Divergent Assembly of Functionalized Pyrazolo[1,5-a]pyridine Derivatives via [3+2] Cyclization of N-Aminopyridinium Salts with Various Unsaturated Hydrocarbons

Two reaction modes for metal-free [3 + 2] cyclization of N-aminopyridinium derivatives with β-alkoxyvinyl trifluoromethylketones have been described through selective C—O or C—O/C—C bond cleavage. This strategy can also be extended to the [3 + 2] cyclization of N-aminopyridinium derivatives with enaminones and bromoalkynes. A broad range of N-aminopyridinium, N-aminoquinolinium, and N-aminoisoquinolinium salts are well tolerated, enabling the divergent synthesis of trifluoroacylated, non-substituted, acylated, and brominated pyrazolo[1,5-a]pyridine derivatives (62 examples).