Disulfide exchange pyridyl disulfides

A very convenient method has been devised [40] for the conversion of thiols to ethyldithio derivatives as a routine procedure for protection of thiol-substituted organic acids. It uses a DMAP-catalysed exchange of the ethylthio group between a thiol and ethyl 2-pyridyl disulfide, prepared from the commercially available 2,2 -dithiobispyridine. Filtration through a macroporous sulfonic acidic resin and evaporation of the solvent yielded the disulfide directly. 

Several other highly efficient coupling reactions have been reported to be of click in nature since they fulfilled all or some of the click chemistry criteria. Several types of metal-free click reactions have been developed. The more prominent ones are thiol-ene, thiol-yne, thiol-para-fluoro, hetero Diels-Alder (HDA) coupling reactions, and pyridyl disulfide exchange reactions (Scheme 8.6) [155-162]. Each of them requires specific initiators, monomers, and/or polymerization conditions. Therefore, the synthetic route should be carefully designed to prepare the desired functional polymers or solid substrates. 

---