Facile One-Pot Synthesis of Polysubstituted Pyridinium Salts by Annulation of Enamines with Alkynes

An efficient and facile synthesis of highly substituted pyridinium salts through the annulation of enamines with alkynes is reported herein. A Ag₂ CO₃ /HNTf₂ synergistically acting catalyst system was developed and used in a condensation reaction between carbonyl substrates and propargylamine to afford structurally diverse pyridinium salts. A mechanistic investigation shows that this one-pot transformation proceeded via selective 6-endo-dig cyclization of the in situ generated propargylenamine and protonolysis of the resulting vinyl-silver intermediate. The reaction conditions are mild, and the substrate scope is broad. During the cyclization, an unusual inversion of the normal reactivity of α,β-unsaturated carbonyl systems was observed.