An Alternative Regioselective Approach for the Synthesis of Highly Functionalized Derivatives of Pyrazolo[5,1‐b]purine Scaffold

A straightforward approach for the regioselective synthesis of highly functionalized pyrazolo5,1‐b]purine from the annulation of 6‐bromo‐3‐cyano‐2‐(ethylthio)‐5‐methyl‐7‐oxo‐6,7‐dihydropyrazolo1,5‐a]pyrimidine with thiourea as a bisnucleophilic reagent under reflux condition in CH₃CN in the presence of Et₃N has been developed. The N‐alkylation of the synthesis compound was also accomplished. The true regioisomer was determined by ²D‐NOESY NMR spectroscopy, as well.