Thio-conjugation of substituted benzofurazans to peptides: molecular sieves catalyze nucleophilic attack on unsaturated fused rings

Bioconjugates of 2,1,3-benzoxadiazole (benzofurazan) and its derivatives have attracted considerable interest due to their biological activities and applications as fluorescent tags. A high-yield, chemoselective, and mild procedure for the S-alkylation of cysteine containing peptides by benzofurazan halogenides is reported. The key feature of this procedure is the use of activated molecular sieves (MS) to catalyze thiol activation for nucleophilic substitution under very mild conditions (room temperature and no need for added bases). To the best of our knowledge, this is the first report about thiol nucleophilic substitution performed on unsaturated and annelated systems catalyzed by activated molecular sieves. Reaction yields were remarkable even with benzofurazans having weakly activating groups or no activating groups at all. The potential of the new methodology was explored by synthesizing fluorescent, hydrophilic benzofurazan/peptide conjugates, also with peptides containing unprotected lysine residues.