p-TSA catalyzed 6-endo-trig/dig cyclization of 5-aminopyrazoles and 3°/2°-propargylic alcohols: access to pyrazolo[1,5-a]dihydropyrimidines and pyrazolo[3,4-b]pyridines†

A facile, straightforward synthesis of fused pyrazolo[1,5-a]dihydropyrimidines and pyrazolo[3,4-b]pyridines is accomplished by using 5-aminopyrazoles, 3°/2°-propargylic alcohols and ynones in the presence of p-TSA. The reaction proceeds through allenylation (N-alkylation)/propargylation (C-alkylation) of 5-aminopyrazoles, followed by intramolecular 6-endo-trig/dig cyclization leading to the title products with the formation of new C–N and C–C bonds. Operationally simple reaction conditions, inexpensive reagents, better yields, and gram-scale synthesis are the advantages of this protocol.