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.

Sulfonate N-Heterocyclic Carbene-Copper Complexes: Uniquely Effective Catalysts for Enantioselective Synthesis of C-C, C-B, C-H, and C-Si Bonds

A copper-based complex that contains a sulfonate N-heterocyclic carbene ligand was first reported 15 years ago. Since then, these organometallic entities have proven to be uniquely effective in catalyzing an assortment of enantioselective transformations, including allylic substitutions, conjugate additions, proto-boryl additions to alkenes, boryl and silyl substitutions, hydride-allyl additions to alkenyl boronates, and additions of boron-containing allyl moieties to N-H ketimines. In this review article, we detail the shortcomings in the state-of-the-art that fueled the development of this air stable ligand class, members of which can be prepared on multigram scale. For each reaction type, when relevant, the prior art at the time of the advance involving sulfonate NHC-Cu catalysts and/or subsequent key developments are briefly analyzed, and the relevance of the advance to efficient and enantioselective total or formal synthesis of biologically active molecules is underscored. Mechanistic analysis of the structural attributes of sulfonate NHC-Cu catalysts that are responsible for their ability to facilitate transformations with high efficiency as well as regio- and enantioselectivity are detailed. This review contains several formerly undisclosed methodological advances and mechanistic analyses, the latter of which constitute a revision of previously reported proposals.

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.

Synthesis, self-assembly, and catalytic activity of histidine-based structured lipopeptides for hydrolysis reactions in water

When interfacial catalysis and organocatalysis meet: self-assembling histidine-based lipopeptides catalyse ester hydrolysis in water, depending on aggregation.

Synthesis of chiral γ-lactones by one-pot sequential enantioselective organocatalytic michael addition of boronic acids and diastereoselective intramolecular passerini reaction

The synthesis of α,γ-substituted chiral γ-lactones was quickly achieved in a one pot sequential process. The procedure involves an enantioselective organocatalysed transfer of boronic acid to 5-hydroxyfuran-2(5H)-one, followed by an intramolecular diastereoselective Passerini-type reaction. The methodology was developed and optimized with N-Boc-indole-2-boronic acid giving access to α-indole-γ-substituted lactones in high yields and good diastereoisomeric and enantiomeric ratios. By applying the process to other boronic acids, the synthesis of structurally diversified α,γ-substituted chiral lactones was also achieved in good yields albeit with lower enantioselectivities.

O-Monoacyltartaric acid catalyzed enantioselective conjugate addition of a boronic acid to dienones: application to the synthesis of optically active cyclopentenones

Enantioselective conjugate addition of styrylboronic acid to dienones was effectively catalyzed by an O-monoacyltartaric acid to afford monostyrylated products with good enantioselectivity. The RCM of the monostyrylated products using the Hoveyda-Grubbs II catalyst afforded optically active cyclopentenones, including a synthetic intermediate of the antitumor agent TEI-9826. The study shows that a diene additive such as 1,6-heptadiene or diallyl ether was essential for the RCM.

A hydrogen bond rationale for the enantioselective β-alkenylboration of enones catalyzed by O-monoacyltartaric acids

DFT calculations suggest that O-monoacyl L-tartaric acids catalyze the asymmetric conjugate alkenylboration of enones through transition structures that are stabilized by hydrogen-bonding interactions. Formation of a five-membered acyloxyborane is proposed. The hydrogen of the free carboxy group derived from the catalyst interacts with the carbonyl group of the cyclic acyloxyborane, stabilizing the transition structure and reducing the flexibility of the system. Additional stabilizing nonclassical CH···O hydrogen-bond interactions seem to determine the observed enantioselectivity.

Transition-metal-free C–C bond forming reactions of aryl, alkenyl and alkynylboronic acids and their derivatives

Transition-metal-free C–C bond forming reactions of boronic acids are new emerging tools in organic synthesis which complement metal-based procedures.

Copper–dienamine catalysis: γ-oxyamination of α,β-unsaturated aldehydes

A multicatalytic system composed of copper complexes and sec-amine compounds is proposed for γ-oxyamination of α,β-unsaturated aldehydes.