Diastereoselective Synthesis of 4,5'-Bis-proline Compounds via Reductive Dimerization of N-Acyloxyiminium Ions

A short, practical synthesis of novel, unsymmetrical 4,5'- bis-proline compounds has been achieved, highlighted by the application of an unprecedented samarium diiodide-driven regio- and diastereocontrolled reductive dimerization of N-acyloxyiminium ions generated from readily available 2-methoxy-5-silyloxymethyl-N-Boc pyrrolidines. The title proline dimers proved to be pertinent metal-free catalysts in aldol and Mannich reactions.