LAUSR

Summary Intramolecular strain is a powerful driving force for rapid and selective chemical reactions, and it is the cornerstone of strain-induced bioconjugation. However, the use of molecules with built-in strain is often complicated as a result of instability or selectivity issues. Here, we show that such strain, and subsequent cycloadditions, can be mediated by visible light via the harvesting of photochemical energy. Through theoretical investigations and molecular engineering of strain-loadable cycloalkenes, we demonstrate the rapid chemoselective cycloaddition of alkyl azides with unstrained cycloalkenes via the transiently (reversibly) formed trans -cycloalkene. We assess this system via the rapid bioconjugation of azide-functionalized insulin. An attractive feature of this process is the cleavable nature of the linker, which makes a catch-and-release strategy possible. In broader terms, we show that conversion of photochemical energy to intramolecular ring strain is a powerful strategy that can facilitate complex chemical transformations, even in biomolecular systems.