Ultrasound-promoted, catalyst-free, greener synthesis of pyrazoles in aqueous medium

A rapid and environmentally benign method for the synthesis of pyrazole derivatives via cyclocondensation reaction between α-oxoketene S,S/N,S-acetals and hydrazine hydrate under ultrasound irradiation in an aqueous medium has been reported. This green and versatile synthetic protocol avoids the use of toxic organic solvents and additional catalysts/ligands, generates no waste, and affords twenty diversely substituted regioselective products with isolated yields ranging from 82 to 93%. The positive impact of sonication led to drastically reduced reaction times (15–45 min), as compared to known conventional methods (10–18 h), which clearly indicated the importance of ultrasound-promoted synthesis in organic transformations. Most of the synthesized compounds possessed –SCH3 as a good leaving group, which is readily accessible for further transformation to generate libraries of biologically important compounds.