Green and efficient synthesis of pyranopyrazoles using [bmim][OH−] as an ionic liquid catalyst in water under microwave irradiation and investigation of their antioxidant activity

Abstract

An imidazole-based ionic liquid, 1-butyl-3-methylimidazolium hydroxide ([bmim][OH⁻]), was utilized as a novel heterogeneous catalyst for the synthesis of pyranopyrazoles. The target pyranopyrazole compounds were isolated, purified and fully characterized. This organo-solid catalyst was used as a novel, mild and fully developed catalysts for the cascade synthesis of colorful pyrano[2,3-c]-pyrazoles via the three component reaction of various benzaldehydes, pyrazolone and malononitrile in the presence of the aforementioned catalyst in a microwave oven at a power of 230 W. Here, we attempt to present a simple and reliable method for the preparation of this type of compound that is environmentally friendly and operationally simple with high yields, short reaction times and an efficient workup method. The antioxidant activity of the products was evaluated using a 2,2-azinobis(3-ethylbenzothiazoline-sulfonate) (ABTS) assay. The antioxidant activity of the synthesized products was compared with both vitamin E and ascorbic acid. The products showed a higher antioxidant activity using this method. Products 4o, 4n, 4a, 4e and 4h were much more active than ascorbic acid and products 4o, 4n and 4a were much more active than vitamin E in the ABTS assay. The high antioxidant activity of product 4o could be due to the longer conjugated system which can stabilize the free radical by resonance through a longer system.