Exploring chemoselective S-to-N acyl transfer reactions in synthesis and chemical biology

Abstract

S-to-N acyl transfer is a high-yielding chemoselective process for amide bond formation. It is widely utilized by chemists for synthetic applications, including peptide and protein synthesis, chemical modification of proteins, protein-protein ligation and the development of probes and molecular machines. Recent advances in our understanding of S-to-N acyl transfer processes in biology and innovations in methodology for thioester formation and desulfurization, together with an extension of the size of cyclic transition states, have expanded the boundaries of this process well beyond peptide ligation. As the field develops, this chemistry will play a central role in our molecular understanding of Biology.

Amide-forming chemical ligation via O -acyl hydroxamic acids

“…Due to the facility of the original native chemical ligation chemistry, many methods have been developed to extend its applicability beyond Xaa-Cys ligation sites, by the use of various cysteine surrogates in place of an N-terminal cysteine (17)(18)(19). The abilities of certain nonthiol side-chain functionalities-including imidazoles (20) and carboxylates (21,22)-to facilitate the aminolysis of a peptide-α thioester by N-terminal histidine or Asp/Glu-peptides have been documented.…”

Cysteine-to-lysine transfer antibody fragment conjugation

“…39 Native chemical ligation (NCL) uses this reactivity to achieve selective amine-acylation on peptides and proteins via an S,Nacyl transfer involving a 5-membered ring intermediate. 40 This concept has also recently been applied to larger, macrocyclic intermediates, 41 such as the use of internal cysteines in peptides to accelerate ligation to the N-terminus. [42][43][44][45] In order for the CLT strategy to work, a thioester would need to react with the Fab fragment only upon reduction of the disulde bond.…”

“…While in NCL the initial transthioesterication is the rate determining step, in ligations proceeding via larger ring sizes, the S,N-acyl transfer becomes rate determining. 40 Given the macrocyclic intermediates involved in the CLT strategy, we anticipated that this reaction would not be rapid and optimization of conditions to limit competing hydrolysis would be required. We analysed reactions up to 72 h and used LC-MS to reveal the alkyne : antibody ratios (AARs) (see ESI Table S2 † for conditions examined).…”

Traceless β-mercaptan-assisted activation of valinyl benzimidazolinones in peptide ligations

“…As we did not observe any lactamization or other obvious side products derived from the activated thioesters, the experimental result underscores the possible instability of peptide segments in the presence of a thiol additive under denaturing conditions, where more optional ligation methods and connection sites would be beneficial for identifying suitable synthetic routes leading to the desired target peptide. 33 Alternatively, when the second ligation segment was extended to Leu 72 ( Fig. 4b ), the corresponding ligation product 14 showed significantly improved stability, and was successfully ligated with peptide 15b containing a γ-thiovaline derivative prepared following a palladium-catalyzed γ-C(sp 3 )–H acetoxylation protocol.…”