Rapid vinyl thianthrenium tetrafluoroborate-promoted thioacid-based native chemical ligation and its applications in chemical protein synthesis

Native chemical ligation (NCL) has emerged as the most extensively employed chemoselective reaction in chemical protein synthesis (CPS). Nevertheless, the inherently low reactivity of peptide alkyl thioesters often necessitates the use of excessive nucleophilic additives in NCL to facilitate the reaction. Herein, we describe a rapid, and additive-free peptide ligation reaction between peptide thioacid and N-terminal cysteinyl peptide without epimerization in the present vinyl thianthrenium tetrafluoroborate (VTT). VTT promotes quantitative and chemoselective activation of fully unprotected C-terminal peptide thioacids into thioester intermediates, which demonstrate exceptional reactivity, facilitating rapid NCL in an additive-free manner with high yields. This additive-free strategy is fully compatible with post-ligation desulfurization, allowing for a streamlined one-pot process that enhances the overall efficiency and simplicity of CPS workflows. The effectiveness of this methodology is demonstrated by synthesizing hyalomin-3 from two fragments through a one-pot thioesterification-ligation-desulfurization protocol and ubiquitin through a one-pot C-to-N sequential three-segment condensation (six steps in one pot). Native chemical ligation is pivotal in chemical protein synthesis but often requires excessive additives due to the low reactivity of peptide alkyl thioesters. Here, the authors introduce a thioacid-based additive-free peptide ligation using vinyl thianthrenium tetrafluoroborate, enhancing efficiency and simplicity in synthesizing complex proteins like hyalomin-3 and ubiquitin.