An Insight into the radical Thiol/Yne Coupling: the Emergence of Arylalkyne-Tagged Sugars for the Direct Photoinduced Glycosylation of Cysteine-Containing Peptides.

An explorative study of the Thiol-Yne Coupling (TYC) reaction has been carried out using an aliphatic (1-octyne) and aromatic alkyne (phenylacetylene) and two alkanethyiols (methyl thioglycolate and N-acetyl-L-cysteine methyl ester). The outcomes of the TYC reactions strongly depend on the experimental conditions (e.g. temperature, solvent, and alkyne/thiol ratio), but these can be properly adjusted to achieve selective production of either mono- or bis-coupling product. With respect to 1-octyne, phenylacetylene undergoes notably easier radical hydrothiolation further showing a notably higher aptititude for mono-hydrothiolation exclusive of bis-hydrothiolation. The overall findings were exploited in glycosylation of cysteine derivatives as well as of cysteine containing peptides. A sugar featuring an arylacetylene moiety gave rise to a true click-reaction, i.e. glycosylation of the tripeptide glutathione in its native form, by means of virtually equimolar amounts of reagents. This reaction was successfully applied, under physiological conditions, to a cysteine-containing nonapeptide with marked advantages over the analogous Thiol-Ene Coupling (TEC) derivatization. A TYC/TEC sequence affording bis-armed cysteine derivatives through dual functionalization of an alkynyl sugar was additionally devised.