Evaluating the Viability of Successive Ring-Expansions Based on Amino Acid and Hydroxyacid Side-Chain Insertion

The outcome of ring expansion reactions based on amino/hydroxyacid side chain insertion is strongly dependent on ring size. This manuscript, which builds upon our previous work on Successive Ring Expansion (SuRE) methods, details efforts to better define the scope and limitations of these reactions on lactam and β-ketoester ring systems with respect to ring size and additional functionality. The synthetic results provide clear guidelines as to which substrate classes are more likely to be successful and are supported by computational results, using a Density Functional Theory (DFT) approach. Calculating the relative Gibbs free energies of the three isomeric species that are formed reversibly during ring expansion enables the viability of new synthetic reactions to be correctly predicted in most cases. The new synthetic and computational results are expected to support the design of new lactam- and β-ketoester-based ring expansion reactions

Selective fluorination: a tool to control peptides' conformation and biological activity

Insertion of fluorine into organic molecules is well-known to affect the molecule's electronic distributions and conformational properties in ways that can be beneficial in drug development. However, the effects of fluorination have not been widely investigated in the context of peptides, which limits the development of peptide-based drugs. In this project, stereoselective fluorination was employed to (i) control the conformations of peptides; and (ii) enhance the potency of a peptide-based enzyme inhibitor.Difluorinated GABA analogues, which are known to adopt different conformations depending on the fluorine stereochemistry, were synthesised through a new synthetic route which improved on the yield and stereoselectivity in comparison to the previously published method. The anti- and syn-difluorinated GABA analogues were then incorporated into γ-peptides through solid-phase peptide synthesis, and it was found through NMR and modelling studies that anti-difluoro-GABA could stabilise certain helical and ribbon-like secondary structures, while syn-difluoro GABA destabilised them.To investigate the effect of fluorine substitution on the biological activity of peptides, analogues of pepstatin A were synthesised. Pepstatin A is a potent inhibitor of aspartic proteases and contains a statine residue which mimics the tetrahedral intermediate of peptide hydrolysis. Docking studies revealed that fluorinated pepstatin analogues should provide close mimics of the tetrahedral intermediate, thus giving more potent enzyme inhibitors. Several fluorinatedstatine analogues were synthesised via either nucleophilic or electrophilic fluorination protocols, and these statine analogues were then elaborated into analogues of pepstatin A. Enzyme inhibitory assays revealed that the monofluorinated pepstatin analogues were more potent than pepstatin A itself against cathepsin D, an enzyme implicated in the progression of breast cancer. The fluorinated pepstatin analogues also showed good stability in liver microsomes.Overall, these studies have developed novel strategies for controlling the conformations and bioactivity of peptides. These advances should facilitate the ongoing design of fluorinated peptide drugs for the treatment of diseases such as cancer

Harmony of Protein Tags and Chimeric Molecules Empowers Targeted Protein Ubiquitination and Beyond

Post-translational modifications (PTMs) are crucial mechanisms that underlie the intricacies of biological systems and disease mechanisms. This review focuses on the latest advancements in the design of heterobifunctional small molecules that hijack PTM machineries for target-specific modifications in living systems. A key innovation in this field is the development of proteolysis-targeting chimeras (PROTACs), which promote the ubiquitination of target proteins for proteasomal degradation. The past decade has seen several adaptations of the PROTAC concept to facilitate targeted (de)phosphorylation and acetylation. Protein fusion tags have been particularly vital in these proof-of-concept studies, aiding in the investigation of the functional roles of post-translationally modified proteins linked to diseases. This overview delves into protein-tagging strategies that enable the targeted modulation of ubiquitination, phosphorylation, and acetylation, emphasizing the synergies and challenges of integrating heterobifunctional molecules with protein tags in PTM research. Despite significant progress, many PTMs remain to be explored, and protein tag-assisted PTM-inducing chimeras will continue to play an important role in understanding the fundamental roles of protein PTMs and in exploring the therapeutic potential of manipulating protein modifications, particularly for targets not yet addressed by existing drugs

Diastereoselective synthesis and conformational analysis of (2R)- and (2S)-fluorostatines : an approach based on organocatalytic fluorination of a chiral aldehyde

Stereoselectively fluorinated analogues of the amino acid statine have been efficiently synthesized. The key step is an organocatalytic electrophilic fluorination of a chiral β-oxygenated aldehyde, which provided a test of both diastereoselectivity and chemoselectivity. The target statine analogues were found to adopt unique conformations influenced by the fluorine gauche effect, rendering them potentially valuable building blocks for incorporation into bioactive peptides.4 page(s

Sequential Deoxyfluorination Approach for the Synthesis of Protected α,β,γ-Trifluoro-δ-amino Acids

Backbone-homologated amino acids have been synthesized, containing three vicinal fluorine atoms placed stereospecifically along the carbon chain. Different trifluoro stereoisomers are found to have contrasting conformations, consistent with known stereoelectronic effects associated with C–F bonds

Internal nucleophilic catalyst mediated cyclisation/ring expansion cascades for the synthesis of medium-sized lactones and lactams

A strategy for the synthesis of medium-sized lactones and lactams from linear precursors is described in which an amine acts as an internal nucleophilic catalyst to facilitate a novel cyclisation/ring expansion cascade sequence. This method obviates the need to use high-dilution conditions usually associated with medium-ring cyclisation protocols, as the reactions operate exclusively via kinetically favourable 'normal' sized cyclic transition states. This same feature also enables biaryl-containing medium-sized rings to be prepared with complete atroposelectivity via point-to-axial chirality transfer

Diastereoselective Synthesis and Conformational Analysis of (2<i>R</i>)- and (2<i>S</i>)‑Fluorostatines: An Approach Based on Organocatalytic Fluorination of a Chiral Aldehyde

Stereoselectively fluorinated analogues of the amino acid statine have been efficiently synthesized. The key step is an organocatalytic electrophilic fluorination of a chiral β-oxygenated aldehyde, which provided a test of both diastereoselectivity and chemoselectivity. The target statine analogues were found to adopt unique conformations influenced by the fluorine <i>gauche</i> effect, rendering them potentially valuable building blocks for incorporation into bioactive peptides

Synthesis and Biological Evaluation of Aurachin D Analogues as Inhibitors of Mycobacterium tuberculosis Cytochrome <i>bd</i> Oxidase

A revised total synthesis of aurachin D (1a), an isoprenoid quinolone alkaloid that targets Mycobacterium tuberculosis (Mtb) cytochrome bd (cyt-bd) oxidase, was accomplished using an oxazoline ring-opening reaction. The ring opening enabled access to a range of electron-poor analogues, while electron-rich analogues could be prepared using the Conrad–Limpach reaction. The aryl-substituted and side-chain-modified aurachin D analogues were screened for inhibition of Mtb cyt-bd oxidase and growth inhibition of Mtb. Nanomolar inhibition of Mtb cyt-bd oxidase was observed for the shorter-chain analogue 1d (citronellyl side chain) and the aryl-substituted analogues 1g/1k (fluoro substituent at C6/C7), 1t/1v (hydroxy substituent at C5/C6) and 1u/1w/1x (methoxy substituent at C5/C6/C7). Aurachin D and the analogues did not inhibit growth of nonpathogenic Mycobacterium smegmatis, but the citronellyl (1d) and 6-fluoro-substituted (1g) inhibitors from the Mtb cyt-bd oxidase assay displayed moderate growth inhibition against pathogenic Mtb (MIC = 4–8 μM)