A Photoredox Thiol-yne Reaction for the Synthesis of Vinyl Sulfide-Based Coumarins and its Effect on Fluorescence Properties.

Coumarins still remain one of the most widely explored fluorescent dyes, with a broad spectrum of applications spanning various fields, such as molecular imaging, bioorganic chemistry, materials chemistry, or medical sciences. Their fluorescence is strongly based on a push-pull mechanism involving an electron-donating group (EDG), mainly located at the C7 or C8 positions of the dye core. Unfortunately, up to now, these positions have been very limited to hydroxyl or amino groups. In this study, we present in detail the synthesis of the first series of coumarins bearing a vinyl sulfide as the EDG at the C7 position. These derivatives were prepared by thiol-yne reaction, promoted by ruthenium- or porphyrin-based photoredox catalysis, enabling rapid late-stage diversification. We also functionalized coumarins with short peptides, and BSA protein as a proof-of-concept study, in a single-step process. This strategy, capable of proceeding under aqueous conditions, overcomes the protection/deprotection steps usually required by traditional methods, which also use strong bases and organic solvents. Moreover, the photophysical properties such as absorption and emission of obtained coumarins (for 3-CF3, 3-benzothiazole, 6-8-difluoro derivatives), predominantly exhibited large Stokes shifts (up to 204 nm) and maintained intramolecular charge transfer (ICT) characteristics.

Metalloenzyme-Mediated Thiol-Yne Addition Towards Photoisomerizable Fluorescent Dyes.

Proteins are able to irreversibly assemble biologically active ligands from building blocks bearing complementary reactive functions due their spatial proximity, through a kinetic target-guided synthetic process (also named in situ click chemistry). Although linkages thus formed are mostly passive, some of them have shown to significantly contribute to the protein binding through for instance hydrogen bonding and stacking interactions. Biocompatible reactions and click chemistry are a formidable source of inspiration for developing such new protein-directed ligations. This study reports a proximity-induced thiol-yne synthesis of carbonic anhydrase inhibitors. Not only this example widens the arsenal of kinetic target-guided synthesis (KTGS) eligible reactions, but the obtained product displayed unsuspected photophysical properties. The corresponding vinyl sulfide linkage conjugated to a coumarin core proved to be engaged in a monodirectional Z to E photoisomerization process. Further investigations guided by theoretical calculations showed that fine-tuning of the nature of the substituents on the coumarin moiety allows to obtain a bidirectional photochemical process, thus discovering a new photoswitching moiety, displaying moreover fluorescence properties. Due to the spectral tunability of coumarin derivatives, this work should open new opportunities for the design of vinyl sulfide-based photoswitch systems with modular photophysical properties.