Acylation of dipyrromethanes at the α and β positions and further development of fluorescent Zn²⁺ probes

The acylation of 5-aryl dipyrromethanes afforded products with interestingly rich substitution modes, i.e.; α- and β-monoacylated (modes a and b), and α, α′-, α, β′- and β, β′-diacylated (modes c-e). Especially, the β- and β, β′-acylation modes are unprecedented. And most of these products can be synthesized at a gram scale. The anisoyl substituted 5-(4-cyanophenyl) dipyrromethanes (1a-1e) were oxidized with DDQ. Thus, 1a and 1b afforded the corresponding dipyrrins 1a-DPR, and 1b-DPR. More interestingly, the diacylated ones 1c-1e could not be oxidized by DDQ. Instead, 1c-OH-1e-OH were obtained with a hydroxyl group attached to the 5-position. 1a-DPR-1e-OH were further developed as fluorescence turn-on Zn²⁺ probes. 1d-OH showed the highest sensitivity, with a detection limit of 1.5 × 10^-8 M, and it was successfully applied in Zn²⁺ imaging in Hela cells. Furthermore, single crystals of two Zn²⁺ complexes were obtained and analyzed by X-ray diffraction.