An expedient four-component domino protocol for the regioselective synthesis of highly functionalized pyranopyrazoles and chromenopyrazoles via nitroketene-N,S-acetal chemistry under solvent-free condition

Combinatorial library of pyranopyrazoles and chromenopyrazoles was regioselectively synthesized in excellent yield from ethylacetoacetate, hydrazinehydrate, substituted aldehyde/salicylaldehyde with nitroketene-N,S-acetal in the presence of piperidine under solvent-free condition (SFC) in an eco-benign manner. This novel strategy excludes tedious extraction, chromatographic separation and recrystallization processes. The final product could be obtained by simple filtration by the addition of ethanol to the reaction mixture. This domino protocol generates biologically significant heterocycles with the formation of CC, CC, CN, CN, CO bonds and one stereo-centre in a single operation via condensation/Knoevenagel/Michael/annulation sequences.