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Scheme 2. Thus, [Bmim]OH promotes all the steps, that is, Knoeve-
nagel condensation and dehydration steps by acting as a strong
base and the ring transformation step owing to the acidic character
of C-2 proton of the imidazolium cation (Scheme 2). The Knoevena-
gel condensation has also been previously studied for its
‘greenness’,32 The reactions were clean and all the synthesized
products were characterized by their 1H NMR, 13C NMR, IR and
mass spectroscopic data.
In summary, we have documented a one-pot high yielding syn-
thetic protocol for 3-benzamidocoumarins using the environmen-
tally benign catalyst [Bmim]OH. In the present investigation
there is no by-product formation and also the ionic liquid [Bmi-
m]OH used could be easily recycled for further use without any
loss of efficiency. Thus, this simple methodology would be a prac-
tical alternative to the existing procedures for the production of
this kind of fine chemical to cater to the need of academic institu-
tions as well as of industries.
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Acknowledgment
We sincerely thank SAIF, Punjab University, Chandigarh, for
providing microanalyses and spectra.
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29. General procedure for the synthesis of 3-benzamidocoumarins 3: A mixture of 2-
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2 (2.0 mmol), salicylaldehyde 1 (2.0 mmol) and
[Bmim]OH (0.4 mmol) in 10 mL of CH3CN was stirred at room temperature
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.
TMS) d 6.91 (s, 1H, 4-H, 7.03–7.65 (m, 6Harom), 7.81–8.08 (m, 2Harom), 8.66 (br
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corresponding compound 3a: The procedure followed was the same as described
above for the synthesis of 3 (Ref. 29) except that the time of stirring in this case
was 8 h instead of 12 h for 3a. The adduct 5a was purified by silica gel column
chromatography (hexane–EtOAc, 3:1) to obtain an analytically pure sample of
5a in 49% yield. A mixture of the intermediate compound 5a (2.0 mmol) and
[Bmim]OH (0.4 mmol) in 10 mL of CH3CN was stirred at rt for 4 h to give the
corresponding product 3a quantitatively and was isolated and purified in the
same way as described above for 3a. The ionic liquid [Bmim]OH was also
recovered by following the same procedure as described in Ref. 29. Physical
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.
(400 MHz; CDCl3/TMS) d 6.27 (br s, 1H, OH, exchangeable with D2O), 6.89–7.85
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