1-Aryl-2-pyridyl-3,4-dihydronaphthalenes: Photofluorogenic Ligands for the Estrogen Receptor

Abstract

Three 1,2-substituted-3,4-dihydronaphthalenes that are pyridine analogs of the antiestrogen desmethylnafoxidine were prepared and evaluated as fluorescent ligands for the estrogen receptor. These compounds represent a class of fluorescent probes that we term “photofluorogenic”, denoting their ability to exist initially as a high affinity though weakly fluorescent stilbazole form which can be photocyclized−oxidized to a highly fluorescent though low affinity azaphenanthrenoid form. These probes also contain an aziridine function that provides a means for their permanent, covalent attachment to the receptor. The three dihydronaphthalene systems were prepared by efficient routes from α-(2-, 3-, and 4-pyridyl)acetophenone precursors. They demonstrate high apparent affinity for the estrogen receptor and show time-dependent irreversible inactivation, consistent with their covalent attachment to the receptor via the aziridine function. Each system is converted into an azaphenanthrene by photocyclization−oxidation of the cis-stilbazole unit. The absorbance and fluorescence emission spectra of the dihydronaphthalene precursors and azaphenanthrene products have been characterized, and they display marked sensitivity to both solvent polarity and pH. The azaphenanthrenoids derived from the 2- and 4-pyridyl isomers exhibit intense emission at wavelengths that exceed 500 nm under certain conditions and appear to be well suited as fluorescent probes for the estrogen receptor.