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.