L.-Q. Kang et al.
oxygen of aldehydes increasing the reactivity of the parent
carbonyl compounds. The group –OH prompts the active
methylene H of ethyl acetoacetate to leave. Many current
studies have established that hydrogen bonding can occur
between the reactant and the cationic or anionic compo-
nents of ILs [26, 27]. Based on this, the following
mechanism for the condensation reaction catalyzed by
ionic liquid 2-HEAA was postulated (Scheme 2).
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In conclusion, we have developed an efficient and clean
approach to the synthesis of 1,4-dihydropyridines using
2-HEAA ionic liquid as catalyst. The catalyst offers several
advantages including non-toxic, non-corrosive, high yield
of the products, simple experimental and isolation proce-
dures. And it follows along the line of green chemistry.
Also, the catalyst was able to be reused easily for three
times without apparent loss of activity.
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Experimental
Melting points were measured on a WRS-IB digital melt-
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500 MHz spectrometer with TMS as an internal standard.
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according to [28].
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General procedure for the synthesis of 4a–4k
Aldehyde (2 mmol), ethyl acetoacetate (4 mmol) or ace-
toacetanilide (4 mmol), NH4OAc (2 mmol), 2-HEAA ionic
liquid (0.1 mmol), and ethanol (5 cm3) were mixed and
stirred at reflux for a certain period of time to complete the
reaction (monitored by TLC). After completion of the
reaction, the mixture was poured into ice cold water. The
resulting solid crude product was filtered and then recrys-
tallized from ethanol–water to obtain pure product 1,4-
DHPs. All compounds were characterized by1H NMR.
Melting points are found to be almost identical with those
that reported in literature elsewhere.
Acknowledgments We are grateful to the Science and Technology
Foundation of Shanghai Institute of Technology for financial support.
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