Halogenated 2,1,3-benzoxadiazoles as potential fluorescent warheads for covalent protease inhibitors

Recently there is a growing interest in covalent protease inhibitors in industry and academia caused by their longer residence times, their higher potency and their high ligand efficiency. Covalently reactive moieties which interact with activated amino acid residues such as serine or cysteine in enzymes like proteases or esterases rarely act through nucleophilic aromatic substitution (SNAr). In our previous work, we presented design and properties of electrophilic "warheads", which contain aromatic(heteroaromaric) or quinoid fragments. Some of them show high inhibition constants for cathepsin L, cathepsin B, rhodesain or dengue-protease and depending on the exact nature of the electrophile, they exhibit reversible covalent or irreversible inhibition modes. In the present work, we demonstrate the synthesis of fluorescent "warhead" candidates based on 2,1,3-benzoxadiazoles and the investigation of their physicochemical and photophysical properties. These molecules shall serve as probes for the detailed analysis of association/dissociation and of the kinetic parameters of the bond forming event.