A one-pot regioselective synthesis of benzo[d]imidazo[2,1-b]thiazoles

Article abstract of DOI:10.1039/c2ob26211h

Benzo[d]imidazo[2,1-b]thiazole analogues were synthesized via a one-pot metal-free procedure. A series of benzene and pyridine substrates were employed to the methodology. The desired products were generated in moderate to good yields. The effect of substituent group had also been studied.

Full text of DOI:10.1039/c2ob26211h

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PAPER
A one-pot regioselective synthesis of benzo[d]imidazo[2,1-b]thiazoles†
Xinhai Zhang, Jiong Jia* and Chen Ma*
Received 26th June 2012, Accepted 9th August 2012
DOI: 10.1039/c2ob26211h
Benzo[d]imidazo[2,1-b]thiazole analogues were synthesized via a one-pot metal-free procedure. A series
of benzene and pyridine substrates were employed to the methodology. The desired products were
generated in moderate to good yields. The effect of substituent group had also been studied.
Introduction
Benzo[d]imidazo[2,1-b]thiazole derivatives are frequently
encountered in bioactive molecules and pharmaceutical chem-
istry. A wide range of compounds possessing benzo[d]imidazo-
[2,1-b]thiazole scaffold have been employed as antimicrobial
agents A,1 antitumor agents B,2 antiallergic agents,3 and antibac-
terial agents.4 Furthermore, labeled molecules (Fig. 1, C) with
this core skeleton have promised utilization for PET imaging of
Alzheimer’s patients brains as well as β-amyloid plaques.5 In
addition, investment on the biological activity of benzo[d]-
imidazo[2,1-b]thiazole derivatives also revealed its potential role
as particular kinase inhibitors and receptors (Fig. 1, D).6
As a class of privileged substructures, the development of the
core scaffold has attracted enormous attention. Benzo[d]thiazol-
2-amine and 2-bromo-1-phenylethanone derivatives could
undergo nucleophilic substitution and nucleophilic addition to
generate the desired product. Wu and co-workers reported a
Fig. 1 Examples of benzo[d]imidazo[2,1-b]thiazole with biological
activity.
copper-catalyzed coupling reaction of 2-iodobenzothiazole with
2-iodoaniline procedure.7 Bakherad developed a method based
on a Sonogashira coupling tandem reaction with aryl iodide and
3-(prop-2-yn-1-yl)benzo[d]thiazol-2(3h)-imine to afford the
target molecule.8 In addition, the microwave promoted Ugi-type
multicomponent reaction of heterocyclic amidines with alde-
hydes and isocyanides approach has been deeply studied.9
Recent achievement on flow chemistry methods and aqueous
phase synthesis provided intriguing access to benzo[d]imidazo-
[2,1-b]thiazole.10 A [4 + 2] type reaction procedure emerged as a
Scheme 1 Proposed procedure for the synthesis of benzo[d]imidazo-
facile way to construct triheterocyclic compounds.11 Despite the
[2,1-b]thiazole.
previous accomplishments to obtain the skeleton, an economical
route still may be required for the synthesis of benzo[d]imidazo-
Binucleophiles13 were frequently employed in our previous
research due to their diversity and high chemical selectivity.
[2,1-b]thiazole derivatives.
Herein, we report a facile one-pot approach based on an SNAr
Our primary synthetic interests focus on transition-metal
free construction of fused heterocycle compounds.12
reaction starting from binucleophile substrates to obtain benzo[d]-
imidazo[2,1-b]thiazole derivatives (Scheme 1).
School of Chemistry and Chemical Engineering, Shandong University,
Result and discussion
Jinan, 250100, P R China. E-mail: chenma@sdu.edu.cn
†Electronic supplementary information (ESI) available. CCDC 897972.
For ESI and crystallographic data in CIF or other electronic format see
DOI: 10.1039/c2ob26211h
We initially optimized the reaction conditions, including solvent,
base and temperature with 2-mercaptobenzimidazole and
7944 | Org. Biomol. Chem., 2012, 10, 7944–7948
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Table 2 One-pot procedure for benzo[d]imidazo[2,1-b]thiazole
Table 1 Optimization of reaction conditionsa
analoguesa
Entry
1
Substrate
Time (h)
6
Product
Yieldb (%)
64
2
3
5
6
68
72
Entry
Base
Solvent
T/°C
Yieldc (%)
1
2
3
4
5
6
7
8
K2CO3
K2CO3
K2CO3
Cs2CO3
K3PO4
NaOH
DMF
DMF
DMF
DMF
DMF
DMF
DMSO
DMSO
80
n.db
Trace
43c
53
41
n.d
64
100
130
130
130
130
130
130
Cs2CO3
K2CO3
4
5
3
4
68
54
47
a The reaction of 1a with 2a was conducted under a nitrogen atmosphere
for 6 hours. b Not detected. c Isolated yields.
2,3-dichloro-5-(trifluoromethyl) pyridine. The results are pre-
sented in Table 1. The yields increased as the temperature rose
(entries 1–3). Several bases were selected and Cs2CO3 was rela-
tively efficient compared to K2CO3 and K3PO4. No desired
product was detected when NaOH was employed as the base.
DMSO was more suitable due to its stability at high temperatures
and remarkable solubility. The optimal reaction conditions are
listed in entry 7. The yields of the expected product could be up
to 64%.
With the optimized conditions in hand, we then examined the
scope of this methodology. Substrates of benzene and pyridine
with double leaving groups at the ortho-position were employed
in the process (Table 2). To our delight, the desired products
were generated in moderate to good yields. The result exhibited
the feasibility to construct benzo[d]imidazo[2,1-b]thiazole and
imidazo[2′,1′ : 2,3]thiazolo[5,4-b]pyridine derivatives. Products
3a and 3b revealed its tolerance to pyridine substrates. Benzene
substrates with halogen and nitro groups were more efficient
than those with double halogen groups at the ortho-position (2i–
2n). This phenomenon could probably be due to the inferior
chemical selectivity at high temperatures for substrates 2i, 2m,
2n. Besides, the conjugative effect makes the sulfur atom more
reactive than the nitrogen atom and the nitro group adjacent to
the halogen makes it easier to be substituted. Substrate 3f, 3g, 3h
shared the same product and the isolated yields had an obvious
difference. The C–F bond was relatively stable compared to the
C–Br and C–Cl bonds. The structure of compound 2a was
confirmed by X-ray diffraction analysis (Fig. 2).†
6
7
5
5
62
77
8
9
6
5
71
69
10
3
85
11
12
5
83
54
12
13
14
10
9
43
53
As we proposed, the methodology possibly followed the
SNAr mechanism and therefore the substituent group might have
a strong impact on the reaction. To verify this hypothesis, we
employed
6-methoxy-1H-benzo[d]imidazole-2-thiol
(1b)
together with 6-nitro-1H-benzo[d]imidazole-2-thiol (1c) to
observe the effect of the electron-donating group and the elec-
tron-withdrawing group. The results are presented in Table 3.
The methoxy group promoted the reaction to obtain the product
with 81% yield (entry 1) while the nitro group greatly weakened
the reaction activity as the yield went down to 27% (entry 2). We
a Reaction conditions: 2-mercaptobenzimidazole 1 (1.1 equiv), 2 (1
equiv), Cs2CO3 (2.5 equiv), DMSO (5 ml), at 130 °C. b Isolated yields.
This journal is © The Royal Society of Chemistry 2012
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also explored the reaction with 1-bromo-2-nitrobenzene (entries
3–4). No desired product was detected for substrate 1c, we could
only obtain the initial substituted intermediate and the sequential
cyclization step could not take place. 1H-imidazole-2-thiol could
readily react with 2,3-dichloro-5-(trifluoromethyl) pyridine to
generate the expected imidazo[2′,1′ : 2,3]thiazolo[5,4-b]pyridine.
Conclusion
In summary, we developed an economical metal-free procedure
to construct benzo[d]imidazo[2,1-b]thiazole analogues. A series
of compounds were synthesized to examine the extension and
limitation of the methodology. This approach may have appli-
cations in the synthesis of biological and pharmaceutical mol-
ecules with benzo[d]imidazo[2,1-b]thiazole skeletons.
Experimental section
General information
All the reagents were commercially available and were used
without further purification. All reactions were monitored by
thin-layer chromatography (TLC). 1H NMR spectra were
recorded on a Bruker Avance 400 or 300 spectrometer at 400 or
Fig. 2 Single crystal structure of 3a.†
Table 3 Effect of various substituent groups on the methodologya
Entry
1
Substrate 1
Substrate 2
Product
Yieldsb (%)
81
2
3
27
79
4
67
a Reaction conditions: 1 (1.1 equiv), 2 (1 equiv), Cs2CO3 (2.5 equiv), DMSO (5 ml), TLC monitored the reaction to completion, reaction time 4–8 h.
b Isolated yields. c The ratios was obtained using NMR.
7946 | Org. Biomol. Chem., 2012, 10, 7944–7948
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Benzo[d]benzo[4,5]imidazo[2,1-b]thiazole (3f, 3g, 3h). White
solid (62%, 77%, 71%), mp 139–141 °C. H NMR (400 MHz,
300 MHz, using CDCl3 or DMSO-d6 as solvent and tetramethyl-
silane (TMS) as internal standard. 13C NMR spectra were run
via the same instrument at 100 or 75 MHz. Melting points were
determined on an XD-4 digital micro melting point apparatus.
HRMS spectra were determined on a Q-TOF6510 spectrograph
(Agilent).
1
CDCl3) δ 7.97 (d, J = 8.36 Hz, 2H), 7.85 (dd, J = 1.36, 1.28 Hz,
1H), 7.76 (d, J = 8 Hz, 1H), 7.54–7.58 (m, 1H), 7.37–7.46 (m,
3H); 13C NMR (100 MHz, CDCl3) δ 110.62 112.41 119.45
122.02 123.68 124.35 124.46 126.74 129.05 130.44 133.22
147.94 155.35. HRMS calcd for (M + H+) 225.0442; found:
225.0468.
General experimental procedure for 3a
3-Chlorobenzo[d]benzo[4,5]imidazo[2,1-b]thiazole (3i). White
solid (69%), mp 210–212 °C. H NMR (400 MHz, CDCl3) δ
A mixture of 1a (1.1 mmol), 2a (1 mmol), Cs2CO3 (2.5 mmol)
was added to 5 ml DMSO, pre-stirred for several minutes, then
the mixture was allowed to heated to 130 °C under the atmos-
phere of N2, TLC monitored the end of the reaction. After the
mixture was cooled, water was added. The solution was extracted
with ethyl acetate (15 × 3). The combined organic phase was
dried with MgSO4 and the solvent was removed in vacuo to
obtain the residue. The residue was purified by column chrom-
atography on silica gel to afford 3a.
1
7.89 (d, J = 7.32 Hz, 1H), 7.83 (d, J = 8.56 Hz, 2H), 7.72 (d,
J = 1.96 Hz, 1H), 7.51 (dd, J = 2 Hz, 1H), 7.36–7.45 (m, 2H);
13C NMR (100 MHz, CDCl3) δ 110.37 112.81 119.68 122.19
123.79 124.07 126.94 129.82 130.25 130.49 131.71 148.08
154.89. HRMS calcd for (M + H+) 259.9989; found: 259.0087.
2-Chlorobenzo[d]benzo[4,5]imidazo[2,1-b]thiazole (3j). White
1
solid (85%), mp 233–236 °C. H NMR (400 MHz, CDCl3) δ
7.93 (dd, J = 1.92, 1.32 Hz, 2H), 7.84 (dd, J = 1.32, 1.48 Hz,
1H), δ 7.65 (d, J = 8.52 Hz, 1H), δ 7.37–7.45 (m, 2H), 7.36 (dd,
J = 1.92 Hz, 1H); 13C NMR (100 MHz, CDCl3) δ 110.64
112.84 119.63 122.40 124.10 124.65 124.99 127.35 130.21
132.88 133.83 147.89 155.54. HRMS calcd for (M + H+)
259.9989; found: 259.0087.
3-(Trifluoromethyl)benzo[4′,5′]imidazo[2′,1′:2,3]thiazolo[5,4-
b]pyridine (3a). White solid (64%), mp 270–271 °C. H NMR
1
(300 MHz, CDCl3) δ 8.76 (s, 1H), 8.25 (s, 1H), 7.88–7.94 (m,
2H), 7.44–7.52 (m, 2H); 13C NMR (100 MHz, CDCl3) δ 110.50
114.62 (d, J = 3.5 Hz) 120.31 121.99 123.15 124.36 124.72
128.57 130.19 141.71 (t, J = 4.3 Hz) 147.24 152.82 156.71.
HRMS calcd for (M + H+) 294.0268; found: 294.0292.
4-Chlorobenzo[d]benzo[4,5]imidazo[2,1-b]thiazole (3k). White
1
Benzo[4′,5′]imidazo[2′,1′:2,3]thiazolo[5,4-b]pyridine
(3b).
solid (83%), mp 198–201 °C. H NMR (400 MHz, CDCl3) δ
Light yellow solid (67%), mp 183–186 °C. 1H NMR (300 MHz,
CDCl3) δ 8.48 (dd, J = 1.2, 1.5 Hz, 1H), 8.11 (d, J = 8.1 Hz,
1H), 7.85 (dd, J = 1.8 Hz, 1H), 7.36–7.49 (m, 4H); 13C NMR
(75 MHz, CDCl3) δ 110.31 118.22 119.77 121.02 122.50 124.07
128.78 130.23 145.16 146.91 152.53 153.02. HRMS calcd for
(M + H+) 226.0394; found: 226.0427.
7.91 (d, J = 8.04 Hz, 1H), 7.84 (t, J = 6.2, 7.64 Hz, 2H), 7.50 (t,
J = 8 Hz, 1H) δ 7.34–7.44 (m, 3H); 13C NMR (100 MHz,
CDCl3) δ 110.27 110.51 119.73 122.27 123.90 124.17 127.60
128.92 129.28. 130.36 134.07 148.05 154.63. HRMS calcd for
(M + H+) 259.9989; found: 259.0083.
2-Nitrobenzo[d]benzo[4,5]imidazo[2,1-b]thiazole (3l). Yellow
solid (54%), mp 274–277 °C. H NMR (400 MHz, CDCl3) δ
2-(Trifluoromethyl)benzo[d]benzo[4,5]imidazo[2,1-b]thiazole (3c).
White solid (72%), mp 211–213 °C. 1H NMR (300 MHz,
CDCl3) δ 8.13 (s, 1H), δ 7.95–7.99 (m, 1H), 7.58–7.88 (m, 1H),
7.63 (d, J = 8.4 Hz, 2H), 7.41–7.49 (m, 2H); 13C NMR
(75 MHz, CDCl3) δ 29.69 109.07 (q, J = 3.75 Hz) 110.64
119.84 121.09 (q, J = 3.75 Hz) 121.92 122.57 124.17 124.64
125.53 129.57 133.32 148.21 155.18. HRMS calcd for
(M + H+) 293.0316; found: 293.0372.
1
8.75 (s, 1H), 8.30 (d, J = 8.32 Hz, 1H), 8.05 (d, J = 4.72 Hz,
1H), 7.87–7.92 (m, 2H), 7.50 (d, J = 3.72 Hz, 2H); 13C NMR
(100 MHz, CDCl3) δ 107.31 110.82 119.44 120.06 123.15
124.54 124.67 130.32 133.27136.75 146.70 147.91 154.95.
HRMS calcd for (M + H+) 270.0293; found: 270.0333.
Benzo[d]benzo[4,5]imidazo[2,1-b]thiazole-4-carbonitrile (3n).
Light yellow solid (53%), mp 232–234 °C. 1H NMR (400 MHz,
CDCl3) δ 9.05 (d, J = 8 Hz, 1H), 7.34 (dd, J = 1.24 Hz, 1H),
7.82 (m, 2H), 7.39–7.48 (m, 3H); 13C NMR (100 MHz, CDCl3)
δ 97.56 114.46 119.10 119.54 122.85 124.28 124.71 128.90
131.21 131.33 132.88 134.91 148.26 154.91. HRMS calcd for
(M + H+) 250.0394; found: 250.0433.
Benzo[d]benzo[4,5]imidazo[2,1-b]thiazole-2-carbonitrile (3d,
3m). White solid (68%, 43%), mp 274–276 °C. 1H NMR
(300 MHz, CDCl3) δ 8.17 (d, J = 1.2 Hz, 1H), 7.94–7.97 (m,
1H), 7.85–7.88 (m, 2H), 7.66 (dd, J = 1.5, 1.2 Hz, 1H),
7.43–7.51 (m, 2H). 13C NMR (75 MHz, CDCl3) δ 112.78
115.89 118.79 121.71 123.80 124.13 124.46 129.94 133.59
144.16 145.45 147.31 147.82 156.43. HRMS calcd for
(M + H+) 250.0394; found: 250.0434.
7-Methoxy-3-(trifluoromethyl)benzo[4′,5′]imidazo[2′,1′:2,3]
thiazolo[5,4-b]pyridine (4b1), 8-methoxy-3-(trifluoromethyl)-
benzo[4′,5′]imidazo[2′,1′:2,3]thiazolo[5,4-b]pyridine (4b2). White
2-Fluorobenzo[d]benzo[4,5]imidazo[2,1-b]thiazole (3e). White
solid (54%), mp 163–166 °C. 1H NMR (300 MHz, DMSO-d6) δ
8.32–8.46 (m, 2H), 8.06–8.14 (m, 1H), 7.76 (dd, J = 2.4,
2.1 Hz, 1H), 7.13–7.52 (m, 3H); 13C NMR (100 MHz, DMSO-
d6) δ 106.54 (d, J = 28.5 Hz) 117.03 (t, J = 16.2 Hz) 123.93
127.15 128.65 (d, J = 2.4 Hz) 131.45 (d, J = 9.7 Hz) 135.08
(d, J = 12 Hz) 152.75 160.90 165.48 167.90. HRMS calcd for
(M + H+) 243.0348; found: 243.0375.
1
solid (81%), mp 225–227 °C. H NMR (300 MHz, CDCl3) δ
8.75 (s, 1H), 8.14 (d, J = 1.5 Hz), 7.77 (dd, J = 4.2 Hz, 1H),
7.35 (dd, J = 2.4 Hz, 1H), 7.09 (dd, J = 2.4 Hz, 1H), 3.97(s,
3H); 13C NMR (75 MHz, CDCl3) δ 56.29 95.09 102.89 110.78
112.56 114.40 (q, J = 3.75 Hz) 120.45 124.57 128.41 130.57
141.66 (t, J = 4.5 Hz) 151.17 156.66 157.82. HRMS calcd for
(M + H+) 324.0374; found: 324.0406.
This journal is © The Royal Society of Chemistry 2012
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7-Nitro-3-(trifluoromethyl)benzo[4′,5′]imidazo[2′,1′:2,3]thiazolo-
[5,4-b]pyridine (4c). Yellow solid (27%), mp 241–244 °C. H
1
5 (a) D. Alagille, H. DaCosta, R. M. Baldwin and G. D. Tamagnan,
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NMR (400 MHz, CDCl3) δ 8.85 (d, J = 0.88 Hz, 1H), 8.75 (d,
J = 2.04 Hz, 1H), 8.51 (d, J = 8.84 Hz, 1H), 7.39–7.48 (m, 2H);
13C NMR (100 MHz, CDCl3) δ 112.78 115.91 118.80 121.73
123.82 124.53 (d, J = 8.4 Hz) 129.94 (d, J = 3.4 Hz) 133.62
144.19 (t, J = 4 Hz) 145.51 147.34 147.86 156.43. HRMS calcd
for (M + H+) 339.0119; found: 339.0151.
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52, 1275; (n) Q. Chao, K. G. Sprankle, R. M. Grotzfeld and A. G. Lai,
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9-Methoxybenzo[d]benzo[4,5]imidazo[2,1-b]thiazole (4g1), 8-
methoxybenzo[d]benzo[4,5]imidazo[2,1-b]thiazole (4g2). White
1
solid (79%), mp 123–125 °C. H NMR (400 MHz, CDCl3) δ
8.85 (dd, J = 3.2 Hz, 2H), 8.75 (d, J = 4.8 Hz, 1H), 7.69–7.72
(m, 3H), 7.52 (t, 2H) δ 7.41 (d, J = 2.24 Hz, 1H), 7.31–7.36 (m,
3H), 6.97–7.05 (m, 2H), 3.92 (s, 6H); 13C NMR (100 MHz,
CDCl3) δ 56.13 95.62 102.28 110.77 111.18 111.59 112.14
119.72 124.25 126.58 128.09 133.08 155.74 156.89. HRMS
calcd for (M + H+) 255.0547 found: 255.0614.
6-(Trifluoromethyl)imidazo[2′,1′:2,3]thiazolo[5,4-b]pyridine (4d).
White solid (67%), mp 128–130 °C. 1H NMR (400 MHz,
CDCl3) δ 8.72 (s, 1H), 8.28 (s, 1H), 7.99 (s, 1H), 7.46 (s, 1H);
13C NMR (75 MHz, CDCl3) δ 112.67 121.40 123.72 125.87
129.89 (q, J = 3.75 Hz) 135.96 143.22 (q, J = 4.5 Hz) 146.32
146.95. HRMS calcd for (M + H+) 244.0112; found: 244.0187.
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Acknowledgements
We are grateful to the National Science Foundation of China
(No. 21172131) for financial support of this research.
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