Structure-Selectivity Relationships and Structure for a Peptide-Based Enantioselective Acylation Catalyst

Ratcheting synthesis

Synthetic chemistry has traditionally relied on reactions between reactants of high chemical potential and transformations that proceed energetically downhill to either a global or local minimum (thermodynamic or kinetic control). Catalysts can be used to manipulate kinetic control, lowering activation energies to influence reaction outcomes. However, such chemistry is still constrained by the shape of one-dimensional reaction coordinates. Coupling synthesis to an orthogonal energy input can allow ratcheting of chemical reaction outcomes, reminiscent of the ways that molecular machines ratchet random thermal motion to bias conformational dynamics. This fundamentally distinct approach to synthesis allows multi-dimensional potential energy surfaces to be navigated, enabling reaction outcomes that cannot be achieved under conventional kinetic or thermodynamic control. In this Review, we discuss how ratcheted synthesis is ubiquitous throughout biology and consider how chemists might harness ratchet mechanisms to accelerate catalysis, drive chemical reactions uphill and programme complex reaction sequences.

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

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

A Combined DFT, Energy Decomposition, and Data Analysis Approach to Investigate the Relationship Between Noncovalent Interactions and Selectivity in a Flexible DABCOnium/Chiral Anion Catalyst System

Developing strategies to study reactivity and selectivity in flexible catalyst systems has become an important topic of research. Herein, we report a combined experimental and computational study aimed at understanding the mechanistic role of an achiral DABCOnium cofactor in a regio- and enantiodivergent bromocyclization reaction. It was found that electron-deficient aryl substituents enable rigidified transition states via an anion-π interaction with the catalyst, which drives the selectivity of the reaction. In contrast, electron-rich aryl groups on the DABCOnium result in significantly more flexible transition states, where interactions between the catalyst and substrate are more important. An analysis of not only the lowest-energy transition state structures but also an ensemble of low-energy transition state conformers via energy decomposition analysis and machine learning was crucial to revealing the dominant noncovalent interactions responsible for observed changes in selectivity in this flexible system.

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

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

Catalytic enantioselective acyl transfer: the case for 4-PPY with a C-3 carboxamide peptide auxiliary based on synthesis and modelling studies

A series of 4-pyrrolidinopyridine (4-PPY) C-3 carboxamides containing peptide-based side chains have been synthesised and evaluated in the kinetic resolution of a small library of chiral benzylic secondary alcohols. A key design element was the incorporation of a tryptophan residue in the peptide side chain for promoting π-stacking between peptide side chain and the pyridinium ring of the N-acyl intermediate, in which modelling was used as a structure-based guiding tool. Together, a catalyst containing a LeuTrp-N-Boc side chain (catalyst 8) was identified that achieved s-values up to and in slight excess of 10. A transition-state model based on the modelling is proposed to explain the origin of enantioselectivity. This study establishes the usefulness of modelling as a structure-based guiding tool for enantioselectivity optimization as well as the potential for developing scalable peptide-based DMAP-type catalysts for large-scale resolution work.

Development of Selective Peptide Catalysts with Secondary Structural Frameworks

Enzymes are biogenic catalysts that enable the vital activity of organisms. Enzymes promote reactions in a selective manner with a high level of substrate recognition ability. The development of such a sophisticated catalyst has been one of the goals for chemists. A synthetic peptide is the prime candidate to realize an enzyme-like catalyst. Considering that the catalytic function of enzymes derives from their molecular structures, the key for the creation of a peptide catalyst might be the introduction of a specific three-dimensional structure. Our motivation was to find a peptide catalyst with a versatile secondary structural framework and apply the peptide to a variety of selective reactions. Although helical-peptide-catalyzed asymmetric epoxidation of enones is popular, no other highly enantioselective reaction with a helical peptide has been reported. It was found that resin-supported α-helical polyleucine promoted asymmetric conjugate addition of a carbon nucleophile to enones via the formation of an iminium intermediate at the N-terminal amino group. By changing the helical chain to a repetitive Leu-Leu-Aib (Aib = α-aminoisobutyric acid) sequence and introducing a few amino acids to the N-terminus, a highly enantioselective peptide catalyst was obtained. The helical peptide catalyst was applicable for a tandem enamine/iminium-mediated reaction and asymmetric epoxidation of enones. Although the extension of the helical peptide to conjugate addition of a nucleophile to an enal was not successful simply by attaching proline to the N-terminus of the helix, the incorporation of a β-turn motif was effective to improve the catalytic performance. In the sequence of such a turn-helix-type peptide, the helical part was seemingly distant from the N-terminal amino group; however, the hydrophobicity, structure, and chirality of the helix largely affected the reaction. The turn-helix-type peptide promoted a wide range of asymmetric reactions: conjugated additions of hydride and carbon nucleophiles to enals via the iminium activation and α-oxyamination of aldehydes via the enamine activation. The peptides with turn-helix and helix frameworks were also employed for several reactions that were difficult to achieve with low-molecular-weight catalysts: enzyme-cocatalyzed asymmetric oxidation in water, diastereo- and enantioselective cyclopropanation, regioselective reduction of dienals, kinetic resolution of planar-chiral compounds, and desymmetrization to induce planar chirality. To explore other types of peptide catalysts, a combinatorial library screening was performed. On the way, it was revealed that a histidyl residue assisted to accelerate a reaction via reversible addition to an iminium intermediate. Through the screening of random peptide libraries, novel peptide sequences for efficient and enantioselective conjugate addition were discovered. Although we have no information about the molecular structure of the newly found peptides, they can be an entry point for establishing a versatile framework of peptide catalysts.

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

X-ray crystallography has been applied to the structural analysis of a series of tetrapeptides that were previously assessed for catalytic activity in an atroposelective bromination reaction. Common to the series is a central Pro-Xaa sequence, where Pro is either l- or d-proline, which was chosen to favor nucleation of canonical β-turn secondary structures. Crystallographic analysis of 35 different peptide sequences revealed a range of conformational states. The observed differences appear not only in cases where the Pro-Xaa loop-region is altered, but also when seemingly subtle alterations to the flanking residues are introduced. In many instances, distinct conformers of the same sequence were observed, either as symmetry-independent molecules within the same unit cell or as polymorphs. Computational studies using DFT provided additional insight into the analysis of solid-state structural features. Select X-ray crystal structures were compared to the corresponding solution structures derived from measured proton chemical shifts, 3J-values, and 1H-1H-NOESY contacts. These findings imply that the conformational space available to simple peptide-based catalysts is more diverse than precedent might suggest. The direct observation of multiple ground state conformations for peptides of this family, as well as the dynamic processes associated with conformational equilibria, underscore not only the challenge of designing peptide-based catalysts, but also the difficulty in predicting their accessible transition states. These findings implicate the advantages of low-barrier interconversions between conformations of peptide-based catalysts for multistep, enantioselective reactions.

An Evolving Insight into Chiral H-Bond Catalyzed Aza-Henry Reactions: A Cooperative Role for Noncovalent Attractive Interactions Unveiled by Density Functional Theory

The role of cooperative effects arising from noncovalent attractive interactions as a vital factor governing stereoinduction in chiral H-bond catalyzed aza-Henry reactions is reported. Supporting this finding were density functional theory (DFT) calculations which revealed a shape and size dependency existed between the catalyst and substrates that when matched lead to high enantioselectivity, as reflected by favorable activation parameters. Associated with optimal catalyst and substrate pairing were a closed catalytic binding pocket and a synclinal orientation of the substrates that reinforced favorable stereoelectronic effects and dispersive type forces. Meanwhile, unfavorable steric interactions were found to be a dominant effect controlling diastereoselection.

Structural studies of β-turn-containing peptide catalysts for atroposelective quinazolinone bromination

We describe herein a crystallographic and NMR study of the secondary structural attributes of a β-turn-containing tetra-peptide, Boc-Dmaa-D-Pro-Acpc-Leu-NMe2, which was recently reported as a highly effective catalyst in the atroposelective bromination of 3-arylquinazolin-4(3H)-ones. Inquiries pertaining to the functional consequences of residue substitutions led to the discovery of a more selective catalyst, Boc-Dmaa-D-Pro-Acpc-Leu-OMe, the structure of which was also explored. This new lead catalyst was found to exhibit a type I'β-turn secondary structure both in the solid state and in solution, a structure that was shown to be an accessible conformation of the previously reported catalyst, as well.

Synthesis and properties of optically active helical polymers from (S)-3-functional-3′-vinyl-BINOL derivatives

Helical vinyl polymers bearing N-heterocycles substituent BINOL derivatives were synthesized. The specific optical rotation and circular dichroism spectra data show the obtained polymers can keep a prevailing helicity of backbone in solution.

A catalyst selection protocol that identifies biomimetic motifs from β-hairpin libraries

Assaying a solid-phase library of histidine-containing β-hairpin peptides by a reactive tagging scheme in organic solvents selects for catalysts that reproduce the strategies used by His-based enzyme active sites to accelerate acyl- and phosphonyl-transfer reactions. Rate accelerations (k(rel)) in organic solvents of up to 2.4 × 10(8) are observed.

Function-Oriented Investigations of a Peptide-Based Catalyst that Mediates Enantioselective Allylic Alcohol Epoxidation

We detail an investigation of a peptide-based catalyst 6 that is effective for the site- (>100:1:1) and enantioselective epoxidation (86% ee) of farnesol. Studies of the substrate scope exhibited by the catalyst are included, along with an exploration of optimized reaction conditions. Mechanistic studies are reported, including relative rate determinations for the catalyst and propionic acid, a historical perspective, truncation studies, and modeling using NMR data. Our compiled data advances our understanding of the inner workings of a catalyst that was identified through combinatorial means.

Conformationally constrained cyclic peptides: powerful scaffolds for asymmetric catalysis

Cyclic peptides containing a disulfide bridge were identified as a simple and versatile coordination sphere for asymmetric catalysis. Upon complexation with Cu(2+) ions they catalyze Diels-Alder and Friedel-Crafts reactions with high enantioselectivities of up to 99% ee and 86% ee, respectively. Moreover, the peptides ligands were systematically optimized with the assistance of "Alanine Scanning". This biomolecular design could greatly expand the choice of peptide scaffolds for artificial metallopeptide catalysts.

Asymmetric catalysis at a distance: catalytic, site-selective phosphorylation of teicoplanin

We report three distinct, peptide-based catalysts that enable site-selective phosphorylation of three distinct hydroxyl groups within the complex glycopeptide antibiotic teicoplanin A2-2. Two of the catalysts are based on a design that capitalizes on a catalyst-substrate interaction that mimics the biological mechanism of action for teicoplanin. These catalysts are based on a DXaa-DXaa peptide motif that is known to target the teicoplanin structure in a specific manner. The third was identified through evaluation of a set of catalysts that had been developed for historically distinct projects. Each catalyst contains additional functionality designed to dispose a catalytic moiety (a nucleophilic alkylimidazole) at a different region of the glycopeptide structure. A combination of mass spectrometry and 2D-NMR spectroscopy allowed structural assignment of the distinct phosphorylated teicoplanin derivatives. Mechanistic studies are also reported that support the hypotheses that led to the discovery of the catalysts. In this manner, small molecule catalysts have been achieved that allow rational, catalytic control over reactions at sites that are separated by 11.6, 16.5, and nearly 17.7 Å, based on the X-ray crystal structure of teicoplanin A2-2. Finally, we report the biological activity of the new phosphorylated teicoplanin analogs and compare the results to the natural product itself.

Enantioselective silyl protection of alcohols promoted by a combination of chiral and achiral Lewis basic catalysts

Catalytic enantioselective mono-silylations of diols and polyols furnish valuable alcohol-containing molecules in high enantiomeric purity. These transformations, however, require high catalyst loadings (20-30 mol%) and long reaction times (2-5 days). Here, we report that a counterintuitive strategy – involving the use of an achiral co-catalyst that is structurally similar to the chiral catalyst – provides an effective solution to this problem. A combination of seemingly competitive Lewis-basic molecules can function in concert such that one serves as an achiral nucleophilic promoter while the other performs as a chiral Brønsted base. Upon addition of 7.5-20 mol % of commercially available N-heterocycle (5-ethylthiotetrazole), reactions typically proceed within one hour, delivering the desired products in high yields and enantiomeric ratios. In some instances, there is no reaction in the absence of the achiral base, yet presence of the achiral co-catalyst results in facile formation of products in high enantiomeric purity.

Desymmetrisation of meso-diols mediated by non-enzymatic acyl transfer catalysts

Complementary to enzymatic methods, catalytic enantioselective desymmetrisation of meso-diols (EDMD) by small molecule catalysts has emerged as a powerful tool that provides highly enantioenriched materials of considerable value in organic synthesis. This review traces the evolution of easily accessible catalysts used in the EDMD and compares their performance with the existing enzymatic methods.

Amine, alcohol and phosphine catalysts for acyl transfer reactions

An overview of the area of organocatalytic asymmetric acyl transfer processes is presented including O- and N-acylation. The material has been ordered according to the structural class of catalyst employed rather than reaction type with the intention to draw mechanistic parallels between the manner in which the various reactions are accelerated by the catalysts and the concepts employed to control transfer of chiral information from the catalyst to the substrates.

Folded biomimetic oligomers for enantioselective catalysis

Many naturally occurring biopolymers (i.e., proteins, RNA, DNA) owe their unique properties to their well-defined three-dimensional structures. These attributes have inspired the design and synthesis of folded architectures with functions ranging from molecular recognition to asymmetric catalysis. Among these are synthetic oligomeric peptide ("foldamer") mimics, which can display conformational ordering at short chain lengths. Foldamers, however, have not been explored as platforms for asymmetric catalysis. This report describes a library of synthetic helical "peptoid" oligomers that enable enantioselective transformations at an embedded achiral catalytic center, as illustrated by the oxidative kinetic resolution of 1-phenylethanol. In an investigation aimed at elucidating key structure-function relationships, we have discovered that the enantioselectivity of the catalytic peptoids depends on the handedness of the asymmetric environment derived from the helical scaffold, the position of the catalytic center along the peptoid backbone, and the degree of conformational ordering of the peptoid scaffold. The transfer of chiral information from a folded scaffold can enable the use of a diverse assortment of embedded achiral catalytic centers, promising a generation of synthetic foldamer catalysts for enantioselective transformations that can be performed under a broad range of reaction environments.

Theoretical study of the catalysis of cyanohydrin formation by the cyclic dipeptide catalyst cyclo[(S)-His-(S)-Phe]

Dipeptide cyclo[(S)-His-(S)-Phe] 1, first applied by Inoue et al. in 1981, catalyzes the hydrocyanation of aromatic aldehydes very efficiently. Enantioselective autoinduction has also been reported for the process. We have employed QM (density functional theory and MP2), molecular mechanics (MM), and molecular dynamics (MD) methods to (i) derive a mechanistic picture for catalysis and (ii) reveal the origin of stereochemistry and autoinduction. A dimer is proposed to be the catalytic species, in which one imidazole group is essential for the delivery of the nucleophile and the second imidazole group acts as an acid, accompanied with pi-interaction for most favorable substrate binding. Hydrogen-bonding via hydroxy groups is crucial for catalysis also. MD studies indicate stability of the dimer only in non-polar media, which is consistent with the need of the experimental (heterogeneous) reaction conditions to achieve high enantioselectivities. DFT and MP2 results suggest the incorporation of the product cyanohydrin via extended edge-to-face pi-interaction over three aromatic units. Transition states derived from this model are in good agreement with experimental findings and enantioselectivities.

Supramolecular bidentate ligands by metal-directed in situ formation of antiparallel beta-sheet structures and application in asymmetric catalysis

The principles of protein structure design, molecular recognition, and supramolecular and combinatorial chemistry have been applied to develop a convergent metal-ion-assisted self-assembly approach that is a very simple and effective method for the de novo design and the construction of topologically predetermined antiparallel beta-sheet structures and self-assembled catalysts. A new concept of in situ generation of bidentate P-ligands for transition-metal catalysis, in which two complementary, monodentate, peptide-based ligands are brought together by employing peptide secondary structure motif as constructing tool to direct the self-assembly process, is achieved through formation of stable beta-sheet motifs and subsequent control of selectivity. The supramolecular structures were studied by (1)H, (31)P, and (13)C NMR spectroscopy, ESI mass spectrometry, X-ray structure analysis, and theoretical calculations. Our initial catalysis results confirm the close relationship between the self-assembled sheet conformations and the catalytic activity of these metallopeptides in the asymmetric rhodium-catalyzed hydroformylation. Good catalyst activity and moderate enantioselectivity were observed for the selected combination of catalyst and substrate, but most importantly the concept of this new methodology was successfully proven. This work presents a perspective interface between protein design and supramolecular catalysis for the design of beta-sheet mimetics and screening of libraries of self-organizing supramolecular catalysts.

Site-selective catalysis of phenyl thionoformate transfer as a tool for regioselective deoxygenation of polyols

We report the application of peptide-embedded imidazoles as catalysts for the site-selective delivery of the phenyl thionoformate unit as a prelude to deoxygenation reactions of polyols. Methodology was developed that allows for the synthesis of thiocarbonyl derivatives based on a combination of additives that include N-alkylimidazoles and FeCl3 as co-catalysts. The use of this reagent combination leads to increased reaction rates and efficient yields relative to those of simple base-mediated reactions. In terms of controlling regioselectivity during the course of polyol modification, we found that histidine-containing peptides, in combination with FeCl3, could lead to modulation of the product distribution. Through screening of peptides and control of reaction conditions, products could be observed that reflected both the inherent preference of substrates and also reversal of inherent selectivity.

Asymmetric catalysis by chiral hydrogen-bond donors

Hydrogen bonding is responsible for the structure of much of the world around us. The unusual and complex properties of bulk water, the ability of proteins to fold into stable three-dimensional structures, the fidelity of DNA base pairing, and the binding of ligands to receptors are among the manifestations of this ubiquitous noncovalent interaction. In addition to its primacy as a structural determinant, hydrogen bonding plays a crucial functional role in catalysis. Hydrogen bonding to an electrophile serves to decrease the electron density of this species, activating it toward nucleophilic attack. This principle is employed frequently by Nature's catalysts, enzymes, for the acceleration of a wide range of chemical processes. Recently, organic chemists have begun to appreciate the tremendous potential offered by hydrogen bonding as a mechanism for electrophile activation in small-molecule, synthetic catalyst systems. In particular, chiral hydrogen-bond donors have emerged as a broadly applicable class of catalysts for enantioselective synthesis. This review documents these advances, emphasizing the structural and mechanistic features that contribute to high enantioselectivity in hydrogen-bond-mediated catalytic processes.

Remote desymmetrization at near-nanometer group separation catalyzed by a miniaturized enzyme mimic

The chirality of biological receptors often requires syntheses of therapeutic compounds in single enantiomer form. The field of asymmetric catalysis addresses enantioselective synthesis with chiral catalysts. Chemical differentiation of sites within molecules that are separated in space by long distances presents special challenges to chiral catalysts. As the distance between enantiotopic sites increases within a substrate, so too may the requirements for size and complexity for the catalyst. The extreme of catalyst complexity could be defined by macromolecular enzymes and their amazing capacity to effect stereospecific reactions over long distances between reactive sites and enzyme-substrate contacts. We report here a synthetic, miniaturized enzyme mimic that catalyzes a desymmetrization reaction over a very long distance.

Efficiency in nonenzymatic kinetic resolution

The Walden memorial at the Technical University in Riga is pictured in the frontispiece to mark the recent centennial of the Walden inversion. This is a rare public monument to key events from the first era of exploration in stereocontrolled synthesis, and may be the only such monument to use the language of organic chemistry expressed at the molecular level. The reaction of racemic substrates with chiral nucleophiles is one of many methods currently known to achieve kinetic resolution, a phenomenon that ranks as the oldest and most general approach for the synthesis of highly enantioenriched substances. The first nonenzymatic kinetic resolutions as well as the original forms of the Walden inversion were studied in the 1890s. All of these investigations were conducted within the first generation following the demonstration that carbon is tetrahedral, and provided abundant evidence that the principles and importance of enantiocontrolled syntheses were understood. However, a reliable, rapid technique to quantify results and guide the optimization process was still lacking. Many decades passed before this problem was solved by the advent of HPLC and GLPC assays on chiral supports, which stimulated explosive growth in the synthesis of nonracemic substances by kinetic resolution. The Walden monument is accessible to passers-by for hands-on inspection as well as for contemplation and learning. In a similar way, kinetic resolution is experimentally accessible and can be thought-provoking at several levels. We follow the story of kinetic resolution from the early discoveries through fascinating historical milestones and conceptual developments, and close with a focus on modern techniques that maximize efficiency.