Desulfurative Borylation of Small Molecules, Peptides, and Proteins

Abstract

We introduce a direct conversion of alkyl thiols into boronic acids, facilitated by a water-soluble phosphine, 1,3,5triaza-7-phosphaadamantane (PTA), in conjunction with tetrahydroxydiboron (B 2 (OH) 4 ), acting as both a radical initiator and a boron source. This desulfurative borylation reaction has been successfully applied to various substrates, including cysteine residues in oligopeptides and small proteins, primary alkyl thiols found in pharmaceutical compounds, disulfides, and selenocysteine. Optimization of reaction conditions was undertaken to reduce the formation of unwanted reactions, such as the reduction of alanyl or other primary radicals, and to prevent deleterious reactions between the phosphine and N-terminal amine that lead to methylene adducts by utilizing a buffer containing glycine−glycine (GG) dipeptide. The developed method is characterized by its operational simplicity and robustness. Moreover, its compatibility with various functional groups present in peptides and proteins makes it a promising tool for late-stage functionalization, extending its potential application across a broad spectrum of chemical and biological targets.

Peptide macrocyclisation via intramolecular interception of visible-light-mediated desulfurisation

“…Several methods for the desulfurisation of Cys residues have been reported in the literature, these include; photo-induced desulfurisation 69 using a ruthenium PC, 60 photo-desulfurisation in flow, 61 accelerated desulfurisation using tetraethylborate (NaBEt 4 ), 63 desulfurative borylation, 64 and the exploitation of phosphite 66 and phosphine-dependent pathways. 65 We attempted to adapt and explore two appropriate examples.…”

“…54,55 Desulfurisation of Cys (and alternative non-proteinogenic thiol-containing amino acids 56–58 ) can be applied post-peptide ligation as an elegant method to access a broad range of ligation junctions and facilitate chemical protein synthesis. 56,59–67 A widely used free-radical-mediated Cys desulfurative protocol, 59 developed by Danishefsky and co-workers, proceeds via a thiophosphoranyl radical species generated using a radical initiator (VA-044) to form a thiyl radical from the thiol sidechain of Cys in the presence of the water-soluble phosphine, tris(2-carboxyethyl)phosphine hydrochloride (TCEP). β-Scission of the thiophosphoranyl radical 62 produces a peptide ‘alanyl’ radical which, in the presence of a suitable thiol additive ( e.g.…”

Peptide and Protein Desulfurization with Diboron Reagents

“…We recently reported a direct borylation of cysteine residues in peptides and proteins forming boronoalanine . During these studies, we observed the formation of small amounts of alanine byproducts, which could be suppressed by a judicious selection of a phosphine additive and by adjusting the concentrations of the reagents.…”

“…Furthermore, experiments with various diboron sources yielded complex mixtures (B 2 pin 2 and B 2 neo 2 , entries 7 and 8, respectively) or reduced the yield of 2a (B 2 cat 2 and B 2 (NMe 2 ) 4 , entries 9 and 10, respectively). Notably, a high concentration of B 2 (OH) 4 without any phosphine could also produce desulfurized product 2a as well as borylated product 2c (entry 11), which implied the possible desulfurize mechanism with only B 2 (OH) 4 . We also ran the reaction in the absence of B 2 (OH) 4 but obtained only trace amounts (<1%) of 2a .…”

Peptide Boronic Acids by Late‐Stage Hydroboration on the Solid Phase

“…To overcome this problem, selective photochemical strategies have been developed to generate boronoalanine from cysteine residues in unprotected peptides and proteins (Figure 1C ). [ 12 ]…”