Transition metal- and oxidant-free [3 + 2] cyclization of azomethine imines utilizing vinylene carbonate as dual synthons

A transition metal- and oxidant-free C–C/C–N annulation of azomethine imines with vinylene carbonate as dual synthons under simple reaction conditions is described herein. Depending on the structure of azinium-N-imines, vinylene carbonate could serve as an ethynol and acetylene surrogate, which results in the formation of pyrazolo[1,5-a]pyridine derivatives and pyrazolo[1,5-a]pyridin-2-ol derivatives through [3 + 2] cyclization. Importantly, this mild system tolerates a diverse array of heterocyclic N-imines such as quinolinium and isoquinolinium salts with good functional group compatibility.