Application of (aza)quinolizinyl-3-diazonium tetrafluoroborates in photocatalytic sulfanylation and in fluorescent labeling of proteins

Photocatalytic sulfanylations of (aza)quinolizine diazonium salts with disulfides and thiols, including N-Boc-protected cysteine and cystine esters, gave the corresponding fluorescent sulfanylation products in 25-85 % yield. Photocatalytic cycle was proposed for the sulfanylation with disulfides using diazonium salts as heteroaryl radical sources. Two representative diazonium salts were evaluated as bioconjugation reagents for fluorescent labeling of proteins using bovine serum albumin (BSA) as a model protein and targeting cysteine residues, and the results showed their applicability in fluorescent labeling under photocatalytic conditions.