EI):
Calc. for C17H15NOS: 281.0874; Found: 281.0874. Anal. Calc. for
C17H15NOS: C, 72.57; H, 5.37; N, 4.98; S, 11.40. Found: C, 72.50;
H, 5.72; N, 5.29; S, 11.25.
2,5-Bis(4-chlorophenyl)-4-methylthiazole (13)
White solid. 1H NMR (300 MHz, CDCl3): d = 7.84 (d, J = 9.0 Hz,
2H), 7.40–7.37 (m, 6H), 2.51 (s, 3H); 13C NMR (75 MHz, CDCl3):
d = 164.1, 149.5, 136.0, 134.1, 132.2, 131.4, 130.6, 130.5, 129.1,
127.6, 16.5. MS (EI) m/z:, 320 (20%, M+), 321 (80), 319 (100);
HRMS (EI) Calc. for C16H11Cl2NS: 318.9989, Found: 318.9931.
Anal. Calc. for C16H11Cl2NS: C, 60.01; H, 3.46; N, 4.37; S, 10.01,
Found: C, 59.51; H, 3.27; N, 4.34; S, 10.27.
Acknowledgements
5-(4-Chlorophenyl)-4-methyl-2-phenylthiazole (8)17
The authors gratefully acknowledge generous financial support
from the Deutsche Forschungsgemeinschaft (Schwerpunktpro-
gramm 1179 “Organokatalyse”) and the BMBF (DLR, Project
MDA 08/010).
White solid. 1H NMR (300 MHz, CDCl3): d = 7.92–7.90 (m, 2H),
7.43–7.39 (m, 7H), 2.52 (s, 3H); 13C NMR (75 MHz, CDCl3): d =
164.9, 148.7, 133.3, 133.0, 130.4, 130.2, 129.9, 129.5, 125.9, 15.9.
MS (EI) m/z: 285 (100%, M+), 182 (75); HRMS (EI): Calc. for
C16H12ClNS: 285.0379; Found: 285.0389.
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White solid. 1H NMR (300 MHz, CDCl3): d = 7.83 (d, J = 9.0 Hz,
2H), 7.45–7.39 (m, 6H), 2.52 (s, 3H), 1.33 (s, 9H); 13C NMR (75
MHz, CDCl3): d = 165.0, 152.9, 148.5, 133.2, 130.4, 130.3, 129.9,
128.4, 125.6, 125.4, 34.4, 30.7, 15.9. MS (EI) m/z: 341 (100%, M+);
HRMS (EI): Calc. for C20H20ClNS: 341.1005; Found: 341.0993.
Anal. Calc. for C20H20ClNS: C, 70.26; H, 5.90; N, 4.10; S, 9.38.
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White solid. 1H NMR (300 MHz, CDCl3): d = 7.92–7.90 (m, 2H),
7.54 (d, J = 9.0 Hz, 2H), 7.43–7.24 (m, 5H), 2.52 (s, 3H); 13C NMR
(75 MHz, CDCl3): d = 164.9, 161.8, 148.7, 133.0, 131.4, 130.7,
130.2, 129.5, 128.5, 125.8, 15.9. MS (EI) m/z: 329 (100%, M+);
HRMS (EI): Calc. for C16H12BrNS: 328.9782; Found: 328.9797.
Anal. Calc. for C16H12BrNS: C, 58.19; H, 3.66; N, 4.24; S, 9.71.
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White solid. 1H NMR (300 MHz, CDCl3): d = 7.85 (d, J = 8.4 Hz,
2H), 7.39–7.31 (m, 4H), 7.04–6.97 (m, 2H); 3.84 (s, 3H), 2.40 (s,
3H); 13C NMR (75 MHz, CDCl3): d = 164.0, 156.5, 150.4, 135.0,
132.0; 131.5, 129.4, 128.6, 127.5, 127.0, 120.1, 120.0, 110.7, 55.1,
15.9. IR (thin film): 3002, 2958, 2919, 2831, 1581, 1537 cm-1. MS
(EI) m/z: 315 (100%, M+); HRMS (EI): Calc. for C17H14ClNOS:
315.0485; Found 315.0467.
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The Royal Society of Chemistry 2011
Org. Biomol. Chem., 2011, 9, 3457–3461 | 3461
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