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

Chemical and Enzymatic Mechanosynthesis of Organocatalytic Peptide Materials Based on Proline and Phenylalanine

In recent years, mechanosynthesis of peptides through either chemical or enzymatic routes has been accomplished. In part, this advancement has been driven due to the organocatalytic properties of peptide-based biomaterials. In this work, we report the merging of chemical and enzymatic protocols under mechanochemical conditions to synthesize peptide materials based on L-proline and L-phenylalanine. Compared to traditional step-by-step peptide synthesis in solution, our mechanochemical approach combining peptide coupling reagents with the proteolytic enzyme papain offers a more sustainable route by reducing the number of synthetic steps, shortening reaction times, increasing chemical yields, and minimizing waste production. Notably, the mechanosynthesized peptides exhibited organocatalytic activity in the asymmetric aldol reaction between cyclohexanone and 4-nitrobenzaldehyde.

Controlling Stereoselectivity with Noncovalent Interactions in Chiral Phosphoric Acid Organocatalysis

Chiral phosphoric acids (CPAs) have emerged as highly effective Brønsted acid catalysts in an expanding range of asymmetric transformations, often through novel multifunctional substrate activation modes. Versatile and broadly appealing, these catalysts benefit from modular and tunable structures, and compatibility with additives. Given the unique types of noncovalent interactions (NCIs) that can be established between CPAs and various reactants─such as hydrogen bonding, aromatic interactions, and van der Waals forces─it is unsurprising that these catalyst systems have become a promising approach for accessing diverse chiral product outcomes. This review aims to provide an in-depth exploration of the mechanisms by which CPAs impart stereoselectivity, positioning NCIs as the central feature that connects a broad spectrum of catalytic reactions. Spanning literature from 2004 to 2024, it covers nucleophilic additions, radical transformations, and atroposelective bond formations, highlighting the applicability of CPA organocatalysis. Special emphasis is placed on the structural and mechanistic features that govern CPA-substrate interactions, as well as the tools and techniques developed to enhance our understanding of their catalytic behavior. In addition to emphasizing mechanistic details and stereocontrolling elements in individual reactions, we have carefully structured this review to provide a natural progression from these specifics to a broader, class-level perspective. Overall, these findings underscore the critical role of NCIs in CPA catalysis and their significant contributions to advancing asymmetric synthesis.

Designing Peptide-Based Nucleophilic Catalysts Possessing Multiple Identical Active Sites for Late-Stage Chlorination of Peptides and Drugs

In the quest for developing catalysts with multiple active sites, we designed a series of methionine-based peptide catalysts prepared by classical peptide coupling. Given the widespread presence of aromatic chloro-substituents and their significant pharmacokinetic properties, the performance of these catalysts were evaluated for the late-stage chlorination of tyrosine residue on peptides up to octamer as well as aromatic drug molecules. The operationally simple reaction conditions, excellent catalyst loading up to 0.25 mol %, and gram-scale reactions are major highlights of this method.

Enantioselective Hydrodifluoroalkylation of Alkenes with Conformationally Tuned Peptidyl Hydrogen Atom Transfer Catalysts

We report the enantioselective hydrodifluoroalkylation of alkenes proceeding via an asymmetric hydrogen atom transfer (HAT) event catalyzed by thiol-containing tetrapeptides. Photocatalytic generation of a difluoroacetyl radical followed by carbon-carbon bond formation results in a prochiral carbon-centered radical that engages with the chiral catalyst. A trialkylamine reductant is proposed to turn over the catalyst in this net-reductive transformation. Notably, incorporating an (S)-β-methyl-substituted cysteine as the N-terminal residue improved selectivity relative to that of the native N-terminal cysteine (Cys) residue, and X-ray crystallographic analysis supports the conformational underpinning of this effect. A range of enantioenriched γ-substituted amides were synthesized in up to a 96:4 enantiomeric ratio, demonstrating the broad functional group tolerance of this method. Models accounting for asymmetric induction are proposed with supporting DFT calculations.

Synthesis of Mono-Boc-2,5-Diketopiperazine: A Key Building Block for Amide and Peptide Synthesis

Diketopiperazine (DKP), a versatile scaffold, is extensively used in the synthesis of complex natural products, bioactive molecules, and smart materials in organic chemistry. Recently, activated DKPs, such as Boc-DKPs, have emerged as key building blocks for peptide elongation in peptide synthesis. In this study, we developed a facile protocol for synthesizing mono-Boc-protected DKPs from readily accessible N-4-methoxybenzyl (N-PMB)-amino acids and amino acid methyl esters. This protocol involved a sequence of reactions encompassing the formation of dipeptides from N-PMB-amino acids and amino acid methyl esters, cyclization of N-PMB-dipeptides to form PMB-DKPs, Boc-protection of PMB-DKPs, and subsequent PMB-deprotection of PMB-DKP-Boc to afford mono-Boc-DKPs. The protocol demonstrated a broad substrate scope, accommodating diverse amino acids with various side chains, affording mono-Boc-DKPs in good yields with excellent stereoselectivities (>20:1 dr). The synthetic utility of mono-Boc-DKPs was showcased in peptide synthesis by synthesizing pentapeptide Boc-l-Tyr(t-Bu)-Gly-l-Phe-Gly-l-Val-OtBu by 2-fold peptide elongation with two mono-Boc-DKPs. Furthermore, we synthesized Leu-enkephalin pentapeptide by reacting cyclo(Boc-l-Tyr(t-Bu)-Gly-) with H-Gly-l-Phe-l-Leu-Ot-Bu, resulting in a good yield and excellent optical purity.

Investigating Interaction Dynamics of an Enantioselective Peptide-Catalyzed Acylation Reaction

Modern nuclear magnetic resonance (NMR) methods like carbon relaxation dispersion in the rotating frame (13C-R1ρ) and proton chemical exchange saturation transfer (1H-CEST) are key methods to investigate molecular recognition in biomacromolecules and to detect molecular motions on the μs to s timescale, revealing transient conformational states. Changes in kinetics can be linked to binding, folding, or catalytic events. Here, we investigated whether these methods allow detection of changes in the dynamics of a small, highly selective peptide catalyst during recognition of its enantiomeric substrates. The flexible tetrapeptide Boc-l-(π-Me)-His-AGly-l-Cha-l-Phe-OMe, used for the monoacetylation of cycloalkane-diols, is probed at natural abundance using 13C-R1ρ and 1H-CEST. Indeed, we detected differences in dynamics of the peptide upon interaction with the diol. Importantly, these differ depending on the enantiomer of the substrate used. These enantiospecific influences of the substrates on the dynamics of the peptide are rationalized using computational techniques. We find that even though one enantiomer reacts faster, as confirmed by reaction monitoring, the other is more tightly bound in DCM (as confirmed by 1H-saturation transfer difference (STD) measurements). These findings provide insights into the recognition of the substrates and explain the selectivity differences observed between the solvents toluene and DCM.

A glycan foldamer that uses carbohydrate-aromatic interactions to perform catalysis

In nature, the ability to catalyse reactions is primarily associated with proteins and ribozymes. Inspired by these systems, peptide-based catalysts have been designed to accelerate chemical reactions and/or ensure regio- and stereoselective transformations. We wondered whether other biomolecules (such as glycans) could be designed to perform catalytic functions, expanding the portfolio of synthetic functional oligomers. Here we report a glycan foldamer inspired by the natural Sialyl Lewis X antigen that acts as catalyst in a chemical reaction. This glycan-based catalyst benefits from structural rigidity and modular adaptability, incorporating a substrate-recognition motif alongside a catalytic active site. Leveraging the inherent ability of carbohydrates to engage in CH-π interactions with aromatic substrates, we demonstrate the recruitment and functionalization of a tryptophan via a Pictet-Spengler transformation. Our modular glycan catalyst accelerates the reaction kinetics, enabling the modification of tryptophan-containing peptides in aqueous environments. Our findings pave the way for the development of glycan-based catalysts and suggest the possibility of catalytic capabilities of glycans in biological contexts.

Rational Design of Bifunctional Imidazoles as Acyl Transfer Catalysts: Dynamic Kinetic Resolution of 5-Hydroxy-furanones/3-Hydroxy-phthalides

A new class of chiral bifunctional imidazole catalysts has been designed and synthesized, utilizing economical amino alcohols as precursors, significantly expanding the diversity of N-1 position catalysts. These catalysts exhibit excellent substrate activation and stereoselectivity control and have been successfully employed in the asymmetric acylation of 5-hydroxy-furanones/3-hydroxy-phthalides through dynamic kinetic resolution, producing a series of chiral furanone and phthalide analogues featuring a quaternary stereocenter. This asymmetric acylation reaction exhibits excellent reactivity and enantioselectivity, has a wide range of applicability, requires a low catalyst loading, and can be readily converted into valuable building blocks. Moreover, DFT calculations revealed the detailed reaction mechanism and demonstrated that the weak N-H···O and C-H···O interactions between the catalyst and substrate are the key factors affecting the stereoselectivity.

Dual Activation Modes Enable Bifunctional Catalysis of Aldol Reactions by Flexible Dihydrazides

Hydrazides are known to catalyze reactions of α,β-unsaturated aldehydes via transient iminium formation. The iminium intermediate displays enhanced electrophilicity, which facilitates conjugate additions and cycloadditions. We observed that a hydrazide embedded in a seven-membered ring catalyzes homoaldol condensation of a simple aldehyde in a process that displays an approximate second-order dependence on the hydrazide. This finding suggests a dual activation mechanism involving both iminium and enamine intermediates. The unexpected nucleophilic activation mode led us to examine a series of simple dihydrazides for bifunctional catalysis of the aldol condensation. The two cyclic hydrazide units were connected via linear polymethylene linkers, which are inherently flexible. Substantial increases in initial reaction rate, relative to a monohydrazide, were observed for polymethylene linkers of sufficient length, with a maximum at 10 methylenes. Reactions promoted by this dihydrazide displayed an approximate first-order dependence on catalyst concentration, which suggested a bifunctional catalytic mechanism. Even for a dihydrazide with an 18-methylene linker, a substantial increase in relative initial rate was observed. These observations show that significant coordination can be achieved between two catalytically active moieties even when these moieties are connected via many flexible bonds.

Natural Cyclic Peptides: Synthetic Strategies and Biomedical Applications

Natural cyclic peptides, a diverse class of bioactive compounds, have been isolated from various natural sources and are renowned for their extensive structural variability and broad spectrum of medicinal properties. Over 40 cyclic peptides or their derivatives are currently approved as medicines, underscoring their significant therapeutic potential. These compounds are employed in diverse roles, including antibiotics, antifungals, antiparasitics, immune modulators, and anti-inflammatory agents. Their unique ability to combine high specificity with desirable pharmacokinetic properties makes them valuable tools in addressing unmet medical needs, such as combating drug-resistant pathogens and targeting challenging biological pathways. Due to the typically low concentrations of cyclic peptides in nature, effective synthetic strategies are indispensable for their acquisition, characterization, and biological evaluation. Cyclization, a critical step in their synthesis, enhances metabolic stability, bioavailability, and receptor binding affinity. Advances in synthetic methodologies-such as solid-phase peptide synthesis (SPPS), chemoenzymatic approaches, and orthogonal protection strategies-have transformed cyclic peptide production, enabling greater structural complexity and precision. This review compiles recent progress in the total synthesis and biological evaluation of natural cyclic peptides from 2017 onward, categorized by cyclization strategies: head-to-tail; head-to-side-chain; tail-to-side-chain; and side-chain-to-side-chain strategies. Each account includes retrosynthetic analyses, synthetic advancements, and biological data to illustrate their therapeutic relevance and innovative methodologies. Looking ahead, the future of cyclic peptides in drug discovery is bright. Emerging trends, including integrating computational tools for rational design, novel cyclization techniques to improve pharmacokinetic profiles, and interdisciplinary collaboration among chemists, biologists, and computational scientists, promise to expand the scope of cyclic peptide-based therapeutics. These advancements can potentially address complex diseases and advance the broader field of biological drug development.

Photochemical Deracemization of Lactams with Deuteration Enabled by Dual Hydrogen Atom Transfer

Photochemical deracemization has emerged as one of the most straightforward approaches to access highly enantioenriched compounds in recent years. While excited-state events such as energy transfer, single electron transfer, and ligand-to-metal charge transfer have been leveraged to promote stereoablation, approaches relying on hydrogen atom transfer, which circumvent the limitations imposed by the triplet energy and redox potential of racemic substrates, remain underexplored. Conceptually, the most attractive method for tertiary stereocenter deracemization might be hydrogen atom abstraction followed by hydrogen atom donation. However, implementing such a strategy poses significant challenges, primarily because the enantioenriched products are also reactive if the chiral catalyst is unable to differentiate between the two enantiomers. Herein we report a distinct dual hydrogen atom transfer strategy for photochemical deracemization of δ- and γ-lactams, achieving high enantioenrichment and deuterium incorporation despite the inherent reactivity of the products. Mechanistic studies reveal that benzophenone enables nonselective hydrogen atom abstraction while a tetrapeptide-derived thiol dictates the enantioselectivity of the hydrogen atom donation step. More importantly, a pyridine-based alcohol was found to play crucial roles in facilitating the hydrogen atom abstraction as well as enhancing the enantioselectivity of the hydrogen atom donation step.

Tackling Undruggable Targets with Designer Peptidomimetics and Synthetic Biologics

The development of potent, specific, and pharmacologically viable chemical probes and therapeutics is a central focus of chemical biology and therapeutic development. However, a significant portion of predicted disease-causal proteins have proven resistant to targeting by traditional small molecule and biologic modalities. Many of these so-called "undruggable" targets feature extended, dynamic protein-protein and protein-nucleic acid interfaces that are central to their roles in normal and diseased signaling pathways. Here, we discuss the development of synthetically stabilized peptide and protein mimetics as an ever-expanding and powerful region of chemical space to tackle undruggable targets. These molecules aim to combine the synthetic tunability and pharmacologic properties typically associated with small molecules with the binding footprints, affinities and specificities of biologics. In this review, we discuss the historical and emerging platforms and approaches to design, screen, select and optimize synthetic "designer" peptidomimetics and synthetic biologics. We examine the inspiration and design of different classes of designer peptidomimetics: (i) macrocyclic peptides, (ii) side chain stabilized peptides, (iii) non-natural peptidomimetics, and (iv) synthetic proteomimetics, and notable examples of their application to challenging biomolecules. Finally, we summarize key learnings and remaining challenges for these molecules to become useful chemical probes and therapeutics for historically undruggable targets.

Cationic Foldamer-Catalyzed Asymmetric Synthesis of Inherently Chiral Cages

The stereochemistry of shape-persistent molecular cages, particularly those resembling prisms, exerts significant influence on their application-specific functionalities. Although methods exist for fabricating inherently chiral prism-like cages, strategies for catalytic asymmetric synthesis of these structures in a diversity-oriented fashion remain unexplored. Herein, we introduce an unprecedented organocatalytic desymmetrization approach for the generation of inherently chiral prism-like cages via phosphonium-containing foldamer-catalyzed SNAr reactions. This methodology establishes a topological connection, enabling the facile assembly of a wide range of versatile stereogenic-at-cage building blocks possessing two highly modifiable groups. Furthermore, subsequent stereospecific transformations of the remaining chlorides and/or ethers afford convenient access to numerous functionally relevant chiral-at-cage molecules.

Advances, opportunities, and challenges in methods for interrogating the structure activity relationships of natural products

Time span in literature: 1985-early 2024Natural products play a key role in drug discovery, both as a direct source of drugs and as a starting point for the development of synthetic compounds. Most natural products are not suitable to be used as drugs without further modification due to insufficient activity or poor pharmacokinetic properties. Choosing what modifications to make requires an understanding of the compound's structure-activity relationships. Use of structure-activity relationships is commonplace and essential in medicinal chemistry campaigns applied to human-designed synthetic compounds. Structure-activity relationships have also been used to improve the properties of natural products, but several challenges still limit these efforts. Here, we review methods for studying the structure-activity relationships of natural products and their limitations. Specifically, we will discuss how synthesis, including total synthesis, late-stage derivatization, chemoenzymatic synthetic pathways, and engineering and genome mining of biosynthetic pathways can be used to produce natural product analogs and discuss the challenges of each of these approaches. Finally, we will discuss computational methods including machine learning methods for analyzing the relationship between biosynthetic genes and product activity, computer aided drug design techniques, and interpretable artificial intelligence approaches towards elucidating structure-activity relationships from models trained to predict bioactivity from chemical structure. Our focus will be on these latter topics as their applications for natural products have not been extensively reviewed. We suggest that these methods are all complementary to each other, and that only collaborative efforts using a combination of these techniques will result in a full understanding of the structure-activity relationships of natural products.

Biocatalytic Synthesis of α-Amino Esters via Nitrene C-H Insertion

α-Amino esters are precursors to noncanonical amino acids used in developing small-molecule therapeutics, biologics, and tools in chemical biology. α-C-H amination of abundant and inexpensive carboxylic acid esters through nitrene transfer presents a direct approach to α-amino esters. Methods for nitrene-mediated amination of the protic α-C-H bonds in carboxylic acid esters, however, are underdeveloped. This gap arises because hydrogen atom abstraction (HAA) of protic C-H bonds by electrophilic metal-nitrenoids is slow: metal-nitrenoids preferentially react with polarity-matched, hydridic C-H bonds, even when weaker protic C-H bonds are present. This study describes the discovery and evolution of highly stable protoglobin nitrene transferases that catalyze the enantioselective intermolecular amination of the α-C-H bonds in carboxylic acid esters. We developed a high-throughput assay to evaluate the activity and enantioselectivity of mutant enzymes together with their sequences using the Every Variant Sequencing (evSeq) method. The assay enabled the identification of enantiodivergent enzymes that function at ambient conditions in Escherichia coli whole cells and whose activities can be enhanced by directed evolution for the amination of a range of substrates.

Photo-induced imine reduction by a photoredox biocatalyst consisting of a pentapeptide and a Ru bipyridine terpyridine complex

Imine reduction is a useful reaction in the preparation of amine derivatives. Various catalysts have been reported to promote this reaction and photoredox catalysts are promising candidates for sustainable amine synthesis. Improvement of this reaction using biomolecule-based reaction scaffolds is expected to increase the utility of the reaction. In this context, we have recently investigated photoredox Ru complexes with pentapeptide scaffolds via coordination bonds as catalysts for photoreduction of dihydroisoquinoline derivatives. First, Ru bipyridine terpyridine complexes coordinated with five different pentapeptides (XVHVV: X = V, F, W, Y, C) were prepared and characterized by mass spectrometry. Catalytic activities of the Ru complexes with XVHVV were evaluated for photoreduction of dihydroisoquinoline derivatives in the presence of ascorbate and thiol compounds as sacrificial reagents and hydrogen sources. Interestingly, the turnover number of the Ru complex with VVHVV is 531, which is two-fold higher than that of a simple Ru complex with an imidazole ligand. The detailed emission lifetime measurements indicate that the enhanced catalytic activity provided by the peptide scaffold is caused by an efficient reaction with the thiol derivative to accelerate reductive quenching of Ru complex. The quenching behavior suggests formation of an active species such as a Ru(I) complex. These findings reveal that the simple pentapeptide serves as an effective scaffold to enhance the photocatalytic activity of a photoactive Ru complex.

Chiral Macrocycles for Enantioselective Recognition

The efficient synthesis of chiral macrocycles with highly enantioselective recognition remains a challenge. We have addressed this issue by synthesizing a pair of chiral macrocycles, namely, R/S-BINOL[2], achieving total isolated yields of up to 62% through a two-step reaction sequence. These macrocycles are readily purified by column chromatography over silica gel without the need for chiral separation, thus streamlining the overall synthesis. R/S-BINOL[2] demonstrated enantioselective recognition toward chiral ammonium salts, with enantioselectivity (KS/KR) values reaching up to 13.2, although less favorable separations were seen for other substrates. R/S-BINOL[2] also displays blue circularly polarized luminescence with a |glum| value of up to 2.2 × 10-3. The R/S-BINOL[2] macrocycles of this study are attractive as chiral hosts in that they both display enantioselective guest recognition and benefit from a concise, high-yielding synthesis. As such, they may have a role to play in chiral separations.

Catalytic Asymmetric Synthesis of Unnatural Axially Chiral Biaryl δ-Amino Acid Derivatives via a Chiral Phenanthroline-Potassium Catalyst-Enabled Dynamic Kinetic Resolution

Axially chiral biaryl δ-amino acids possess significantly different conformational properties and chiral environment from centrally chiral amino acids, therefore, have drawn considerable attention in the fields of synthetic and medicinal chemistry. Herein, a novel chiral phenanthroline-potassium catalyst has been developed by constructing a well-organized axially chiral ligand composed of one 1,10-phenanthroline unit and two axially chiral 1,1'-bi-2-naphthol (BINOL) units. In the presence of this catalyst, good to excellent yields and enantioselectivities (up to 99 % yield, 98 : 2 er) have been achieved in the ring-opening alcoholytic dynamic kinetic resolution of a variety of biaryl lactams, thereby providing an efficient protocol for catalytic asymmetric synthesis of unnatural axially chiral biaryl δ-amino acid derivatives.

Catalytic Enantioselective Sulfoxidation of Functionalized Thioethers Mediated by Aspartic Acid-Containing Peptides

A peptide-catalyzed enantioselective oxidation of sulfides to yield pharmaceutically relevant chiral sulfoxides is reported. Experimental evidence suggesting that a hydrogen bond-donating moiety must be present in the substrate to achieve high levels of enantioinduction is supported by computational modeling of transition states. These models also indicate that dual points of contact between the peptidic catalyst and substrate are likely responsible for the formation of one desired sulfoxide in 94:6 er.

Catalytic Asymmetric Aza-Electrophilic Additions of 1,1-Disubstituted Styrenes

Electrophilic addition of alkenes is a textbook reaction that plays a pivotal role in organic chemistry. In the past decades, catalytic asymmetric variants of this important type of reaction have witnessed great achievements by the development of novel catalytic systems. However, enantioselective aza-electrophilic additions of unactivated alkenes, which could provide a transformative strategy for the preparation of synthetically significant nitrogen-containing compounds, still remain a formidable challenge. Herein, we have developed unprecedented Au(I)/NHC-catalyzed asymmetric aza-electrophilic additions of unactivated 1,1-disubstituted styrenes by the utilization of readily available dialkyl azodicarboxylates as electrophilic nitrogen sources. Based on this approach, a series of transformations, including [2 + 2] cycloaddition, intermolecular 1,2-oxyamination, and several types of intramolecular hydrazination-induced cyclizations, have been realized. These transformations provide a previously unattainable platform for the divergent synthesis of hydrazine derivatives, which could also be converted to other nitrogen-containing chiral synthons. Experimental and computational studies support the idea that carbocation intermediates are involved in reaction pathways.

Catalysis of an SN2 pathway by geometric preorganization

Bimolecular nucleophilic substitution (SN2) mechanisms occupy a central place in the historical development and teaching of the field of organic chemistry1. Despite the importance of SN2 pathways in synthesis, catalytic control of ionic SN2 pathways is rare and notably uncommon even in biocatalysis2,3, reflecting the fact that any electrostatic interaction between a catalyst and the reacting ion pair necessarily stabilizes its charge and, by extension, reduces polar reactivity. Nucleophilic halogenase enzymes navigate this tradeoff by desolvating and positioning the halide nucleophile precisely on the SN2 trajectory, using geometric preorganization to compensate for the attenuation of nucleophilicity4. Here we show that a small-molecule (646 Da) hydrogen-bond-donor catalyst accelerates the SN2 step of an enantioselective Michaelis-Arbuzov reaction by recapitulating the geometric preorganization principle used by enzymes. Mechanistic and computational investigations show that the hydrogen-bond donor diminishes the reactivity of the chloride nucleophile yet accelerates the rate-determining dealkylation step by reorganizing both the phosphonium cation and the chloride anion into a geometry that is primed to enter the SN2 transition state. This new enantioselective Arbuzov reaction affords highly enantioselective access to an array of H-phosphinates, which are in turn versatile P-stereogenic building blocks amenable to myriad derivatizations. This work constitutes, to our knowledge, the first demonstration of catalytic enantiocontrol of the phosphonium dealkylation step, establishing a new platform for the synthesis of P-stereogenic compounds.

Application of artificial backbone connectivity in the development of metalloenzyme mimics

Metal-dependent enzymes are abundant and vital catalytic agents in nature. The functional versatility of metalloenzymes has made them common targets for improvement by protein engineering as well as mimicry by de novo designed sequences. In both strategies, the incorporation of non-canonical cofactors and/or non-canonical side chains has proved a useful tool. Less explored-but similarly powerful-is the utilization of non-canonical covalent modifications to the polypeptide backbone itself. Such efforts can entail either introduction of limited artificial monomers in natural chains to produce heterogeneous backbones or construction of completely abiotic oligomers that adopt defined folds. Herein, we review recent research applying artificial protein-like backbones in the construction of metalloenzyme mimics, highlighting progress as well as open questions in this emerging field.

Asymmetric Synthesis of β,β-Disubstituted Alanines via a Sequential C(sp2)-C(sp3) Cross-Coupling-Hydrogenation Strategy

We report the development of a sequential C(sp2)-C(sp3) Suzuki cross-coupling-asymmetric hydrogenation strategy which allows access to a diverse array of valuable β,β-disubstituted alanine derivatives. This synthesis exhibits broad functional group tolerance, and permits efficient access to β-aryl-β-alkyl, and the more rarely reported β,β-dialkyl Ala derivatives with high yield and excellent enantioselectivity. This transformation has been exhibited on decagram quantity, and can be used to generate Fmoc amino acid derivatives which are useful for SPPS.

De Novo Engineering of Pd-Metalloproteins and Their Use as Intracellular Catalysts

The development of transition metal-based catalytic platforms that promote bioorthogonal reactions inside living cells remains a major challenge in chemical biology. This is particularly true for palladium-based catalysts, which are very powerful in organic synthesis but perform poorly in the cellular environment, mainly due to their rapid deactivation. We now demonstrate that grafting Pd(II) complexes into engineered β-sheets of a model WW domain results in cell-compatible palladominiproteins that effectively catalyze depropargylation reactions inside HeLa cells. The concave shape of the WW domain β-sheet proved particularly suitable for accommodating the metal center and protecting it from rapid deactivation in the cellular environment. A thorough NMR and computational study confirmed the formation of the metal-stapled peptides and allowed us to propose a three-dimensional structure for this novel metalloprotein motif.

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.

6-Strand to Stable 10/12 Helix Conformational Switch by Incorporating Flexible β-hGly in the Homooligomers of Camphor Derived β-Amino Acid: NMR and X-Ray Crystallographic Evidence

Rational design of unnatural amino acid building blocks capable of stabilizing predictable secondary structures similar to protein fragments is pivotal for foldamer chemistry/catalysis. Here, we introduce novel β-amino acid building blocks: [1S,2R,4R]exoCDA and [1S,2S,4R]endoCDA, derived from the abundantly available R(+)-camphor, which is traditionally known for its medicinal value. Further, we demonstrate that the homooligomers of exoCDA adopt 6-strand conformation, which switches to a robust 10/12-helix simply by inserting flexible β-hGly spacer at alternate positions (1 : 1 β-hGly/exoCDA heterooligomers), as evident by DFT-calculations, solution-state NMR spectroscopy and X-ray crystallography. To the best of our knowledge, this is the first example of crystalline-state structure of left-handed 10/12-mixed helix, that is free from the conventional approach of employing β-amino acids of either alternate chirality or alternate β2/β3 substitutions, to access the 10/12-helix. The results also show that the homooligomers of heterochiral exoCDA don't adopt helical fold, instead exhibit banana-shaped strands, whereas the homodimers of the other diastereomer endoCDA, nucleate 8-membered turns. Furthermore, the homo-exoCDA and hetero-[β-hGly-exoCDA] oligomers are found to exhibit self-association properties with distinct morphological features. Overall, the results offer new possibilties of constructing discrete stable secondary and tertiary structures based on CDAs, which can accommodate flexible residues with desired side-chain substitutions.

Friedel-Crafts reactions for biomolecular chemistry

Chemical tools and principles have become central to biological and medical research/applications by leveraging a range of classical organic chemistry reactions. Friedel-Crafts alkylation and acylation are arguably some of the most well-known and used synthetic methods for the preparation of small molecules but their use in biological and medical fields is relatively less frequent than the other reactions, possibly owing to the notion of their plausible incompatibility with biological systems. This review demonstrates advances in Friedel-Crafts alkylation and acylation reactions in a variety of biomolecular chemistry fields. With the discoveries and applications of numerous biomolecule-catalyzed or -assisted processes, these reactions have garnered considerable interest in biochemistry, enzymology, and biocatalysis. Despite the challenges of reactivity and selectivity of biomolecular reactions, the alkylation and acylation reactions demonstrated their utility for the construction and functionalization of all the four major biomolecules (i.e., nucleosides, carbohydrates/saccharides, lipids/fatty acids, and amino acids/peptides/proteins), and their diverse applications in biological, medical, and material fields are discussed. As the alkylation and acylation reactions are often fundamental educational components of organic chemistry courses, this review is intended for both experts and nonexperts by discussing their basic reaction patterns (with the depiction of each reaction mechanism in the ESI) and relevant real-world impacts in order to enrich chemical research and education. The significant growth of biomolecular Friedel-Crafts reactions described here is a testament to their broad importance and utility, and further development and investigations of the reactions will surely be the focus in the organic biomolecular chemistry fields.

Design of Functional Globular β-Sheet Miniproteins

The design of discrete β-sheet peptides is far less advanced than e. g. the design of α-helical peptides. The reputation of β-sheet peptides as being poorly soluble and aggregation-prone often hinders active design efforts. Here, we show that this reputation is unfounded. We demonstrate this by looking at the β-hairpin and WW domain. Their structure and folding have been extensively studied and they have long served as model systems to investigate protein folding and folding kinetics. The resulting fundamental understanding has led to the development of hyperstable β-sheet scaffolds that fold at temperatures of 100 °C or high concentrations of denaturants. These have been used to design functional miniproteins with protein or nucleic acid binding properties, in some cases with such success that medical applications are conceivable. The β-sheet scaffolds are not always completely rigid, but can be specifically designed to respond to changes in pH, redox potential or presence of metal ions. Some engineered β-sheet peptides also exhibit catalytic properties, although not comparable to those of natural proteins. Previous reviews have focused on the design of stably folded and non-aggregating β-sheet sequences. In our review, we now also address design strategies to obtain functional miniproteins from β-sheet folding motifs.

Peptide and Enzyme Catalysts Work in Concert in Stereoselective Cascade Reactions-Oxidation followed by Conjugate Addition

Enzymes and peptide catalysts consist of the same building blocks but require vastly different environments to operate best. Herein, we show that an enzyme and a peptide catalyst can work together in a single reaction vessel to catalyze a two-step cascade reaction with high chemo- and stereoselectivity. Abundant linear alcohols, nitroolefins, an alcohol oxidase, and a tripeptide catalyst provided chiral γ-nitroaldehydes in aqueous buffer. High yields (up to 92 %) and stereoselectivities (up to 98 % ee) were achieved for the cascade through the rational design of the peptide catalyst and the identification of common reaction conditions.

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.

Carbene organic catalytic planar enantioselective macrolactonization

Macrolactones exhibit distinct conformational and configurational properties and are widely found in natural products, medicines, and agrochemicals. Up to now, the major effort for macrolactonization is directed toward identifying suitable carboxylic acid/alcohol coupling reagents to address the challenges associated with macrocyclization, wherein the stereochemistry of products is usually controlled by the substrate's inherent chirality. It remains largely unexplored in using catalysts to govern both macrolactone formation and stereochemical control. Here, we disclose a non-enzymatic organocatalytic approach to construct macrolactones bearing chiral planes from achiral substrates. Our strategy utilizes N-heterocyclic carbene (NHC) as a potent acylation catalyst that simultaneously mediates the macrocyclization and controls planar chirality during the catalytic process. Macrolactones varying in ring sizes from sixteen to twenty members are obtained with good-to-excellent yields and enantiomeric ratios. Our study shall open new avenues in accessing macrolactones with various stereogenic elements and ring structures by using readily available small-molecule catalysts.

Organocatalytic activation of hydrogen peroxide: towards green and sustainable oxidations

The advent of organocatalysis provided an additional option in every researcher's arsenal, towards the development of elegant and sustainable protocols for various organic transformations. Oxidation reactions are considered to be key in organic synthesis since oxygenated functionalities appear in many natural products. Hydrogen peroxide is categorized as a green oxidant, since its only by-product is water, offering novel opportunities for the development of green and sustainable protocols. In this review article, we intend to present recent developments in the field of the organocatalytic activation of hydrogen peroxide, providing useful insight into the applied oxidative protocols. At the same time, we will present some interesting mechanistic studies, providing information on the oxygen transfer processes.

Quantifying Intermolecular Interactions in Asymmetric Peptide Organocatalysis as a Key toward Understanding Selectivity

The kinetic resolution of trans-cyclohexane-1,2-diol with a lipophilic oligopeptide catalyst shows extraordinary selectivities. To improve our understanding of the factors governing selectivity, we quantified the Gibbs free energies of interactions of the peptide with both enantiomers of trans-cyclohexane-1,2-diol using nuclear magnetic resonance (NMR) spectroscopy. For this, we use advanced methods such as transverse relaxation (R2), diffusion measurements, saturation transfer difference (STD), and chemical shift (δ) analysis of peptide-diol mixtures upon varying their composition (NMR titrations). The methods employed give comparable and consistent results. The molecular recognition by the catalyst is approximately 3 kJ mol-1 in favor of the preferentially acetylated (R,R)-enantiomer in the temperature range studied. Interestingly, the difference of 3 kJ mol-1 is also confirmed by results from reaction monitoring of the acylation step under catalytic conditions, indicating that this finding is true regardless of whether the investigation is performed on the acetylated species or on the free catalyst. To arrive at these conclusions, the self-association of both the catalyst and the substrate in toluene was found to play an important role and thus needs to be taken into account in reaction screening.

Enantioselective conjugate addition to nitroolefins catalysed by helical peptides with a single remote stereogenic centre

Two short pentapeptides rich in α-aminoisobutyric acid (Aib) residues have been shown to act as enantioselective organocatalysts for the conjugate addition of nucleophiles to nitroolefins. An L-alanine terminated peptide, (Aib)4(L-Ala)NHtBu, which has neither functionalised sidechains nor a highly designed reactive site, used an exposed N-terminal primary amine and the amide bonds of the backbone to mediate catalysis. Folding of this peptide into a 310 helical structure was observed by crystallography. Folding into a helix relays the conformational preference of the chiral alanine residue at the C-terminus to the primary amine at the N-terminus, 0.9 nm distant. The chiral environment and defined shape produced by the 310 helix brings the amine site into proximity to two exposed amide NHs. Reaction scope studies implied that the amine acts as a Brønsted base and the solvent-exposed NH groups of the helix, shown to weakly bind β-nitrostyrene, are needed to obtain an enantiomeric excess. Replacement of L-alanine with D-phenylalanine gave (Aib)4(D-Phe)NHtBu, a peptide that now catalysed the benchmark reaction with the opposite enantioselectivity. These studies show how achiral residues can play a key role in enantioselective catalysis by peptides through the promotion of folding.

A tutorial on asymmetric electrocatalysis

Electrochemistry has emerged as a powerful means to enable redox transformations in modern chemical synthesis. This tutorial review delves into the unique advantages of electrochemistry in the context of asymmetric catalysis. While electrochemistry has historically been used as a green and mild alternative for established enantioselective transformations, in recent years asymmetric electrocatalysis has been increasingly employed in the discovery of novel asymmetric methodologies based on reaction mechanisms unique to electrochemistry. This tutorial review first provides a brief tutorial introduction to electrosynthesis, then explores case studies on homogenous small molecule asymmetric electrocatalysis. Each case study serves to highlight a key advance in the field, starting with the historic electrification of known asymmetric transformations and culminating with modern methods relying on unique electrochemical mechanistic sequences. Finally, we highlight case studies in the emerging reasearch areas at the interface of asymmetric electrocatalysis with biocatalysis and heterogeneous catalysis.

Multiscale Modeling Approach for the Aldol Addition Reaction in Multicompartment Micelle-Based Nanoreactor

Water has emerged as a versatile solvent for organic chemistry in recent years due to its abundance, low cost, and environmental friendliness. However, one of the most important reactions, the aldol reaction, in the presence of excess water exhibits low yields and poor enantioselectivities. In this regard, we have employed a multiscale modeling approach to investigate the aldol addition reaction catalyzed by l-proline in the hydrophobic compartment of multicompartment micelle (MCM) nanoreactor consisting of amphiphilic bottlebrush copolymer, which minimizes the water content at the reactive site. Through performing dissipative particle dynamics (DPD) simulation, it is found that the "clover-like" morphology of the MCM is formed from multiblock copolymer with a sequence of ethylene oxide-based hydrophilic blocks, styrene lipophilic blocks, l-proline catalyst blocks, and a pentafluorostyrene fluorophilic block in aqueous media. We find that the vicinity of the catalyst in the clover-like MCM has a low dielectric environment, which could facilitate the aldol addition reaction. Our DFT calculations demonstrate that the asymmetric aldol addition of l-proline-catalyzed acetone and 4-nitrobenzaldehyde is energetically more favorable under the low dielectric environment in MCM compared with other commonly used solvents such as DMSO, water, and vacuum condition.

Sculpting Secondary Structure of a Cyclic Peptide: Conformational Analysis of a Cyclic Hexapeptide Containing a Combination of l-Leu, d-Leu, and Aib Residues

We have previously reported that cyclo(l-Leu-d-Leu-Aib-l-Leu-d-Leu-Aib) (2), a cyclic hexapeptide consisting of heterochiral l-Leu and d-Leu (l-Leu-d-Leu) residues with achiral 2-aminoisobutyric acid (Aib) residues, forms a figure-8 conformation. In this study, we newly designed cyclo(l-Leu-d-Leu-Aib-d-Leu-l-Leu-Aib)+ (4), an epimer of 2, and examined the conformational differences between 2 and 4 by X-ray crystallographic analysis. Peptide 4 formed a planar cyclic conformation with an antiparallel β-sheet hydrogen-bonding pattern. This investigation demonstrates the potential to manipulate the molecular conformation of cyclic peptides by simply arranging the l- and d-amino acids and emphasizes that diverse conformations can be obtained by using cyclic peptides. Harnessing cyclic peptides as platforms for distinct molecular structures is a promising approach to expanding the chemical space for various applications.

Design and Application of Peptide-Mimic Phosphonium Salt Catalysts in Asymmetric Synthesis

Chiral phosphonium salt catalysis, traditionally classified as a type of phase transfer catalysis, has proven to be a powerful strategy for the stereoselective preparation of diverse optically active molecules. However, there still remain numerous forbidding issues of reactivity and selectivity in such well-known organocatalysis system. Accordingly, the development of new and high-performance phosphonium salt catalysts with unique chiral backbones is highly desirable, yet challenging. This Minireview describes the prominent endeavours in the development of a new family of chiral peptide-mimic phosphonium salt catalysts with multiple hydrogen-bonding donors and their applications in a plethora of enantioselective synthesis during the past few years. Hopefully, this minireview will pave a way for further developing much more efficient and privileged chiral ligands/catalysts featuring exclusively catalytic ability in asymmetric synthesis.

Enantioselective Access to Chiral 2,5-Diketopiperazines via Stereogenic-at-Cobalt(III)-Catalyzed Ugi-4CRs/Cyclization Sequences

An asymmetric synthesis of chiral 2,5-diketopiperazines by the Ugi-4CR/cyclization is exhibited. The employment of catalytic anionic chiral Co(III) complexes delivered α-propiolyl aminoamides in high yields with excellent enantioselectivities (31 examples, up to 95% ee). The following treatment of Ugi-adducts with PPh3 leads to chiral 2,5-DKPs without significant loss of enantioselectivities (26 examples, up to 91% ee).

Highly asymmetric aldol reaction of isatins and ketones catalyzed by chiral bifunctional primary-amine organocatalyst on water

Herein, we have reported an environmentally friendly asymmetric aldol reaction between isatins and ketones catalyzed by double-hydrogen-bonded primary amine organocatalysts on water under mild conditions. Enantioenriched 3-hydroxy-2-oxindoles were obtained in high yields (up to 99%) and excellent stereoselectivities (up to 99 : 1 dr and 99% ee) under optimal conditions. Furthermore, the model reaction involving isatin and cyclohexanone was successfully scaled to 10 mmol with no reduction in yield or stereoselectivity. In addition, the catalyst was recovered via simple filtration and was subsequently reused on water, which highlights its good application potential.

A Simple Screening and Optimization Bioprocess for Long-Chain Peptide Catalysts Applied to Asymmetric Aldol Reaction

Peptides have demonstrated their efficacy as catalysts in asymmetric aldol reactions. But the constraints inherent in chemical synthesis have imposed limitations on the viability of long-chain peptide catalysts. A noticeable dearth of tools has impeded the swift and effective screening of peptide catalysts using biological methods. To address this, we introduce a straightforward bioprocess for the screening of peptide catalysts for asymmetric aldol reactions. We synthesized several peptides through this method and obtained a 15-amino acid peptide. This peptide exhibited asymmetric aldol catalytic activity, achieving 77% ee in DMSO solvent and 63% ee with over an 80.8% yield in DMSO mixed with a pH 9.0 buffer solution. The successful application of our innovative approach not only represents an advancement but also paves the way for currently unexplored research avenues.

Reversal of Enantioselectivity for the Desymmetrization of meso-1,2-Diols Catalyzed by Pyridine-N-oxides

The desymmetrization of meso-vic-diols with a reversal of enantioselectivity catalyzed by chiral pyridine-N-oxides with l-proline as a single source of chirality is reported. With chiral 3-substituted ArPNO C2c and 2-substituted 4-(dimethylamino)pyridine-N-oxide C3b as catalysts, a wide range of monoesters were obtained with satisfactory results with a complete and controlled switch in stereoselectivity (up to 97:3 and 1:99 er). Chiral six-membered carbocyclic uracil nucleosides were generated with excellent enantioselectivities after derivatization. A series of control experiments and density functional theory (DFT) calculations supported that the reaction proceeded in a bifunctional activated manner, where the N-oxide groups and N-H proton of the amides were vital for catalytic reactivity and stereocontrol. The DFT calculation also supported the distance-directed switching of enantioselectivity, in which the l-prolinamide moiety moved from the C3 to C2 position on the pyridine ring, resulting in the H-bond interaction between the amide N-H and OH group of meso-vic-diol also shifted from one hydroxyl group to another.

Synthesis of a glycan hairpin

The primary sequence of a biopolymer encodes the essential information for folding, permitting to carry out sophisticated functions. Inspired by natural biopolymers, peptide and nucleic acid sequences have been designed to adopt particular three-dimensional (3D) shapes and programmed to exert specific functions. In contrast, synthetic glycans capable of autonomously folding into defined 3D conformations have so far not been explored owing to their structural complexity and lack of design rules. Here we generate a glycan that adopts a stable secondary structure not present in nature, a glycan hairpin, by combining natural glycan motifs, stabilized by a non-conventional hydrogen bond and hydrophobic interactions. Automated glycan assembly enabled rapid access to synthetic analogues, including site-specific 13C-labelled ones, for nuclear magnetic resonance conformational analysis. Long-range inter-residue nuclear Overhauser effects unequivocally confirmed the folded conformation of the synthetic glycan hairpin. The capacity to control the 3D shape across the pool of available monosaccharides has the potential to afford more foldamer scaffolds with programmable properties and functions.

On the Ability of the N-O Bond to Support a Stable Stereogenic Axis: Peptide-Catalyzed Atroposelective N-Oxidation

During studies of atroposelective, peptide-catalyzed N-oxidations of pyridines, we observed lower-than-expected barriers to atropisomerization for these stereodynamic processes under the reaction conditions. Mechanistic studies indicate a hydrogen bond-assisted racemization mechanism intrinsic to both the starting materials and products. We also identified a protonation-dependent barrier to rotation that operates for the starting materials alone. Nonetheless, several substrates were amenable to atroposelective N-oxidations via kinetic resolution, yielding krel values of up to 12.6 and the isolation of one N-oxide with >99:1 er after recrystallization.

Short Peptides for Hydrolase Supramolecular Mimicry and Their Potential Applications

Hydrolases are enzymes that have found numerous applications in various industrial sectors spanning from pharmaceuticals to foodstuff and beverages, consumers' products such as detergents and personal care, textiles, and even for biodiesel production and environmental bioremediation. Self-assembling and gelling short peptides have been designed for their mimicry so that their supramolecular organization leads to the creation of hydrophobic pockets for catalysis to occur. Catalytic gels of this kind can also find numerous industrial applications to address important global challenges of our time. This concise review focuses on the last 5 years of progress in this fast-paced, popular field of research with an eye towards the future.

Kinetics and Mechanism of Azole n-π*-Catalyzed Amine Acylation

Azole anions are highly competent in the activation of weak acyl donors, but, unlike neutral (aprotic) Lewis bases, are not yet widely applied as acylation catalysts. Using a combination of in situ and stopped-flow 1H/19F NMR spectroscopy, kinetics, isotopic labeling, 1H DOSY, and electronic structure calculations, we have investigated azole-catalyzed aminolysis of p-fluorophenyl acetate. The global kinetics have been elucidated under four sets of conditions, and the key elementary steps underpinning catalysis deconvoluted using a range of intermediates and transition state probes. While all evidence points to an overarching mechanism involving n-π* catalysis via N-acylated azole intermediates, a diverse array of kinetic regimes emerges from this framework. Even seemingly minor changes to the solvent, auxiliary base, or azole catalyst can elicit profound changes in the temporal evolution, thermal sensitivity, and progressive inhibition of catalysis. These observations can only be rationalized by taking a holistic view of the mechanism and a set of limiting regimes for the kinetics. Overall, the analysis of 18 azole catalysts spanning nearly 10 orders of magnitude in acidity highlights the pitfall of pursuing ever more nucleophilic catalysts without regard for catalyst speciation.

Amino Acid-Derived Ionic Chiral Catalysts Enable Desymmetrizing Cross-Coupling to Remote Acyclic Quaternary Stereocenters

Synthetic application of asymmetric catalysis relies on strategic alignment of bond construction to creation of chirality of a target molecule. Remote desymmetrization offers distinctive advantages of spatial decoupling of catalytic transformation and generation of a stereogenic element. However, such spatial separation presents substantial difficulties for the chiral catalyst to discriminate distant enantiotopic sites through a reaction three or more bonds away from a prochirality center. Here, we report a strategy that establishes acyclic quaternary carbon stereocenters through cross-coupling reactions at distal positions of aryl substituents. The new class of amino acid-derived ionic chiral catalysts enables desymmetrizing (enantiotopic-group-selective) Suzuki-Miyaura reaction, Sonogashira reaction, and Buchwald-Hartwig amination between diverse diarylmethane scaffolds and aryl, alkynyl, and amino coupling partners, providing rapid access to enantioenriched molecules that project substituents to widely spaced positions in the three-dimensional space. Experimental and computational investigations reveal electrostatic steering of substrates by the C-terminus of chiral ligands through ionic interactions. Cooperative ion-dipole interactions between the catalyst's amide group and potassium cation aid in the preorganization that transmits asymmetry to the product. This study demonstrates that it is practical to achieve precise long-range stereocontrol through engineering the spatial arrangements of the ionic catalysts' substrate-recognizing groups and metal centers.

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.