Background:
Chromenes and pyran annulated heterocycles are the most common frameworks
existing in various biologically active molecules. Due to their beneficial and biological properties, they are eyecatching
synthetic targets in the arsenal of organic chemistry. Thus, finding green and efficient methods, as
well as searching for a new catalyst for the synthesis of these heterocycles is of interest to organic chemistry
researchers.
Objective:
Sodium malonate as a readily available catalyst was employed aimed at the multicomponent
synthesis of numerous 2-amino-4H-chromenes and 2-amino-4H-pyrans in water as a green medium reaction.
Methods:
The reaction was performed via treatment of aldehydes (1 mmol) with malononitrile/ethyl
cyanoacetate (1 mmol) and β-dicarbonyl compounds (1 mmol)/or resorcinol (1 mmol) in water (4 mL) in the
presence of sodium malonate (10 mol %) at 70°C. On completion of the reaction (monitored by TLC analysis),
the reaction mixture was gradually cooled at room temperature, and the resulting precipitates were collected by
filtration, washed with cold ethanol and air-dried to give the corresponding pure products. The solvent was
evaporated from the filtrate to recover the catalyst, and the catalyst was reused for subsequent reactions.
Results:
In the initial stages, we explored the best reaction conditions using three-component reaction of
benzaldehyde, malononitrile, and dimedone as the model reaction. The effects of catalyst loading, temperature,
and solvents were explored for this reaction. It was found that the best results were obtained for the synthesis
of 2-amino-4H-chromenes and 2-amino-4H-pyran when the three-component reaction was carried out with
equivalent molar quantities of each of the reactants in water containing 10 mol% sodium malonate at 70°C for
15 min in 96% yield. After finding optimal conditions, these conditions apply to other reactants and the target
heterocyclic products were obtained with excellent yields.
Conclusion:
This study describes an efficient, environmentally benign, and clean one-pot, three-component
synthesis of 2-amino-4H-chromenes and 2-amino-4H-pyran-3-carboxylates in the presence of sodium malonate
as the commercially available catalyst in an aqueous medium at 70°C. High yields, mild reaction conditions,
relatively shorter reaction times, use of simple reagents, and no requirement of the ultrasound, microwave, and
ball milling techniques are the salient features and benefits of the present method. In addition, the present
process is smooth and green.
Heterogeneous acidic catalysts for the tetrahydropyranylation of alcohols and phenols in green ethereal solvents