Synthesis of Naphthalene-Based Push-Pull Molecules with a Heteroaromatic Electron Acceptor

Naphthalene derivatives bearing electron-accepting and electron-donating groups at the 2,6-positions belong to the family of D--A push-pull dyes. It has been found that these compounds, e.g., 2-(1-(6-((2-(fluoro)ethyl)(methyl)amino)naphthalen-2-yl)ethylidene)malononitrile (FDDNP), show not only interesting optical properties, such as solvatochromism, but they have the potential to label protein aggregates of different compositions formed in the brain of patients suffering from neurodegenerative diseases like Alzheimer's (AD). In continuation of our research we set our goal to find new FDDNP analogs, which would inherit optical and binding properties but hopefully show better specificity for tau protein aggregates, which are characteristic for neurodegeneration caused by repetitive mild trauma. In this work we report on the synthesis of new FDDNP analogs in which the acceptor group has been formally replaced with an aromatic five- or six-membered heterocycle. The heterocyclic moiety was annealed to the central naphthalene ring either by classical ring closure reactions or by modern transition metal-catalyzed coupling reactions. The chemical characterization, NMR spectra, and UV/vis properties of all new compounds are reported.