Kinetic Resolution of Alcohols Catalyzed by Tripeptides Containing the N-Alkylimidazole Substructure

Cross-strand histidine-aromatic interactions enhance acyl-transfer rates in beta-hairpin peptide catalysts

A reactive tagging methodology was used to select the species most reactive to an acylation reagent from a solid phase library of beta hairpin peptides. Hits bearing an electron-rich aromatic residue across strand from a reactive histidine were found to competitively become N-acylated. In addition to displaying rapid N-acylation rates the hit peptide was additionally deacylated in the presence of a nucleophile, thus closing a putative catalytic cycle. Variants of the hit peptide were studied to elucidate both the magnitude (up to 18,000-fold over background, kcat/kuncat = 94,000,000, or 45-fold over Boc-histidine methyl ester) and mechanism of acyl transfer catalysis. A combination of CH-π, cation-π and HisH(+)-O interactions in the cationic imidazole transition state is implicated in the rate acceleration, in addition to the fidelity of the beta hairpin fold. Moreover, NMR structural data on key intermediates or models thereof suggest that a key feature of this catalyst is the ability to access several different stabilizing conformations along the catalysis reaction coordinate.

Cation–π interactions in iminium ion activation: correlating quadrupole moment & enantioselectivity

A cation–π interaction is operational in the addition of uncharged nucleophiles to iminium salts derived from MacMillan's 1st generation catalyst.

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.

Identification and optimization of short helical peptides with novel reactive functionality as catalysts for acyl transfer by reactive tagging

Herein we describe the screening and subsequent optimization of peptide catalysts for ester activation. A combinatorial methodology using dye-tagged substrate analogs is described for determining which components of a His-containing helical library display acyl transfer activity. We found that helical peptides display high activity, and amino acids that reinforce this propensity are advantaged. Through this approach two new structural motifs have been discovered that are capable of activating esters in organic solvents. Unlike most acyl transfer catalysts functioning in organic solvents, these catalysts are histidine- rather than N-alkyl histidine-based. Longer peptides with localization of reactive groups on the C-terminal end of the peptide were found to further enhance catalytic activity up to ∼2800-fold over background.

Catalyst control over regio- and enantioselectivity in Baeyer-Villiger oxidations of functionalized ketones

We report a peptide-based catalyst that can strongly influence the regio- and enantioselectivity of the Baeyer–Villiger (BV) oxidation of cyclic ketones bearing amide, urea, or sulfonamide functional groups. Both types of selectivity are thought to arise from a catalyst–substrate hydrogen-bonding interaction. Furthermore, in selected cases, the reactions exhibit the hallmarks of parallel kinetic resolution. The capacity to use catalysis to select between BV products during an asymmetric process may have broad utility for both the synthesis and diversification of complex molecules, including natural products.

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.

Dynamic asymmetric organocatalysis: cooperative effects of weak interactions and conformational flexibility in asymmetric organocatalysts

This review describes recent advances by our research group in the design and applications of conformationally flexible guanidine/bisthiourea organocatalysts to facilitate unique bond-forming processes, including retro-free, enantiodivergent, entropy-controlled, and cycle-specific organocatalysis. Special emphasis is placed on the key requirements to attain high selectivity and on functional switching by exploiting the structural flexibility of our organocatalysts.

Asymmetric catalysis with peptides

Over the past decade several peptides have been developed as effective asymmetric catalysts for a range of synthetically useful reactions. Many have properties that are difficult to achieve with other catalysts. The article highlights features that render peptidic organocatalysts unique and attractive for future applications. Challenges such as the design of peptidic catalysts are discussed.

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.

Structure-enantioselectivity effects in 3,4-dihydropyrimido[2,1-b]benzothiazole-based isothioureas as enantioselective acylation catalysts

The catalytic activity and enantioselectivity in the kinetic resolution of (±)-1-naphthylethanol with a range of structurally related 3,4-dihydropyrimido[2,1-b]benzothiazole-based catalysts is examined. Of the isothiourea catalysts screened, (2S,3R)-2-phenyl-3-isopropyl substitution proved optimal, giving good levels of selectivity in the kinetic resolution of a number of secondary alcohols (S values up to >100 at ~50% conversion). Low catalyst loadings (0.10-0.25 mol%) of the optimal isothiourea can be used to generate enantiopure alcohols (>99% ee) in good yields.

A highly selective ferrocene-based planar chiral PIP (Fc-PIP) acyl transfer catalyst for the kinetic resolution of alcohols

Novel planar chiral ferrocene nucleophilic catalysts (Fc-PIP) containing both central and planar chiral elements were designed and synthesized for catalytic enantioselective acyl transfer of secondary alcohols. A remarkably efficient catalyst with high selectivity factors (up to S = 1892) was identified. Comparing the combination of central and planar chirality revealed a strong requirement for the "matched" chiral elements, indicating that the stereogenic center of the imidazole rings should present itself on the same face as the ferrocenyl fragment; otherwise, the catalyst is completely inactive. An exclusively stacked transition state that accounts for the high selectivity of the kinetic resolution of secondary alcohols is proposed. Notably, this newly designed catalyst family is suitable for the catalytic kinetic resolution of bulky arylalkyl carbinols, producing esters with extremely high ee (>99%).

Chiral bicycle imidazole nucleophilic catalysts: rational design, facile synthesis, and successful application in asymmetric Steglich rearrangement

A new type of chiral bicycle imidazole nucleophilic catalyst was rationally designed, facilely synthesized, and successfully applied in an asymmetric Steglich rearrangement with good to excellent yield and enantioselectivity at ambient temperature. Moreover, it can be easily recycled with almost no reduction of catalytic efficiency. This is the first example for the successful chiral imidazole nucleophilic catalyst without H-bonding assistance.

Sulfonylation-induced N- to O-acetyl migration in 2-acetamidoethanol derivatives

The first example of sulfonylation-induced N- to O-acetyl migration of 2-acetamidoethanol derivatives is described. This type of reaction could happen with any 2-acetamidoethanol derivatives under typical sulfonylation conditions (TsCl or MsCl, pyridine) and might be a common side reaction of significance. Furthermore, the results reveal that 2-acetamidoethanol derivatives with a sterically encumbered hydroxyl group result in the migration products in high yields. The mechanism of the migration reaction is discussed.

Synergistic organocatalysis in the kinetic resolution of secondary thiols with concomitant desymmetrization of an anhydride

Kinetic resolution is an important method for the separation of racemates into their component enantiomers. Thiols are precursors to a variety of organosulfur compounds, with high utility in both chemistry and chemical biology, yet there is a surprising dearth of methodologies for their direct and efficient catalytic kinetic resolution. Here, we demonstrate an organocatalytic process involving the highly enantioselective desymmetrization of an achiral electrophile with the simultaneous kinetic resolution of a racemic thiol. The preparative potential of the methodology is exemplified by the synthesis of a drug precursor antipode in excellent yield and enantioselectivity as a by-product of a process that also resolves a sec-thiol substrate with a selectivity of S = 226 (that is, both thiol antipodes produced in >95% ee at 51% conversion). In a second example a racemic sec-thiol representing the stereocentre-containing core of the anti-asthma drug (R)-Montelukast was resolved with synthetically useful selectivity under mild conditions.