A Mild Aqueous Sonogashira Reaction as a Fluorescent Labeling Strategy for 5-Bromide-2'-Deoxyuridine.

C5-modified uridines are a valuable class of nucleoside analogues, both as potent chemotherapy agents and through their use as the conjunction site in DNA labeling strategies. As an important C5-modified uridine, BrdU has been used in cell proliferation assays since the 1980s. Currently, the detection of BrdU relies on traditional immunostaining; however, this approach has its limitations. Thus, it is desirable, albeit difficult, to develop chemistry methods to fluorescently label BrdU in a cellular context. In the present study, we report our efforts toward developing a robust chemistry methodology for BrdU fluorescent labeling. The Sonogashira reaction was chosen as the key reaction, and various alkynyl groups (aliphatic or aryl) containing fluorescent dyes were synthesized to cross-couple with BrdU. Various bases and catalyst systems were screened to evaluate the optimum conditions. A mild aqueous Sonogashira reaction (K₂PdCl₄, S-Phos, n-Bu₄N⁺OH-, Sodium d-isoascorbate, EtOH/H₂O = 1:1, 37 °C, Ar) was obtained to enable high-yielding BrdU fluorescent labeling.