Asymmetric 1,4-Addition Reactions Catalyzed by N-Terminal Thiourea-Modified Helical l-Leu Peptide with Cyclic Amino Acids

Conformational Analysis and Organocatalytic Activity of Helical Stapled Peptides Containing α-Carbocyclic α,α-Disubstituted α-Amino Acids

Conformational freedom-restricted peptides, such as stapled peptides, play a crucial role in the advancement of functional peptide development. We synthesized stapled octapeptides using α-carbocyclic α,α-disubstituted α-amino acids, particularly 3-allyloxy-1-aminocyclopentane-1-carboxylic acid, as the crosslink motifs. The organocatalytic capabilities of the synthesized stapled peptides were assessed in an asymmetric nucleophilic epoxidation reaction because the catalytic activities are known to be proportional to α-helicity. Despite incorporating side-chain crosslinks, the enantioselectivities of the epoxidation reaction catalyzed by stapled octapeptides were found to be comparable to those obtained using unstapled peptides. Interestingly, the stapled peptides using α-carbocyclic α,α-disubstituted α-amino acids demonstrated higher reactivities and stereoselectivities (up to 99% ee) compared to stapled peptides derived from (S)-α-(4-pentenyl)alanine, a commonly used motif for stapled peptides. These differences could be attributed to the increased α-helicity of the former stapled peptide in contrast to the latter, as evidenced by the X-ray crystallographic structures of their N-tert-butoxycarbonyl derivatives.

Fluoride Additive as a Simple Tool to Qualitatively Improve Performance of Nickel-Catalyzed Asymmetric Michael Addition of Malonates to Nitroolefins

Nowadays, design of the new chiral ligands for organometallic catalysts is often based on the step-by-step increase in their complexity to improve efficiency. Herein we describe that simple in situ addition of the fluoride source to the asymmetric organometallic catalyst can improve not only activity but also enantioselectivity. Bromide-nickel diimine complexes were found to catalyze asymmetric Michael addition in low yields and ee, but activation with fluoride leads to a significant improvement in catalyst performance. The developed approach was applied to prepare several enantioenriched GABA analogues.

Insight into Substrate Recognition by Urea-Based Helical Foldamer Catalysts Using a DFT Global Optimization Approach

Peptides and foldamers have recently gained increasing attention as chiral catalysts to achieve challenging (asymmetric) transformations. We previously reported that short helically folded aliphatic oligoureas in combination with achiral Brønsted bases are effective H-bonding catalysts for C-C bond-forming reactions─i.e., the conjugate addition of 1,3-dicarbonyl pronucleophiles to nitroalkenes─with high reactivity and selectivity and at remarkably low chiral catalyst/substrate molar ratios. This theoretical investigation at the density functional theory level of theory, aims to both analyze how the substrates of the reaction interact with the foldamer catalyst and rationalize a chain-length dependence effect on the catalytic properties. We confirm that the first two ureas are the only H-bond donors available to interact with external molecules. Moreover, each urea site interacts with one of the two reactants allowing a short distance between the two reacting carbons, thus facilitating the conjugated addition. Additionally, it was observed that the molecular recognition and catalyst-substrate interactions are mainly governed by electrostatic interactions but not orbital interactions (see from NBO if this is finally true). On these grounds, an electrostatic potential (ESP) analysis showed an important internal charge separation in the catalyst, the positive ESP region being concentrated around the first two ureas, with its area extending as the number of residues increases.

An Orthogonal Dynamic Covalent Chemistry Tool for Ring-Opening Polymerization of Cyclic Oligochalcogenides on Detachable Helical Peptide Templates

A model system is introduced as a general tool to elaborate on orthogonal templation of dynamic covalent ring-opening polymerization (ODC-TROP). The tool consists of 310 helical peptides as unprecedented templates and semicarbazones as orthogonal dynamic covalent linkers. With difficult-to-control 1,2-dithiolanes, ODC-TROP on the level of short model oligomers occurs with high templation efficiency, increasing and diminishing upon helix stabilization and denaturation, respectively. Further, an anti-templated conjugate with mispositioned monomers gave reduced templation upon helix twisting. Even with the "unpolymerizable" 1,2-diselenolanes, initial studies already afford mild templation efficiency. These proof-of-principle results promise that the here introduced tool, recyclable and enabling late-stage side chain modification, will be useful to realize ODC-TROP of intractable or unknown cyclic dynamic covalent monomers for dynamer materials as well as cellular uptake and signaling applications.

Helical Foldamers and Stapled Peptides as New Modalities in Drug Discovery: Modulators of Protein-Protein Interactions

A “foldamer” is an artificial oligomeric molecule with a regular secondary or tertiary structure consisting of various building blocks. A “stapled peptide” is a peptide with stabilized secondary structures, in particular, helical structures by intramolecular covalent side-chain cross-linking. Helical foldamers and stapled peptides are potential drug candidates that can target protein-protein interactions because they enable multipoint molecular recognition, which is difficult to achieve with low-molecular-weight compounds. This mini-review describes a variety of peptide-based foldamers and stapled peptides with a view to their applications in drug discovery, including our recent progress.

E-Selective Ring-Closing Metathesis in α-Helical Stapled Peptides Using Carbocyclic α,α-Disubstituted α-Amino Acids

We present an E-selective ring-closing metathesis reaction in α-helical stapled peptides at positions i and i + 4. The use of two chiral carbocyclic α,α-disubstituted α-amino acids, (1S,3S)-Ac5c3OAll and (1R,3S)-Ac5c3OAll, provides a high E-selectivity of a ≤59:1 E:Z ratio, while mixtures with E:Z ratios of 2.1-0.5:1 were produced with standard acyclic (S)-(4-pentenyl)alanine amino acids. A stapled octapeptide composed of (1S,3S)- and (1R,3S)-Ac5c3OAll amino acids showed a right-handed α-helical crystal structure.

A Novel Dual Organocatalyst for the Asymmetric Pinder Reaction and a Mechanistic Proposal Consistent with the Isoinversion Effect Thereof

In general, the Pinder reaction concerns the reaction between an enolisable anhydride and an aldehyde proceeding initially through a Knoevenagel reaction followed by the ring closing process generating lactones with at least two chiral centers. These scaffolds are frequently present in natural products and synthetic bioactive molecules, hence it has attracted intense interest in organic synthesis and medicinal chemistry, particularly with respect to controlling the diastereo- and enantioselectivity. To the best of our knowledge, there has been only one attempt prior to this work towards the development of a catalytic enantioselective Pinder reaction. In our approach, we designed, synthesized, and tested dual chiral organocatalysts by combining BIMAH amines, (2-(α-(alkyl)methanamine)-1H-benzimidazoles, and a Lewis acid motif, such as squaramides, ureas and thioureas. The optimum catalyst was the derivative of isopropyl BIMAH bearing a bis(3,5-trifluoromethyl) thiourea, which afforded the Pinder products from various aromatic aldehydes with diastereomeric ratio >98:2 and enatioselectivity up to 92 ee%. Interestingly, the enantioselectivity of this catalyzed process is increased at higher concentrations and exhibits an isoinversion effect, namely an inverted "U" shaped dependency with respect to the temperature. Mechanistically, these features, point to a transition state involving an entropy-favored heterodimer interaction between a catalyst/anhydride and a catalyst/aldehyde complex when all other processes leading to this are much faster in comparison above the isoinversion temperature.