The diketopiperazine-fused tetrahydro-β-carboline scaffold as a model peptidomimetic with an unusual α-turn secondary structure.

Aiming at restricting the conformational freedom of tryptophan-containing peptide ligands, we designed a THBC (tetrahydro-β-carboline)-DKP (diketopiperazine)-based peptidomimetic scaffold capable of arranging in an unusual α-turn conformation. The synthesis is based on a diastereoselective Pictet-Spengler condensation to give the THBC core, followed by an intramolecular lactamization to complete the tetracyclic THBC-DKP fused ring system. The presence of conformers bearing the intramolecular thirteen-membered hydrogen bond that characterizes the α-turn structure is confirmed by (1)H NMR conformational studies. To the best of our knowledge, this scaffold represents one of the rare examples of a designed constrained α-turn mimic.