Fluorogenic bioorthogonal reactions enable biomolecule visualization in real time. These reactions comprise reporters that "light up" upon reaction with complementary partners. While the spectrum of fluorogenic chemistries is expanding, few transformations are compatible with live cells due to cross-reactivities or insufficient signal turn-on. To address the need for more suitable chemistries for cellular imaging, we developed a fluorogenic reaction featuring cyclopropenone reporters and phosphines. The transformation involves regioselective activation and cyclization of cyclopropenones to form coumarin products. With optimal probes, the reaction provides >1600-fold signal turn-on, one of the highest fluorescence enhancements reported to date. The bioorthogonal motifs were evaluated in vitro and in cells. The reaction was also found to be compatible with other common fluorogenic transformations, enabling multicomponent, real-time imaging. Collectively, these data suggest that the cyclopropenone-phosphine reaction will bolster efforts to track biomolecule targets in their native settings.

We report a proximity-driven crosslinking strategy featuring bioorthogonal cyclopropenones. These motifs react with phosphines to form electrophilic ketene-ylides. Such intermediates can be trapped by neighboring proteins to form covalent adducts. Successful crosslinking was achieved using a model split reporter, and the rate of crosslinking could be tuned using different phosphine triggers. We further demonstrated that the reaction can be performed in cell lysate. Based on these features, we anticipate that cyclopropenones will enable unique studies of protein-protein and other biomolecule interactions.

A general method to synthesize substituted butenolides from hydroxymethylcyclopropenones is reported. Functionalized cyclopropenones undergo ring-opening reactions with catalytic amounts of phosphine, forming reactive ketene ylides. These intermediates can be trapped by pendant hydroxy groups to afford target butenolide scaffolds. The reaction proceeds efficiently in diverse solvents and with low catalyst loadings. Importantly, the cyclization is tolerant of a broad range of functional groups, yielding a variety of α- and γ-substituted butenolides.