β-Hydroxynorbornane amino acid derivatives: valuable synthons for the diastereoselective preparation of substituted cyclopentylglycine derivatives

Self-assembled hydrophobic Ala-Aib peptide encapsulating curcumin: a convenient system for water insoluble drugs

The exploitation of self-assembled systems to improve the solubility of drugs is getting more and more attention. Among the different types of self-assembled biomaterials, peptides and in particular peptides containing non-coded amino acids (NCAPs) are promising because their use opens the door to more stable materials inducing increased stability to proteolysis. New classes of NCAP, Ac-Ala-X-Ala-Aib-AlaCONH2 (X = alpha-aminoisobutyric acid (Aib) or X = cyclopentane amino acid (Ac5c)) have been prepared and the correlation between the different secondary peptide structure and solvent (i.e. CD3CN, CD3OH, H2O/D2O) verified by NMR. Furthermore, the formation of a nanocolloidal system in water was deeply studied by DLS and the morphology of the obtained spherical aggregates with nanometric dimensions was assessed by TEM. Aib containing pentapeptide was selected for greater ease of synthesis. Its ability to encapsulate curcumin, as a model insoluble drug molecule, was investigated using fluorescence emission and confocal microscopy analyses. Two different approaches were used to study the interaction between curcumin and peptide aggregates. In the first approach peptide aggregates were formed in the presence of curcumin, while in the second approach curcumin was added to the already formed peptide aggregates. We succeeded in our challenge by using the second approach and 53.8% of added curcumin had been encapsulated.

Fluoro-Aryl Substituted α,β2,3-Peptides in the Development of Foldameric Antiparallel β-Sheets: A Conformational Study

α,β2,3-Disteroisomeric foldamers of general formula Boc(S-Ala-β-2R,3R-Fpg)nOMe or Boc(S-Ala-β-2S,3S-Fpg)nOMe were prepared from both enantiomers of syn H-2-(2-F-Phe)-h-PheGly-OH (named β-Fpg) and S-alanine. Our peptides show two appealing features for biomedical applications: the presence of fluorine, attractive for non-covalent interactions, and aryl groups, crucial for π-stacking. A conformational study was performed, using IR, NMR and computational studies of diastereoisomeric tetra- and hexapeptides containing the β2,3-amino acid in the R,R- and S,S-stereochemistry, respectively. We found that the stability of peptide conformation is dependent on the stereochemistry of the β-amino acid. Combining S-Ala with β-2R,3R-Fpg, a stable extended β-strand conformation was obtained. Furthermore, β-2R,3R-Fpg containing hexapeptide self-assembles to form antiparallel β-sheet structure stabilized by intermolecular H-bonds and π,π-interactions. These features make peptides containing the β2,3-fluoro amino acid very appealing for the development of bioactive proteolytically stable foldameric β-sheets as modulators of protein-protein interaction (PPI).

From glucose to enantiopure morpholino β-amino acid: a new tool for stabilizing γ-turns in peptides

“Environmentally sustainable” synthesis of a new enantiopure morpholino β-amino acid from glucose: a new tool for exotic peptide architectures.

Synthesis of Enantiopure Constrained α,β-Cycloaliphatic Cystines via Diels-Alder Reaction with Homochiral Thiazolines

The behavior of homochiral 2,3-dihydrothiazoles, easily available from l-cysteine in Diels-Alder reaction with different dienes, "en route" to sterically constrained modified cystines, has been studied. The oxidation level of the sulfur atom of the heterocyclic ring was crucial for the course of the reaction. Whereas 2,3-dihydrothiazoles did not lead to Diels-Alder adducts, 1-oxide and 1,1-dioxide derivatives afforded the exo adduct enantiopurely in high yields and diastereoselectivities. Further elaboration of the resulting adducts provided conformationally restricted quaternary cystines. DFT calculations correctly predict both the reactivity and stereoselectivity observed experimentally.

Tandem Tetrahydroisoquinoline-4-carboxylic Acid/β-Alanine as a New Construct Able To Induce a Flexible Turn

Tetrahydroisoquinoline-4-carboxylic acid, a constrained β² -amino acid named β-TIC, was synthesised for the first time in enantiopure form. The biocatalytic route applied herein represents one of the few successful examples of enzymatic resolution of β² -amino acids. Model tetrapeptides, namely, Fmoc-l-Ala-β-TIC-β-Ala-l-Val-OBn (Fmoc=fluorenylmethyloxycarbonyl, Bn=benzyl), containing both isomers of β-TIC, were prepared. Both computational and NMR spectroscopy studies were performed. A reverse-turn conformation was observed in the case of (R)-β-TIC enantiomer that was obtained in 99 % enantiomeric excess by enzymatic resolution. The β-TIC/β-Ala construct represents the first example of a flexible turn mimetic containing a cyclic and an acyclic β-amino acid. Furthermore, the presence of an aromatic ring of β-TIC could facilitate non-covalent interactions to increase the potential of this scaffold for the preparation of protein-protein interaction modulators.

Mechanism of stabilization of helix secondary structure by constrained Cα-tetrasubstituted α-amino acids

The theoretical basis behind the ability of constrained Cα-tetrasubstituted amino acids (CTAAs) to induce stable helical conformations has been studied through Replica Exchange Molecular Dynamics Potential of Mean Force Quantum Theory of Atoms In Molecules calculations on Ac-l-Ala-CTAA-l-Ala-Aib-l-Ala-NHMe peptide models. We found that the origin of helix stabilization by CTAAs can be ascribed to at least two complementary mechanisms limiting the backbone conformational freedom: steric hindrance predominantly in the (+x,+y,-z) sector of a right-handed 3D Cartesian space, where the z axis coincides with the helical axis and the Cα of the CTAA lies on the +y axis (0,+y,0), and the establishment of additional and relatively strong C-H···O interactions involving the CTAA.

Model peptides containing the 3-sulfanyl-norbornene amino acid, a conformationally constrained cysteine analogue effective inducer of 310-helix secondary structures

Two model peptides containing the 3-benzylsulfanylnorbornene amino acid (NRB) was prepared. Theoretical calculations, spectroscopic and X-ray analyses confirmed that both NRB enantiomers possess a strong right-handed helicogenic effect.

Strain-promoted retro-Dieckmann-type condensation on [2.2.2]- and [2.2.1]bicyclic systems: a fragmentation reaction for the preparation of functionalized heterocycles and carbocycles

The fragmentation reaction of differently functionalized [2.2.2]- and [2.2.1]bicyclic systems that leads to substituted five membered heterocycles and five/six membered carbocycles is broadly studied. This reaction is carried out through a retro-Dieckmann-type condensation on strained [2.2.1]bicyclic β-ketosulfones and their counterparts β-ketoesters under very mild catalytic acid or basic conditions and short reaction times. The same reaction is also achieved on [2.2.2]bicyclic β-ketosulfones requiring harsher reaction conditions.

1H-Azepine-4-amino-4-carboxylic acid: a new α,α-disubstituted ornithine analogue capable of inducing helix conformations in short Ala-Aib pentapeptides

A very efficient synthesis of orthogonally protected 1H-azepine-4-amino-4-carboxylic acid, abbreviated as Azn, a conformationally restricted analogue of ornithine, was realized. It was obtained on a gram scale in good overall yield in five steps, three of which did not require isolation of the intermediates, starting from the readily available 1-amino-4-oxo-cyclohexane-4-carboxylic acid. Both enantiomers were used for the preparation of pentapeptide models containing Ala, Aib, and Azn. Conformational studies using both spectroscopic techniques (NMR, CD) and molecular dynamics on model 5-mer peptides showed that the (R)-Azn isomer possesses a marked helicogenic effect.

Exploiting the ring strain in bicyclo[2.2.1]heptane systems for the stereoselective preparation of highly functionalized cyclopentene, dihydrofuran, pyrroline, and pyrrolidine scaffolds

The high strain of bicyclic systems drives retro-condensation reactions on bridgehead substituted bicyclo[2.2.1]hept-2-enes giving rise to orthogonally functionalized cyclopentene, 2,5-dihydrofuran, and 3-pyrroline scaffolds. Retro-Dieckman reactions were easily carried out on 3-tosyl-(7-carba/7-oxa/7-aza)bicyclo[2.2.1]hept-5-en-2-ones. Retro-aldol reactions of N-Boc-3-tosyl-7-azabicyclo[2.2.1]hept-5-en-2-ol and functionalized N-Boc-3-tosyl-7-azabicyclo[2.2.1]heptan-2-ols yield functionalized pyrrolidine scaffolds stereoselectively. The same reaction does not work with corresponding norbornene and 7-oxanorbornene derivatives.

Recent Progress on the Stereoselective Synthesis of Cyclic Quaternary alpha-Amino Acids

The most recent papers describing the stereoselective synthesis of cyclic quaternary alpha-amino acids are collected in this review. The diverse synthetic approaches are classified according to the size of the ring and taking into account the bond that is formed to complete the quaternary skeleton.