Nano-titania sulfuric acid-promoted synthesis of tetrahydrobenzo[b]pyran and 1,4-dihydropyrano[2,3-c]pyrazole derivatives under ultrasound irradiation

An efficient and convenient procedure has been described for one-pot synthesis of tetrahydrobenzo[b]pyran and 1,4-dihydropyrano[2,3-c]pyrazole derivatives in the presence of nano-titania sulfuric acid (15-nm TSA) as a heterogeneous catalyst. The catalyst is easily prepared from the reaction of nano-titania and chlorosulfonic acid in dichloromethane as solvent. The transmission electron microscopy image of the catalyst showed no detectable changes on the size of the nano-catalyst as a result of chemisorptions of the sulfuric acid. To examine the effect of minimizing the size of TSA particles, the pH analysis of the catalyst with different particle sizes was carried out. The catalyst can be easily recovered and reused for several successive fresh runs without significant loss of activity. Under ultrasound irradiation condition, the reactions proceed smoothly to afford the corresponding products in quantitative yields.