Stereoselectivity of Proline/Cyclobutane Amino Acid-Containing Peptide Organocatalysts for Asymmetric Aldol Additions: A Rationale

Several α,β,α- or α,γ,α-tripeptides, consisting of a central cyclobutane β- or γ-amino acid being flanked by two d- or l-proline residues, have been synthesized and tested as organocatalysts in asymmetric aldol additions. High yields and enantioselectivities have been achieved with α,γ,α-tripeptides, being superior to peptides containing a cyclobutane β-amino acid residue. This is probably due to their high rigidity, which hinders some of the peptide catalysts to adopt the proper active conformation. This reasoning correlates with the major conformation of the peptides in the ground state, as suggested by ¹H NMR and computational calculations. The configuration of the aldol products is controlled by the proline chirality, and consequently, the R/S configuration of aldol products can be tuned by the use of either commercially available d- or l-proline. The enantioselectivity in the aldol reactions is reversed if the reactions are carried out in the presence of water or other protic solvents such as methanol. Spectroscopic and theoretical investigations revealed that this effect is not the consequence of conformational changes in the catalyst but rather caused by the participation of a water molecule in the rate determining transition state, in such a way that the preferential nucleophilic attack is oriented to the opposite enantiotopic aldehyde face.

Chemoselective, Substrate-directed Fluorination of Functionalized Cyclopentane β-Amino Acids

This work describes a substrate-directed fluorination of some highly functionalized cyclopentane derivatives. The cyclic products incorporating CH₂ F or CHF₂ moieties in their structure have been synthesized from diexo- or diendo-norbornene β-amino acids following a stereocontrolled strategy. The synthetic study was based on an oxidative transformation of the ring carbon-carbon double bond of the norbornene β-amino acids, followed by transformation of the resulted "all cis" and "trans" diformyl intermediates by fluorination with "chemodifferentiation".

Stereo- and Regiocontrolled Syntheses of Exomethylenic Cyclohexane β-Amino Acid Derivatives

Cyclohexane analogues of the antifungal icofungipen [(1R,2S)-2-amino-4-methylenecyclopentanecarboxylic acid] were selectively synthesized from unsaturated bicyclic β-lactams by transformation of the ring olefinic bond through three different regio- and stereocontrolled hydroxylation techniques, followed by hydroxy group oxidation and oxo-methylene interconversion with a phosphorane. Starting from an enantiomerically pure bicyclic β-lactam obtained by enzymatic resolution of the racemic compound, an enantiodivergent procedure led to the preparation of both dextro- and levorotatory cyclohexane analogues of icofungipen.

Efficient regio- and stereoselective access to novel fluorinated β-aminocyclohexanecarboxylates

A regio- and stereoselective method has been developed for the synthesis of novel fluorinated 2-aminocyclohexanecarboxylic acid derivatives with the fluorine attached to position 4 of the ring. The synthesis starts from either cis- or trans-β-aminocyclohex-4-enecarboxylic acids and involves regio- and stereoselective transformation of the ring C–C double bond through iodooxazine formation and hydroxylation, followed by hydroxy–fluorine or oxo–fluorine exchange.

Synthesis of highly functionalized fluorinated cispentacin derivatives

Fluorinated highly functionalized cispentacin derivatives were synthetised starting from an unsaturated bicyclic β-lactam through C=C bond functionalization via the dipolar cycloaddition of a nitrile oxide, isoxazoline opening, and fluorination by OH/F exchange.

Low-molecular-weight gelators consisting of hybrid cyclobutane-based peptides

Some hybrid tetrapeptides consisting of (1R,2S)-2-aminocyclobutane-1-carboxylic acid and glycine, β-alanine, or γ-aminobutyric acid (GABA) joined in alternation, compounds 1-3, respectively, have been investigated to gain information on the non-covalent interactions responsible for their self-assembly to form ordered aggregates, as well as on parameters such as their morphology and size. All three peptides formed nice gels in many organic solvents and significant difference in their behaviour was not observed. Scanning electron microscopy (SEM) and circular dichroism (CD) pointed out that peptide 1, which contains the shortest C2 linear residue, presented the most defined fibril network and afforded nanoscale helical aggregates. Tetrapeptide 3, with C4 linear residues in its structure, also showed bundles of fibres whereas a homogeneous spherulitic network was observed for tetrapeptide 2, with a C3 spacer between cyclobutane residues. Computational calculations for 1 allowed us to model the self-assembly of the molecules and suggested a head-to-head arrangement to give helical structures corresponding to hydrogen-bonded single chains. These features were corroborated by a high-resolution NMR spectroscopy study of the dynamics of the gelation process in toluene-d8 which evidenced that molecules self-assemble to afford ordered aggregates with a supramolecular chirality.

Searching for new cell-penetrating agents: hybrid cyclobutane-proline γ,γ-peptides

Two generations of hybrid γ,γ-peptides containing cyclobutane amino acids and cis-γ-amino-L-proline joined in alternation have been synthesized and their capacity to cross the eukaryotic cell membrane has been evaluated. The first generation consists of di-, tetra- and hexapeptides, and their properties have been analyzed as well as the influence of peptide length and chirality of the cyclobutane residues. Results have shown that the absolute configuration of the cyclobutane amino acid does not have a relevant influence. The second generation consists of hybrid γ,γ-hexapeptides with a common backbone and distinct side chains introduced with different linkage types through the α-amino group (N(α)) of the proline monomers. These peptides have been shown to be non-toxic towards HeLa cells and to internalize them effectively, the best results being obtained for the peptides with a spacer of five carbons between the N(α) atom and the guanidinium group. The introduction of cyclobutane residues inside the sequence affords a good balance between charge and hydrophobicity, reducing the number of positive charges. This results in lower toxicity and similar cell-uptake properties when compared to previously described peptide agents.