Introducing <i>N</i>-Carboxyethyl-Ynamides, Precursors of Metalated Ketenimines: Protecting Group Swap in Ynamides and Olefination of Aldehydes

<i>N</i>-Carboxyethyl-ynamides are shown to be useful and versatile precursors of metalated ketenimines. Upon simple treatment with a strong base, a retro-Michael addition indeed affords these highly reactive ambident electrophilic intermediates that can hardly be generated otherwise. Their reaction with chloroformates, acyl chlorides, chlorophosphates, and chlorosilanes regioselectively occurs at the nitrogen atom and provides a useful entry to novel ynamides possessing two electron-withdrawing groups on the nitrogen atom by an unprecedented protecting group swap. In sharp contrast, their condensation with aldehydes exclusively takes place on the distal carbon atom, resulting in an efficient and stereoselective olefination.