Design and synthesis of chiral alpha,alpha-disubstituted amino acids and conformational study of their oligopeptides

Anodic Oxidation for the Stereoselective Synthesis of Heterocycles

Stereodefined aliphatic heterocycles are one of the fundamental structural motifs observed in natural products and biologically active compounds. Various strategies for the synthesis of these building blocks based on transition metal catalysis, organocatalysis, and noncatalytic conditions have been developed. Although electrosynthesis has also been utilized for the functionalization of aliphatic heterocycles, stereoselective transformations under electrochemical conditions are still a challenging field in electroorganic chemistry. This Account consists of four main topics related to our recent efforts on the diastereo- and/or enantioselective synthesis of aliphatic heterocycles, especially N-heterocycles, using anodic oxidations as key steps. The first topic is the development of stereoselective synthetic methods for multisubstituted piperidines and pyrrolidines from anodically prepared α-methoxy cyclic amines. Our strategies were based primarily on N-acyliminium ion chemistry, and the key electrochemical transformations were diastereoselective anodic methoxylation, diastereoselective arylation, and anodic deallylative methoxylation. Furthermore, we found a unique property of the N-cyano protecting group that enabled the electrochemical α-methoxylation of α-substituted cyclic amines. The second topic of investigation is memory of chirality in electrochemical decarboxylative methoxylation. We observed that the electrochemical decarboxylative methoxylation of oxazolidine and thiazolidine derivatives with the appropriate N-protecting group occurred in a stereospecific manner even though the reaction proceeded through an sp² planar carbon center. Our findings demonstrated the first example of memory of chirality in N-acyliminium ion chemistry. The third topic is the synthesis of chiral azabicyclo-N-oxyls and their application to chiral organocatalysis in the electrochemical oxidative kinetic resolution of secondary alcohols. The final topic is stereoselective transformations utilizing anodically generated halogen cations. We investigated the oxidative kinetic resolution of amino alcohol derivatives using anodically generated bromo cations. We also developed an intramolecular C-C bond formation of keto amides, a diastereoselective bromoiminolactonization of α-allyl malonamides, and an oxidative ring expansion reaction of allyl alcohols. It is noteworthy that most of the electrochemical reactions were performed in undivided cells under constant-current conditions, which avoided a complicated reaction setup and was beneficial for a large-scale reaction. In addition, we developed some enantioselective electrochemical transformations that are still challenges in electroorganic chemistry. We hope that our research will contribute to the further development of diastereo- and/or enantioselective transformations and the construction of valuable heterocyclic compounds using an electrochemical approach.

Self-assembly and hydrogel formation ability of Fmoc-dipeptides comprising α-methyl-L-phenylalanine

Various biofunctional hydrogel materials can be fabricated in aqueous media through the self-assembly of peptide derivatives, forming supramolecular nanostructures and their three-dimensional networks. In this study, we describe the self-assembly of new Fmoc-dipeptides comprising α-methyl-L-phenylalanine. We found that the position and number of methyl groups introduced onto the α carbons of the Fmoc-dipeptides by α-methyl-L-phenylalanine have a marked influence on the morphology of the supramolecular nanostructure as well as the hydrogel (network) formation ability.

Insulin Hot-Spot Analogs Formed with N-Methylated Amino Acid Residues Inhibit Aggregation of Native Hormone

In this study, N-methylated analogs of hot-spots of insulin were designed and synthesized, in the expectation that they would inhibit the aggregation of both insulin hot-spots and the entire hormone. Synthesis of insulin “amyloidogenic” analogs containing N-methylated amino acid residues was performed by microwave-assisted solid phase according to the Fmoc/tert-Bu strategy. As a coupling reagent 4-(4,6-dimethoxy-1,3,5-triazin-2-yl)-4-methylmorpholinium toluene-4-sulfonate (DMT/NMM/TosO^-) was used. Three independent methods were applied in aggregation studies of the complexes of insulin with its N-methylated peptides. Additionally, circular dichroism (CD) measurements were used to confirm that aggregation processes did not occur in the presence of the N-methylated analogs of hot-spot insulin fragments, and that insulin retains its native conformation. Of the seven N-methylated analogs of the A- and B-chain hot-spots of insulin, six inhibited insulin aggregation (peptides 1 and 3–7). All tested peptides were found to have a lower ability to inhibit the aggregation of insulin hot-spots compared to the capability to inhibit native hormone aggregation.

Isolated α-turn and incipient γ-helix

The unique abilities of homo-oligo-adamantyl peptides to adopt α- and γ-turn conformations are demonstrated by X-ray diffraction, and NMR and FT-IR absorption spectroscopies. Assembled by an Ugi multiple component reaction strategy, N α-formyl-adamantyl tripeptide iso-propyl and tert-butyl amides are respectively found to adopt an isolated α-turn and an incipient γ-helix conformation by X-ray diffraction crystallography. The shortest example of a single α-turn with ideal geometry is observed in the crystalline state. In solution both peptides predominantly assume γ-helical structures.

Cell-Penetrating Peptide Foldamers: Drug-Delivery Tools

Highly efficient drug-delivery tools for membrane-impermeable compounds, proteins, and nucleic acids in living cells are useful in the fields of chemical biology and drug discovery, and such tools have been widely studied. One strategy in the development of novel drug-delivery tools is to utilize cell-penetrating peptide (CPP) foldamers. CPP foldamers are folded oligopeptides that possess cell membrane permeability. In recent decades, a wide variety of CPP foldamers have been reported by many groups. Herein, CPP foldamers are introduced and discussed from the viewpoints of component monomers (amino acids) and their application as drug-delivery tools.

[Peptide Foldamers: Structural Control and Cell-penetrating Ability]

α,α-Disubstituted α-amino acids (dAAs), in which the α-hydrogen of the α-amino acid is replaced with an alkyl substituent, stabilize peptide secondary structures and have been utilized as a tool for building blocks of peptide foldamers. Peptides composed of acyclic dAAs with two bulky substituents equal to or larger than ethyl groups are more likely to form an extended planar conformation, whereas peptides with cyclic dAAs are more likely to adopt a helical structure. Based on these conformational properties of dAA-containing peptides, we developed a novel methodology using cyclic dAAs with an acetal side chain for conformational changes in peptides from a helical to a random structure with acidic treatment. Furthermore, peptide foldamers containing dAAs are useful for the design of functional peptides. In addition to the stabilization properties of peptide secondary structures, peptides foldamers exhibit resistance to degradation by proteases and thus are expected to be useful for development into bioactive peptides. In this presentation, I introduce cell-penetrating peptide foldamers as an application for dAAs in functional peptides. Peptide foldamers with appropriate functional groups at target positions show excellent, continuous cell membrane permeability and the ability to deliver biomacromolecules, such as plasmid DNA, into cells efficiently.

Secondary structures and cell-penetrating abilities of arginine-rich peptide foldamers

Foldamers, which are folded oligomers with well-defined conformations, have been recently reported to have a good cell-penetrating ability. α,α-Disubstituted α-amino acids are one such promising tool for the design of peptide foldamers. Here, we prepared four types of L-arginine-rich nonapeptides containing L-leucine or α,α-disubstituted α-amino acids, and evaluated their secondary structures and cell-penetrating abilities in order to elucidate a correlation between them. Peptides containing α,α-disubstituted α-amino acids had similar resistance to protease digestion but showed different secondary structures. Intracellular uptake assays revealed that the helicity of peptides was important for their cell-penetrating abilities. These findings suggested that a peptide foldamer with a stable helical structure could be promising for the design of cell-penetrating peptides.

Bifunctional Brønsted base catalyzed inverse-electron-demand aza-Diels–Alder reactions of saccharin-derived 1-azadienes with azlactones

An enantioselective inverse-electron-demand aza-Diels–Alder reaction of saccharin-derived 1-azadienes and azlactones was developed using a phenylalanine-derived bifunctional Brønsted base-squaramide catalyst.

Left-Handed Helix of Three-Membered Ring Amino Acid Homopeptide Interrupted by an N-H···Ethereal O-Type Hydrogen Bond

A chiral three-membered ring C^α,α-disubstituted α-amino acid ( R, R)-Ac₃c^dMOM, in which the α-carbon is not a chiral center, but two side chain β-carbons are chiral centers, was synthesized from dimethyl l-(+)-tartrate, and its homopeptides were prepared. X-ray crystallographic analysis of ( R, R)-Ac₃c^dMOM pentapeptide showed bent left-handed ( M) 3₁₀-helical structures with an unusual intramolecular hydrogen bond of the N-H···O (ethereal) type. The left-handedness of the bent helices was exclusively controlled by the side-chain β-carbon chiral centers.

Cell-Penetrating Peptides Using Cyclic α,α-Disubstituted α-Amino Acids with Basic Functional Groups

In the delivery of cell-impermeable molecules, cell-penetrating peptides (CPPs) have been attracting increasing attention as intracellular delivery tools. In the present study, we designed four types of cyclic α,α-disubstituted α-amino acids (dAAs) with basic functional groups on their five-membered rings and different chiralities at the α-position and introduced them into arginine (Arg)-rich peptides. The evaluation of cell-penetrating abilities indicated that these peptides exhibited better cell permeabilities than an Arg nonapeptide. Furthermore, peptides containing dAAs delivered plasmid DNA (pDNA) better than a commercially available transfection reagent with a longer incubation time. These results demonstrate that the introduction of cyclic dAAs with basic functional groups into Arg-rich peptides is an effective strategy for the design of CPPs as a pDNA delivery tool.

Ir/PTC cooperatively catalyzed asymmetric umpolung allylation of α-imino ester enabled synthesis of α-quaternary amino acid derivatives bearing two vicinal stereocenters

A novel Ir/PTC (phase-transfer catalyst) cooperatively catalyzed asymmetric umpolung allylation of simple α-imino esters is developed and it provides facile access to α-quaternary amino acid derivatives bearing two vicinal stereocenters. Both the metal catalyst and the phase-transfer catalyst are crucial for this methodology. The reaction features good yields, high diastereo- and enantio-selectivities, ease of scale-up to gram scale and further transformation of the products (e.g. to quaternary proline analogues with multi stereocenters).

Effects of five-membered ring amino acid incorporation into peptides for coiled coil formation

A five-membered ring amino acid (Ac₅c), the peptides of which exhibit a preference for helical secondary structures, was introduced into peptides for the purpose of designing coiled coil peptides with high binding affinities. We prepared five types of peptides containing Ac₅c with different numbers or at different positions. The incorporation of Ac₅c into peptides enhanced their α-helicities; however, in contrast to our expectations, it did not result in stable coiled coil formation. The structures of side chains in hydrophobic amino acids, not α-helicities appeared to be important for stable hydrophobic interactions between peptides. Although we were unable to develop coiled coil peptides with high binding affinities, the present results will be useful for designing novel coiled coil peptides.

Biocatalytic approaches towards the stereoselective synthesis of vicinal amino alcohols

The global need for clean manufacturing technologies and the management of hazardous chemicals and waste present new research challenges to both chemistry and biotechnology.

Highly enantioselective synthesis of α-tertiary chiral amino acid derivatives through rhodium-catalyzed asymmetric arylation of cyclic N-sulfonyl α-ketimino esters

A simple sulfur-olefin ligand promoted Rh-catalyzed highly enantioselective arylation of cyclic α-ketimino esters with arylboronic acids is described.

Asymmetric Synthesis of Less Accessible α-Tertiary Amines from Alkynyl Z-Ketimines

A highly stereoselective synthesis of hitherto less accessible chiral α-tertiary amines with multiple structurally similar linear carbon chains was achieved through chiral auxiliary mediated addition of organolithium reagents to the geometrically well-controlled alkynyl Z-ketimines. This stereoselective nucleophilic addition offers a general approach to the asymmetric synthesis of nitrogen-containing chiral materials.

Diastereomeric Right- and Left-Handed Helical Structures with Fourteen (R)-Chiral Centers

The relationship between chiral centers and the helical-screw control of their peptides has already been reported, but it has yet to be elucidated in detail. A chiral four-membered ring α,α-disubstituted α-amino acid with a (R,R)-butane-2,3-diol acetal moiety at the γ-position, but no α-chiral carbon, was synthesized. X-ray crystallographic analysis unambiguously revealed that its homo-chiral heptapeptide formed right-handed (P) and left-handed (M) 3₁₀ -helical structures at a ratio of 1:1. They appeared to be enantiomeric at the peptide backbone, but diastereomeric with fourteen (R)-configuration chiral centers. Conformational analyses of homopeptides in solution also indicated that diastereomeric (P) and (M) helices existed at approximately equal amounts, with a slight preference toward right-handedness, and they quickly interchanged at room temperature. The circumstances of chiral centers are important for the control of their helical-screw direction.

Enhanced and Prolonged Cell-Penetrating Abilities of Arginine-Rich Peptides by Introducing Cyclic α,α-Disubstituted α-Amino Acids with Stapling

Cell-penetrating peptides are receiving increasing attention as drug delivery tools, and the search for peptides with high cell-penetrating ability and negligible cytotoxicity has become a critical research topic. Herein, cyclic α,α-disubstituted α-amino acids were introduced into arginine-rich peptides and an additional staple was provided in the side chain. The peptides designed in the present study showed more enhanced and prolonged cell-penetrating abilities than an arginine nonapeptide due to high resistance to protease and conformationally stable helical structures.

Synthesis of chiral five-membered carbocyclic ring amino acids with an acetal moiety and helical conformations of its homo-chiral homopeptides

Chiral five-membered carbocyclic ring amino acids bearing various diol acetal moieties were synthesized starting from l-malic acid, and homo-chiral homopeptides composed of cyclic amino acid (S)-Ac5 c(3EG) bearing an ethylene glycol acetal, up to an octapeptide, were prepared. A conformational analysis revealed that (S)-Ac5 c(3EG) homopeptides formed helical structures. (S)-Ac5 c(3EG) homopeptides, up to hexapeptides, formed helical structures without controlling the helical screw direction, while (S)-Ac5 c(3EG) hepta- and octapeptides formed helical structures with a preference for the left-handed (M) helical-screw direction. © 2015 Wiley Periodicals, Inc. Biopolymers (Pept Sci) 106: 555-562, 2016.

Copper-catalyzed asymmetric alkynylation of cyclic N-sulfonyl ketimines

A Cu-catalyzed asymmetric alkynylation of cyclic N-sulfonyl ketimines was developed, providing the corresponding chiral α-tertiary amines with up to 98% ee.

Low pH-triggering changes in peptide secondary structures

An acidic treatment of cyclic α,α-disubstituted α-amino acid-containing peptides changes their conformation from a helical to a random structure.

Converting disulfide bridges in native peptides to stable methylene thioacetals

Disulfide bridges play a crucial role in defining and rigidifying the three-dimensional structure of peptides. However, disulfides are inherently unstable in reducing environments. Consequently, the development of strategies aiming to circumvent these deficiencies - ideally with little structural disturbance - are highly sought after. Herein, we report a simple protocol converting the disulfide bond of peptides into highly stable methylene thioacetal. The transformation occurs under mild, biocompatible conditions, enabling the conversion of unprotected native peptides into analogues with enhanced stability. The developed protocol is applicable to a range of peptides and selective in the presence of a multitude of potentially reactive functional groups. The thioacetal modification annihilates the reductive lability and increases the serum, pH and temperature stability of the important peptide hormone oxytocin. Moreover, it is shown that the biological activities for oxytocin are retained.

Therapeutic Potential of Foldamers: From Chemical Biology Tools To Drug Candidates?

Over the past decade, foldamers have progressively emerged as useful architectures to mimic secondary structures of proteins. Peptidic foldamers, consisting of various amino acid based backbones, have been the most studied from a therapeutic perspective, while polyaromatic foldamers have barely evolved from their nascency and remain perplexing for medicinal chemists due to their poor drug-like nature. Despite these limitations, this compound class may still offer opportunities to study challenging targets or provide chemical biology tools. The potential of foldamer drug candidates reaching the clinic is still a stretch. Nevertheless, advances in the field have demonstrated their potential for the discovery of next generation therapeutics. In this perspective, the current knowledge of foldamers is reviewed in a drug discovery context. Recent advances in the early phases of drug discovery including hit finding, target validation, and optimization and molecular modeling are discussed. In addition, challenges and focus areas are debated and gaps highlighted.

Thermodynamic and Structural Impact of α,α-Dialkylated Residue Incorporation in a β-Hairpin Peptide

Peptides containing α,α-dialkylated α-amino acids, owing to their ability to disrupt aggregation of β-amyloid proteins, have therapeutic potential in the treatment of neurodegenerative diseases. Thermodynamic and structural analyses are reported for a series of β-hairpin peptides containing α,α-dialkylated α-amino acids with varying side-chain lengths. The results of these experiments show that α,α-dialkylated α-amino acids with side-chain lengths longer than one carbon unit are tolerated in a β-hairpin, although at a moderate cost to folded stability.

Helical-Peptide-Catalyzed Enantioselective Michael Addition Reactions and Their Mechanistic Insights

Helical peptide foldamer catalyzed Michael addition reactions of nitroalkane or dialkyl malonate to α,β-unsaturated ketones are reported along with the mechanistic considerations of the enantio-induction. A wide variety of α,β-unsaturated ketones, including β-aryl, β-alkyl enones, and cyclic enones, were found to be catalyzed by the helical peptide to give Michael adducts with high enantioselectivities (up to 99%). On the basis of X-ray crystallographic analysis and depsipeptide study, the amide protons, N(2)-H and N(3)-H, at the N terminus in the α-helical peptide catalyst were crucial for activating Michael donors, while the N-terminal primary amine activated Michael acceptors through the formation of iminium ion intermediates.

Building Bridges: Biocatalytic C-C-Bond Formation toward Multifunctional Products

Carbon-carbon bond formation is the key reaction for organic synthesis to construct the carbon framework of organic molecules. The review gives a selection of biocatalytic C-C-bond-forming reactions which have been investigated during the last 5 years and which have already been proven to be applicable for organic synthesis. In most cases, the reactions lead to products functionalized at the site of C-C-bond formation (e.g., α-hydroxy ketones, aminoalcohols, diols, 1,4-diketones, etc.) or allow to decorate aromatic and heteroaromatic molecules. Furthermore, examples for cyclization of (non)natural precursors leading to saturated carbocycles are given as well as the stereoselective cyclopropanation of olefins affording cyclopropanes. Although many tools are already available, recent research also makes it clear that nature provides an even broader set of enzymes to perform specific C-C coupling reactions. The possibilities are without limit; however, a big library of variants for different types of reactions is required to have the specific enzyme for a desired specific (stereoselective) reaction at hand.

Enantioselective Synthesis of Quaternary α-Amino Acids via l-tert-Leucine-Derived Squaramide-Catalyzed Conjugate Addition of α-Nitrocarboxylates to Enones

Enantioselective Michael addition of tertiary α-nitroesters to β-unsubstituted vinyl ketones has been carried out in the presence of an l-tert-leucine-derived squaramide as organocatalyst. The products, quaternary α-nitroesters, were formed in excellent yield and moderate to good ee's in most cases. Scale-up of the reaction and synthetic applications of the products, including transformation to representative quaternary α-amino acids, have also been demonstrated.