Selective exploitation of acetoacetate carbonyl groups using imidazolium based ionic liquids: synthesis of 3-oxo-amides and substituted benzimidazoles

Article abstract of DOI:10.1016/j.tetlet.2016.06.048

An unprecedented Br?nsted base ionic liquid tuned selective aminolysis of ester carbonyl of acetoacetates is demonstrated to achieve acetoacetamide derivatives. Other imidazolium ionic liquid performs an efficient cyclization catalysis involving acetoacetate-carbonyl groups and o-phenylenediamine at elevated temperature to produce benzimidazoles via C–C bond cleavage of intermediate 1,5-benzodiazepinones under solvent-free conditions.

Full text of DOI:10.1016/j.tetlet.2016.06.048

Accepted Manuscript
Selective exploitation of acetoacetate carbonyl groups using imidazolium based
ionic liquids: Synthesis of 3-oxo-amides and substituted benzimidazoles
Ankita Chakraborty, Swapan Majumdar, Dilip K Maiti
PII:
S0040-4039(16)30715-8
DOI:
http://dx.doi.org/10.1016/j.tetlet.2016.06.048
TETL 47775
Reference:
Tetrahedron Letters
To appear in:
Received Date:
Revised Date:
Accepted Date:
17 May 2016
9 June 2016
12 June 2016
Please cite this article as: Chakraborty, A., Majumdar, S., Maiti, D.K., Selective exploitation of acetoacetate carbonyl
groups using imidazolium based ionic liquids: Synthesis of 3-oxo-amides and substituted benzimidazoles,
Tetrahedron Letters (2016), doi: http://dx.doi.org/10.1016/j.tetlet.2016.06.048
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Graphical Abstract
Selective exploitation of acetoacetate carbonyl groups using
imidazolium based ionic liquids: Synthesis of 3-oxo-amides
Leave this area blank for abstract info.
O
O
and substituted benzimidazoles
Ankita Chakraborty, Swapan Majumdar* and
Dilip K. Maiti
An unprecedented imidazolium based ionic liquid tuned
selective aminolysis of ester-carbonyl and cyclization catalysis
involving carbonyl group of acetoacetates is demonstrated to
achieve acetoacetas, their chiral analogues
R1
OR
1
H
N
Bu
O
120oC
N
N
N
120oC
OH
Bu
Br
R1
120oC
R4
N
N
N
H
R2NHR3
N
R4
R1
4
6
O
O
120oC
R1
NR2R3
and benzimidazoles.
2

1
Tetrahedron Letters
Tetrahedron Letters
journal homepage: www.els evier.com
Selective exploitation of acetoacetate carbonyl groups using imidazolium based ionic
liquids: Synthesis of 3-oxo-amides and substituted benzimidazoles
Ankita Chakraborty,a Swapan Majumdar∗a and Dilip K Maitib
aDepartment of Chemistry, Tripura University, Suryamaninagar, 799 022, INDIA,
bDepartment of Chemistry, University of Calcutta, 92, A. P. C. Road, Kolkata 700 009, INDIA
ARTICLE INFO
ABSTRACT
An unprecedented Brønsted base ionic liquid tuned selective aminolysis of ester
Article history:
Received
Received in revised form
Accepted
Available online
carbonyl of acetoacetates is demonstrated to achieve acetoacetamide derivatives.
Other imidazolium ionic liquid performs an efficient cyclization catalysis
involving acetoacetate-carbonyl groups and o-phenylenediamine at elevated
temperature to produce benzimidazoles via C-C bond cleavage of intermediate
1,5-benzodiazepinones under solvent-free conditions.
2009 Elsevier Ltd. All rights reserved.
Keywords:
β-ketoamides
ionic liquid
C-C bond cleavage
benzodiazepinones
benzimidazole
β-Ketoamides (3-oxo-amides) are recognized as highly versatile
synthons in organic synthesis because of the presence of several
reactive sites in one molecule diversifies its application in
various synthetic strategies1 to multifunctional entities. Synthesis
of a wide variety of such multi-functionalized molecules through
development of a new domino reaction2 is an important addition
to the existing synthetic approaches in organic chemistry. A large
number of heterocyclic compounds having different
pharmacological activities3 have been synthesized involving
multiple bond-forming transformations4 using substituted
acetoacetamides. For these reason, it is always of current interest
to find new and simple procedures for the synthesis of N-
substituted acetoacetamides. Usually two protocols for the
synthesis of N-substituted acetoacetamides (2) are commonly
followed: (i) acylation of amide enolates or their synthetic
equivalents5a and (ii) two components coupling of amines with β-
ketoacids or their synthetic analogues such as β-ketoesters,5b β-
ketothioesters,5c,d ketene dimer,5e,f and acylketens5g-j obtained
from thermal decomposition of 2,2,6 trimethyl-4H-1,3-dioxin-4-
one. Apparently, the direct aminolysis of inexpensive β-ketoester
Scheme 1: Synthetic strategy for the amidation of acetoacetate
Moreover huge quantity of xylene is required for such
transformation, which is objectionable in respect to green
chemistry principles. Other methods for the synthesis of β-
ketoamides from acylated Meldrum’s acids,7 lipase catalysis8 and
enamine-based9 domino strategy of acetoacetamides are also
known. Although some of the methods are very effective in terms
of yield but synthesis of starting materials are cumbersome and
many of them are posed to serious negative impact to health and
environment. Thus, the development of a method that can suppress
the formation of intermediate iminoester (i) or eneminoester (ii)
thereby shield the keto-carbonyl from the nucleophilic attack of
amine (Scheme 1), which would be more viable and attractive to
achieve desired β-ketoamide (2). As a part of our ongoing
program to design and develop solvent and metal-free strategies in
(1) would be a straightforward pathway to β-ketoamides (2) but organic synthesis,10 we were interested to observe the effect of
this synthesis generally involves low yields of the product due to
the presence of more reactive keto-carbonyl group leading to co- amidation of alkyl acetoacetate and synthesis of benzimidazoles
imidazolium-based basic ionic liquid as catalyst in the direct
occurrence of side product enamino esters2b,6 (3, Scheme 1).
———
by C-C bond cleavage of acetoacetate. It is expected that basic
∗ Corresponding author. Tel: +91-381-237-9070; Fax: +91-381-2374802; e-mail: smajumdar@tripurauniv.in

2
Tetrahedron Letters
ionic liquid I ([BMIm]OH)11 will facilitate enolization of 1 that
can be stabilized by imidazolium cation and simultaneously the
ester group also activated. Thus it will favor the amidation
reaction (Scheme 2). We also screened two more ionic liquids
namely 1-butyl imidazolium trifluoroacetate (II) and 1-butyl-3-
Table 1: Optimization of amidation of acetoacetate by the catalysis
of ionic liquid
OH
Me
Bu
N
N
Me
Bu
N
N
O
O
O
O
I
p-toluidine (3a)
O
O
reaction condition
Me
N
H
OR
Me
Table 1
Me
OMe
2a
1
methyl imidazolium bromide (III) in the present study(Scheme 2).
C-O cleavage and C-N coupling
masked carbonyl
Me
Bu
Entry
R
BIL
(mol%)
Temp
(oC)
80
Solvent
Yield (%)a,b
N
O
N
O
O
O
R"NH2 (3)
O
O
I
R
NHR"
reaction
conditions
R
OR'
2
1
2
3
Me (1a)
Me
Me
no
50
50
Toluene
Toluene
Toluene
toluene
-
Dioxane
CH3CN
Glycerol
Toluene
Toluene
toluene
mixture
mixture
R OR'
masked keto group
(iii) X = H, Me
1
β-keto amide
80
120
120
120
100
90
120
120
120
120
82
90
50
75
40
NR
88
70
Me
Bu
N
OH
4
5
Me
Me
Me
Me
100
100
100
100
100
100
100
100
Me
Bu
N
Br
H
Bu
N
N
N
N
TFA
I
6
7
8
II
III
Scheme 2: Basic ionic liquid catalyzed synthesis of 2
Me
In our initial experiments, the amidation of methyl acetoacetate
with p-toluidine (3a) was chosen as a model reaction to optimize
the reaction conditions (Table 1). Few drops toluene were used
9
10
11c
Et (1b)
iPr (1c)
Me
88, 87, 87,
85
o
for smooth stirring of the mixture at 80 C and the reaction was
a yields refer to purified yield; 0.5 mL toluene was used in each
entry; c using recycled basic ionic liquid (I)
b
unsuccessful in the absence of basic ionic liquid. Whereas in the
presence of 50mol% of basic ionic liquid (I) it provided a
mixture of products (Table 1, entry 2). Although the expected β-
rate with respect to aliphatic amines with better yield. Aniline, o-
toluidine, naphthylamine and 4-methoxyaniline all undergo
smooth amidation with β-ketoester to form β-ketoamide (2b-e,
Fig. 1) in good to excellent yield. Among the aromatic amines,
the electron releasing aromatic amine proceeded faster reaction
than the aromatic amines containing electron withdrawing
substituents such as nitro, fluoro- and pyridine (amide 2f-i, Fig.
1). Aliphatic amines like morpholine, piperidine, benzylamine
and tert-butyl amine also produced corresponding amides (2j-m,
Fig. 1) in high yield without any difficulty. Ethyl
benzoylacetate12 produced corresponding β-ketoamides 2n-q in
excellent yield with shorter reaction time as compared to the
1
ketoamide (2a) was found at 120 oC, but H NMR spectra of the
crude products indicated the presence of very small amount of
enemine of the β-ketoamide. Acidic hydrolysis of the mixture
with 10% aqueous HCl followed by purification over silica-gel
column chromatography afforded 82% of the desired product (2a,
Table 1, entry 3). Increased amount of amines did not
significantly improve the yield; however using 100 mol% of
ionic liquid afforded the desired product in excellent yield (90%,
Table 4, entry 4) after 4hrs. Without solvent or changing solvents
from nonpolar to polar the yield was drastically dropped (Table
1, entries 5-7) and the reaction was arrested in protic solvent
glycerol (Table 1, entry 8). To ascertain the role of ionic liquid
we have carried out the same reaction without basic ionic liquid,
and with KOH and 18-crown-6 under similar reaction conditions.
Our observed essentiality of ionic liquid for the reaction to occur
because only a trace amount of the desired acetoacetanilide was
detected (not included in Table 1). For further improvement of
yield we were curious about the effect of alkyl group present in
acetoacetate. However, only a little difference was observed
between methyl or ethyl acetoacetate (Table 1, entries 4 vs 9).
However considerable discrimination was observed in the case of
isopropyl acetoacetate that produced only 70% of the
corresponding product (Table 1, entry 10). After isolation of
products by solvent extraction, the aqueous layer containing ionic
liquid can be collected (concentrated and vacuum dried) and
recycled at least 4 times without significant loss of catalytic
activity (Table 1, entry 11). It is noteworthy to mention that
protic ionic liquid N-butylimidazolium trifluoroacetate (II) and
neutral ionic liquid 1-butyl-3-methyl imidazolium bromide (III)
are favoring the formation of imine (i) - enamine (ii) tautomeric
form of β-ketoester (1) and only a little amount of desired
acetoacetanilides (2a) was detected in both the reactions (not
shown in the Table 1). On the basis of successful results shown
in Table 1 (entry 4) we decided to perform subsequent reactions
with varieties of amines and β-ketoesters for establishing it as
general strategy. As listed in Figure 1, different kinds of amines
were acetoacetylated using methyl or ethyl acetoacetate in good
to excellent yields. Aromatic amines followed slower reaction
R
OMe
O
O
O
O
O
O
N
N
N
H
H
H
2e: 77%, 4.0h
2b: 80%, 4.0h, R = H
2c: 84%, 3.0h, R =Me
O2N
2d: 77%, 4.0h
NO2
F
O
O
O
O
O
O
N
N
N
H
2g: 72%, 5.0h
H
2f: 67%, 5.0h
H
2h: 73%, 4.0h
O
O
O
O
O
O
N
N
N
H
N
O
2k: 72%, 3.0h
2i: 70%, 4.0h
2j: 68%, 3.5h
O
O
O
O
O
O
N
H
Ph
N
H
Ph
N
H
2l: 68%, 3.5h
2m: 72%, 3.0h
2n: 91%, 2.0h
NO2
OMe
O
O
O
O
O
O
Ph
N
H
Ph
N
H
Ph
N
H
2p: 96%, 2.0h
2o: 95%, 2.0h
2q: 85%, 3.0h
O
O
O
O
O
O
Ph
N
H
Ph
N
H
Ph
Ph
N
H
Ph
2t: 85%, 2.5h
2s: 85%, 2.5h
2r: 82%, 3.0h
Figure 1: Description of various synthesized acetoacetamides

3
Tetrahedron Letters
methyl or ethyl acetoacetate. Versatility of the strategy is
examined by synthesizing valuable chiral acetoacetamides (2r-t,
Fig. 1) under the same reaction conditions. It was known for a
long time that o-phenylenediamine reacts with ethyl acetoacetate
(EAA) under acidic conditions13 or microwave irradiation14 to
affords 1,5-benzodiazepinone derivatives (4). Since under basic
condition ketone carbonyl of β-ketoester becomes masked, we
reasoned that o-phenylenediamine may react with two molecules
of ethyl acetoacetate to produce bis-acetoacetamide, which could
be a potential ligand for the detection/sensing of metal ions or
other interesting application. Accordingly o-phenylenediamine
(5a) was allowed to reacts with EAA under the catalysis using
basic ionic liquid I in toluene under refluxing conditions. This
reaction results mainly 2-methyl benzimidazole (6a) as one of the
products with small amount of unidentified other by-products.
However a very clean reaction occurs in the presence of 10 mol%
of ionic liquid 1-butyl-3-methyl-imidazolium bromide (III) under
solvent-free conditions affording 2-methyl benzimidazole (6a) in
95% isolated yield (Table 2, entry 1). To the best of our
knowledge there is no report for such transformation performed
under neutral conditions. This interesting observation prompted
us to explore the synthetic utility and mechanism of the
benzimidazole formation under solvent-free protocol. To this
end, we have utilized series of aromatic diamines (5)
Table 2: Ionic liquid III promoted synthesis of substituted benzimidazole
ionic liquid III
Me
N
N
O
O
NH2
+
(10 mol%)
R
R"
toluene, reflux
N
H
Br
Bu
R
X
1: X = OR'
2: X = NHR'
N
NH2
R"
6
5
III
Entry
1
Diamine (5)a
Ketoester/amides
Time (min)
90
Product (6)
Yield (%)b
95
NH2
N
O
O
O
O
O
OEt
OEt
N
NH2
1b
H
5a
6a
NH2
N
N
2
3
4
5
60
95
90
91
75
NH2
1b
1b
5b
H
6b
N
NH2
O
O
180
180
N
H
O2N
OEt
OEt
O2N
NH2
NH2
6c
5c
N
O
N
H
HO2C
NH2
HO2C
N
5d
1b
6d
Ph
O
NH2
O
O
N
Me
OEt
N
NH2
NH2
NH2
N
N
H
4e
120
45
1b
5e
N
N
6e
H
(14%)
N
O
O
Ph
6
7
92
95
Ph
OEt
N
H
1d
6f
5a
N
NH2
O
O
Ph
60
Ph
N
H
OEt
6g
NH2
NH2
1d
5b
N
O
O
O
O
Ph
8
60
84
Ph
OEt
OEt
N
H
Cl
6h
Cl
NH2
NH2
1d
1d
5f
N
Ph
9
45
88
95
Ph
N
H
NH2
5g
6i
N
N
NH2
10
120
O
O
N
H
H2N
H
2a
6a
NH2
NH2
90%
5a
OMe
OMe
NO2
11
12
60
N
N
95
92
O
O
O
N
H
2e
H2N
NH2
NH2
H
6a
6a
93%
5a
NO2
N
O
60
90
N
N
H
H2N
2g
2p
NH2
NH2
H
90%
5a
5a
OMe
OMe
N
O
O
Ph
13
14
97
Ph
N
H
N
H
NH2
NH2
H2N
6h
92%
N
O
O
120
93c
OEt
N
H
NH2
1b
5a
6a
a reaction carried out in one mmol scale; b yield refers to purified yield of benzimidazoles; c reaction was conducted
using 2-methyl-1,3-dibutyl imidazolium bromide as activator

4
Tetrahedron Letters
and several β-ketoesters for the synthesis of multi substituted
benzimidazoles (6, Table 2). When ethyl acetoacetate (1b) was
indicated that basic ionic liquid mask the carbonyl of β-ketoesters
as well benzidiazepinone through enolization thereby retard the
reaction to complete, whereas other ionic liquids facilitate
nucleophilic attack through simultaneous activation of carbonyls
of β-ketoesters or amides to form benzodiazepinone 4 followed
by activation of other nitrogen that helps for ring closure
followed by C-C bond cleavage to form benzimidazole 6
(Scheme 4). Involvement of activated hydrogen at C-2 of ionic
liquid may be ruled out because on use of 2-methyl-1,3-dibutyl
imidazolium bromide as activator also produced the desired
product 2-methyl benzimidazoles (6a) in 93% yield (Table 2,
entry 14).
allowed to react with aromatic amine 5b-e in the presence of
o
ionic liquid III under solvent-free conditions at 120 C smooth
transformation to highly substituted benzimidazole 6b-e were
observed in high yield of 75-95% (Table 2, entries 2-5). No 1,5-
benzodiazepinoe was detected except in the case 2,3-diamino
pyridine (Table 2, entry 5). In this case 1,5-benzodiazepinone
was isolated in small quantity (14% yield), which showed the
presence of amide carbonyl in its FTIR spectrum, but in NMR it
exists as four tautomeric forms (ESI). Ethyl benzoylacetate (1d)
also reacted with aromatic diamine 5a-b, 5f, 5g (Table 2, entries
6-9) to afforded 6f-i in excellent yield. All the synthesized
products (6a-i) were characterized by spectral analyses and the
data were compared with reported compounds (ESI). Similar
trends were observed in the cases of N-aryl acetoacetamides (2a,
e, g, p, Table 2, entries 10-13) and 2-methyl benzimidazole
(Table 2, entries 10-12) and 2-phenyl benzimidazole (Table 2,
entry 13) were obtained with concomitant formation of the parent
amines. Formation of free amine (ArNH2) is strongly suggested
that 1,5 benzodiazepinone forms as initial intermediate in these
reactions. Although we were unable to isolate intermediate 1,5-
benzo-diazepinones from the reaction of diamines with
acetoacetic ester or amides (Table 2) even quenching of reaction
mixture after short time ( except entry 5), however the reaction of
ethyl benzoylacetate (1d) with 4-methyl-1,2-diaminobenzene
(5b) with catalyst ionic liquid I and II a variable mixture of 2-
phenyl-5(6)-methyl benzimidazole (6g) and 4-phenyl-8-methyl-
2,3-dihydro-1H-1,5-benzodiaze-pin-2-one (4g) were obtained
(Scheme 3). With the use of basic ionic liquid I reaction yields a
mixture benzimidazole (6g) and benzodiazepinone (4g) in a ratio
of 1:3 whereas reverse effect was observed in the case of protic
ionic liquid II (6g:4g = 6:1). In contrast to the previous result
exclusive formation of 6g (95%) was observed in a shorter
reaction time on employing ionic liquid 1-butyl-3-methyl
imidazolium bromide (III) (Table 2, entry 7). Appearance of
infrared stretching frequency at 1677cm-1 indicated the presence
of -NHCO- group in 4g whereas NMR spectra in CDCl3 (1H, 13C)
of 4g showed the existence of four tautomeric forms (Scheme 3).
Thus with these experimental results in hand we proposed that at
high temperature in the presence of ionic liquid 1,2-
diaminobenzene or its derivative reacts with β-ketoester or amide
to produce substituted 1,5-benzodiazepinone, which than undergo
facile thermolysis to benzimidazoles and ketene (Scheme 4).
Involvement of thermolytic decomposition in the formation of
benzimidazole (6) from benzodiazepinone was also further
confirmed by heating of 4-phenyl benzodiazepinone (4g) in the
presence of ionic liquid III at 120 oC (Scheme 3). These results
Bu
N
Me
Bu
Me
Bu
N
N
O
N
O
N
O
Br
N
III
H
O
I
TFA
II
O
O
R
XR'
R
XR'
R
XR'
C
A
B
has a little tendency to mask
ketone carbonyl by enol formation
O
O
NH
both carbonyls activated
for nucleophilic attack
ketone carbonyl
is masked by BIL
CH2=C=O
NH2
NH
N
N
NH2
"R
R
"R
N
N
"R
"R
R
IL
Scheme 4: Proposed mechanism of benzimidazole formation
In conclusion, this article described15 a very fast, simple and
efficient method to synthesize functionalized synthons β-
ketoamides and their chiral analogues, and substituted
benzimidazoles from β-ketoesters through selective aminolysis of
less reactive ester carbonyl by masking of more active carbonyl
group with powerful basic ionic liquid. On treatment of 1,2-
diaminobenzenes with β-ketoesters or amides benzimidazoles
were formed by exploiting both carbonyls of via formation of
1,5-benzodiazepinone intermediate under the influence of a
neutral ionic liquid. Details mechanistic investigation, trapping of
intermediate ketene and other substrate scope is underway and
results will be communicated in due course.
Acknowledgments
Financial supports by Council of Scientific and Industrial
Research (CSIR), Govt. of India is gratefully acknowledged
(Project no. 02(0235)/15/EMR-II). A.C. is thankful to DST, India
for providing DST-INSPIRE research fellowship.
Notes and references
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Ph
H
N
O
N
N
NH2
O
O
IL
Ph
120oC
+
120oC
N
H
+
NH2
2
3
Ph
OEt
N
H
N
Ph
4g
O
1d
H
N
6g
5b
OH
Ph
N
H
H
Entry Ionic liquids Time (h) 6g (%) 4g (%)
N
N
H
H
N
Ph
OH
OH
Ph
1
2
3
I
3
2
1
25
84
95
75
13
0
N
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II
III
H
H
N
H
N
Ph
OH
Scheme 3: Effect of IL in the formation of benzodiazepinone

5
Tetrahedron Letters
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(a) Yip, K.-T.; Li, J.-H.; Lee O.-Y.; Yang, D. Org. Lett. 2005,
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15 Representative experimental procedure: (a) For aceto-
acetamide derivatives-
A
mixture of methyl/ethyl
acetoacetate (0.58g, 5.0 mmol), basic ionic liquid (0.78g,
5.0 mmol) and an p-toluidine (0.64g, 6.0 mmol) in toluene
(1 mL) was refluxed for 4 hrs. The mixture was diluted
with water (30 mL) and extracted with ethylacetate or ether
(3 x 10mL). The aqueous layer containing basic ionic
liquid was recycled. The organic layer was stirred with
10% HCl (10 mL) at room temperature for 2hrs. The
organic layer was collected, washed with water (3 x 10
mL), dried over anhydrous sodium sulphate, and
concentrated under reduced pressure. The crude product
was purified over silica-gel (60–120 mesh) using ethyl
6
7
(a) Xin, D.; Burgess, K. Org. Lett. 2014, 16, 2108-2110; (b)
Bartoli, G.; Bosco, M.; Locatelli, M.; Marcantoni E.;
Melchiorre, P.; Sambri, L. Synlett 2004, 239–242; (c) Khodaei,
M. M.; Khosropour A. R.; Kookhazadeh, M. Synlett 2004,
1980–1984.
acetate-hexane (3:7) as eluent to afford 2a as colourless
needles. Similar procedure was followed for other esters.
(b) For benzimidazole derivatives- A stirred mixture of
aromatic diamine (1 mmol) and β-ketoester or β-ketoamide
(1 mmol) was heated under inert condition at 120 oC in the
presence of 10mol% of ionic liquid III. After completion
of reaction as revealed by TLC, product was crystallized
from ethyl acetate–hexane or passes through short pad
silica-gel to remove any colour impurities from the product
to obtain analytically pure benzimidazoles.
(a) Cantin, A.; Moya, P.; Castillo, M.; Primo, J.; Miranda, M.
A.; Primo-Yufera, E. Eur. J. Org. Chem. 1999, 221-226; (b)
Moya, P.; Cantin, A.; Castillo, M.-A.; Primo, J.; Miranda, M.
A.; Primo-Yufera, E. J. Org. Chem. 1998, 63, 8530-8533.
Garcia, M. J.; Rebolledo, F.; Gotor, V. Tetrahedron 1994, 50,
6935-6940.
8
9
Angelov, P. Synlett 2010, 1273–1275.
10 (a) Majumdar, S.; Chakraborty, M.; Maiti, D. K.; Chowdhury,
S.; Hossain, J. RSC Adv. 2014, 4, 16497-16502; (b) Majumdar,
S.; De, J.; Chakraborty A.; Maiti, D. K. RSC Adv. 2014, 4,
24544-24550;
11 (a) Mehnert, C. P.; Dispenziere, N. C.; Cook, R. A. Chem.
Commun. 2002, 1610 - 1611; (b) Ranu, B. C.; Banerjee, S.
Org. Letts. 2005, 7, 3049-3052.

6
Tetrahedron Letters
HIGHLIGHTS
• Basic ionic liquid [BMIm]OH
catalyzed amidation of 3-oxo esters
were developed.
• [BMIm]OH acts as masking agent of
ketone carbonyl and activator of ester
carbonyl.
• Benzimidazole was formed via 1,5-
benzodiazepinone followed by C-C
bond cleavage.