Lactic acid-catalyzed Eco-friendly Cyclization Reaction for the Synthesis of 4H-benzo[b]pyrans and 3,4- dihydropyrano[c]chromenes in EtOH/H2O as an Efficient Green Reaction Medium

An environmentally friendly, green, and highly efficient process for the synthesis of biologically and pharmacologically 4H-benzo[b]pyrans and 3,4-dihydropyrano[c]chromenes is developed by condensing cyclic aromatic aldehydes, malononitrile, and dimedone or 4‐hydroxycoumarin in the presence of lactic acid as an efficient and green catalyst. The corresponding products have been synthesized in 50–97% yield in middle condition reaction which confirmed the excellent catalytic activity of the lactic acid. Furthermore, this methodology shows unique advantages, such as operational simplicity, short reaction time, high yields, low cost, absence of any tedious workup or purification and avoidance of hazardous or toxic reagents/catalysts/solvents.