An efficient ultrasound-assisted one-pot chemoselective synthesis of pyrazolo[3,4-b] pyridine-5-carbonitriles in aqueous medium using NaCl as a catalyst†
A simple, convenient and green synthetic protocol for the chemoselective synthesis of pyrazolo[3,4-b]pyridine derivatives via three-component reaction of 3-amino-5-methylpyrazole, ethyl cyanoacetate, and aldehydes catalyzed by sodium chloride under ultrasound irradiation in aqueous medium is described. The method showed remarkable selectivity for pyrazolo[3,4-b]pyridine over dehydrogenated pyrazolopyridines, bis-pyrazolopyridines and pyrazolo[1,5-a]pyrimidine derivatives. The reactions were carried out under both conventional and ultrasonic irradiation conditions. Under ultrasound radiation, the catalytic activity of NaCl was about 15-fold higher as compared to the conventional method. The sonochemical procedure offers several advantages including cleaner reaction profile, use of easily available, cheap and environmentally benign catalyst, high yields, and simple experimental and work-up procedures.