Short and scalable synthesis of an anhydride precursor of the environment-sensitive fluorophore 6-dimethylaminonaphthalimide

Environment-sensitive fluorophores have different quantum yields in different solvents. 6-Dimethylaminonaphthalimide (6-DMN), for example, has a low quantum yield in aqueous solutions, but is highly fluorescent in nonpolar solvents or when bound to hydrophobic sites in proteins or membranes. 6-DMN has been used to investigate protein-protein interactions, as well as the in-tissue distribution and internalization of δ-receptors. This protocol describes a highly efficient three-stage synthesis of 6-dimethylamino-2,3- naphthalenedicarboxylic anhydride (compound 1), which is a stable precursor that can be converted to 6-DMN. The three stages are (i) photo-bromination of 4-nitro-o-xylene (yield 82%), (ii) Diels-Alder reaction followed by base hydrolysis, resulting in 6-nitro-2,3-naphthalenedicarboxylic acid (yield 87%) and (iii) reductive amination followed by dehydration (yield 76.5%) to form compound 1. The synthesis can be performed on a gram scale (in 55 h over 3 d) with an overall yield of about 55%. It can easily be modified to prepare related compounds by, for example, performing different reactions using 6-nitro-2,3-naphthalenedicarboxylic acid.