Color-Tunable Indolizine-Based Fluorophores and Fluorescent pH Sensor

Development of Highly Fluorogenic Styrene Probes for Visualizing RNA in Live Cells

Styrene dyes are useful imaging probes and fluorescent sensors due to their strong fluorogenic responses to environmental changes or binding macromolecules. Previously, indole-containing styrene dyes have been reported to selectively bind RNA in the nucleolus and cytoplasm. However, the application of these indole-based dyes in cell imaging is limited by their moderate fluorescence enhancement and quantum yields, as well as relatively high background associated with these green-emitting dyes. In this work, we have investigated the positional and electronic effects of the electron donor by generating regioisomeric and isosteric analogues of the indole ring. Select probes exhibited large Stokes shifts, enhanced molar extinction coefficients, and bathochromic shifts in their absorption and fluorescence wavelengths. In particular, the indolizine analogues displayed high membrane permeability, strong fluorogenic responses upon binding RNA, compatibility with fluorescence lifetime imaging microscopy (FLIM), low cytotoxicity, and excellent photostability. These indolizine dyes not only give rise to rapid, sensitive, and intense staining of nucleoli in live cells but can also resolve subnucleolar structures enabling highly detailed studies of nucleolar morphology. Furthermore, our dyes can partition into RNA coacervates and resolve the formation of multiphase complex coacervate droplets. These indolizine-containing styrene probes offer the highest fluorescence enhancement among the RNA-selective dyes reported in the literature; thus, these new dyes are excellent alternatives to the commercially available RNA dye, SYTO RNASelect, for visualizing RNA in live cells and in vitro.

The Eschenmoser's Salt as a Formylation Agent for the Synthesis of Indolizinecarbaldehydes and Their Use for Colorimetric Nitrite Detection

C-H bond formylation is the most immediate way to incorporate the versatile formyl group into (hetero)aromatics. However, the type of reagents and severe conditions involved in the classical formylation methods often curtail their application, especially in the presence of other functional groups. Herein, we present the Eschenmoser's salt, a commercially available (dimethylamino)methylating chemical, as a useful reagent for the C-H formylation of indolizines and other compounds. The method is straightforward and mild, furnishing indolizinecarbaldehydes in modest-to-good yields with exclusive and remote regioselectivity. Furthermore, these compounds can be easily transformed into push-pull dyes and are highly selective in the colorimetric detection of nitrite, a substance extensively employed as preservative in the food industry, the concentration of which is crucial to control to prevent harmful effects in living organisms. The assay is simple, allowing the naked-eye detection of nitrite in solution or on a cotton swab for a wide range of concentrations.

Indolizin-1-ols with Charged Electron Acceptors: A Direct Way to 3H-Indolizinium-1-olates with Donor Functions

The reaction of cyclopropenones with pyridines, having an attached integer-charged electron-withdrawing group (pyridinium, imidazolium, and phosphonium) was discovered to afford novel indolizin-1-ol derivatives in high yields with no chromatographic purification required. While being stable as solids, these indolizin-1-ols have a limited lifetime in solution. The study of reasons for such instability uncovered an aerobic oxidative pathway, eventually resulting in indolizine-1,7-dione dimers. The exploration of N-(1-hydroxyindolizin-7-yl)pyridinium salts' chemistry led to a reaction discovery, affording a new type of rare pseudo-cross-conjugated mesomeric betaines (3H-indolizin-4-ium-1-olates with an electron-donating function at C7 position) inaccessible by other means. In this reaction, a sequential introduction of nucleophiles takes place: the first one (Nu₁) is represented by simple anilines, whereas Nu₂ extends to primary, secondary, aliphatic, aromatic amines, and phenols. For the obtained betaines having unsymmetrical aliphatic amino groups at C7 position an increased order of the C7-Nu₂ bond resulting in existence of amide type E/Z-forms (∼1:1 at room temperature) was demonstrated. For aryl amino groups, with typically reduced nitrogen's lone-pair donation, the barrier of rotation around the C7-Nu₂ bond was lower, and for the C7-oxy betaines, no such E/Z-isomerism was revealed. Although primary amines (as Nu₂) introduce a hydrogen atom in the conjugated betaine system, allowing prototropic tautomerism in this way, non-zwitterionic tautomers (3-amino-7-iminoindolizin-1-ones) were rejected by nuclear Overhauser effect spectroscopy experiments.