Caffeine-catalyzed green synthesis of biscoumarins

Abstract

Biscoumarin derivatives are an important group of bis-heterocyclic compounds that have attracted much attention due to their useful biological and medicinal properties. Caffeine was extracted from black tea and characterized using infrared (IR), ¹H, and ¹³C nuclear magnetic resonance (NMR) spectra. The extracted caffeine was used as an efficient, useful, and environmentally friendly organo-catalyst to synthesize biscoumarins. Optimizing the reaction conditions showed that 10 mol% of catalyst, water, and 80 °C is enough to synthesize the desired bis-heterocyclic products. Under the optimized reaction conditions, the pseudo-three component reaction of aryl/hetaryl aldehydes with 4-hydroxycoumarin was used to synthesize 3,3′-(arylmethylene)-bis(4-hydroxycoumarins. The 3,3′-(arylmethylene)-bis(4-hydroxycoumarin)s have been synthesized in good to excellent reaction yields and relatively shorter reaction times. The synthesis may proceed via a Knoevenagel condensation, Michael addition, and tautomerization. Some of the advantages of this method include the ease of separation of products, utilizing available starting materials, and the relatively simple implementation of the reaction. This work is also important from the point of view of green chemistry and economic savings. Furthermore, the method works well for substituted benzaldehydes containing electron-donating and electron-withdrawing groups and is compatible with heteroaryl aldehydes such as thiophene-2-carboxaldehyde. The structure of the synthesized biscoumarins was characterized using melting point measurements and spectroscopic data.