Peptides Containing β-Amino Acid Patterns: Challenges and Successes in Medicinal Chemistry

Catalytic asymmetric synthesis of 5-membered alicyclic α-quaternary β-amino acids via [3 + 2]-photocycloaddition of α-substituted acrylates

The photocatalytically active salt of a cationic iridium polypyridyl complex and a chiral borate is competent to promote a highly stereoselective [3 + 2]-cycloaddition of cyclopropylurea with α-substituted acrylates. This protocol provides straightforward access to a variety of stereochemically defined 5-membered alicyclic α-quaternary β-amino acids, useful building blocks of β-peptides and peptidomimetics.

Peptidomimetics - An infinite reservoir of metal binding motifs in metabolically stable and biologically active molecules

The involvement of metal ions in interactions with therapeutic peptides is inevitable. They are one of the factors able to fine-tune the biological properties of antimicrobial peptides, a promising group of drugs with one large drawback - a problematic metabolic stability. Appropriately chosen, proteolytically stable peptidomimetics seem to be a reasonable solution of the problem, and the use of D-, β-, γ-amino acids, unnatural amino acids, azapeptides, peptoids, cyclopeptides and dehydropeptides is an infinite reservoir of metal binding motifs in metabolically stable, well-designed, biologically active molecules. Below, their specific structural features, metal-chelating abilities and antimicrobial potential are discussed.

Synthesis, Antitumor and Antibacterial Studies of New Shortened Analogues of (KLAKLAK)2-NH2 and Their Conjugates Containing Unnatural Amino Acids

(1) Background: (KLAKLAK)2 is a representative of the antimicrobial peptide group which also shows good anticancer properties. (2) Methods: Herein, we report synthesis using SPPS and characterization by HPLC/MS of a series of shortened analogues of (KLAKLAK)2. They contain single sequence KLAKLAK as C-terminal amides. In addition, substitution of some natural amino acids with unnatural β-Ala and nor-Leu is realized. In addition, these structures are conjugated with second pharmacophore with well proven anticancer properties 1,8-naphthalimide or caffeic acid. Cytotoxicity, antiproliferative effect and antimicrobial activity of newly synthesized structures were studied. (3) Results: The obtained experimental results reveal significant selective index for substances with common chemical structure KLβAKLβAK-NH2. The antibacterial properties of newly synthesized analogues at two different concentrations 10 μM and 20 μM, were tested against Gram-negative microorganisms Escherichia coli K12 407. Only two of the studied compounds KLAKLAK-NH2 and the one conjugated with second pharmacophore 1,8-naphthalimide and unnatural amino acid nor-Leu showed moderate activity against tested strains at concentration of 20 μM. (4) Conclusions: The obtained results reveal that the introducing of 1,8-naphthalimideGly- and Caf- increase the cytotoxicity and antiproliferative activity of the peptides but not their selectivity. Only two compounds KLAKLAK-NH2 and 1,8-naphthalimideGKnLAKnLAK-NH2 show moderate activity against Escherichia coli K12 at low concentration of 20μM.

1-Aminobicyclo[2.2.2]octane-2-carboxylic Acid and Derivatives As Chiral Constrained Bridged Scaffolds for Foldamers and Chiral Catalysts

The improvement of molecular diversity is one of the major concerns of chemists since the continuous development of original synthetic molecules provides unique scaffolds usable in organic and bioorganic chemistry. The challenge is to develop versatile platforms with highly controlled chemical three-dimensional space thanks to controlled chirality and conformational restraints. In this respect, cyclic β-amino acids are of great interest with applications in various fields of chemistry. In addition to their intrinsic biological properties, they are important precursors for the synthesis of new generations of bioactive compounds such as antibiotics, enzyme inhibitors, and antitumor agents. They have also been involved in asymmetric synthesis as efficient organo-catalysts in their free form and as derivatives. Finally, constrained cyclic β-amino acids have been incorporated into oligomers to successfully stabilize original structures in foldamer science with recent successes in health, material science, and catalysis. Over the last ∼10 years, we focused on bicyclic β-amino acids possessing a bicyclo[2.2.2]octane structure. This latter is a structural key element in numerous families of biologically active natural and synthetic products and is an interesting template for asymmetric synthesis. Nonetheless, reported studies on bicyclic carbo-bridged compounds are rather limited compared to those on bicyclic-fused and heterobridged derivatives. In this Account, we particularly focused on the synthesis and applications of the 1-aminobicyclo[2.2.2]octane-2-carboxylic acid, named, ABOC, and its derivatives. This highly constrained bicyclic β-amino acid, with a sterically hindered bridgehead primary amine and an endocyclic chiral center, displays drastically reduced conformational freedom. In addition, its high bulkiness strongly impacts the spatial orientation of the appended functionalities and the conformation of adjacent building blocks. Thus, we have first expanded a fundamental synthetic work by a wide ranging study in the field of foldamers, in the design of various stable peptide/peptidomimetic helical structures incorporating the ABOC residue (11/9-, 18/16-, 12/14/14-, and 12/10-helices). In addition, such bicyclic residue was fully compatible with and stabilized the canonical oligourea helix, whereas very few cyclic β-amino acids have been incorporated into oligoureas. In addition, we have pursued with the synthesis of some ABOC derivatives, in particular the 1,2-diaminobicyclo[2.2.2]octane chiral diamine, named DABO, and its investigation in chiral catalytic systems. Covalent organo-catalysis of the aldol reaction using ABOC-containing tripeptide catalysts provided a range of aldol products with high enantioselectivity. Moreover, the double reductive condensation of DABO with various aldehydes allowed the building of new chiral ligands that proved their efficiency in the copper-catalyzed asymmetric Henry reaction.

Selected β2-, β3- and β2,3-Amino Acid Heterocyclic Derivatives and Their Biological Perspective

Heterocyclic moieties, especially five and six-membered rings containing nitrogen, oxygen or sulfur atoms, are broadly distributed in nature. Among them, synthetic and natural alike are pharmacologically active compounds and have always been at the forefront of attention due to their pharmacological properties. Heterocycles can be divided into different groups based on the presence of characteristic structural motifs. The presence of β-amino acid and heterocyclic core in one compound is very interesting; additionally, it very often plays a vital role in their biological activity. Usually, such compounds are not considered to be chemicals containing a β-amino acid motif; however, considering them as this class of compounds may open new routes of their preparation and application as new drug precursors or even drugs. The possibility of their application as nonproteinogenic amino acid residues in peptide or peptide derivatives synthesis to prepare a new class of compounds is also promising. This review highlights the actual state of knowledge about β-amino acid moiety-containing heterocycles presenting antiviral, anti-inflammatory, antibacterial compounds, anaplastic lymphoma kinase (ALK) inhibitors, as well as agonist and antagonists of the receptors.

"Doubly Customizable" Unit for the Generation of Structural Diversity: From Pure Enantiomeric Amines to Peptide Derivatives

Readily available, low-cost 4R-hydroxy-l-proline (Hyp) is introduced as a "doubly customizable" unit for the generation of libraries of structurally diverse compounds. Hyp can be cleaved at two points, followed by the introduction of new functionalities. In the first cycle, the removal and replacement of the carboxylic group are carried out, followed (second cycle) by the scission of the 4,5-position and manipulation of the resulting chains. In this way, three new chains are generated and can be transformed independently to afford a diversity of products with tailored substituents, such as β-amino aldehydes, diamines, β-amino acid derivatives, including N-alkylated ones, or modified peptides. Many of these products are high-profit compounds but, in spite of their commercial value, are still scarce. Moreover, the process takes place with stereochemical control, and either pure R or S isomers can be obtained with small variations of the synthetic route.

Constrained beta-amino acid-containing miniproteins

The impact of constrained beta-amino acid substitutions on the conformational stability of two model miniproteins was evaluated.

Cyclic RGD and isoDGR Integrin Ligands Containing cis-2-amino-1-cyclopentanecarboxylic (cis-β-ACPC) Scaffolds

Integrin ligands containing the tripeptide sequences Arg-Gly-Asp (RGD) and iso-Asp-Gly- Arg (isoDGR) were actively investigated as inhibitors of tumor angiogenesis and directing unit in tumor-targeting drug conjugates. Reported herein is the synthesis, of two RGD and one isoDGR cyclic peptidomimetics containing (1S,2R) and (1R,2S) cis-2-amino-1-cyclopentanecarboxylic acid (cis-β-ACPC), using a mixed solid phase/solution phase synthetic protocol. The three ligands were examined in vitro in competitive binding assays to the purified α_vβ₃ and α₅β₁ receptors using biotinylated vitronectin (α_vβ₃) and fibronectin (α₅β₁) as natural displaced ligands. The IC50 values of the ligands ranged from nanomolar (the two RGD ligands) to micromolar (the isoDGR ligand) with a pronounced selectivity for α_vβ₃ over α₅β₁. In vitro cell adhesion assays were also performed using the human skin melanoma cell line WM115 (rich in integrin α_vβ₃). The two RGD ligands showed IC₅₀ values in the same micromolar range as the reference compound (cyclo[RGDfV]), while for the isoDGR derivative an IC₅₀ value could not be measured for the cell adhesion assay. A conformational analysis of the free RGD and isoDGR ligands by NMR (VT-NMR and NOESY experiments) and computational studies (MC/EM and MD), followed by docking simulations performed in the α_Vβ₃ integrin active site, provided a rationale for the behavior of these ligands toward the receptor.

Peptides and pseudopeptide ligands: a powerful toolbox for the affinity purification of current and next-generation biotherapeutics

Following the consolidation of therapeutic proteins in the fight against cancer, autoimmune, and neurodegenerative diseases, recent advancements in biochemistry and biotechnology have introduced a host of next-generation biotherapeutics, such as CRISPR-Cas nucleases, stem and car-T cells, and viral vectors for gene therapy. With these drugs entering the clinical pipeline, a new challenge lies ahead: how to manufacture large quantities of high-purity biotherapeutics that meet the growing demand by clinics and biotech companies worldwide. The protein ligands employed by the industry are inadequate to confront this challenge: while featuring high binding affinity and selectivity, these ligands require laborious engineering and expensive manufacturing, are prone to biochemical degradation, and pose safety concerns related to their bacterial origin. Peptides and pseudopeptides make excellent candidates to form a new cohort of ligands for the purification of next-generation biotherapeutics. Peptide-based ligands feature excellent target biorecognition, low or no toxicity and immunogenicity, and can be manufactured affordably at large scale. This work presents a comprehensive and systematic review of the literature on peptide-based ligands and their use in the affinity purification of established and upcoming biological drugs. A comparative analysis is first presented on peptide engineering principles, the development of ligands targeting different biomolecular targets, and the promises and challenges connected to the industrial implementation of peptide ligands. The reviewed literature is organized in (i) conventional (α-)peptides targeting antibodies and other therapeutic proteins, gene therapy products, and therapeutic cells; (ii) cyclic peptides and pseudo-peptides for protein purification and capture of viral and bacterial pathogens; and (iii) the forefront of peptide mimetics, such as β-/γ-peptides, peptoids, foldamers, and stimuli-responsive peptides for advanced processing of biologics.

Forming All-Carbon Quaternary Stereocenters by Organocatalytic Aminomethylation: Concise Access to β2,2 -Amino Acids

The asymmetric synthesis of β^2,2 -amino acids remains a formidable challenge in organic synthesis. Here a novel organocatalytic enantioselective aminomethylation of ketenes with stable and readily available N,O-acetals is reported, providing β^2,2 -amino esters bearing an all-carbon quaternary stereogenic center in high enantiomeric ratios with a catalytic amount of chiral phosphoric acid. Typically, this transformation probably proceeds through an asymmetric counter-anion-directed catalysis. As a result, a concise, practical, and atom-economic protocol toward rapidly access to β^2,2 -amino acids has been developed.

Ribosomal synthesis and de novo discovery of bioactive foldamer peptides containing cyclic β-amino acids

Peptides that contain β-amino acids display stable secondary structures, such as helices and sheets, and are often referred to as foldamers. Cyclic β^2,3-amino acids (cβAAs), such as 2-aminocyclohexanecarboxylic acid (2-ACHC), are strong helix/turn inducers due to their restricted conformations. Here we report the ribosomal synthesis of foldamer peptides that contain multiple, up to ten, consecutive cβAAs via genetic code reprogramming. We also report the de novo discovery of macrocyclic cβAA-containing peptides capable of binding to a protein target. As a demonstration, potent binders with low-to-subnanomolar K_D values were identified for human factor XIIa (hFXIIa) and interferon-gamma receptor 1, from a library of their 10¹² members. One of the anti-hFXIIa macrocyclic peptides that exhibited a high inhibitory activity and serum stability was co-crystallized with hFXIIa. The X-ray structure revealed that it adopts an antiparallel β-sheet structure induced by a (1S,2S)-2-ACHC residue via the formation of two γ-turns. This work demonstrates the potential of this platform to explore the previously inaccessible sequence space of cβAA-containing peptides.

Covalent and noncovalent constraints yield a figure eight-like conformation of a peptide inhibiting the menin-MLL interaction

The interaction between menin and mixed lineage leukemia (MLL) was identified as an interesting target for treating some cancers including acute leukemia. On the basis of the known crystal structure of the MBM1-menin complex (MBM - menin binding motif), several cyclic peptides were designed. Elaboration of the effective cyclization strategy using a metathesis reaction allowed for a successfully large number of derivatives to be obtained. Subsequent optimization of the loop size, as well as N-terminal, central and C-terminal parts of the studied peptides resulted in structures exhibiting low nanomolar activities. A crystal structure of an inhibitor-menin complex revealed a compact conformation of the ligand molecule, which is stabilized not only by the introduction of a covalent linker but also three intramolecular hydrogen bonds. The inhibitor adopts a figure eight-like conformation, which perfectly fits the cleft of menin. We demonstrated that the development of compact, miniprotein-like structures is a highly effective approach for inhibition of protein-protein interactions.

Silver-Catalyzed Enantioselective Propargylic C-H Bond Amination through Rational Ligand Design

Asymmetric C-H amination via nitrene transfer is a powerful tool to prepare enantioenriched amine precursors from abundant C-H bonds. Herein, we report a regio- and enantioselective synthesis of γ-alkynyl γ-aminoalcohols via a silver-catalyzed propargylic C-H amination. The protocol was enabled by a new bis(oxazoline) (BOX) ligand designed via a rapid structure-activity relationship (SAR) analysis. The method utilizes accessible carbamate esters bearing γ-propargylic C-H bonds and furnishes versatile products in good yields and excellent enantioselectivity (90-99% ee). The putative Ag-nitrene is proposed to undergo enantiodetermining hydrogen-atom transfer (HAT) during the C-H amination event. Density functional theory calculations shed insight into the origin of enantioselectivity in the HAT step.

Synthesis and biological evaluation of S-lipidated lipopeptides of a connexin 43 channel inhibitory peptide

The synthesis and biological activity of 42 novel S-lipidated analogues of a connexin 43 channel inhibitory Peptide5 is described. Unmodified Peptide5 moderates hemichannels and gap junctions that are both implicated in the progression of neurological disease. Peptide5 was site-specifically modified with a cysteine residue, which then underwent thiol-ene mediated S-lipidation to afford S-lipidated Peptide5 analogues containing straight-chain, branched, or aromatic lipids. The modified peptides were assessed for their effect on hemichannel opening and the most promising candidates were evaluated in serum stability studies.

Organocatalytic Multicomponent Synthesis of α/β-Dipeptide Derivatives

A straightforward multicomponent Knoevenagel-aza-Michael-cyclocondensation reaction involving readily available hydroxamic acid-derived from naturally occurring α-amino acids allows a diversity-oriented synthesis of novel isoxazolidin-5-ones possessing an N-protected α-amino acid pendant with good to high diastereoselectivities thanks to a match effect with a chiral organocatalyst. These diversely substituted heterocycles, easily isolated as a single diastereoisomer, proved to be versatile platforms for the formation of an array of α/β-dipeptide fragments.

New Morphiceptin Peptidomimetic Incorporating (1S,2R,3S,4S,5R)-2-Amino-3,4,5-trihydroxycyclopen-tane-1-carboxylic acid: Synthesis and Structural Study

We present the synthesis and structural study of a new peptidomimetic of morphiceptin, which can formally be considered as the result of the replacement of the central proline residue of this natural analgesic drug with a subunit of (1S,2R,3S,4S,5R)-2-amino-3,4,5-trihydroxycyclopentane-1-carboxylic acid, previously obtained from L-idose. An optimized synthesis of this trihydroxylated cispentacin derivative is also reported. Molecular docking calculations on the target receptor support a favorable role of the hydroxy substituents of the non-natural β-amino acid incorporated into the peptidomimetic.

β2-Homo-Amino Acid Scan of µ-Selective Opioid Tetrapeptide TAPP

TAPP (H-Tyr-d-Ala-Phe-Phe-NH₂) is a potent, µ-selective opioid ligand. In order to gain further insights into pharmacophoric features of this tetrapeptide, we have performed a β²-Homo-amino acid (β²hAA) scan of the TAPP sequence. To this aim, 10 novel analogues have been synthesized and evaluated for µ-opioid and δ-opioid receptor affinity as well as for stability in human plasma. The derivatives included compounds in which a (R)- or (S)-β²-Homo-Homologue replaced the amino acids in the TAPP sequence. The derivatives with (R)- or (S)-β²hPhe⁴ turned out to bind µOR with affinities equal to that of the parent. β²hAAs in position 1 and 3 resulted in rather large affinity decreases, but the change differed depending on the stereochemistry. β²-Homologation in the second position gave derivatives with very poor µOR binding. According to molecular modelling, the presented α/β-peptides adopt a variety of binding poses with their common element being an ionic interaction between a protonable amine of the first residue and Asp147. A feature required for high µOR affinity seems the ability to accommodate the ring in the fourth residue in a manner similar to that found for TAPP. Contrary to what might be expected, several compounds were significantly less stable in human plasma than the parent compound.

Convergent Route to β-Amino Acids and to β-Heteroarylethylamines: An Unexpected Vinylation Reaction

Various protected β²-amino acids can be prepared by radical addition of β-phthalimido-α-xanthyl propionic acid, both as the free acid or as the ethyl ester. Successive radical additions provide access to more complex structures. In the case of the free acid, addition to certain heteroaromatics leads directly to β-heteroarylethylamines through spontaneous decarboxylation of the intermediate adduct. Forcing the decarboxylation in some cases generated a vinyl group by decarboxylative elimination of the phthalimido group.

Conformational preference of bicyclic β-amino acid dipeptides

Bridged bicyclic amino acids have high potential applicability as self-organized, conformationally constrained synthetic building blocks that do not require assistance from hydrogen bond formation. We systematically investigated the intrinsic conformational propensities of dipeptides of bridged bicyclic β-amino acids by means of accelerated molecular dynamics simulation and density functional theory (DFT) calculations in methanol, chloroform, and water. While the main-chain conformation, represented by φ and θ values, is fixed by the nature of the bicyclic ring structure, rotation of the C-terminal carbonyl group (ψ) is also restricted, converging to one or two minima. In endo-type dipeptides, in which the two N- and C-terminal amides are spatially close to each other, the C-terminal amide plane is placed horizontally. In exo-type dipeptides, in which the two amides are on opposite sides of the ring plane, the C-terminal carbonyl group can take two types of positions: either parallel/antiparallel with the N-terminal carbonyl or beneath the bicyclic ring, forcing the amide NHMe moiety to lie outside of the ring. We also examined the cis-trans preference of model bicyclic amides. Although the parent amides exhibit cis-trans equilibrium without any preference, addition of a methyl group on one of the bridgehead positions tips the equilibrium towards trans.

A bicyclic unit reversal to stabilize the 12/14-helix in mixed homochiral oligoureas

Incorporation of highly constrained building blocks into oligoureas: a simple bicycle reversal leads to tunable 12/14-helices formation.

Asymmetric acyl-Mannich reaction of isoquinolines with α-(diazomethyl)phosphonate and diazoacetate catalyzed by chiral Brønsted acids

α-Diazo-β-isoquinoline derivatives were obtained in excellent yields and enantioselectivities by asymmetric acyl-Mannich reaction of (diazomethyl)phosphonate or diazoacetate with isoquinolines.

Synthesis of Unprotected Spirocyclic β-Prolines and β-Homoprolines by Rh-Catalyzed C-H Insertion

A series of unprotected spirocyclic β-prolines and β-homoprolines are prepared by Rh-catalyzed C-H insertion. The key intermediate, a Rh nitrenoid, is generated by the N-O bond cleavage of a substituted isoxazolidin-5-one. The reaction proceeds on a gram scale with a catalyst loading of as little as 0.1 mol %, affording spirocyclic β-amino acids that are otherwise difficult to obtain. The building blocks prepared in this work will likely find applications in medicinal chemistry.

Impact of Dehydroamino Acids on the Structure and Stability of Incipient 310-Helical Peptides

A comparative study of the impact of small, medium-sized, and bulky α,β-dehydroamino acids (ΔAAs) on the structure and stability of Balaram's incipient 3₁₀-helical peptide (1) is reported. Replacement of the N-terminal Aib residue of 1 with a ΔAA afforded peptides 2a-c that maintained the 3₁₀-helical shape of 1. In contrast, installation of a ΔAA in place of Aib-3 yielded peptides 3a-c that preferred a β-sheet-like conformation. The impact of the ΔAA on peptide structure was independent of size, with small (ΔAla), medium-sized (Z-ΔAbu), and bulky (ΔVal) ΔAAs exerting similar effects. The proteolytic stabilities of 1 and its analogs were determined by incubation with Pronase. Z-ΔAbu and ΔVal increased the resistance of peptides to proteolysis when incorporated at the 3-position and had negligible impact on stability when placed at the 1-position, whereas ΔAla-containing peptides degraded rapidly regardless of position. Exposure of peptides 2a-c and 3a-c to the reactive thiol cysteamine revealed that ΔAla-containing peptides underwent conjugate addition at room temperature, while Z-ΔAbu- and ΔVal-containing peptides were inert even at elevated temperatures. These results suggest that both bulky and more accessible medium-sized ΔAAs should be valuable tools for bestowing rigidity and proteolytic stability on bioactive peptides.

Organocatalytic Enantioselective Mukaiyama-Mannich Reaction of Isatin-Derived Ketimines for the Synthesis of Oxindolyl-β3, 3 -Amino Acid Esters

Mukaiyama-Mannich reactions of ester enolate equivalents with aldimines have been elegantly used for the asymmetric synthesis of β-amino acids; nevertheless, the corresponding asymmetric reaction employing ketimines are unexplored. Herein, the first organocatalytic enantioselective Mukaiyama-Mannich reaction employing isatin-derived ketimines with unsubstituted silyl ketene acetals is disclosed towards the scalable synthesis of 2-oxoindolinyl-β^3, 3 -amino acid esters at room temperature with excellent enantioselectivities (ee >99.5 %). Ultra-low catalyst loadings (as low as 250 ppm) could be used for the quantitative product formation with high enantiopurity. The synthetic utility of this protocol has been showcased in the short formal synthesis of pharmaceutically demanded (+)-AG-041R, a potent gastrin/CCK-B receptor antagonist.

A gold mine for drug discovery: Strategies to develop cyclic peptides into therapies

As a versatile therapeutic modality, peptides attract much attention because of their great binding affinity, low toxicity, and the capability of targeting traditionally "undruggable" protein surfaces. However, the deficiency of cell permeability and metabolic stability always limits the success of in vitro bioactive peptides as drug candidates. Peptide macrocyclization is one of the most established strategies to overcome these limitations. Over the past decades, more than 40 cyclic peptide drugs have been clinically approved, the vast majority of which are derived from natural products. The de novo discovered cyclic peptides on the basis of rational design and in vitro evolution, have also enabled the binding with targets for which nature provides no solutions. The current review summarizes different classes of cyclic peptides with diverse biological activities, and presents an overview of various approaches to develop cyclic peptide-based drug candidates, drawing upon series of examples to illustrate each strategy.

Metal-supported and -assisted stereoselective cooperative photoredox catalysis

In this perspective, we review those stereoselective photocatalytic reactions that use synergy between photoredox catalysts and transition metal catalysts. In particular, we highlight the orchestrated interaction between two and more metals which not only enhance the turnover numbers, but also lead to increased selectivities. Aspects of green chemistry and sustainable developments are included. In this review, C-C, C-O, C-N and C-S forming reactions are discussed and a perspective on future developments is given.

Lewis Base Assisted Lithium Brønsted Base Catalysis: A New Entry for Catalytic Asymmetric Synthesis of β2,2-Amino Acids

A new catalytic system comprising chiral Ag complex and Li aryloxide/bisphosphine oxide is developed for the synthesis of β^2,2-amino acids via direct asymmetric Mannich-type reaction of 4-subsituted isoxazolidin-5-ones. The Mannich adduct is a direct precursor of β-peptidic compounds otherwise difficult to obtain.

Catalytic Enantio- and Diastereoselective Cyclopropanation of 2-Azadienes for the Synthesis of Aminocyclopropanes Bearing Quaternary Carbon Stereogenic Centers

We report the catalytic enantio- and diastereoselective preparation of aminocyclopropanes by the cyclopropanation of terminal and (Z)-internal 2-azadienes with donor/acceptor carbenes derived from α-diazoesters. The resulting cyclopropanes bear quaternary carbon stereogenic centers vicinal to the amino-substituted carbon and are formed as a single diastereomer in up to 99:1 er and 97% yield with 0.5 mol % of Rh2(DOSP)4 and only 1.5 equiv of the diazo reagent. Transformations with internal azadienes afford cyclopropanes with three contiguous stereogenic centers.

Regiodivergent enantioselective C-H functionalization of Boc-1,3-oxazinanes and application to the synthesis of β2 and β3-amino acids

β²- and β³-Amino acids are important chiral building blocks for the design of new pharmaceuticals and peptidomimetics. Here we report a straightforward regio- and enantiodivergent access to these compounds using a one-pot reaction composed of sparteine-mediated enantioselective lithiation of a Boc-1,3-oxazinane, transmetallation to zinc and direct or migratory Negishi coupling with an organic electrophile. The regioselectivity of the Negishi coupling was highly ligand-controlled and switchable to obtain the C4- or the C5-functionalized product exclusively. High enantioselectivities were achieved on a broad range of examples, and a catalytic version in chiral diamine was developed using the (+)-sparteine surrogate. Selected C4- and C5-functionalized Boc-1,3-oxazinanes were subsequently converted to highly enantio-enriched β²- and β³-amino acids with the (R) or (S) configuration, depending on the sparteine enantiomer employed in the lithiation step.

Stereocontrolled Preparation of Diversely Trifunctionalized Cyclobutanes

The expedient and stereoselective syntheses of small libraries of trifunctionalized cyclobutane scaffolds bearing an acid, an amine, and a third functional group are described. Starting from a single precursor, the readily available protected derivative of all-cis-2-amino-3-hydroxycyclobutane-1-carboxylic acid, cis-trans stereoisomers are obtained following an S_N2-type reaction, while all-trans stereoisomers are obtained using the same strategy preceded by a C1 epimerization reaction.

Carbon Isotope Labeling Strategy for β-Amino Acid Derivatives via Carbonylation of Azanickellacycles

A series of 4-membered azametallacycles have been prepared by the oxidative addition of Ni(0) with aziridines. Stoichiometric ¹³C-labeled carbon monoxide could be efficiently incorporated via Ni-C bond insertion to generate air stable and isolable cyclic Ni-acyl complexes. Upon subjection to a range of C-, N-, O-, and S-nucleophiles, ¹³C-labeled β-amino acids and derivatives thereof, as well as β-aminoketones, could be rapidly accessed. The methodology proved highly adaptable for the synthesis of the antidiabetic drug, sitagliptin, with a single carbon isotope label.

A Comprehensive Review on Current Advances in Peptide Drug Development and Design

Protein-protein interactions (PPIs) execute many fundamental cellular functions and have served as prime drug targets over the last two decades. Interfering intracellular PPIs with small molecules has been extremely difficult for larger or flat binding sites, as antibodies cannot cross the cell membrane to reach such target sites. In recent years, peptides smaller size and balance of conformational rigidity and flexibility have made them promising candidates for targeting challenging binding interfaces with satisfactory binding affinity and specificity. Deciphering and characterizing peptide-protein recognition mechanisms is thus central for the invention of peptide-based strategies to interfere with endogenous protein interactions, or improvement of the binding affinity and specificity of existing approaches. Importantly, a variety of computation-aided rational designs for peptide therapeutics have been developed, which aim to deliver comprehensive docking for peptide-protein interaction interfaces. Over 60 peptides have been approved and administrated globally in clinics. Despite this, advances in various docking models are only on the merge of making their contribution to peptide drug development. In this review, we provide (i) a holistic overview of peptide drug development and the fundamental technologies utilized to date, and (ii) an updated review on key developments of computational modeling of peptide-protein interactions (PepPIs) with an aim to assist experimental biologists exploit suitable docking methods to advance peptide interfering strategies against PPIs.

Catalytic asymmetric cycloaddition reactions of enoldiazo compounds

This review describes catalytic asymmetric cycloaddition reactions of silyl-protected enoldiazo compounds for the construction of highly functionalized carbo- and heterocycles which possess one or more chiral center(s). The enoldiazo compound or its derivative, donor-acceptor cyclopropene, form electrophilic vinylogous metal carbene intermediates that combine stepwise with nucleophilic dipolar reactants to form products from [3 + 1]-, [3 + 2]-, [3 + 3]-, [3 + 4]-, and [3 + 5]-cycloaddition, generally in high yield and with exceptional stereocontrol and regioselectivity.

Inhibitory effect of four novel synthetic peptides on food spoilage yeasts

The spoilage of foods caused by the growth of undesirable yeast species is a problem in the food industry. Yeast species such as Zygosaccharomyces bailii, Zygosaccharomyces rouxii, Debaryomyces hansenii, Kluyveromyces lactis and Saccharomyces cerevisiae have been encountered in foods such as high sugar products, fruit juices, wine, mayonnaise, chocolate and soft drinks. The demand for new methods of preservations has increased because of the negative association attached to chemical preservatives. The sequence of a novel short peptide (KKFFRAWWAPRFLK-NH2) was modified to generate three versions of this original peptide. These peptides were tested for the inhibition of the yeasts mentioned above, allowing for the better understanding of their residue modifications. The range of the minimum inhibitory concentration was between 25 and 200 μg/mL. Zygosaccharomyces bailii was the most sensitive strain to the peptides, while Zygosaccharomyces rouxii was the most resistant. Membrane permeabilisation was found to be responsible for yeast inhibition at a level which was a two-fold increase of the MIC (400 μg/mL). The possibility of the production of reactive oxygen species was also assessed but was not recognised as a factor involved for the peptides' mode of action. Their stability in different environments was also tested, focusing on high salt, pH and thermal stability. The newly designed peptides showed good antifungal activity against some common food spoilage yeasts and has been proven effective in the application in Fanta Orange. These efficient novel peptides represent a new source of food preservation that can be used as an alternative for current controversial preservatives used in the food industry.

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

α,β^2,3-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 β^2,3-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 β^2,3-fluoro amino acid very appealing for the development of bioactive proteolytically stable foldameric β-sheets as modulators of protein-protein interaction (PPI).