Fluorogenic Protein Labeling by Generation of Fluorescent Quinoliziniums Using [Cp*RhCl2]2

Modular synthesis of pentacyclic-fused pyranoquinoliziniums as organelle-selective fluorescent probes

On basis of their unique chemical and photophysical properties, and excellent biological activities, quinoliziniums have been widely used in various research fields. Herein, modular synthetic strategies for efficient synthesis of novel fluorescent quinoliziniums by using one-pot and stepwise rhodium(III)-catalyzed C-H annulations were developed. In the one-pot synthesis, the reaction between 2-aryl-4-quinolones (1) and 1,2-diarylalkynes (2) proceeded in a chemo- and regioselective manner to give quinolinone-fused isoquinolines (3) and pentacyclic-fused pyranoquinoliziniums (4). The structural diversity of pentacyclic-fused pyranoquinoliziniums (4) was expanded by the stepwise synthesis from 3 and 2, allowing the strategic incorporation of electron-donating (OMe and OH) and electron-withdrawing (Cl) substituents on the top and bottom parts of the pyranoquinoliziniums (4). These newly synthesized pyranoquinoliziniums (4) exhibited tunable absorptions (455-532 nm), emissions (520-610 nm), fluorescence lifetime (0.3-5.6 ns), large Stokes shifts (up to 120 nm), and excellent fluorescence quantum yields (up to 0.73) upon adjusting the different substituents. The the unique arrangement of N and O atoms and extended π-conjugation of 4 could cause the relocation of HOMO comparing with our previous quinoliziniums. Importantly, pyranoquinoliziniums (4a-4g and 4i) targeted the mitochondria, while 4h was localized in lysosome. Due to the remarkable photophysical properties and the potential for organelle targeting of the novel class of quinoliziniums, they could be further applied for biological, chemical and material applications.

Synthesis of Azonia Aromatic Heterocycles Bearing 6-6-6-5-6 Pentacyclic Core via Intramolecular [4 + 2]-Cycloaddition and Oxidative Aromatization Reaction Sequence in One Pot

Cationic aza-heterocycle-fused compounds have gained wide applications in materials science, biological applications, and synthetic organic chemistry. In this report, synthesis of benzothiazolochromenopyridinium tetrafluoroborates, a novel molecular scaffold, bearing 6-6-6-5-6 pentacyclic core is described that proceeds via (i) piperidine-catalyzed Knoevenagel condensation between 2-propargyloxyarylaldehydes bearing internal alkynes and 2-benzothiazoleacetonitrile, (ii) intramolecular formal [4 + 2]-cycloaddition, and (iii) crucial molecular oxygen-mediated oxidative aromatization reaction sequence in one pot. These quaternary pyridinium salts are obtained at ambient temperature in good to high yields.

Isoindolium-Based Allenes: Reactivity Studies and Applications in Fluorescence Temperature Sensing and Cysteine Bioconjugation

The reaction of a series of electron-deficient isoindolium-based allenes with sulfhydryl compounds has been studied, leading to the formation of isoindolium-based vinyl sulfides. The vinyl sulfides generated could be readily converted into the corresponding indanones and amines upon heating at 30-70 °C with good yields up to 61 %. The thermal cleavage reaction of vinyl sulfides was further studied for developing temperature-sensitive systems. Notably, a novel FRET-based fluorescent temperature sensor was designed and synthesized for temperature sensing at 50 °C, giving a 6.5-fold blue fluorescence enhancement. Moreover, chemoselective bioconjugation of cysteine-containing peptides with the isoindolium-based allenes for the construction of multifunctional peptide bioconjugates was investigated. Thermal cleavage of isoindoliums on the modified peptides at 35-70 °C gave indanone bioconjugates with up to >99 % conversion. These results indicated the biocompatibility of this novel temperature-sensitive reaction.

Copper-Catalyzed Cyclization of 2-Alkynylanilines to Give 2-Haloalkoxy-3-alkyl(aryl)quinolines

Herein we describe a method to produce 2-haloalkoxy-3-substituted quinolines via the cyclization of 2-alkynylanilines with TMSCF₃ and THF. This synthetic method uses inexpensive and easy-to-handle TMSCF₃ and employs a commercially available CuI catalyst to transform a broad range of 2-alkynylanilines into versatile 2-difluoromethoxy-3-substituted quinolines and 2-iodoalkoxy-3-substituted quinolines with excellent chemoselectivity.