Enantiopure alkaloid analogues and iminosugars from proline derivatives: stereocontrol in sequential processes

An alternative to the use of expensive auxiliaries or catalysts in the synthesis of chiral 2-substituted pyrrolidines is described. Thus, commercial, cheap 4-(S)-hydroxyproline was readily transformed into optically pure pyrrolidines, using a one-pot decarboxylation–alkylation reaction as the key step. In this reaction, an acyliminium intermediate was generated, which was trapped by carbon nucleophiles. As postulated by Woerpel, the addition of the nucleophiles to the five-membered ring iminium ions took place stereoselectively, affording 2,4-cis-disubstituted pyrrolidines in high de. The hydroxy group at C-4 can then be removed, or alternatively, it can be used to create new functionalities in the molecule. In this way, optically pure alkaloid analogues and iminosugars were prepared.