ZnO nanoparticles as a highly efficient heterogeneous catalyst for the synthesis of various chromene and pyrano[4,3-b]pyran derivatives under solvent-free conditions

 Three- component reaction of 4-hydroxy-2H-pyran-2-one, aldehydes, and malononitrile or ethyl cyanoacetate in the presence of ZnO nanoparticles as the catalyst is described as an efficient and green synthetic method for obtaining of pyrano[4,3-b]pyran derivatives in excellent yield. These reactions were also tested with 2'-hydroxyacetophenone under similar conditions and produced chromene derivatives.