Imidazole/Cyanuric Acid as an Efficient Catalyst for the Synthesis of 2-Amino-4H-chromenes in Aqueous Media at Ambient Temperature

Background: 2-Amino-4H-chromenes are an important class of heterocyclic compounds having important biological activities. Many of the methods reported for the synthesis of these compounds are associated with the use of hazardous organic solvents, long reaction time, use of toxic amine-based catalysts, and lack of general applicability. Thus, the development of an inexpensive, mild, general, environmentally and commercially available catalyst for MCRs remains an issue of interest. In continuation of our work on one-pot multicomponent reactions catalyzed by organocatalysts for the synthesis of various heterocyclic compounds, here we report a general and highly efficient one-pot multicomponent reactions using an inexpensive, clean, safe, environmentally and commercially available imidazole/cyanuric acid as a mixture of organic and inorganic catalyst in aqueous media. Methods: 15 mol % of catalyst imidazole/cyanuric acid (3:1 mol %), dissolved in water (2 mL) at room temperature by magnetic stirring. The mixture of aromatic aldehyde (1 mmol), malononitrile (1 mmol, 0.066 mg) and resorcinol (1 mmol, 0.110 mg) or dimedone (1 mmol, 0.140 mg) was added, after completion of the reaction which was monitored by TLC, the precipitate was washed with distilled water and dried, then was crystallized in ethanol 96%. Results: The preparation of 2-amino-4-(4-chlorophenyl)-7-hydroxy-4H-chromene-3-carbonitrile was studied at different amounts of imidazole and cyanuric acid as catalyst. A longer reaction time would be necessary and lower yield was obtained when used Cyanuric acid or Imidazole alone and the best result was obtained by using 15 mol% of imidazole and cyanuric acid (3: 1) in aqueous media at room temperature under the optimized reaction conditions, a series of substituted 2-amino-4H-chromene derivatives were synthesized. The reactions worked well with a variety of aryl aldehydes including those bearing electron-withdrawing and electron-donating groups and the desired compounds were obtained in high to excellent yields. In all of the reactions, yields were dependent on substituents of aldehyde and their positions and enolizable C-H acid. Reaction for benzaldehyde with electron-withdrawing groups such as chloro-or nitro-, proceeded at faster rates than those with electron-donating groups such as methoxy-, methyl-, etc. Conclusion: We have demonstrated a simple and efficient protocol for the synthesis of 2-amino-4H-chromenes by the reaction between various benzaldehydes, malononitrile and enolizable C-H acidic compounds (resorcinol and dimedon), using imidazole/cyanuric acid as an efficient catalyst in aqueous media at room temperature. Compared with some other reported methods, the main advantages of this procedure are shorter reaction times, high to excellent yields, inexpensive operation, simple work-up and environment friendliness.