Enantiopure C-alpha-tetrasubstituted alpha-amino acids. Chemoenzymatic synthesis and application to turn-forming peptides

By a chemo-enzymatic approach we carried out a large-scale synthesis of four enantiopure, sterically constrained, Cα-tetrasubstituted α-amino acids, all characterized by a sidechain CγCδ double bond. By using one of them (l-Mag), we prepared an Nα-protected tetrapeptide benzylamide which was shown to adopt a β-turn conformation and to efficiently undergo ring-closing olefin metathesis.