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 fou r enantiopure, sterically constrained, C-alpha-tetra-substituted alpha -ami no acids, all characterized by a sidechain (CCdelta)-C-gamma double bond. B y using one of them (L-Mag), we prepared an N-alpha-protected tetrapeptide benzylamide which was shown to adopt a beta -turn conformation and to effic iently undergo ring-closing olefin metathesis. (C) 2001 Elsevier Science Le d. All rights reserved.