Domino Approach for the Synthesis of Pyridinium Salts and 1,2,3,8a-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-ß-enaminoester fragment, easily undergo a domino reaction to form pyridinium salts with ß-(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 1,2,3,8a-tetrahydroimidazo[1,2-a]pyridines. Reaction of the latter compounds with acid chlorides affords pyridinium salts with ß-(alkylamido)ethyl moiety at the nitrogen atom.