Kinetic acidity of iminium ions. 2-Alkynyl- and 2,5-dialkynyl-pyrrolidines via the iminium ion route to azomethine ylides

Condensation of carboxaldehydes with appropriate secondary propargylamines furnishes iminium ions which undergo regioselective deprotonation at the propargylic methylene group to generate azomethine ylides. Interception of the latter by N-methyl or N-phenyl maleimide furnishes 2- and 2,5-dialkynyl-pyrrolidines.