Oxidative Annulation of Aldehydes, 5-Aminopyrazoles, and Nitriles: Synthesis and Applications of Pyrazolo[3,4-d]Pyrimidines

The oxidative [3+2+1] annulation reaction of aldehydes, 5-aminopyrazoles, and nitriles is achieved under the oxidation of I-2/O-2, affording various valuable pyrazolo[3,4-d]pyrimidines in 27%-91% yields. The salient feature of this reaction is transition-metal free, operationally simple, broad substrate scope, good functional group tolerance, and gram-scalable. Remarkably, the corresponding products exhibited intriguing photophysical properties such as Aggregation-Induced Emission (AIE) and intramolecular charge transfer (ICT) and showed good potential to specifically light up lipid droplets (LDs) in living cells with bright fluorescence, low cytotoxicity, and good photostability.