T3P-Mediated N–N Cyclization for the Synthesis of 1,2,4-Triazolo[1,5-a]pyridines

Propylphosphonic anhydride has been shown to be an effective reagent for the synthesis of substituted [1,2,4]triazolo[1,5-a]pyridines from the corresponding N′-hydroxy-N-formimidamides. The reactions worked under mild conditions and exhibited wide functional group tolerance, delivering the triazolopyridines in good to excellent yields and purities.