Synthesis and conformational studies of peptidomimetics containing a new bifunctional diketopiperazine scaffold acting as a β-hairpin inducer

A practical synthesis of a new bifunctional diketopiperazine (DKP) scaffold 1, formally derived from the cyclization of L-aspartic acid and (S)-2,3-diaminopropionic acid, is reported. DKP-1 bears a carboxylic acid and an amino functionalities in a cis relationship, which have been used to grow peptide sequences. Tetra-, penta-, and hexapeptidomimetic sequences were prepared by solution-phase peptide synthesis (Boc strategy). Conformational analysis of these derivatives was carried out by a combination of H-1 NMR spectroscopy, IR spectroscopy, CD spectroscopy, and computer modeling, and reveals the formation of P-hairpin mimics involving 10-membered and 18-membered H-bonded rings and a reverse turn of the growing peptide chain.