Catalysis with phosphine-containing amino acids in various "turn" motifs

We have been actively involved in the development of parallel approaches for the discovery of phosphine ligands. Our approach has been based on the incorporation of phosphine-containing amino acids into peptide sequences that are designed to have stable secondary structures. We have examined helical and turn secondary structures and have reported that alkylation of cyclopentenyl acetate with dimethylmalonate can be catalyzed in high enantiomeric excess (ee) with a beta-turn-based ligand. The importance of the peptide secondary structure was demonstrated through the synthesis of a series of peptide ligands where the nature of the turn-forming residues was probed. Additionally, other turn-forming units and a variety of different phosphine-containing amino acids have been examined for their ability to control the selectivity of the allylation reaction. This paper reports the results obtained through the examination of different turn motifs as well as different phosphine substitutions on the "best" turn sequence, Pps-Pro-D-Xxx-Pps.