[Molecular Conjugation for the "Diazido Probe" Method].

Target identification of screening hit compounds with unknown mechanisms of action obtained from chemical libraries by phenotypic assays has played an important role in the development of innovative drugs that work based on novel mechanisms. To improve the usability of the target identification based on the photoaffinity labeling method, we have studied the "diazido probe" method, wherein a photoreactive aromatic azido and relatively photostable aliphatic azido groups are sequentially used for photoreaction and introduction of a latent detectable tag via the Staudinger ligation or click reactions, and related synthetic methods that enable expeditious preparation of molecular probes. To facilitate the development of diazido probes, we established short synthetic routes to diazido building blocks with different connectable groups based on sequential iridium-catalyzed C-H borylation and copper-catalyzed azidation of 1,3-disubstituted benzenes, and subsequent diverse functional group transformations leaving the azido groups untouched. To improve the utility of click chemistry for efficient introduction of a latent detectable tag, we developed a transient protection method of cyclooctynes from cycloaddition with an azide by 1 : 1 complexation with a cationic copper(I) salt. Application of this protection method using a cationic copper salt to a cyclooctyne bearing a terminal alkyne allowed for the selective click conjugation with an azide at the terminal alkyne moiety, which rendered functionalized cyclooctyne derivatives easily synthesizable.