The use of small-molecule fluorophores to label proteins with minimal perturbation in response to an external stimulus is a powerful tool to probe chemical and biochemical environments. Herein we describe first the use of a coumarin modified triazabutadiene that can deliver aryl diazonium ions to fluorescently label proteins via tyrosine selective modification. The labeling can be triggered by low pH induced liberation of the diazonium species, making the fluorophore specifically useful in labeling biochemical surroundings such as those found within the late endosome. Additionally, we show that a variety of coumarin triazabutadienes may also be prone to releasing their diazonium cargo via irradiation with UV light.
A concise one-pot three-component reaction that affords fluorescent indolizines, benzo[d]pyrrolo[2,1-b]thiazoles, and pyrrolo[1,2-a]pyrazines is reported. The methodology involves the formation of a heterocyclic 1-aza-1,3-diene derived from a Knoevenagel condensation between an aldehyde and 2-methyl-ene-cyano aza-heterocycles, followed by [4 + 1] cycloaddition of acetyl cyanide behaving as a non-classical isocyanide replacement.
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