Effect of the Helical Tether of a Resin-Supported Peptide Catalyst for Friedel-Crafts-Type Alkylation in Water

Recent advances in enzymatic carbon-carbon bond formation

Enzymatic carbon-carbon (C-C) bond formation reactions have become an effective and invaluable tool for designing new biological and medicinal molecules, often with asymmetric features. This review provides a systematic overview of key C-C bond formation reactions and enzymes, with the focus of reaction mechanisms and recent advances. These reactions include the aldol reaction, Henry reaction, Knoevenagel condensation, Michael addition, Friedel-Crafts alkylation and acylation, Mannich reaction, Morita-Baylis-Hillman (MBH) reaction, Diels-Alder reaction, acyloin condensations via Thiamine Diphosphate (ThDP)-dependent enzymes, oxidative and reductive C-C bond formation, C-C bond formation through C1 resource utilization, radical enzymes for C-C bond formation, and other C-C bond formation reactions.

Development of fructose-1,6-bisphosphate aldolase enzyme peptide mimics as biocatalysts in direct asymmetric aldol reactions

This study describes the design and synthesis of mimetic peptides modelled on the catalytic active site of the fructose-1,6-bisphosphate aldolase (FBPA) enzyme. The synthesized peptides consisting of the turn motifs and catalytic site amino acids of FBPA enzyme were evaluated for catalytic activity in direct asymmetric aldol reactions of ketones and aldehydes. The influence of substrate scope, catalyst loading and solvents including water, on the reaction were also investigated. Nuclear magnetic resonance (NMR) and circular dichroism (CD) were used to determine the secondary structure of the peptides to provide an understanding of the structure–activity relationship. The peptides showed catalytic activity and the aldol products were obtained in low yields (up to 44%), but excellent enantioselectivity (up to 93%) and moderate diastereoselectivity (65 : 35).

Novel asymmetric aldol reaction catalysing fructose-1,6-bisphosphate aldolase peptide mimics with secondary structural motifs.

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.

Design and Synthesis of Helical N-Terminal L-Prolyl Oligopeptides Possessing Hydrocarbon Stapling

We designed and synthesized helical short oligopeptides with an L-proline on the N-terminus and hydrocarbon stapling on the side chain. Side-chain stapling is a frequently used method for the development of biologically active peptides. Side-chain stapling can stabilize the secondary structures of peptides, and, therefore, stapled peptides may be applicable to peptide-based organocatalysts. Olefin-tethered cis-4-hydroxy-L-proline 1 and L-serine 2 and 8, and (R)-α-allyl-proline 18 were used as cross-linking motifs and incorporated into helical peptide sequences. The Z- and E-selectivities were observed for the ring-closing metathesis reactions of peptides 3 and 11 (i,i+1 series), respectively, while no E/Z-selectivity was observed for that of 19 (i,i+3 series). The stapled peptide B' catalyzed the Michael addition reaction of 1-methylindole to α,β-unsaturated aldehyde, which was seven times faster than that of unstapled peptide B. Furthermore, the high catalytic activity was retained even at lower catalyst loadings (5 mol %) and lower temperatures (0 °C). The circular dichroism spectra of stapled peptide B' showed a right-handed helix with a higher intensity than that of unstapled peptide B. These results indicate that the introduction of side-chain stapling is beneficial for enhancing the catalytic activity of short oligopeptide catalysts.

Asymmetric one-pot reactions using heterogeneous chemical catalysis: recent steps towards sustainable processes

Asymmetric one-pot reactions applying heterogeneous chemical catalysts and unifying the benefits of these catalytic materials with the advantages of one-pot methods, are surveyed.

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.

Strategies for the asymmetric functionalization of indoles: an update

During the past four years, the research of new synthetic methodologies for the rapid construction of enantiomerically pure substituted indole has had a fruitful and important growth.

Histidine-containing peptide catalysts developed by a facile library screening method

Although peptide catalysts have a high potential for the use as organocatalysts, the optimization of peptide sequences is laborious and time-consuming. To address this issue, a facile screening method for finding efficient aminocatalysts from a peptide library has been developed. In the screening for the Michael addition of a malonate to an enal, a dye-labeled product is immobilized on resin-bound peptides through reductive amination to visualize active catalysts. This procedure allows for the monitoring of the reactivity of entire peptides without modifying the resin beads beforehand. Peptides containing histidine at an appropriate position were identified by this method. A novel function of the histidyl residue, which enhances the binding of a substrate to the catalyst by capturing an iminium intermediate, was indicated.

Peptide-catalyzed consecutive 1,6- and 1,4-additions of thiols to α,β,γ,δ-unsaturated aldehydes

Regio- and enantioselective addition of thiols to α,β,γ,δ-unsaturated aldehydes was performed with a resin-supported peptide catalyst. It was shown that a 1,4-adduct was generated mainly at the initial stage of the reaction, and this was eventually converted to a thermodynamically stable 1,6- and 1,4-diadduct through retro-addition/addition reactions.

Experimental lineage and functional analysis of a remotely directed peptide epoxidation catalyst

We describe mechanistic investigations of a catalyst (1) that leads to selective epoxidation of farnesol at the 6,7-position, remote from the hydroxyl directing group. The experimental lineage of peptide 1 and a number of resin-bound peptide analogues were examined to reveal the importance of four N-terminal residues. We examined the selectivity of truncated analogues to find that a trimer is sufficient to furnish the remote selectivity. Both 1D and 2D ¹H NMR studies were used to determine possible catalyst conformations, culminating in proposed models showing possible interactions of farnesol with a protected Thr side chain and backbone NH. The models were used to rationalize the selectivity of a modified catalyst (17) for the 6,7-position relative to an ether moiety in two related substrates.

Peptide-catalyzed kinetic resolution of planar-chiral metallocenes

Kinetic resolution of racemic planar-chiral metallocenes was performed by a resin-supported peptide catalyst, in which low-molecular-weight organocatalysts were not effective.

Rational design of mimetic peptides based on aldo-ketoreductase enzyme as asymmetric organocatalysts in aldol reactions

Excellent yield (up to 97%), enantioselectivity (up to 99%) and diastereoselectivity (up to 99/1) were obtained using mimetic peptides.

Asymmetric one-pot sequential Friedel-Crafts-type alkylation and α-oxyamination catalyzed by a peptide and an enzyme

In the presence of a peptide catalyst and the oxidative enzyme laccase, a one-pot sequential reaction including a Friedel–Crafts-type alkylation of α,β-unsaturated aldehydes followed by an α-oxyamination was realized. The reaction in aqueous solvent to promote the enzymatic oxidation, and the use of a peptide catalyst compatible with such conditions, were essential. The present sequential reaction afforded oxygen-functionalized indole or pyrrole derivatives in a highly enantioselective manner.