A catalyst- and solvent-free protocol for the sustainable synthesis of fused 4-pyran derivatives.

An efficient and cost-effective method was developed for the synthesis of two kinds of fused 4-pyran derivatives, namely, dihydropyrano[2,3-]pyrazole 4 and pyrano[3,2-]chromenone 6. The reactions of 3-methyl-1-phenyl-5-pyrazolone/4-hydroxycoumarin with aromatic aldehydes and ()--methyl-1-(methylthio)-2-nitroethenamine (NMSM), involving the Knoevenagel, Michael-addition, -cyclization and elimination reactions under thermal heating, afforded the desired products. The synthesized compounds were characterized by standard spectroscopic techniques. Further, the structures of pyrazole-fused 4-pyran 4a and coumarin-fused 4-pyran 6b were confirmed by single-crystal XRD analysis. The short reaction time, good-to-excellent yields, elimination of the use of expensive, metallic and toxic catalysts or hazardous organic solvents and high atom-economy are some noteworthy features of this protocol.