Enantioselective Desymmetrization of Methylenedianilines via Enzyme-Catalyzed Remote Halogenation

Identifying and Engineering Flavin Dependent Halogenases for Selective Biocatalysis

ConspectusOrganohalogen compounds are extensively used as building blocks, intermediates, pharmaceuticals, and agrochemicals due to their unique chemical and biological properties. Installing halogen substituents, however, frequently requires functionalized starting materials and multistep functional group interconversion. Several classes of halogenases evolved in nature to enable halogenation of a different classes of substrates; for example, site-selective halogenation of electron rich aromatic compounds is catalyzed by flavin-dependent halogenases (FDHs). Mechanistic studies have shown that these enzymes use FADH2 to reduce O2 to water with concomitant oxidation of X- to HOX (X = Cl, Br, I). This species travels through a tunnel within the enzyme to access the FDH active site. Here, it is believed to interact with an active site lysine proximal to bound substrate, enabling electrophilic halogenation with selectivity imparted via molecular recognition, rather than directing groups or strong electronic activation.The unique selectivity of FDHs led to several early biocatalysis efforts, preparative halogenation was rare, and the hallmark catalyst-controlled selectivity of FDHs did not translate to non-native substrates. FDH engineering was limited to site-directed mutagenesis, which resulted in modest changes in site-selectivity or substrate preference. To address these limitations, we optimized expression conditions for the FDH RebH and its cognate flavin reductase (FRed), RebF. We then showed that RebH could be used for preparative halogenation of non-native substrates with catalyst-controlled selectivity. We reported the first examples in which the stability, substrate scope, and site selectivity of a FDH were improved to synthetically useful levels via directed evolution. X-ray crystal structures of evolved FDHs and reversion mutations showed that random mutations throughout the RebH structure were critical to achieving high levels of activity and selectivity on diverse aromatic substrates, and these data were used in combination with molecular dynamics simulations to develop predictive model for FDH selectivity. Finally, we used family wide genome mining to identify a diverse set of FDHs with novel substrate scope and complementary regioselectivity on large, three-dimensionally complex compounds.The diversity of our evolved and mined FDHs allowed us to pursue synthetic applications beyond simple aromatic halogenation. For example, we established that FDHs catalyze enantioselective reactions involving desymmetrization, atroposelective halogenation, and halocyclization. These results highlight the ability of FDH active sites to tolerate different substrate topologies. This utility was further expanded by our recent studies on the single component FDH/FRed, AetF. While we were initially drawn to AetF because it does not require a separate FRed, we found that it halogenates substrates that are not halogenated efficiently or at all by other FDHs and provides high enantioselectivity for reactions that could only be achieved using RebH variants after extensive mutagenesis. Perhaps most notably, AetF catalyzes site-selective aromatic iodination and enantioselective iodoetherification. Together, these studies highlight the origins of FDH engineering, the utility and limitations of the enzymes developed to date, and the promise of FDHs for an ever-expanding range of biocatalytic halogenation reactions.

Molecular Insights into Converting Hydroxide Adenosyltransferase into Halogenase

Halogenation plays a unique role in the design of agrochemicals. Enzymatic halogenation reactions have attracted great attention due to their excellent specificity and mild reaction conditions. S-adenosyl-l-methionine (SAM)-dependent halogenases mediate the nucleophilic attack of halide ions (X-) to SAM to produce 5'-XDA. However, only 11 SAM-dependent fluorinases and 3 chlorinases have been reported, highlighting the desire for additional halogenases. SAM-dependent hydroxide adenosyltransferase (HATase) has a similar reaction mechanism as halogenases but uses water as a substrate instead of halide ions. Here, we explored a HATase from the thermophile Thermotoga maritima MSB8 and transformed it into a halogenase. We identified a key dyad W8L/V71T for the halogenation reaction. We also obtained the best performing mutants for each halogenation reaction: M1, M2 and M4 for Cl-, Br- and I-, respectively. The M4 mutant retained the thermostability of HATase in the iodination reaction at 80 °C, which surpasses the natural halogenase SalL. QM/MM revealed that these mutants bind halide ions with more suitable angles for nucleophilic attack of C5' of SAM, thus conferring halogenation capabilities. Our work achieved the halide ion specificity of halogenases and generated thermostable halogenases for the first time, which provides new opportunities to expand the halogenase repertoire from hydroxylase.

Selective C-H Halogenation of Alkenes and Alkynes Using Flavin-Dependent Halogenases

Single component flavin-dependent halogenases (FDHs) possess both flavin reductase and FDH activity in a single enzyme. We recently reported that the single component FDH AetF catalyzes site-selective bromination and iodination of a variety of aromatic substrates and enantioselective bromolactonization and iodoetherification of styrenes bearing pendant carboxylic acid or alcohol substituents. Given this inherent reactivity and selectivity, we explored the utility of AetF as catalyst for alkene and alkyne C-H halogenation. We find that AetF catalyzes halogenation of a range of 1,1-disubstituted styrenes, often with high stereoselectivity. Despite the utility of haloalkenes for cross-coupling and other applications, accessing these compounds in a stereoselective manner typically requires functional group interconversion processes, and selective halogenation of 1,1'-disubstituted olefins remains rare. We also establish that AetF and homologues of this enzyme can halogenate terminal alkynes. Mutagenesis studies and deuterium kinetic isotope effects are used to support a mechanistic proposal involving covalent catalysis for halogenation of unactivated alkynes by AetF homologues. These findings expand the scope of FDH catalysis and continue to show the unique utility of single component FDHs for biocatalysis.

Non-Native Site-Selective Enzyme Catalysis

The ability to site-selectively modify equivalent functional groups in a molecule has the potential to streamline syntheses and increase product yields by lowering step counts. Enzymes catalyze site-selective transformations throughout primary and secondary metabolism, but leveraging this capability for non-native substrates and reactions requires a detailed understanding of the potential and limitations of enzyme catalysis and how these bounds can be extended by protein engineering. In this review, we discuss representative examples of site-selective enzyme catalysis involving functional group manipulation and C-H bond functionalization. We include illustrative examples of native catalysis, but our focus is on cases involving non-native substrates and reactions often using engineered enzymes. We then discuss the use of these enzymes for chemoenzymatic transformations and target-oriented synthesis and conclude with a survey of tools and techniques that could expand the scope of non-native site-selective enzyme catalysis.

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.

Asymmetric catalysis by flavin-dependent halogenases

In nature, flavin-dependent halogenases (FDHs) catalyze site-selective chlorination and bromination of aromatic natural products. This ability has led to extensive efforts to engineer FDHs for selective chlorination, bromination, and iodination of electron rich aromatic compounds. On the other hand, FDHs are unique among halogenases and haloperoxidases that exhibit catalyst-controlled site selectivity in that no examples of enantioselective FDH catalysis in natural product biosynthesis have been characterized. Over the past several years, our group has established that FDHs can catalyze enantioselective reactions involving desymmetrization, atroposelective halogenation, and halocyclization. Achieving high activity and selectivity for these reactions has required extensive mutagenesis and mitigation of problems resulting from hypohalous acid generated during FDH catalysis. The single-component flavin reductase/FDH AetF is unique among the wild type enzyme we have studied in that it provides high activity and selectivity toward several asymmetric transformations. These results highlight the ability of FDH active sites to tolerate different substrate topologies and suggest that they could be useful for a broad range of oxidative halogenations.

Organocatalyzed enantio- and diastereoselective isomerization of prochiral 1,3-cyclohexanediones into nonalactones bearing distant stereocenters

The lactonization of 2-(2-nitrophenyl)-1,3-cyclohexanediones containing an alcohol side chain and up to three distant prochiral elements is reported by isomerization under the mediation of simple organocatalysts such as quinidine. Through a process of ring expansion, strained nonalactones and decalactone are produced with up to three stereocenters in high er and dr (up to 99 : 1). Distant groups, including alkyl, aryl, carboxylate and carboxamide moieties, were examined.

The Single-Component Flavin Reductase/Flavin-Dependent Halogenase AetF is a Versatile Catalyst for Selective Bromination and Iodination of Arenes and Olefins

Flavin-dependent halogenases (FDHs) natively catalyze selective halogenation of electron rich aromatic and enolate groups. Nearly all FDHs reported to date require a separate flavin reductase to supply them with FADH2 , which complicates biocatalysis applications. In this study, we establish that the single component flavin reductase/flavin dependent halogenase AetF catalyzes halogenation of a diverse set of substrates using a commercially available glucose dehydrogenase to drive its halogenase activity. High site selectivity, activity on relatively unactivated substrates, and high enantioselectivity for atroposelective bromination and bromolactonization was demonstrated. Site-selective iodination and enantioselective cycloiodoetherification was also possible using AetF. The substrate and reaction scope of AetF suggest that it has the potential to greatly improve the utility of biocatalytic halogenation.

Directed Evolution of Flavin-Dependent Halogenases for Site- and Atroposelective Halogenation of 3-Aryl-4(3H)-Quinazolinones via Kinetic or Dynamic Kinetic Resolution

In this study, we engineer a variant of the flavin-dependent halogenase RebH that catalyzes site- and atroposelective halogenation of 3-aryl-4(3H)-quinazolinones via kinetic or dynamic kinetic resolution. The required directed evolution uses a combination of random and site-saturation mutagenesis, substrate walking using two probe substrates, and a two-tiered screening approach involving the analysis of variant conversion and then enantioselectivity of improved variants. The resulting variant, 3-T, provides >99:1 e.r. for the (M)-atropisomer of the major brominated product, 25-fold improved conversion, and 91-fold improved site selectivity relative to the parent enzyme on the probe substrate used in the final rounds of evolution. This high activity and selectivity translate well to several additional substrates with varied steric and electronic properties. Computational modeling and docking simulations are used to rationalize the effects of key mutations on substrate binding. Given the range of substrates that have been used for atroposelective synthesis via electrophilic halogenation in the literature, these results suggest that flavin-dependent halogenases (FDHs) could find many additional applications for atroposelective catalysis. More broadly, this study highlights how RebH can be engineered to accept structurally diverse substrates that enable its use for enantioselective catalysis.

Distal Ionic Substrate-Catalyst Interactions Enable Long-Range Stereocontrol: Access to Remote Quaternary Stereocenters through a Desymmetrizing Suzuki-Miyaura Reaction

Spatial distancing of a substrate's reactive group and nonreactive catalyst-binding group from its pro-stereogenic element presents substantial hurdles in asymmetric catalysis. In this context, we report a desymmetrizing Suzuki-Miyaura reaction that establishes chirality at a remote quaternary carbon. The anionic, chiral catalyst exerts stereocontrol through electrostatic steering of substrates, even as the substrate's reactive group and charged catalyst-binding group become increasingly distanced. This study demonstrates that precise long-range stereocontrol is achievable by engaging ionic substrate-ligand interactions at a distal position.

Reaction pathway engineering converts a radical hydroxylase into a halogenase

Fe^II/α-ketoglutarate (Fe^II/αKG)-dependent enzymes offer a promising biocatalytic platform for halogenation chemistry owing to their ability to functionalize unactivated C-H bonds. However, relatively few radical halogenases have been identified to date, limiting their synthetic utility. Here, we report a strategy to expand the palette of enzymatic halogenation by engineering a reaction pathway rather than substrate selectivity. This approach could allow us to tap the broader class of Fe^II/αKG-dependent hydroxylases as catalysts by their conversion to halogenases. Toward this goal, we discovered active halogenases from a DNA shuffle library generated from a halogenase-hydroxylase pair using a high-throughput in vivo fluorescent screen coupled to an alkyne-producing biosynthetic pathway. Insights from sequencing halogenation-active variants along with the crystal structure of the hydroxylase enabled engineering of a hydroxylase to perform halogenation with comparable activity and higher selectivity than the wild-type halogenase, showcasing the potential of harnessing hydroxylases for biocatalytic halogenation.

Engineered RebH Halogenase Variants Demonstrating a Specificity Switch from Tryptophan towards Novel Indole Compounds

Activating industrially important aromatic hydrocarbons by installing halogen atoms is extremely important in organic synthesis and often improves the pharmacological properties of drug molecules. To this end, tryptophan halogenase enzymes are potentially valuable tools for regioselective halogenation of arenes, including various industrially important indole derivatives and similar scaffolds. Although endogenous enzymes show reasonable substrate scope towards indole compounds, their efficacy can often be improved by engineering. Using a structure-guided semi-rational mutagenesis approach, we have developed two RebH variants with expanded biocatalytic repertoires that can efficiently halogenate several novel indole substrates and produce important pharmaceutical intermediates. Interestingly, the engineered enzymes are completely inactive towards their natural substrate tryptophan in spite of their high tolerance to various functional groups in the indole ring. Computational modelling and molecular dynamics simulations provide mechanistic insights into the role of gatekeeper residues in the substrate binding site and the dramatic switch in substrate specificity when these are mutated.

Enzymkatalysierte späte Modifizierungen: Besser spät als nie

Die Enzymkatalyse gewinnt zunehmend an Bedeutung in der Synthesechemie. Die durch Bioinformatik und Enzym‐Engineering stetig wachsende Zahl von Biokatalysatoren eröffnet eine große Vielfalt selektiver Reaktionen. Insbesondere für späte Funktionalisierungsreaktionen ist die Biokatalyse ein geeignetes Werkzeug, das oftmals der konventionellen De‐novo‐Synthese überlegen ist. Enzyme haben sich als nützlich erwiesen, um funktionelle Gruppen direkt in komplexe Molekülgerüste einzuführen sowie für die rasche Diversifizierung von Substanzbibliotheken. Biokatalytische Oxyfunktionalisierungen, Halogenierungen, Methylierungen, Reduktionen und Amidierungen sind von besonderem Interesse, da diese Strukturmotive häufig in Pharmazeutika vertreten sind. Dieser Aufsatz gibt einen Überblick über die Stärken und Schwächen der enzymkatalysierten späten Modifizierungen durch native und optimierte Enzyme in der Synthesechemie. Ebenso werden wichtige Beispiele in der Wirkstoffentwicklung hervorgehoben.

Enzymatic Late-Stage Modifications: Better Late Than Never

Enzyme catalysis is gaining increasing importance in synthetic chemistry. Nowadays, the growing number of biocatalysts accessible by means of bioinformatics and enzyme engineering opens up an immense variety of selective reactions. Biocatalysis especially provides excellent opportunities for late-stage modification often superior to conventional de novo synthesis. Enzymes have proven to be useful for direct introduction of functional groups into complex scaffolds, as well as for rapid diversification of compound libraries. Particularly important and highly topical are enzyme-catalysed oxyfunctionalisations, halogenations, methylations, reductions, and amide bond formations due to the high prevalence of these motifs in pharmaceuticals. This Review gives an overview of the strengths and limitations of enzymatic late-stage modifications using native and engineered enzymes in synthesis while focusing on important examples in drug development.

Flavin-dependent halogenases catalyze enantioselective olefin halocyclization

Halocyclization of alkenes is a powerful bond-forming tool in synthetic organic chemistry and a key step in natural product biosynthesis, but catalyzing halocyclization with high enantioselectivity remains a challenging task. Identifying suitable enzymes that catalyze enantioselective halocyclization of simple olefins would therefore have significant synthetic value. Flavin-dependent halogenases (FDHs) catalyze halogenation of arene and enol(ate) substrates. Herein, we reveal that FDHs engineered to catalyze site-selective aromatic halogenation also catalyze non-native bromolactonization of olefins with high enantioselectivity and near-native catalytic proficiency. Highly selective halocyclization is achieved by characterizing and mitigating the release of HOBr from the FDH active site using a combination of reaction optimization and protein engineering. The structural origins of improvements imparted by mutations responsible for the emergence of halocyclase activity are discussed. This expansion of FDH catalytic activity presages the development of a wide range of biocatalytic halogenation reactions.

Catalytic enantioselective halocyclization of alkenes is an important bond forming tool and a key step in natural product biosynthesis, but so far no examples of the enzymatic counterpart of this reaction on simple achiral olefins have been reported. Here, the authors describe examples of engineered flavin-dependent halogenases that catalyze halolactonization of olefins with high enantioselectivity and near-native catalytic activity.

Synthetic C6-Functionalized Aminoflavin Catalysts Enable Aerobic Bromination of Oxidation-Prone Substrates

Flavoenzymes catalyze oxidations via hydroperoxide intermediates that result from activation of molecular O₂. These reactions—such as hydroxylation and halogenation—depend on the additional catalytic activity of functional groups in the peptide environment of the flavin cofactor. We report synthetic flavin catalysts that contain C6 amino modifications at the isoalloxazine core and are consequently capable of mediating halogenations outside the peptide surrounding. The catalysts are competent in the selective, biomimetic bromination of oxidation‐prone phenols, flavones, and flavanones using a halide salt in combination with 2,6‐lutidinium oxalate as a flavin reductant under visible‐light irradiation. Our studies show the beneficial effect of stacked bisflavins as well as the catalytic activity of the flavin modifications. The designed flavin catalysts outperform isolated natural (−)‐riboflavin and contribute to the continuing search for tailored flavins in oxidation reactions.

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.

An Fe2+ - and α-Ketoglutarate-Dependent Halogenase Acts on Nucleotide Substrates

While halogenated nucleosides are used as common anticancer and antiviral drugs, naturally occurring halogenated nucleosides are rare. Adechlorin (ade) is a 2'-chloro nucleoside natural product first identified from Actinomadura sp. ATCC 39365. However, the installation of chlorine in the ade biosynthetic pathway remains elusive. Reported herein is a Fe²⁺ -α-ketoglutarate halogenase AdeV that can install a chlorine atom at the C2' position of 2'-deoxyadenosine monophosphate to afford 2'-chloro-2'-deoxyadenosine monophosphate. Furthermore, 2',3'-dideoxyadenosine-5'-monophosphate and 2'-deoxyinosine-5'-monophosphate can also be converted, albeit 20-fold and 2-fold, respectively, less efficiently relative to the conversion of 2'-deoxyadenosine monophosphate. AdeV represents the first example of a Fe²⁺ -α-ketoglutarate-dependent halogenase that converts nucleotides into chlorinated analogues.

Structure and Activity of the Thermophilic Tryptophan-6 Halogenase BorH

Flavin-dependent halogenases carry out regioselective aryl halide synthesis in aqueous solution at ambient temperature and neutral pH using benign halide salts, making them attractive catalysts for green chemistry. BorH and BorF, two proteins encoded by the biosynthetic gene cluster for the chlorinated bisindole alkaloid borregomycin A, are the halogenase and flavin reductase subunits of a tryptophan-6-halogenase. Quantitative conversion of l-tryptophan (Trp) to 6-chlorotryptophan could be achieved using 1.2 mol % BorH and 2 mol % BorF. The optimal reaction temperature for Trp chlorination is 45 °C, and the melting temperatures of BorH and BorF are 48 and 50 °C respectively, which are higher than the thermal parameters for most other halogenases previously studied. Steady-state kinetic analysis of Trp chlorination by BorH determined parameters of k_cat =4.42 min^-1 , and K_M of 9.78 μm at 45 °C. BorH exhibits a broad substrate scope, chlorinating and brominating a variety of aromatic substrates with and without indole groups. Chlorination of Trp at a 100 mg scale with 52 % crude yield, using 0.2 mol % BorH indicates that industrial scale biotransformations using BorH/BorF are feasible. The X-ray crystal structure of BorH with bound Trp provides additional evidence for the model that regioselectivity is determined by substrate positioning in the active site, showing C6 of Trp juxtaposed with the catalytic Lys79 in the same binding pose previously observed in the structure of Thal.

Recent Advances in Flavin-Dependent Halogenase Biocatalysis: Sourcing, Engineering, and Application

The introduction of a halogen atom into a small molecule can effectively modulate its properties, yielding bioactive substances of agrochemical and pharmaceutical interest. Consequently, the development of selective halogenation strategies is of high technological value. Besides chemical methodologies, enzymatic halogenations have received increased interest as they allow the selective installation of halogen atoms in molecular scaffolds of varying complexity under mild reaction conditions. Today, a comprehensive library of aromatic halogenases exists, and enzyme as well as reaction engineering approaches are being explored to broaden this enzyme family’s biocatalytic application range. In this review, we highlight recent developments in the sourcing, engineering, and application of flavin-dependent halogenases with a special focus on chemoenzymatic and coupled biosynthetic approaches.

Site-Selective C-H Halogenation Using Flavin-Dependent Halogenases Identified via Family-Wide Activity Profiling

Enzymes are powerful catalysts for site-selective C-H bond functionalization. Identifying suitable enzymes for this task and for biocatalysis in general remains challenging, however, due to the fundamental difficulty of predicting catalytic activity from sequence information. In this study, family-wide activity profiling was used to obtain sequence-function information on flavin-dependent halogenases (FDHs). This broad survey provided a number of insights into FDH activity, including halide specificity and substrate preference, that were not apparent from the more focused studies reported to date. Regions of FDH sequence space that are most likely to contain enzymes suitable for halogenating small-molecule substrates were also identified. FDHs with novel substrate scope and complementary regioselectivity on large, three-dimensionally complex compounds were characterized and used for preparative-scale late-stage C-H functionalization. In many cases, these enzymes provide activities that required several rounds of directed evolution to accomplish in previous efforts, highlighting that this approach can achieve significant time savings for biocatalyst identification and provide advanced starting points for further evolution.

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.