Synthesis and conformational preferences of unnatural tetrapeptides containing l-valine units

The stereoselective synthesis of non-proteinogenic tetrapeptides 7, 16 and 17 containing two l-valine units and two modified α-amino acids (proline and aspartic acid) has been accomplished starting from the l-valine derived chiral synthon 1. Investigations of the conformational preference and structure of these unnatural peptides were carried out using 1H NMR and IR spectroscopic techniques and a conformational analysis based on quenched molecular dynamics (QMD).