Diversity of Secondary Structure in Catalytic Peptides with β-Turn-Biased Sequences

Scaffold-Oriented Asymmetric Catalysis: Conformational Modulation of Transition State Multivalency during a Catalyst-Controlled Assembly of a Pharmaceutically Relevant Atropisomer

A new class of superbasic, bifunctional peptidyl guanidine catalysts is presented, which enables the organocatalytic, atroposelective synthesis of axially chiral quinazolinediones. Computational modeling unveiled the conformational modulation of the catalyst by a novel phenyl urea N-cap, that preorganizes the structure into the active, folded state. A previously unanticipated noncovalent interaction involving a difluoroacetamide acting as a hybrid mono- or bidentate hydrogen bond donor emerged as a decisive control element inducing atroposelectivity. These discoveries spurred from a scaffold-oriented project inspired from a fascinating investigational BTK inhibitor featuring two stable chiral axes and relies on a mechanistic framework that was foreign to the extant lexicon of asymmetric catalysis.

Recent Advances in the Field of Amino Acid-Conjugated Aminoferrocenes-A Personal Perspective

The development of turn-based inhibitors of protein-protein interactions has attracted considerable attention in medicinal chemistry. Our group has synthesized a series of peptides derived from an amino-functionalized ferrocene to investigate their potential to mimic protein turn structures. Detailed DFT and spectroscopic studies (IR, NMR, CD) have shown that, for peptides, the backbone chirality and bulkiness of the amino acid side chains determine the hydrogen-bond pattern, allowing tuning of the size of the preferred hydrogen-bonded ring in turn-folded structures. However, their biological potential is more dependent on their lipophilicity. In addition, our pioneering work on the chiroptical properties of aminoferrocene-containing peptides enables the correlation of their geometry with the sign of the CD signal in the absorption region of the ferrocene chromophore. These studies have opened up the possibility of using aminoferrocene and its derivatives as chirooptical probes for the determination of various chirality elements, such as the central chirality of amino acids and the helicity of peptide sequences.

Machine Learning to Develop Peptide Catalysts-Successes, Limitations, and Opportunities

Peptides have been established as modular catalysts for various transformations. Still, the vast number of potential amino acid building blocks renders the identification of peptides with desired catalytic activity challenging. Here, we develop a machine-learning workflow for the optimization of peptide catalysts. First-in a hypothetical competition-we challenged our workflow to identify peptide catalysts for the conjugate addition reaction of aldehydes to nitroolefins and compared the performance of the predicted structures with those optimized in our laboratory. On the basis of the positive results, we established a universal training set (UTS) containing 161 catalysts to sample an in silico library of ∼30,000 tripeptide members. Finally, we challenged our machine learning strategy to identify a member of the library as a stereoselective catalyst for an annulation reaction that has not been catalyzed by a peptide thus far. We conclude with a comparison of data-driven versus expert-knowledge-guided peptide catalyst optimization.

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.

An accurate DFT study within conformational survey of the D-form serine-alanine protected dipeptide

The conformational analysis of N-formyl-D-serine-D-alanine-NH2 dipeptide was studied using density functional theory methods at B3LYP, B3LYP‒D3, and M06‒2X levels using 6‒311 + G (d,p) basis set in the gas and water phases. 87 conformers of 243 stable ones were located and the rest of them were migrated to the more stable geometries. Migration pattern suggests the more stable dipeptide model bears serine in βL, γD, γL and the alanine in γL and γD configurations. The investigation of side‒chain‒backbone interactions revealed that the most stable conformer, γD-γL, is in the β‒turn region of Ramachandran map; therefore, serine-alanine dipeptide model should be adopted with a β‒turn conformation. Intramolecular hydrogen bonding in β‒turns consideration by QTAIM disclosed γD-γL includes three hydrogen bonds. The computed UV‒Vis spectrum alongside of NBO calculation showed the five main electronic transition bands derived of n → n* of intra‒ligand alanine moiety of dipeptide structure.

19 F VT NMR: Novel Tm3+ and Ce3+ Complexes Provide New Insight into Temperature Measurement Using Molecular Sensors

In the past few decades, magnetic resonance spectroscopy (MRS) and MR imaging (MRI) have developed into a powerful non-invasive tool for medical diagnostic and therapy. Especially 19 F MR shows promising potential because of the properties of the fluorine atom and the negligible background signals in the MR spectra. The detection of temperature in a living organism is quite difficult, and usually external thermometers or fibers are used. Temperature determination via MRS needs temperature-sensitive contrast agents. This article reports first results of solvent and structural influences on the temperature sensitivity of 19 F NMR signals of chosen molecules. By using this chemical shift sensitivity, a local temperature can be determined with a high precision. Based on this preliminary study, we synthesized five metal complexes and compared the results of all variable temperature measurements. It is shown that the highest 19 F MR signal temperature dependence is detectable for a fluorine nucleus in a Tm3+ -complex.

Catalytic Asymmetric Hydrogen Atom Transfer: Enantioselective Hydroamination of Alkenes

We report a highly enantioselective radical-based hydroamination of enol esters with sulfonamides jointly catalyzed by an Ir photocatalyst, Brønsted base, and tetrapeptide thiol. This method is demonstrated for the formation of 23 protected β-amino-alcohol products, achieving selectivities up to 97:3 er. The stereochemistry of the product is set through selective hydrogen atom transfer from the chiral thiol catalyst to a prochiral C-centered radical. Structure-selectivity relationships derived from structural variation of both the peptide catalyst and olefin substrate provide key insights into the development of an optimal catalyst. Experimental and computational mechanistic studies indicate that hydrogen-bonding, π-π stacking, and London dispersion interactions are contributing factors for substrate recognition and enantioinduction. These findings further the development of radical-based asymmetric catalysis and contribute to the understanding of the noncovalent interactions relevant to such transformations.

Structural Analysis of Non-native Peptide-Based Catalysts Using 2D NMR-Guided MD Simulations

Proteins and enzymes generally achieve their functions by creating well-defined 3D architectures that pre-organize reactive functionalities. Mimicking this approach to supramolecular pre-organization is leading to the development of highly versatile artificial chemical environments, including new biomaterials, medicines, artificial enzymes, and enzyme-like catalysts. The use of β-turn and α-helical motifs is one approach that enables the precise placement of reactive functional groups to enable selective substrate activation and reactivity/selectivity that approaches natural enzymes. Our recent work has demonstrated that helical peptides can serve as scaffolds for pre-organizing two reactive groups to achieve enzyme-like catalysis. In this study, we used CYANA and AmberTools to develop a computational approach for determining how the structure of our peptide catalysts can lead to enhancements in reactivity. These results support our hypothesis that the bifunctional nature of the peptide enables catalysis by pre-organizing the two catalysts in reactive conformations that accelerate catalysis by proximity. We also present evidence that the low reactivity of monofunctional peptides can be attributed to interactions between the peptide-bound catalyst and the helical backbone, which are not observed in the bifunctional peptide.

Aspartyl β-Turn-Based Dirhodium(II) Metallopeptides for Benzylic C(sp3)-H Amination: Enantioselectivity and X-ray Structural Analysis

Amination of C(sp3)-H bonds is a powerful tool to introduce nitrogen into complex organic frameworks in a direct manner. Despite significant advances in catalyst design, full site- and enantiocontrol in complex molecular regimes remain elusive using established catalyst systems. To address these challenges, we herein describe a new class of peptide-based dirhodium(II) complexes derived from aspartic acid-containing β-turn-forming tetramers. This highly modular system can serve as a platform for the rapid generation of new chiral dirhodium(II) catalyst libraries, as illustrated by the facile synthesis of a series of 38 catalysts. Critically, we present the first crystal structure of a dirhodium(II) tetra-aspartate complex, which unveils retention of the β-turn conformation of the peptidyl ligand; a well-defined hydrogen-bonding network is evident, along with a near-C4 symmetry that renders the rhodium centers inequivalent. The utility of this catalyst platform is illustrated by the enantioselective amination of benzylic C(sp3)-H bonds, in which state-of-the-art levels of enantioselectivity up to 95.5:4.5 er are obtained, even for substrates that present challenges with previously reported catalyst systems. Additionally, we found these complexes to be competent catalysts for the intermolecular amination of N-alkylamides via insertion into the C(sp3)-H bond α to the amide nitrogen, yielding differentially protected 1,1-diamines. Of note, this type of insertion was also observed to occur on the amide functionalities of the catalyst itself in the absence of the substrate but did not appear to be detrimental to reaction outcomes when the substrate was present.

Generality-oriented optimization of enantioselective aminoxyl radical catalysis

Catalytic enantioselective methods that are generally applicable to a broad range of substrates are rare. We report a strategy for the oxidative desymmetrization of meso-diols predicated on a nontraditional catalyst optimization protocol by using a panel of screening substrates rather than a singular model substrate. Critical to this approach was rational modulation of a peptide sequence in the catalyst incorporating a distinct aminoxyl-based active residue. A general catalyst emerged, providing high selectivity in the delivery of enantioenriched lactones across a broad range of diols, while also achieving up to ~100,000 turnovers.

Explainable Deep Hypergraph Learning Modeling the Peptide Secondary Structure Prediction

Accurately predicting peptide secondary structures remains a challenging task due to the lack of discriminative information in short peptides. In this study, PHAT is proposed, a deep hypergraph learning framework for the prediction of peptide secondary structures and the exploration of downstream tasks. The framework includes a novel interpretable deep hypergraph multi-head attention network that uses residue-based reasoning for structure prediction. The algorithm can incorporate sequential semantic information from large-scale biological corpus and structural semantic information from multi-scale structural segmentation, leading to better accuracy and interpretability even with extremely short peptides. The interpretable models are able to highlight the reasoning of structural feature representations and the classification of secondary substructures. The importance of secondary structures in peptide tertiary structure reconstruction and downstream functional analysis is further demonstrated, highlighting the versatility of our models. To facilitate the use of the model, an online server is established which is accessible via http://inner.wei-group.net/PHAT/. The work is expected to assist in the design of functional peptides and contribute to the advancement of structural biology research.

Catalyst-Substrate Helical Character Matching Determines the Enantioselectivity in the Ishihara-Type Iodoarenes Catalyzed Asymmetric Kita-Dearomative Spirolactonization

Catalyst design has traditionally focused on rigid structural elements to prevent conformational flexibility. Ishihara's elegant design of conformationally flexible C₂-symmetric iodoarenes, a new class of privileged organocatalysts, for the catalytic asymmetric dearomatization (CADA) of naphthols is a notable exception. Despite the widespread use of the Ishihara catalysts for CADAs, the reaction mechanism remains the subject of debate, and the mode of asymmetric induction has not been well established. Here, we report an in-depth computational investigation of three possible mechanisms in the literature. Our results, however, reveal that this reaction is best rationalized by a fourth mechanism called "proton-transfer-coupled-dearomatization (PTCD)", which is predicted to be strongly favored over other competing pathways. The PTCD mechanism is consistent with a control experiment and further validated by applying it to rationalize the enantioselectivities. Oxidation of the flexible I(I) catalyst to catalytic active I(III) species induces a defined C₂-symmetric helical chiral environment with a delicate balance between flexibility and rigidity. A match/mismatch effect between the active catalyst and the substrate's helical shape in the dearomatization transition states was observed. The helical shape match allows the active catalyst to adapt its conformation to maximize attractive noncovalent interactions, including I(III)···O halogen bond, N-H···O hydrogen bond, and π···π stacking, to stabilize the favored transition state. A stereochemical model capable of rationalizing the effect of catalyst structural variation on the enantioselectivities is developed. The present study enriches our understanding of how flexible catalysts achieve high stereoinduction and may serve as an inspiration for the future exploration of conformational flexibility for new catalyst designs.

Investigation of the Effect of Turn Residues on Tetrapeptide Aldol Catalysts with β-Turn Propensity

Peptide catalysts for a wide diversity of reaction types contain a common motif-residues that bias the sequence toward β-turn secondary structure. In this work, we explore what role that secondary structure plays in the catalysis of aldol reactions for primary amine tetrapeptide aldol catalysts. Using a lead tetrapeptide β-turn catalytic sequence, we varied the i + 1 and i + 2 residues to amino acids that would affect the β-turn propensity. We then studied the correlation between secondary structure, aldol rate enhancement, and stereoselectivity of the reaction between hydroxyacetone and 4-nitrobenzaldehyde. Using the i + 3 amide chemical shift as a measure of β-turn character, we found a rough correlation between the peptide structure and reaction kinetics but minimal effect on stereoselectivity. These trends may help aid the design of future catalytic sequences.

Mechanistic Investigations on N-Cα Bond Cleavages in Dibasic Peptides Containing Internal Lys and Arg Residues

The model nonapeptide AAARAAKAG* (* indicates amide) is used to explore N-C^α bond fragmentation under CID-MS conditions. Neighboring group participation and the effect of positioning of Lys and Arg residues on N-C^α bond cleavage is established using a library of synthetic peptide analogues. The importance of the Arg residue at position 4 and the i to i+3 spacing between Arg and Lys residues in determining the formation of the N-C^α bond cleaved product ions (c_n) is demonstrated by a comparative MS study of positional variants in analogue peptides. The effect of shortening of the Lys side chain has been established using ornithine (Orn) and diaminobutyric acid (Dab) analogues. The involvement of the Lys residue in mediating the N-C^α bond cleavage is further probed using N^ε-dimethyl and isotopically labeled ¹⁵N^α, ε lysine residues. MSⁿ experiments reveal that the c₆ ion originates from a doubly charged dehydrated b₈ ion [b₈-18]²⁺. The mechanism of this unusual fragmentation process has been probed by using position 8 analogues (Gly, Ala, and Aib). A plausible mechanism is proposed for the origin of the c₆ ion, which involves C-terminus lactam formation followed by transannular cyclization and dehydration. The results presented in this study highlight the role of reactive side chain functionalities in promoting noncanonical fragmentation pathways.

Visible-light mediated catalytic asymmetric radical deuteration at non-benzylic positions

Site- and enantioselective incorporation of deuterium into organic compounds is of broad interest in organic synthesis, especially within the pharmaceutical industry. While catalytic approaches relying on two-electron reaction manifolds have allowed for stereoselective delivery of a formal deuteride (D^–) or deuteron (D⁺) at benzylic positions, complementary strategies that make use of one-electron deuterium atom transfer and target non-benzylic positions remain elusive. Here we report a photochemical approach for asymmetric radical deuteration by utilizing readily available peptide- or sugar-derived thiols as the catalyst and inexpensive deuterium oxide as the deuterium source. This metal-free platform enables four types of deuterofunctionalization reactions of exocyclic olefins and allows deuteration at non-benzylic positions with high levels of enantioselectivity and deuterium incorporation. Computational studies reveal that attractive non-covalent interactions are responsible for stereocontrol. We anticipate that our findings will open up new avenues for asymmetric deuteration.

Catalytic asymmetric deuterations rely largely on two-electron reaction manifolds and are mostly limited to benzylic positions. Here, a metal-free platform using peptide- or sugar-derived chiral thiols and deuterium oxide allows for asymmetric open-shell deuteration at non-benzylic positions under visible-light irradiation.

Impact of Host Flexibility on Selectivity in a Supramolecular Host-Catalyzed Enantioselective aza-Darzens Reaction

A highly enantioselective aza-Darzens reaction (up to 99% ee) catalyzed by an enantiopure supramolecular host has been discovered. To understand the role of host structure on reaction outcome, nine new gallium(III)-based enantiopure supramolecular assemblies were prepared via substitution of the external chiral amide. Despite the distal nature of the substitution in these catalysts, changes in enantioselectivity (61 to 90% ee) in the aziridine product were observed. The enantioselectivities were correlated to the flexibility of the supramolecular host scaffold as measured by the kinetics of exchange of a model cationic guest. This correlation led to the development of a best-in-class catalyst by substituting the gallium(III)-based host with one based on indium(III), which generated the most flexible and selective catalyst.

Synthesis and Structural Characterization of β-Turn Mimics Containing (Z)-Chloroalkene Dipeptide Isosteres

Described here is the synthetic, spectroscopic, crystallographic, and computational analysis of a series of peptidomimetics containing l-Xaa-d-Yaa-type (Z)-chloroalkene dipeptide isosteres (CADIs) that were measured in an investigation of the β-turn mimicry of this peptide bond surrogate. We found that the 1,3-allylic strain across the chloroalkene moiety engenders the hyperconjugative interactions between the chloroalkene moiety and the C-H bonding or antibonding orbitals of the C-H bonds in allylic positions. These effects contribute significantly to the stabilization of β-turn structures.

Mariz B, Moreira IP, Kalafatovic D, Morais Faustino BM, Narang V, Wang T, Pappas CG, Ferreira I, Roque ACA, Ulijn RV. Discovery of phosphotyrosine-binding oligopeptides with supramolecular target selectivity

Phage-display screening on self-assembled tyrosine-phosphate ligands enables the identification of oligopeptides selective to dynamic supramolecular targets, with the lead peptide showing a preferred hairpin-like conformation and catalytic activity.

Conformational Preferences and Antiproliferative Activity of Peptidomimetics Containing Methyl 1'-Aminoferrocene-1-carboxylate and Turn-Forming Homo- and Heterochiral Pro-Ala Motifs

The concept of peptidomimetics is based on structural modifications of natural peptides that aim not only to mimic their 3D shape and biological function, but also to reduce their limitations. The peptidomimetic approach is used in medicinal chemistry to develop drug-like compounds that are more active and selective than natural peptides and have fewer side effects. One of the synthetic strategies for obtaining peptidomimetics involves mimicking peptide α-helices, β-sheets or turns. Turns are usually located on the protein surface where they interact with various receptors and are therefore involved in numerous biological events. Among the various synthetic tools for turn mimetic design reported so far, our group uses an approach based on the insertion of different ferrocene templates into the peptide backbone that both induce turn formation and reduce conformational flexibility. Here, we conjugated methyl 1'-aminoferrocene-carboxylate with homo- and heterochiral Pro-Ala dipeptides to investigate the turn formation potential and antiproliferative properties of the resulting peptidomimetics 2-5. Detailed spectroscopic (IR, NMR, CD), X-ray and DFT studies showed that the heterochiral conjugates 2 and 3 were more suitable for the formation of β-turns. Cell viability study, clonogenic assay and cell death analysis showed the highest biological potential of homochiral peptide 4.

Unusual Pseudopeptides: Syntheses and Structural Analyses of Ethylenediprolyl Peptides and Their Metal Complexes with Cu(II) Ion

The synthetic unnatural amino acids and their peptides as peptidomimetics have shown remarkable structural and functional properties. In the repertoire of synthetic peptides, pseudopeptides have emerged as attractive small peptidomimetics that are capable of forming the characteristic secondary structures in the solid/solution phase, as in natural peptides. This report describes the synthesis and structural analyses of novel pseudopeptides as ethylenediprolyl (etpro) tetra/hexapeptides, comprising a chiral diaminedicarboxylate scaffold. Their NMR and CD spectral analyses strongly support the formation of the β-turn-type structures in organic solvents (ACN/MeOH). Further, the single-crystal X-ray studies of tetrapseudopeptide confirm the formation of a unique self-assembly structure as β-strand type in the solid state through hydrogen bonding. Importantly, their diamine moiety influences the formation of Cu-complexes with Cu(II) ions. A tetrapseudopeptide monocarboxylate-Cu(II) complex forms the single crystal that is studied by the single-crystal X-ray diffractometer. The crystal structure of the tetrapseudopeptide-Cu(II) complex confirms the formation of the distorted square planar geometry structure, almost like the amyloid β(Aβ)-peptide-Cu(II) complex structural geometry. Hence, these etpro-pseudopeptides are emerging peptidomimatics that form β-turn types of structures and metal complexes mainly with Cu(II) ions. These molecules could be considered for the development of peptide-based catalysts and peptide-based therapeutic drug candidates.

Tunable and Cooperative Catalysis for Enantioselective Pictet-Spengler Reaction with Varied Nitrogen-Containing Heterocyclic Carboxaldehydes

Herein we report an organocatalytic enantioselective functionalization of heterocyclic carboxaldehydes via the Pictet-Spengler reaction. Through careful pairing of novel squaramide and Brønsted acid catalysts, our method tolerates a breadth of heterocycles, enabling preparation of a series of heterocycle conjugated β-(tetrahydro)carbolines in good yield and enantioselectivity. Careful selection of carboxylic acid co-catalyst is essential for toleration of a variety of regioisomeric heterocycles. Utility is demonstrated via the three-step stereoselective preparation of pyridine-containing analogues of potent selective estrogen receptor downregulator and U.S. FDA approved drug Tadalafil.

Macrocylases as synthetic tools for ligand synthesis: enzymatic synthesis of cyclic peptides containing metal-binding amino acids

Improving the sustainability of synthesis is a major goal in green chemistry, which has been greatly aided by the development of asymmetric transition metal catalysis. Recent advances in asymmetric catalysis show that the ability to control the coordination sphere of substrates can lead to improvements in enantioselectivity and activity, in a manner resembling the operation of enzymes. Peptides can be used to mimic enzyme structures and their secondary interactions and they are easily accessible through solid-phase peptide synthesis. Despite this, cyclic peptides remain underexplored as chiral ligands for catalysis due to synthetic complications upon macrocyclization. Here, we show that the solid-phase synthesis of peptides containing metal-binding amino acids, bipyridylalanine (1), phenyl pyridylalanine (2) and N,N-dimethylhistidine (3) can be combined with peptide macrocylization using peptide cyclase 1 (PCY1) to yield cyclic peptides under mild conditions. High conversions of the linear peptides were observed (approx. 90%) and the Cu-bound cyclo(FSAS(1)SSKP) was shown to be a competent catalyst in the Friedel-Crafts/conjugate addition of indole. This study shows that PCY1 can tolerate peptides containing amino acids with classic inorganic and organometallic ligands as side chains, opening the door to the streamlined and efficient development of cyclic peptides as metal ligands.

Acyclic Twisted Amides

In this contribution, we provide a comprehensive overview of acyclic twisted amides, covering the literature since 1993 (the year of the first recognized report on acyclic twisted amides) through June 2020. The review focuses on classes of acyclic twisted amides and their key structural properties, such as amide bond twist and nitrogen pyramidalization, which are primarily responsible for disrupting n_N to π*_C═O conjugation. Through discussing acyclic twisted amides in comparison with the classic bridged lactams and conformationally restricted cyclic fused amides, the reader is provided with an overview of amidic distortion that results in novel conformational features of acyclic amides that can be exploited in various fields of chemistry ranging from organic synthesis and polymers to biochemistry and structural chemistry and the current position of acyclic twisted amides in modern chemistry.

δ-Valerolactamic Quaternary Amino Acid Derivatives: Enantiodivergent Synthesis and Evidence for Stereodifferentiated β-Turn-Inducing Properties

Enantiopure (R) and (S) cyclic α,α-disubstituted amino acid derivatives displaying a δ-valerolactam side chain were prepared from a common isoxazolidine precursor. The (R)-configured δ-valerolactam 11 was converted into diastereoisomeric pseudopeptides to investigate its ability to induce secondary structures in peptidomimetics. Conformational studies of these pseudopeptides were carried out in the solid state (X-ray diffraction), in solution (NMR analyses), and in silico (computer-aided conformational analysis), which demonstrated that such quaternary amino acids induce β-turn conformations stable enough to be retained in polar media (DMSO). Incorporation of this new type of α,α-disubstituted amino acid into a representative pseudopeptidic sequence by N- then C-elongation and N-debenzylation is also described herein and could serve for the synthesis of various structured peptidomimetics.

Catalytic Enantioselective Synthesis of Pyridyl Sulfoximines

With unique chemical and biological activity, sulfoximines have attracted enormous attention in the past decades, whereas limited reports exist for their synthesis via asymmetric catalysis. We report the synthesis of chiral sulfoximines through the desymmetrizing N-oxidation of pyridyl sulfoximines using an aspartic acid-containing peptide catalyst. Various mono- and bis-pyridyl sulfoximine oxides are obtained with up to 99:1 er. The directing group introduced on the substrate highly enhances the enantioinduction and could be easily removed to give the free N-H sulfoximines. Additionally, peptides with methyl ester and the methyl amide C-terminal protecting group give the opposite enantiomers of the product. A binding model is proposed to explain this phenomenon.

Heterocycles as a Peptidomimetic Scaffold: Solid-Phase Synthesis Strategies

Peptidomimetics are a privileged class of pharmacophores that exhibit improved physicochemical and biological properties. Solid-phase synthesis is a powerful tool for gaining rapid access to libraries of molecules from small molecules to biopolymers and also is widely used for the synthesis of peptidomimetics. Small molecules including heterocycles serve as a core for hundreds of drugs, including peptidomimetic molecules. This review covers solid-phase synthesis strategies for peptidomimetics molecules based on heterocycles.

Isolating Conformers to Assess Dynamics of Peptidic Catalysts Using Computationally Designed Macrocyclic Peptides

Studying the relationship between catalyst conformational dynamics and selectivity in an asymmetric reaction is a challenge. In this study, cyclic peptides were computationally designed to stabilize different ground state conformations of a highly effective, flexible tetrapeptide catalyst for the atroposelective bromination of N-aryl quinazolinones. Through a combination of computational and experimental techniques, we have determined that dynamic movement of the lead catalyst plays a crucial role in achieving high enantioselectivity in the reaction of study. This approach may also serve as a valuable method for investigating the mechanism of other peptide-catalyzed transformations.

Two-Dimensional Barriers for Probing Conformational Shifts in Macrocycles

We describe the development and use of composite two-dimensional barriers in macrocyclic backbones. These tunable constructs derive their mode of action from heterocyclic rearrangements. The Boulton-Katritzky reaction has been identified as a particularly versatile means to effect a composite barrier, allowing the examination of the influence of heterocycle translocation on conformation. Kinetic studies using ¹H NMR have revealed that the in-plane atom movement is fast in 17, 18, 19-membered rings but slows down in 16-membered rings. The analysis by NMR and MD simulation experiments is consistent with the maintenance of rare cis-amide motifs during conformational interconversion. Taken together, our investigation demonstrates that heterocyclic rearrangement reactions can be used to control macrocyclic backbones and provides fundamental insights that may be applicable to the development of a wide range of other conformational control elements.

The Size-Accelerated Kinetic Resolution of Secondary Alcohols

The factors responsible for the kinetic resolution of alcohols by chiral pyridine derivatives have been elucidated by measurements of relative rates for a set of substrates with systematically growing aromatic side chains using accurate competitive linear regression analysis. Increasing the side chain size from phenyl to pyrenyl results in a rate acceleration of more than 40 for the major enantiomer. Based on this observation a new catalyst with increased steric bulk has been designed that gives enantioselectivity values of up to s=250. Extensive conformational analysis of the relevant transition states indicates that alcohol attack to the more crowded side of the acyl-catalyst intermediate is favoured due to stabilizing CH-π-stacking interactions. Experimental and theoretical results imply that enantioselectivity enhancements result from accelerating the transformation of the major enantiomer through attractive non-covalent interactions (NCIs) rather than retarding the transformation of the minor isomer through repulsive steric forces.

Hybrid Peptide-Thiourea Catalyst for Asymmetric Michael Additions of Aldehydes to Heterocyclic Nitroalkenes

Bifunctional organocatalysis combining covalent and noncovalent activation is presented. The hybrid peptide-thiourea catalyst features a N-terminal proline moiety for aldehyde activation and a thiourea unit for electrophile activation. This catalyst effectively promotes asymmetric Michael additions of aldehydes to challenging but biologically relevant heterocycle-containing nitroalkenes. The catalyst can be used under solvent-free conditions. Spectroscopic and density functional theory studies elucidate the catalyst structure and mode of action.

Traceless solid-phase synthesis and β-turn propensity of 1,3-thiazole-based peptidomimetics

The design and solid-phase synthesis of 1,3-thiazole-based peptidomimetic molecules is described. The solid-phase synthesis was based on the utilization of a traceless linker strategy. The synthesis starts from the conversion of chloromethyl polystyrene resin to the resin with a sulfur linker unit. The key intermediate 4-amino-thiazole-5-carboxylic acid resin is prepared in three steps from Merrifield resin. The amide coupling proceeded at the C4 and C5 positions via an Fmoc solid-phase peptide synthesis strategy. After cleavage, the final compounds were obtained in moderate yields (average 9%, 11-step overall yields) with high purities (≥87%). Geometric measurements of Cα distances and dihedral angles along with an rmsd of 0.5434 for attachment with Cα of the β-turn template suggest type IV β-turn structural motifs. Additionally, the physicochemical properties of the molecules have been evaluated.

Effects of turn-structure on folding and entanglement in artificial molecular overhand knots

The length and constitution of spacers linking three 2,6-pyridinedicarboxamide units in a molecular strand influence the tightness of the resulting overhand (open-trefoil) knot that the strand folds into in the presence of lanthanide(iii) ions. The use of β-hairpin forming motifs as linkers enables a metal-coordinated pseudopeptide with a knotted tertiary structure to be generated. The resulting pseudopeptide knot has one of the highest backbone-to-crossing ratios (BCR)—a measure of knot tightness (a high value corresponding to looseness)—for a synthetic molecular knot to date. Preorganization in the crossing-free turn section of the knot affects aromatic stacking interactions close to the crossing region. The metal-coordinated pseudopeptide knot is compared to overhand knots with other linkers of varying tightness and turn preorganization, and the entangled architectures characterized by NMR spectroscopy, ESI-MS, CD spectroscopy and, in one case, X-ray crystallography. The results show how it is possible to program specific conformational properties into different key regions of synthetic molecular knots, opening the way to systems where knotting can be systematically incorporated into peptide-like chains through design.

Spacers linking 2,6-pyridinedicarboxamide units influence the tightness of the corresponding lanthanide-coordinated overhand knot. β-Hairpin forming motifs generate a metal-coordinated pseudopeptide with a knotted tertiary structure.

Asymmetric Catalysis Mediated by Synthetic Peptides, Version 2.0: Expansion of Scope and Mechanisms

Low molecular weight synthetic peptides have been demonstrated to be effective catalysts for an increasingly wide array of asymmetric transformations. In many cases, these peptide-based catalysts have enabled novel multifunctional substrate activation modes and unprecedented selectivity manifolds. These features, along with their ease of preparation, modular and tunable structures, and often biomimetic attributes make peptides well-suited as chiral catalysts and of broad interest. Many examples of peptide-catalyzed asymmetric reactions have appeared in the literature since the last survey of this broad field in Chemical Reviews (Chem. Rev. 2007, 107, 5759-5812). The overarching goal of this new Review is to provide a comprehensive account of the numerous advances in the field. As a corollary to this goal, we survey the many different types of catalytic reactions, ranging from acylation to C-C bond formation, in which peptides have been successfully employed. In so doing, we devote significant discussion to the structural and mechanistic aspects of these reactions that are perhaps specific to peptide-based catalysts and their interactions with substrates and/or reagents.

Catalytic Dynamic Kinetic Resolutions in Tandem to Construct Two-Axis Terphenyl Atropisomers

The defined structure of molecules bearing multiple stereogenic axes is of increasing relevance to materials science, pharmaceuticals, and catalysis. However, catalytic enantioselective approaches to control multiple stereogenic axes remain synthetically challenging. We report the catalytic synthesis of two-axis terphenyl atropisomers, with complementary strategies to both chlorinated and brominated variants, formed with high diastereo- and enantioselectivity. The chemistry proceeds through a sequence of two distinct dynamic kinetic resolutions: first, an atroposelective ring opening of Bringmann-type lactones produces a product with one established axis of chirality, and second, a stereoselective arene halogenation delivers the product with the second axis of chirality established. In order to achieve these results, a class of Brønsted basic guanidinylated peptides, which catalyze an efficient atroposelective chlorination, is reported for the first time. In addition, a complementary bromination is reported, which also establishes the second stereogenic axis. These bromo-terphenyls are accessible following the discovery that chiral anion phase transfer catalysis by C₂-symmetric phosphoric acids allows catalyst control in the second stereochemistry-determining event. Accordingly, we established the fully catalyst-controlled stereodivergent synthesis of all possible chlorinated stereoisomers while also demonstrating diastereodivergence in the brominated variants, with significant levels of enantioselectivity in all cases.

Turn Conformation of β-Amino Acid-Based Short Peptides Promoted by an Amidothiourea Moiety at C-Terminus

A C-terminal amidothiourea motif is shown to promote a β-turn-like folded conformation in a series of β-amino acid-based short peptides in both the solid state and solution phase by an intramolecular 11-membered ring hydrogen bond.

Asymmetric Catalysis upon Helically Chiral Loratadine Analogues Unveils Enantiomer-Dependent Antihistamine Activity

Analogues of the conformationally dynamic Claritin (loratadine) and Clarinex (desloratadine) scaffolds have been enantio- and chemoselectively N-oxidized using an aspartic acid containing peptide catalyst to afford stable, helically chiral products in up to >99:1 er. The conformational dynamics and enantiomeric stability of the N-oxide products have been investigated experimentally and computationally with the aid of crystallographic data. Furthermore, biological assays show that rigidifying the core structure of loratadine and related analogues through N-oxidation affects antihistamine activity in an enantiomer-dependent fashion. Computational docking studies illustrate the observed activity differences.

Balancing interactions in proline-based receptors for chiral recognition of l-/d-DOPA

Proline based receptors (1-14) attached with phenylboronic acid and benzaldehyde binding groups at the N-/C- or C-/N-termini of the proline residue were created for chiral recognition of l-/d-DOPA, in an attempt to examine if balancing the two binding events would influence the recognition. By changing the positions of boronic acid and aldehyde groups substituted on the phenyl rings (1-4, 5-8) and the site at which phenylboronic acid and benzaldehyde moieties attached respectively to the N- and C-termini or C- and N-termini of the proline residue (1-4vs.5-8), and by introducing an electron-withdrawing fluorine atom in the phenyl ring of the weaker binder the benzaldehyde moiety (11vs.1, 14vs.5), we were able to show that a better balance of the two binding events does improve the chiral recognition. This finding can only be made with the current version of receptors that were equipped with two different binding groups. Together with the finding that the chiral recognition performance in mixed organic-aqueous solutions is tunable by varying the solvent composition, we have now arrived at a protocol for designing proline based receptors for extended applications in chiral recognition.

Recent advances in reactions promoted by amino acids and oligopeptides

During the last 20 years, Organocatalysis has become one of the major fields of Catalysis. Herein, we provide a recent overview on reactions where the use of amino acids and peptides as the organocatalysts was employed. All aspects regarding aldol reactions, Michael reactions, epoxidation, Henry reactions and many others that are crucial for the reaction conditions and reaction mechanisms are discussed.

Peptide-Catalyzed Highly Asymmetric Cross-Aldol Reaction of Aldehydes to Biomimetically Synthesize 1,4-Dicarbonyls

β-Turn tetrapeptides were demonstrated to catalyze asymmetric aldol reaction of α-branched aldehydes and α-carbonyl aldehydes, i.e. glyoxylates and α-ketoaldehydes, to biomimetically synthesize acyclic all-carbon quaternary center-bearing 1,4-dicarbonyls in high yield and excellent enantioselectivity under mild conditions. The spatially restricted environment of the tetrapeptide warrants high enantioselectivity and yield with broad substrates. Using this protocol, (R)-pantolactone, the key intermediate of vitamin B5, was readily accessed in a practical, efficient, and environmentally benign process from inexpensive starting materials.

Effect on the Conformation of a Terminally Blocked, (E) β,γ-Unsaturated δ-Amino Acid Residue Induced by Carbon Methylation

Peptides are well-known to play a fundamental therapeutic role and to represent building blocks for numerous useful biomaterials. Stabilizing their active 3D-structure by appropriate modifications remains, however, a challenge. In this study, we have expanded the available literature information on the conformational propensities of a promising backbone change of a terminally blocked δ-amino acid residue, a dipeptide mimic, by replacing its central amide moiety with an (E) C^β═C^γ alkene unit. Specifically, we have examined by DFT calculations, X-ray diffraction in the crystalline state, and FT-IR absorption/NMR spectroscopies in solution the extended vs folded preferences of analogues of this prototype system either unmodified or possessing single or multiple methyl group substituents on each of its four -CH₂-CH═CH-CH₂- main-chain carbon atoms. The theoretical and experimental results obtained clearly point to the conclusion that increasing the number of adequately positioned methylations will enhance the preference of the original sequence to fold, thus opening interesting perspectives in the design of conformationally constrained peptidomimetics.