Conformational Properties of a Peptidic Catalyst: Insights from NMR Spectroscopic Studies

Catalytic Enantioconvergent Alkylation Reactions That Construct Chiral Quaternary Carbon Centers and Tune C(sp3)-C(sp2) Bond Rotation

In molecules with central chiral centers functionalized with aryl groups, the rotation of the single bond between the aryl group and the central chiral center may be hindered. Enantioconvergent alkylation reactions of racemic diastereomers to construct all-carbon quaternary stereocenters resulting in easing of the rotation around the axes are described. We demonstrated that the rotation of the bond between a central chiral carbon center and a dihydrobenzofuranone can be tuned by selection of either a tertiary carbon bearing a hydrogen or a quaternary carbon at the central chiral center. The products have the benefits of a central chiral center and a flexibly rotatable bond, which can facilitate interactions with other molecules.

Peptide Catalyzed Conjugate Additions to β-Nitroacrylates - Steric Bulk Increases the Reaction Rate

The organocatalytic conjugate addition of aldehydes to β-nitroacrylates provides direct access to β-ester-γ-nitroaldehydes and, thereby, common structural motifs of many bioactive compounds. However, the deactivation of amine-based catalysts by alkylation with the highly electrophilic nitroacrylates hampers this reaction. Here, we show that the peptide H-Mep-dPro-dGlu-NH2, which is reluctant to undergo alkylation, catalyzes this reaction at low catalyst loading (0.5-1 mol %) within short reaction times (15-60 min) to yield a broad range of β-ester-γ-nitroaldehydes with high stereoselectivity. Kinetic studies revealed that increased steric bulk on the β-nitroacrylate enhances the reaction rate by hindering catalyst alkylation.

Kinetic Resolution of β-Branched Aldehydes through Peptide-Catalyzed Conjugate Addition Reactions

The catalytic kinetic resolution of racemic β-branched aldehydes offers a straightforward stereoselective entry to aldehydes and addition products. Yet, control over stereoselectivity is difficult due to the conformational flexibility of β-branched aldehydes. Here, we show that the peptide catalyst H-dPro-αMePro-Glu-NH2 resolves β-branched aldehydes through reaction with nitroolefins and provides γ-nitroaldehydes with three consecutive stereogenic centers in high yields and stereoselectivities. Kinetic, NMR spectroscopic, and computational studies provided insights into the selectivity-determining step and origins of the kinetic resolution.

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.

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.

Temperature-Controllable Liquid Crystalline Medium for Stereochemical Elucidation of Organic Compounds via Residual Chemical Shift Anisotropies

The configuration elucidation of organic molecules continues to pose significant challenges in studies involving stereochemistry. Nuclear magnetic resonance (NMR) techniques are powerful for obtaining such structural information. Anisotropic NMR techniques, such as measurement of residual dipolar couplings (RDCs) and residual chemical shift anisotropies (RCSAs), complementing isotropic NMR parameters, provide relative configuration information. RCSAs provide valuable structural information, especially for nonprotonated carbons, yet have been severely underutilized due to the lack of an easily operational alignment medium capable of rapid transition from anisotropic to isotropic environments, especially in aqueous conditions. In this study, an oligopeptide-based alignment media (FK)4 is presented for RCSA measurements. Temperature variation manipulates the assembly of (FK)4, yielding tunable anisotropic and isotropic phases without the requirement of any special devices or time-consuming correction procedures during data analysis. Decent observed ΔΔRCSA values from sp3 carbons benefit the utilization of RCSA measurements in the structural elucidation of organic molecules highly composed with sp3 carbons. Moreover, the (FK)4 alignment medium is applicable for both RDC and RCSA measurements in one sample, further advancing the configuration analysis of molecules of interest.

Overcoming Deactivation of Amine-Based Catalysts: Access to Fluoroalkylated γ-Nitroaldehydes

Organocatalytic conjugate addition reactions of aldehydes to fluoroalkylated nitroolefins with chiral amine catalysts offer a straightforward stereoselective path to fluoroalkylated γ-nitroaldehydes and downstream derivatives. However, amine-based catalysts suffer from deactivation by reaction with electron-poor fluoroalkylated nitroolefin. Here, we show that catalyst deactivation can be overcome by catalysts that bear an intramolecular acid for protonation and release of the alkylated catalyst through ß-elimination of the nitroolefin. NMR spectroscopic, kinetic, and molecular modeling studies provided detailed structural and mechanistic insights into the factors that control reversible catalyst alkylation and facilitate efficient catalysis.

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.

Organocatalytic Synthesis of Triflones Bearing Two Non-Adjacent Stereogenic Centers

Trifluoromethylsulfones (triflones) are useful compounds for synthesis and beyond. Yet, methods to access chiral triflones are scarce. Here, we present a mild and efficient organocatalytic method for the stereoselective synthesis of chiral triflones using α-aryl vinyl triflones, building blocks previously unexplored in asymmetric synthesis. The peptide-catalyzed reaction gives rise to a broad range of γ-triflylaldehydes with two non-adjacent stereogenic centers in high yields and stereoselectivities. A catalyst-controlled stereoselective protonation following a C-C bond formation is key to control over the absolute and relative configuration. Straightforward derivatization of the products into, e.g., disubstituted δ-sultones, γ-lactones, and pyrrolidine heterocycles highlights the synthetic versatility of the products.

Site-Selective Acylation of Pyranosides with Immobilized Oligopeptide Catalysts in Flow

We report the site-selective acetylation of partially protected monosaccharides using immobilized oligopeptide catalysts, which are readily accessible via solid-phase peptide synthesis. The catalysts are able to invert the intrinsic selectivity, which was determined using N-methylimidazole, for a variety of pyranosides. We demonstrate that the catalysts are stable for multiple reaction cycles and can be easily reused after separation from the reaction solution. The catalysts can also be used in flow without loss of reactivity and selectivity.

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.

Palladium-peptide oxidative addition complexes for bioconjugation

The synthesis of palladium oxidative addition complexes derived from unprotected peptides is described. Incorporation of 4-halophenylalanine into a peptide during solid phase peptide synthesis allows for subsequent oxidative addition at this position upon treatment with a palladium precursor and suitable ligand. The resulting palladium-peptide complexes are solid, storable, water-soluble, and easily purified via high-performance liquid chromatography. These complexes react with thiols in aqueous buffer, offering an efficient method for bioconjugation. Using this strategy, peptides can be functionalized with small molecules to prepare modified aryl thioether side-chains at low micromolar concentrations. Additionally, peptide-peptide and peptide-protein ligations are demonstrated under dilute aqueous conditions.

Controlling the Enantioselectivity in an Adaptable Ligand by Biomimetic Intramolecular Interlocking

For the preparation of chiral drugs, both stereochemically stable and flexible catalysts in combination with chiral auxiliaries can be used. Here, chiral induction plays an important role in generating an enantiomerically pure catalyst. We demonstrate a successful approach to the spontaneous deracemization of tropos ligands for asymmetric catalysis. Three different constitutional isomers of a bisphosphinite ligand decorated with l-valine moieties (interaction units) linked to the flexible biphenyl system by a phenylene bridge for inducing a chiral switch were prepared. The substitution pattern's influence on the attached intermolecular recognition sites was systematically investigated. We can show that biomimetic intramolecular hydrogen bonding leads to a pronounced diastereoselective enrichment of one of the ligand stereoisomers. As a result, in the asymmetric Rh-catalyzed hydrogenation of prochiral olefins using these ligands, enantiomeric ratios of up to 95.8:4.2 (S) were obtained. Of particular note is the inversion of enantioselectivity relative to the previously reported BIBIPHOS-Rh catalyst due to the altered orientation of the biphenyl moiety from (R_ax) to (S_ax). The enantioselectivities achieved by appropriate intramolecular interlocking are remarkable for a tropos ligand/catalyst. The strategy presented here represents a powerful approach for the spontaneous alignment of tropos ligands, yielding high enantioselectivities in asymmetric catalysis.

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.

Peptide-Catalyzed Stereoselective Conjugate Addition Reaction of Aldehydes to C-Substituted Maleimides

Catalytic stereoselective additions with maleimides are useful one-step reactions to yield chiral succinimides, molecules that are widespread among therapeutically active compounds but challenging to prepare when the maleimide is C-substituted. We present the tripeptide H-Pro-Pro-Asp-NHC12 H25 as a catalyst for conjugate addition reactions between aldehydes and C-substituted maleimides to form succinimides with three contiguous stereogenic centers in high yields and stereoselectivities. The peptidic catalyst is so chemoselective that no protecting group is needed at the imide nitrogen of the maleimides. Derivatization of the succinimides was straightforward and provided access to chiral pyrrolidines, lactones, and lactams. Kinetic studies, including a Hammett plot, provided detailed insight into the reaction mechanism.

Systems chemistry of peptide-assemblies for biochemical transformations

During the billions of years of the evolutionary journey, primitive polymers, involved in proto metabolic pathways with low catalytic activity, played critical roles in the emergence of modern enzymes with remarkable substrate specificity. The precise positioning of amino acid residues and the complex orchestrated interplay in the binding pockets of evolved enzymes promote covalent and non-covalent interactions to foster a diverse set of complex catalytic transformations. Recent efforts to emulate the structural and functional information of extant enzymes by minimal peptide based assemblies have attempted to provide a holistic approach that could help in discerning the prebiotic origins of catalytically active binding pockets of advanced proteins. In addition to the impressive sets of advanced biochemical transformations, catalytic promiscuity and cascade catalysis by such small molecule based dynamic systems can foreshadow the ancestral catalytic processes required for the onset of protometabolism. Looking beyond minimal systems that work close to equilibrium, catalytic systems and compartments under non-equilibrium conditions utilizing simple prebiotically relevant precursors have attempted to shed light on how bioenergetics played an essential role in chemical emergence of complex behaviour. Herein, we map out these recent works and progress where diverse sets of complex enzymatic transformations were demonstrated by utilizing minimal peptide based self-assembled systems. Further, we have attempted to cover the examples of peptide assemblies that could feature promiscuous activity and promote complex multistep cascade reaction networks. The review also covers a few recent examples of minimal transient catalytic assemblies under non-equilibrium conditions. This review attempts to provide a broad perspective for potentially programming functionality via rational selection of amino acid sequences leading towards minimal catalytic systems that resemble the traits of contemporary enzymes.

Stereoselective Peptide Catalysis in Complex Environments – From River Water to Cell Lysates

Many stereoselective peptide catalysts have been established. They consist, like nature's catalysts, of amino acids but have significantly lower molecular weights than enzymes. Whereas enzymes operate with exquisite chemoselectivity in complex biological environments, peptide catalysts are used in pure organic solvents and at higher concentrations. Can a peptide catalyst exhibit chemoselectivity reminiscent of enzymes? Here, we investigated the properties of tripeptide catalysts in complex mixtures in hydrophobic and aqueous solvents. We challenged the catalysts with biomolecules bearing functional groups that could interfere by coordination or reaction with the peptide, the substrates, or intermediates. H-dPro-αMePro-Glu-NHC₁₂H₁₅ emerged through tailoring of the trans/cis ratio of the tertiary amide as a conformationally well-defined tripeptide that catalyzes C–C bond formations with high reactivity and stereoselectivity – regardless of the solvent and compound composition. The chemoselectivity of the tripeptide is so high that it even catalyzes reactions in cell lysates. The findings provoke the question of the potential role of peptide catalysis in nature and during the evolution of enzymes.

The reactivity, stereo-, and chemoselectivity of a tripeptide are so high that it catalyzes conjugate addition reactions with high stereoselectivity in complex compound mixtures—even in cell lysates.

Synergistic Peptide and Gold Catalysis: Enantioselective Addition of Branched Aldehydes to Allenamides

The combination of a peptide catalyst and a gold catalyst is presented for enantioselective addition reactions between branched aldehydes and allenamides. The two catalysts act in concert to provide γ,δ-enamide aldehydes bearing a fully substituted, benzylic stereogenic center - a structural motif common in many natural products and therapeutically active compounds - with good yields and enantioselectivities. The reaction tolerates a variety of alkyl and alkoxy substituted aldehydes and the products can be elaborated into several chiral building blocks bearing either 1,4- or 1,5- functional group relationships. Mechanistic studies showed that the conformational features of the peptide are important for both the catalytic efficiency and stereochemistry, while a balance of acid/base additives is key for ensuring formation of the desired product over undesired side reactions.

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.

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.

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.

Amine Catalysis with Substrates Bearing N-Heterocyclic Moieties Enabled by Control over the Enamine Pyramidalization Direction

Stereoselective organocatalytic C-C bond formations that tolerate N-heterocycles are valuable since these moieties are common motifs in numerous chiral bioactive compounds. Such transformations are, however, challenging since N-heterocyclic moieties can interfere with the catalytic reaction. Here, we present a peptide that catalyzes conjugate addition reactions between aldehydes and nitroolefins bearing a broad range of different N-heterocyclic moieties with basic and/or H-bonding sites in excellent yields and stereoselectivities. Tuning of the pyramidalization direction of the enamine intermediate enabled the high stereoselectivity.

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.

N-Primary-amine tetrapeptide-catalyzed highly asymmetric Michael addition of aliphatic aldehydes to maleimides

The highly asymmetric Michael addition reaction between maleimides and aliphatic aldehydes catalyzed by low-loading β-turn tetrapeptides with excellent yields and enantioselectivities at room temperature was reported. α-Branched and α-unbranched aldehydes both are suitable nucleophiles. N-Aryl, alkyl and hydrogen maleimides all are well tolerated and led to high yields and enantioselectivities. The transformation can be enlarged to the gram scale without decrease in the yield and enantioselectivity. Furthermore, the succinimides were converted into γ-lactams and γ-lactones, showing good practicality of this work. Some reaction intermediates in the proposed reaction mechanism can be captured with the HR-MS method.

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.

Catalysis-Enabled Access to Cryptic Geldanamycin Oxides

Catalytic, selective modifications of natural products can be a fertile platform for not only unveiling new natural product analogues with altered biological activity, but also for revealing new reactivity and selectivity hierarchies for embedded functional groups in complex environments. Motivated by these intersecting aims, we report site- and stereoselective oxidation reactions of geldanamycin facilitated by aspartyl-peptide catalysts. Through the isolation and characterization of four new geldanamycin oxides, we discovered a synergistic effect between lead peptide-based catalysts and geldanamycin, resulting in an unexpected reaction pathway. Curiously, our discoveries would likely not have been possible absent the attractive noncovalent interactions intrinsic to both the catalysts and the natural product. The result is a set of new "meta" catalytic reactions that deliver both unknown and previously incompletely characterized geldanamycin analogues. Enabled by the catalytic, site-selective epoxidation of geldanamycin, biological assays were carried out to document the bioactivities of the new compounds.

Stereochemical Elucidation of Natural Products from Residual Chemical Shift Anisotropies in a Liquid Crystalline Phase

Determination of the stereochemistry of organic molecules still represents one of the major obstacles in the structure elucidation procedure in drug discovery. Although the application of residual dipolar couplings (RDCs) has revolutionized this field, residual chemical shift anisotropies (RCSAs) which contain valuable structural information for nonprotonated carbons have only been scarcely employed so far. In this study, we present a simple but highly effective solution to extract RCSAs of the analytes in a liquid crystalline phase formed by AAKLVFF oligopeptides. This method does not require any special instruments, devices, or correction during postacquisition data analysis and thus can be easily applied in any chemistry laboratory. To illustrate the potential of this method, the relative configurations of four known natural products (1-4) belonging to different structural classes were confirmed. Moreover, we unambiguously elucidated the stereochemistry of spiroepicoccin A (5), a rare thiodiketopiperazine marine natural product whose configuration could not be assigned based on conventional NMR methods.

Effect of the enamine pyramidalization direction on the reactivity of secondary amine organocatalysts

Chiral secondary amines are valuable catalysts for reactions that proceed through an enamine intermediate. Here, we explored the importance of the pyramidalization direction of the enamine-N on the reactivity of chiral enamines with a combination of computational, NMR spectroscopic, and kinetic experiments. Studies with peptidic catalysts that bear cyclic amines with different ring sizes revealed that endo-pyramidalized enamines are significantly more reactive compared to exo-pyramidalized analogs. The results show that the pyramidalization direction can have a greater effect than n→π* orbital overlap on the reactivity of chiral enamines. The data enabled the development of a catalyst with higher reactivity compared to the parent catalyst.

Endo-pyramidalisation at nitrogen bestows enamines derived from α-substituted amines with higher reactivity compared to exo-pyramidalisation.

Organocatalytic Asymmetric Addition of Aldehyde to Nitroolefin by H-d-Pro-Pro-Glu-NH2: A Mechanistic Study

The mechanism of the asymmetric addition of aldehyde (butanal) to nitroolefin (β-nitrostyrene) catalyzed by H-d-Pro-Pro-Glu-NH₂ (dPPE-NH₂; 1) was explored using density functional theory methods in chloroform. By conformational search, it was confirmed that catalyst 1 and its enamine intermediate adopted a dominant conformation with a βI structure stabilized by a C₁₀ H-bond between the C=O of d-Pro1 and C-terminal NH₂ proton and by an additional H-bond between the side chain and the backbone of Glu3. This βI turn structure was conserved all along the catalytic cycle. Consistently with the kinetic studies, the C-C bond formation between the enamine and electrophile was also confirmed as the rate-determining step. The stereoselectivity results from a re → re prochiral approach of enamine and β-nitrostyrene with a gauche^- orientation of the double bonds. Although it was suggested as the possible formation of dihydrooxazine oxide species, this process was confirmed to be kinetically less accessible than the formation of acyclic nitronate. In particular, our calculated results supported that the carboxylic acid group of Glu3 in 1 played a central role by acting as general acid/base all along the catalytic cycle and orienting the asymmetric C-C bond formation.

Conformational Dynamics in Asymmetric Catalysis: Is Catalyst Flexibility a Design Element?

Traditionally, highly selective low molecular weight catalysts have been designed to contain rigidifying structural elements. As a result, many proposed stereochemical models rely on steric repulsion for explaining the observed selectivity. Recently, as is the case for enzymatic systems, it has become apparent that some flexibility can be beneficial for imparting selectivity. Dynamic catalysts can reorganize to maximize attractive non-covalent interactions that stabilize the favored diastereomeric transition state, while minimizing repulsive non-covalent interactions for enhanced selectivity. This Short Review discusses catalyst conformational dynamics and how these effects have proven beneficial for a variety of catalyst classes, including tropos ligands, cinchona alkaloids, hydrogen-bond donating catalysts, and peptides.

Peptide-Based Catalysts Reach the Outer Sphere through Remote Desymmetrization and Atroposelectivity

Nature’s catalytic machinery has provided endless inspiration for chemists. While the enzymatic ideal has yet to be fully realized, the field has made tremendous strides toward synthetic, small-molecule catalysts for a wide array of transformations, often drawing upon biological concepts in their design. One strategy that has been particularly influenced by enzymology is peptide catalysis, wherein oligopeptides are implemented as chiral catalysts in synthetically relevant reactions. The fundamental goal has been to mimic enzymatic active sites by taking advantage of secondary structures that allow for multifunctional activation of substrates within a framework of significantly reduced molecular complexity.

Our group has now been studying peptide-based catalysis for over two decades. At the outset, there were many reasons to be concerned that general contributions might not be possible. Precedents existed, including the Juliá–Colonna epoxidations mediated by helical oligopeptides, among others. However, we sought to explore whether peptide catalysts could find broad applications in organic synthesis despite what was expected to be their principal liability: conformational flexibility. Over time, we have been able to identify peptidic catalysts for a variety of site- and enantioselective transformations ranging from hydroxyl group and arene functionalizations to redox and C–C bond forming reactions. The peptides often exhibited excellent catalytic activities, in many cases enabling never-before-seen patterns of selectivity. Recent studies even suggest that, in certain situations, the conformational flexibility of these catalysts may be advantageous for asymmetric induction.

In the course of our studies, opportunities to employ peptide-based catalysis to solve long-standing and stereochemically intriguing problems in asymmetric synthesis presented themselves. For example, we have found that peptides provide exceptional enantiotopic group differentiation in catalytic desymmetrization reactions. Early results with symmetrical polyol substrates, such as myo-inositols and glycerols, eventually spurred the development of remote desymmetrizations of diarylmethanes, in which the enantiotopic groups are separated from the prochiral center by ∼6 Å and from one another by nearly 1 nm. Various hydroxyl group functionalizations and electrophilic brominations, as well as C–C, C–O, and C–N cross-coupling reactions using peptidic ligands on copper(I) have now been developed within this reaction archetype. Additionally, the preponderance of axially chiral, atropisomeric compounds as ligands, organocatalysts, and pharmacophores encouraged us to employ peptides as atroposelective catalysts. We have developed peptide-catalyzed brominations of pharmaceutically relevant biaryl, non-biaryl, and hetero-biaryl atropisomers that take advantage of dynamic kinetic resolution schemes. These projects have vastly expanded the reach of our original hypotheses and raised new questions about peptide-based catalysts and the extent to which they might mimic enzymes.

Herein, we recount the development and optimization of these stereochemically complex reactions, with a particular focus on structural and mechanistic aspects of the peptide-based catalysts that make them well-suited for their respective functions. The ability of these peptides to address important yet fundamentally challenging issues in asymmetric catalysis, combined with their modularity and ease-of-synthesis, make them primed for future use in organic synthesis.

Cyclic Peptide Design Guided by Residual Dipolar Couplings, J-Couplings, and Intramolecular Hydrogen Bond Analysis

Cyclic peptides have long tantalized drug designers with their potential ability to combine the best attributes of antibodies and small molecules. An ideal cyclic peptide drug candidate would be able to recognize a protein surface like an antibody while achieving the oral bioavailability of a small molecule. It has been hypothesized that such cyclic peptides balance permeability and solubility using their solvent-dependent conformational flexibility. Herein we report a conformational deconvolution NMR methodology that combines residual dipolar couplings, J-couplings, and intramolecular hydrogen bond analysis along with conformational analysis using molecular dynamics simulations and density functional theory calculations for studying cyclic peptide conformations in both low-dielectric solvent (chloroform) and high-dielectric solvent (DMSO) to experimentally study the solvent-dependent conformational change hypothesis. Taken together, the combined experimental and computational approaches can illuminate conformational ensembles of cyclic peptides in solution and help identify design opportunities for better permeability.

Phosphothreonine (pThr)-Based Multifunctional Peptide Catalysis for Asymmetric Baeyer-Villiger Oxidations of Cyclobutanones

Biologically inspired phosphothreonine (pThr)-embedded peptides that function as chiral Brønsted acid catalysts for enantioselective Baeyer-Villiger oxidations (BV) of cyclobutanones with aqueous H₂O₂ are reported herein. Complementary to traditional BINOL-derived chiral phosphoric acids (CPAs), the functional diversity of the peptidic scaffold provides the opportunity for multiple points of contact with substrates via hydrogen bonding, and the ease of peptide synthesis facilitates rapid diversification of the catalyst structure, such that numerous unique peptide-based CPA catalysts have been prepared. Utilizing a hypothesis-driven design, we identified a pThr-based catalyst that contains an N-acylated diaminopropionic acid (Dap) residue, which achieves high enantioselectivity with catalyst loadings as low as 0.5 mol%. The power of peptide-based multi-site binding is further exemplified through reversal in the absolute stereochemical outcome upon repositioning of the substrate-directing group (ortho- to meta). Modifications to the i+3 residue (^LDap to ^LPhe) lead to an observed enantiodivergence without inversion of any stereogenic center on the peptide catalyst, due to noncovalent interactions. Structure-selectivity and ¹H-¹H-ROESY studies revealed that the proposed hydrogen bonding interactions are essential for high levels of enantioinduction. The ability for the phosphopeptides to operate as multifunctional oxidation catalysts expands the scope of pThr catalysts and provides a framework for the future selective diversification of more complex substrates, including natural products.