Further applications of classical amide coupling reagents: Microwave-assisted esterification on solid phase

On-Resin Ugi Reaction for C-Terminally Modified and Head-to-Tail Cyclized Antibacterial Peptides

Here we report a method to synthesize C-terminally modified peptides on resin. A four-component Ugi reaction of isocyanide resin, an Fmoc-protected amino acid, an amine, and a 6-nitroveratrylaldehyde gives C-terminal photocaged peptide amides, which can be photolyzed to generate C-terminal peptide amides. Changing the amine component in the Ugi reaction gives peptides with different C-terminal modifications including substituted anilides, alkyne, and azide. By installing an N-terminal azide and C-terminal alkyne, we synthesized a head-to-tail cyclized antibacterial peptide through copper(I)-catalyzed azide-alkyne cycloaddition (CuAAC). The cyclized peptide exhibited higher proteolytic stability and antibacterial activity than the linear peptide.

Total Synthesis and Antimicrobial Evaluation of Pagoamide A

Natural products are often the starting point for drug development and also the testing ground for synthetic methods. Herein we describe the total synthesis and antimicrobial evaluation of a marine natural product, pagoamide A, which is a macrocyclic depsipeptide with two backbone thiazole units and a dimethylated N-terminus. The two thiazole building blocks were synthesized from commercially available materials in four or fewer steps and employed directly in solid-phase peptide synthesis (SPPS) to afford pagoamide A. The use of SPPS ensured that the synthetic sequence is operationally straightforward and, if needed, permits modular substitution of building blocks to easily access diverse structural analogs. Our antimicrobial assays showed that pagoamide A has moderate activity against Bacillus subtilis.

DIC/Oxyma-based accelerated synthesis and oxidative folding studies of centipede toxin RhTx

Coupling reagents play crucial roles in the iterative construction of amide bonds for the synthesis of peptides and peptide-based derivatives. The novel DIC/Oxyma condensation system featured with the low risk of explosion displayed remarkable abilities to inhibit racemization, along with efficient coupling efficiency in both manual and automated syntheses. Nevertheless, an ideal reaction molar ratio in DIC/Oxyma condensation system and the moderate reaction temperature by manual synthesis remain to be further investigated. Herein, the synthetic efficiencies of different reaction ratios between DIC and Oxyma under moderate reaction temperature were systematically evaluated. The robustness and efficiency of DIC/Oxyma condensation system are validated by the rapid synthesis of linear centipede toxin RhTx. Different folding strategies were applied for the construction of disulfide bridges in RhTx, which was further confirmed in assays of circular dichroism and patch-clamp electrophysiology evaluation. This work establishes the DIC/Oxyma-based accelerated synthesis of peptides under moderate condensation conditions, which is especially useful for the manual synthesis of peptides. Besides, the strategy presented here provides robust technical supports for the large-scale synthesis and oxidative folding of RhTx.

Solid-phase esterification between poly(vinyl alcohol) and malonic acid and its function in toughening hydrogels

Solid-phase esterification reactions occur in the poly(vinyl alcohol)-malonic acid (PVA-MA) hydrogel by a simple drying treatment without using any catalyst under ambient conditions, which largely strengthen the hydrogel.

Synthesis of O-Acyl Isopeptides: Stepwise and Convergent Solid-Phase Synthesis

The O-acyl isopeptide method was developed for the preparation of difficult sequence-containing peptides, whose hydrophobic nature hampers both peptide chain construction on resin and purification with HPLC after deprotection. In the O-acyl isopeptide method, the target peptide is synthesized in an O-acyl isopeptide form, which contains an O-acyl isopeptide bond instead of the native N-acyl peptide bond at a hydroxy group-containing amino acid residue, such as Ser or Thr. The hydrophilic O-acyl isopeptide can be isolated, e.g., as a lyophilized TFA salt. The target peptide can be quantitatively obtained by a final O-to-N intramolecular acyl migration reaction with exposure to neutral conditions. Additionally, the O-acyl isopeptide is important as a hydrophilic precursor peptide for biological peptide assays that are difficult to handle. This chapter describes the synthesis of such O-acyl isopeptides by stepwise and convergent Fmoc solid-phase peptide synthesis.

On-Resin Passerini Reaction toward C-Terminal Photocaged Peptides

An on-resin, three-component Passerini reaction was developed to synthesize C-terminal photocaged peptides. Highly compatible with conventional Fmoc SPPS, this reaction produces peptides with a C-terminal o-amido-6-nitroveratryl (αANV) ester in one pot with conserved chirality. Under physiological conditions, the C-terminal αANV ester rapidly photolyzed to revert to carboxylate, offering a convenient method for optical control of cellular signals by modulating the C-terminal carboxylate.