Domino Approach for the Synthesis of Pyridinium Salts and 1,2,3,8 a -Tetrahydroimidazo[1,2- a ]pyridines from 2-Imidazolines and Propiolic Acid Esters.

Adducts of 1-alkyl-2-imidazolines and two molecules of alkyl propiolate, possessing an N -propargyl-β-enaminoester fragment, easily undergo a domino reaction to form pyridinium salts with β-(alkylammonio)ethyl group at the nitrogen atom in the presence of 2 equiv of a protic acid. Treatment of the above reaction mixture with a base gives 1,2,3,8 a -tetrahydroimidazo[1,2- a ]pyridines. Reaction of the latter compounds with acid chlorides affords pyridinium salts with β-(alkylamido)ethyl moiety at the nitrogen atom.