Synthesis of Quinuclidines by Intramolecular Silver-Catalysed Amine Additions to Alkynes

A new method has been developed for the synthesis of 2-​alkylidenequinuclidines based on a silver triflate catalyzed intramol. hydroamination of 4-​(prop-​2-​ynyl)​piperidines. Monosubstituted piperidines reacted less efficiently than cis-​disubstituted piperidines, and the reaction was selective for an alkyne moiety, even in the presence of a vinyl group at the 3-​position. The hydroamination occurred readily with a terminal alkyne, as well as with an internal alkyne bearing an aliph. or arom. group at the terminal carbon atom. Using this silver-​catalyzed cyclization, a short procedure was developed for the relay synthesis of the cinchona alkaloids dihydroquinidine and dihydroquinine.