Epimerisation in Peptide Synthesis

Epimerisation is basically a chemical conversion that includes the transformation of an epimer into another epimer or its chiral partner. Epimerisation of amino acid is a side reaction that sometimes happens during peptide synthesis. It became the most avoided reaction because the process affects the overall conformation of the molecule, eventually even altering the bioactivity of the peptide. Epimerised products have a high similarity of physical characteristics, thus making it difficult for them to be purified. In regards to amino acids, epimerisation is very important in keeping the chirality of the assembled amino acids unchanged during the peptide synthesis and obtaining the desirable product without any problematic purification. In this review, we report several factors that induce epimerisation during peptide synthesis, including how to characterise and affect the bioactivities. To avoid undesirable epimerisation, we also describe several methods of suppressing the process.

Trimethylaluminum-mediated one-pot peptide elongation

Efficient and straightforward peptide bond formation of N-, and C-terminal unprotected amino acids was successfully achieved by using trimethylaluminum. The coupling reaction was accomplished by pre-reaction of N-, and C-terminal unprotected amino acids and trimethylaluminum to form a five-membered ring that smoothly reacted with nucleophilic amino acid esters. This simple and highly efficient reaction system allows one-pot tripeptide synthesis without the need for expensive coupling reagents. Furthermore, peptide bond formation can be effectively achieved even for amino acids with bulky substituents at the side chain to afford the corresponding tripeptides in high yields in a one-pot manner. In addition, the reaction can be applied for further peptide elongation by the subsequent addition of amino acids and trimethylaluminum. We anticipate that this cost-effective, straightforward, and efficient protocol will be useful for the synthesis of a wide variety of peptides.

Mechanochemistry: New Tools to Navigate the Uncharted Territory of "Impossible" Reactions

Mechanochemical transformations have made chemists enter unknown territories, forcing a different chemistry perspective. While questioning or revisiting familiar concepts belonging to solution chemistry, mechanochemistry has broken new ground, especially in the panorama of organic synthesis. Not only does it foster new "thinking outside the box", but it also has opened new reaction paths, allowing to overcome the weaknesses of traditional chemistry exactly where the use of well-established solution-based methodologies rules out progress. In this Review, the reader is introduced to an intriguing research subject not yet fully explored and waiting for improved understanding. Indeed, the study is mainly focused on organic transformations that, although impossible in solution, become possible under mechanochemical processing conditions, simultaneously entailing innovation and expanding the chemical space.

Peptide Bond Formation of Amino Acids by Transient Masking with Silylating Reagents

A one-pot peptide bond-forming reaction has been developed using unprotected amino acids and peptides. Two different silylating reagents, HSi[OCH(CF₃)₂]₃ and MTBSTFA, are instrumental for the successful implementation of this approach, being used for the activation and transient masking of unprotected amino acids and peptides at C-termini and N-termini, respectively. Furthermore, CsF and imidazole are used as catalysts, activating HSi[OCH(CF₃)₂]₃ and also accelerating chemoselective silylation. This method is versatile as it tolerates side chains that bear a range of functional groups, while providing up to >99% yields of corresponding peptides without any racemization or polymerization.

Recent advances in the synthesis of C-terminally modified peptides

C-Terminally modified peptides are important for the development and delivery of peptide-based pharmaceuticals because they impact peptide activity, stability, hydrophobicity, and membrane permeability. Additionally, the vulnerability of C-terminal esters to cleavage by endogenous esterases makes them excellent pro-drugs. Methods for post-SPPS C-terminal functionalization potentially enable access to libraries of modified peptides, facilitating tailoring of their solubility, potency, toxicity, and uptake pathway. Apparently minor structural changes can significantly impact the binding, folding, and pharmacokinetics of the peptide. This review summarizes developments in chemical methods for C-terminal modification of peptides published since the last review on this topic in 2003.

N-Pyrazinoyl Substituted Amino Acids as Potential Antimycobacterial Agents-The Synthesis and Biological Evaluation of Enantiomers

Tuberculosis is an infectious disease caused by Mycobacterium tuberculosis (Mtb), each year causing millions of deaths. In this article, we present the synthesis and biological evaluations of new potential antimycobacterial compounds containing a fragment of the first-line antitubercular drug pyrazinamide (PZA), coupled with methyl or ethyl esters of selected amino acids. The antimicrobial activity was evaluated on a variety of (myco)bacterial strains, including Mtb H37Ra, M. smegmatis, M. aurum, Staphylococcus aureus, Pseudomonas aeruginosa, and fungal strains, including Candida albicans and Aspergillus flavus. Emphasis was placed on the comparison of enantiomer activities. None of the synthesized compounds showed any significant activity against fungal strains, and their antibacterial activities were also low, the best minimum inhibitory concentration (MIC) value was 31.25 µM. However, several compounds presented high activity against Mtb. Overall, higher activity was seen in derivatives containing ʟ-amino acids. Similarly, the activity seems tied to the more lipophilic compounds. The most active derivative contained phenylglycine moiety (PC-ᴅ/ʟ-Pgl-Me, MIC < 1.95 µg/mL). All active compounds possessed low cytotoxicity and good selectivity towards Mtb. To the best of our knowledge, this is the first study comparing the activities of the ᴅ- and ʟ-amino acid derivatives of pyrazinamide as potential antimycobacterial compounds.

Stereoselective C-terminal peptide elongation from Chan-Lam-Evans reaction generated isopropenyl esters

C-Terminal dipeptide isopropenyl esters were synthesised by a Cu(ii)-mediated Chan-Lam-Evans enol esterification of peptide carboxylic acids and isoprenyl boroxine. These shelf stable peptide esters could be coupled stereoselectively with a variety of amino acid and dipeptide nucleophiles in high yield and purity in the presence of pyrazole/DBU as the catalyst.

Sequence Diversification by Divergent C-Terminal Elongation of Peptides

Sequence diversification at the C terminus is traditionally limited by significant epimerization of the C-terminal residue during its activation toward nucleophilic attack, thus mandating repetition of the peptide synthesis for each targeted variation. Here, we accomplish divergent C-terminal elongation of a single peptide substrate with concomitant resin cleavage via displacement of an N-acyl urea moiety. Sterically hindered amino acids such as Ile and Pro are well-tolerated in this approach, which proceeds reasonable conversion and no detectable epimerization of the starting peptide's C-terminal amino acid.

Aerobic Copper-Catalyzed O-Methylation with Methylboronic Acid

The oxidative coupling of alkylboronic acids with oxygen nucleophiles offers a strategy for replacing toxic, electrophilic alkylating reagents. Although the Chan-Lam reaction has been widely applied in the arylation of heteroatom nucleophiles, O-alkylation with boronic acids is rare. We report a Cu-catalyzed nondecarboxylative methylation of carboxylic acids with methylboronic acid that proceeds in air with no additional oxidant. An isotope-labeling study supports an oxidative cross-coupling mechanism, in analogy to that proposed for Chan-Lam arylation.

Peptide 2-formylthiophenol esters do not proceed through a Ser/Thr ligation pathway, but participate in a peptide aminolysis to enable peptide condensation and cyclization

Peptide thiol salicylaldehyde esters unexpectedly do not follow a Ser/Thr ligation pathway, but proceed towards a peptide aminolysis in DMSO.

A rapid and efficient one-pot method for the reduction of N-protected α-amino acids to chiral α-amino aldehydes using CDI/DIBAL-H

N-Protected α-amino acids can be easily converted directly into chiral α-amino aldehydes in a one-pot procedure by activation with CDI followed by reduction with DIBAL-H.