Solid-Phase Synthesis of Head to Side-Chain Tyr-Cyclodepsipeptides Through a Cyclative Cleavage From Fmoc-MeDbz/MeNbz-resins

Safety-Catch Linkers for Solid-Phase Peptide Synthesis

Solid-phase peptide synthesis (SPPS) is the preferred strategy for synthesizing most peptides for research purposes and on a multi-kilogram scale. One key to the success of SPPS is the continual evolution and improvement of the original method proposed by Merrifield. Over the years, this approach has been enhanced with the introduction of new solid supports, protecting groups for amino acids, coupling reagents, and other tools. One of these improvements is the use of the so-called "safety-catch" linkers/resins. The linker is understood as the moiety that links the peptide to the solid support and protects the C-terminal carboxylic group. The "safety-catch" concept relies on linkers that are totally stable under the conditions needed for both α-amino and side-chain deprotection that, at the end of synthesis, can be made labile to one of those conditions by a simple chemical reaction (e.g., an alkylation). This unique characteristic enables the simultaneous use of two primary protecting strategies: tert-butoxycarbonyl (Boc) and fluorenylmethoxycarbonyl (Fmoc). Ultimately, at the end of synthesis, either acids (which are incompatible with Boc) or bases (which are incompatible with Fmoc) can be employed to cleave the peptide from the resin. This review focuses on the most significant "safety-catch" linkers.

A versatile o-aminoanilide linker for native chemical ligation

Chemical protein synthesis (CPS) is a consolidated field founded on the high chemospecificity of amide-forming reactions, most notably the native chemical ligation (NCL), but also on new technologies such as the Ser/Thr ligation of C-terminal salicylaldehyde esters and the α-ketoacid-hydroxylamine (KAHA) condensation. NCL was conceptually devised for the ligation of peptides having a C-terminal thioester and an N-terminal cysteine. The synthesis of C-terminal peptide thioesters has attracted a lot of interest, resulting in the invention of a wide diversity of different methods for their preparation. The N-acylurea (Nbz) approach relies on the use of the 3,4-diaminobenzoic (Dbz-COOH) and the 3-amino-(4-methylamino)benzoic (MeDbz-COOH) acids; the latter disclosed to eliminate the formation of branching peptides. Dbz-COOH has been also used for the development of the benzotriazole (Bt)-mediated NCL, in which the peptide-Dbz-CONH2 precursor is oxidized to a highly acylating peptide-Bt-CONH2 species. Here, we have brought together the Nbz and Bt approaches in a versatile linker, the 1,2-diaminobenzene (Dbz). The Dbz combines the robustness of MeDbz-COOH and the flexibility of Dbz-COOH: it can be converted into the Nbz or Bt C-terminal peptides. Both are ligated in high yields, and the reaction intermediates can be conveniently characterized. Our results show that the Bt precursors have faster NCL kinetics that is reflected by a rapid transthioesterification (<5 min). Taking advantage of this major acylating capacity, peptide-Bt can be transselenoesterified in the presence of selenols to afford peptide selenoesters which hold enormous potential in NCL.

Selective Conversion of Unactivated C-N Amide Bond to C-C bond via Steric and Electronic Resonance Destabilization

The chemo- and site-selective reaction at the particular C-N amide bond among a sea of other amides is a significant and long-standing challenge. Although the use of twisted amides has been demonstrated for modifications of inert C-N amide bonds, none of these methods can selectively activate a particular amide bond for C-C bond formation in the presence of similar amides. Using density functional theory as a guide, we report the first site-selective C-C bond modification of a particular C-N amide bond in polyamides with a low twist angle by combining ground-state steric distortion with electronic activation.

On-Resin Peptide Cyclization Using the 3-Amino-4-(Methylamino)Benzoic Acid MeDbz Linker

Cyclic peptides are becoming increasingly important in drug discovery due to their specific binding properties, larger surface area compared to small molecules, and their ready and modular synthetic accessibility. In this protocol, we describe an on-resin, cleavage-inducing cyclization methodology for the synthesis of cyclic thiodepsipeptides and cyclic homodetic peptides using the 3-amino-4-(methylamino)benzoic acid (MeDbz) linker. We further describe three post-cyclization one-pot procedures, which include desulfurization, disulfide bond formation, and S-alkylation of cysteine residues.

Metal-Free Selective Modification of Secondary Amides: Application in Late-Stage Diversification of Peptides

Here we solve a long-standing challenge of the site-selective modification of secondary amides and present a simple two-step, metal-free approach to selectively modify a particular secondary amide in molecules containing multiple primary and secondary amides. Density functional theory (DFT) provides insight into the activation of C-N bonds. This study encompasses distinct chemical advances for late-stage modification of peptides thus harnessing the amides for the incorporation of various functional groups into natural and synthetic molecules.

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.