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 pyrazolo[5,1-b]purine from the annulation of 6-bromo-3-cyano-2-(ethylthio)-5-methyl-7-oxo-6,7-dihydropyrazolo[1,5-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 D-2-NOESY NMR spectroscopy, as well.