Synthesis of Pipecolic Acid-Based Spiro Bicyclic Lactam Scaffolds as β-Turn Mimics

β-Turn Induction by a Diastereopure Azepane-Derived Quaternary Amino Acid

β-Turns are one of the most common secondary structures found in proteins. In the interest of developing novel β-turn inducers, a diastereopure azepane-derived quaternary amino acid has been incorporated into a library of simplified tetrapeptide models in order to assess the effect of the azepane position and peptide sequence on the stabilization of β-turns. The conformational analysis of these peptides by molecular modeling, NMR spectroscopy, and X-ray crystallography showed that this azepane amino acid is an effective β-turn inducer when incorporated at the i + 1 position. Moreover, the analysis of the supramolecular self-assembly of one of the β-turn-containing peptide models in the solid state reveals that it forms a supramolecular helical arrangement while maintaining the β-turn structure. The results here presented provide the basis for the use of this azepane quaternary amino acid as a strong β-turn inducer in the search for novel peptide-based bioactive molecules, catalysts, and biomaterials.

Recent developments in the utility of saturated azaheterocycles in peptidomimetics

To a large extent, the physical and chemical properties of peptidomimetic molecules are dictated by the integrated heterocyclic scaffolds they contain. Heterocyclic moieties are introduced into a majority of peptide-mimicking molecules to modulate conformational flexibility, improve bioavailability, and fine-tune electronics, and in order to achieve potency similar to or better than that of the natural peptide ligand. This mini-review delineates recent developments, limited to the past five years, in the utility of selected saturated 3- to 6-membered heterocyclic moieties in peptidomimetic design. Also discussed is the chemistry involved in the synthesis of the azaheterocyclic scaffolds and the structural implications of the introduction of these azaheterocycles in peptide backbones as well as side chains of the peptide mimics.

Synthesis of enantiopure α-Tfm-proline and α-Tfm-pipecolic acid from oxazolo-pyrrolidines and -piperidines

Enantiopure α-Tfm-proline and α-Tfm-pipecolic acid were synthesized starting from commercially available diesters and ethyl trifluoroacetate. A Strecker type reaction on intermediate chiral Tfm-oxazolo-pyrrolidine and -piperidine provided the corresponding nitrile precursor of enantiopure (R) and (S) α-Tfm-proline and α-Tfm-pipecolic acid. The C-terminal peptide coupling reaction of α-Tfm-pipecolic acid has been successfully achieved.

Synthesis and evaluation of a (3R,6S,9S)-2-oxo-1-azabicyclo[4.3.0]nonane scaffold as a mimic of Xaa-trans-Pro in poly-l-proline type II helix conformation

Stereoselective synthesis of a (3R,6S,9S)-2-oxo-1-azabicyclo[4.3.0]nonane mimic of Xaa-trans-Pro in poly-l-proline type II helix conformation is reported.

Tetrahydro-β-carboline-based spirocyclic lactam as type II' β-turn: application to the synthesis and biological evaluation of somatostatine mimetics

The synthesis of novel spirocyclic lactams, embodying D-tryptophan (Trp) amino acid as the central core and acting as peptidomimetics, is presented. It relies on the strategic combination of Seebach's self-reproduction of chirality chemistry and Pictet-Spengler condensation as key steps. Investigation of the conformational behavior by molecular modeling, X-ray crystallography, and NMR and IR spectroscopies suggests very stable and highly predictable type II' β-turn conformations for all compounds. Relying on this feature, we also pursued their application to two potential mimetics of the hormone somatostatin, a pharmaceutically relevant natural peptide, which contains a Trp-based type II' β-turn pharmacophore.

Cytotoxicity of semisynthetic acetal triterpenes from one-pot vicinal diol cleavage following by lactolization: reaction promoted by NaIO4/SiO2 gel in THF

In situ C-C bond cleavage of vicinal diol following by the lactolisation resulted from separated treatment of Arjunolic acid (1), 24-hydroxytormentic acid (2) and 3-O-β-D-glucopyranosylsitosterol (3) with sodium periodate and silica gel in dried THF according to the strategic position of hydroxyl functions in the molecule. The reaction led to a lactol pentacyclic triterpenes 1A, 2A and a bicyclotriacetal of β-sitosterol 3A. These products were further acetylated and the cytotoxicity of all molecules was evaluated against human fibrosarcoma HT1080 cancer cells lines.

Trishomocubane amino acid as a beta-turn scaffold

The synthesis and X-ray structure of two diasteriomeric heptapeptides [Ac-Ala-Ala-Ala-(R/S)-Cage-Ala-Ala-Ala-NH2] with a trishomocubane amino acid as a beta-turn scaffold are reported. The amino acid was synthesized as a racemate and two diastereomeric peptides were obtained. The two peptides were separated by preparative high-pressure liquid chromatography and crystals suitable for X-ray analysis were grown for both diasteriomeric peptides. In general, both the peptides satisfy the criteria for beta-turn conformations. Five of the six Ala residues of both cage peptide crystals satisfy the criteria for 3(10)-helix characteristics and the cage amino acid residue satisfied the alpha-helix classification. These experimental results confirm previous theoretical studies in our laboratory which predicted that the cage moiety would be a strong/active beta-turn inducer.