Copper-Mediated Dimerization to Access 3a,3a′-Bis­pyrrolidino­indoline: Diastereoselective Synthesis of (+)-WIN 64821 and (−)-Ditrypto­phenaline

A copper-mediated cyclization and dimerization of tryptamine or tryptophan was developed to generate a C₂-symmetry C₃(sp³)–C₃(sp³) bridge with two contiguous stereogenic quaternary carbons in one step. Impressively, the ratio between exo and endo cyclization products varies when different protecting groups of Nb are utilized. This dimerization reaction could be conducted in gram scale. With this dimerization method, both endocyclo­tryptophan (+)-WIN 64821 and exocyclo­tryptophan (−)-ditrypto­phenaline were synthesized in 5 steps.