Direct Assembly of a Fully Functionalized-Fragment Library for Photoaffinity Labeling Applications via Mild A3-Coupling in Water

Abstract In classical photoaffinity labeling (PAL) probes, diazirine and alkyne handles are attached via a flexible spacer, which inherently modulates biological activity. We herein report a diazirine-tolerant A3-coupling (aldehyde–amine–alkyne) catalyzed by AuCl3 in water at room temperature, enabling direct assembly of fully functionalized fragments (FFFs) with rigidly positioned cross-linking and retrieval tags. This mild protocol accommodates diverse aldehydes endowed with benzenes or aliphatic systems, and so far underdeveloped heteroaromatic moieties such as pyridines, quinolines, and indoles, thus delivering FFFs exhibiting more complex structural scaffolds, which are of interest in medicinal chemistry. Postfunctionalization via a mild Suzuki coupling demonstrates how chemical diversity can be further expanded. A library of over 60 compounds were accessed, establishing the methodology’s broad scope. Furthermore, biological testing tentatively showed that these new probes exhibit reactivity with the proteome, enabling labeling in human cells.