Synthesis of cis-2,5-disubstituted pyrrolidines via diastereoselective reduction of N-acyl iminium ions

A new procedure for forming cis-2,5-disubstituted pyrrolidines having unsaturated side chains has been developed that features the diastereoselective reduction of N-acyl iminium ions, which were formed in situ by acid-catalyzed cyclizations of unsaturated γ-keto carbamates, with triphenylsilane. The sequence was applied to a very concise synthesis of 16, a subunit in the nonpeptide cholecystokinin antagonist (+)-RP-66803.