Arylidene Meldrum's Acid: A Versatile Structural Motif for the Design of Enzyme-Responsive "Covalent-Assembly" Fluorescent Probes with Tailor-Made Properties.

Latent cyclic C-nucleophiles are recently proving their value in the field of reaction-based fluorescent probes, far beyond their primary utility in organic synthesis. They are typically used to introduce a Michael acceptor moiety acting as a recognition/reaction site for analyte to be detected or as a kinetic promoter of fluorogenic cascade reactions triggered by a reactive species. C-nucleophiles bearing a further reactive handle offer an additional opportunity for tuning the physicochemical/targeting properties or providing drug-releasing capabilities to these probes, through the covalent attachment of ad hoc chemical moiety. In order to implement such strategy to fluorogenic enzyme substrates based on the "covalent-assembly" principle, we have explored the potential of some functionalized derivatives of barbituric acid, piperidine-2,4-dione and Meldrum's acid. Our investigations based on the rational design and validations of enzyme-responsive caged precursors of fluorescent pyronin dyes and 7-(diethylamino)coumarin-3-carboxylic acid, led to identify a versatile candidate suitable for this late-stage structural optimization approach. This Meldrum's acid derivative enables to either enhance water solubility or achieve the reversible conjugation of a targeting ligand, while promoting in situ formation of fluorophore upon enzymatic activation. This study opens the way to novel multifunctional fluorescence imaging probes and optically modulated small conjugate-based theranostics.