Copper(I)-Catalyzed Synthesis of 4,5-Dihydropyrazolo[1,5-a]pyrimidines via Cascade Transformation of N-Propargylic Sulfonylhydrazones with Sulfonyl Azides

A rapid and effective approach to 4-tosyl-4,5-dihydropyrazolo[1,5-a]pyrimidines via copper-catalyzed one-pot cascade transformation of N-propargylic sulfonylhydrazones with sulfonyl azides under mild conditions has been developed. This process relies on the copper-catalyzed azide-alkyne cycloaddition (CuAAC) and intramolecular [4+2] cycloaddition of α,β-unsaturated hydrazone with the in situ generated ketenimine intermediate, affording the useful bicyclization products with high efficiency.