Synthesis and Evaluation of a 1,3a,6a-Triazapentalene (TAP)-Bonded System

SOMOphilic alkyne vs radical-polar crossover approaches: The full story of the azido-alkynylation of alkenes

We report the detailed background for the discovery and development of the synthesis of homopropargylic azides by the azido-alkynylation of alkenes. Initially, a strategy involving SOMOphilic alkynes was adopted, but only resulted in a 29% yield of the desired product. By switching to a radical-polar crossover approach and after optimization, a high yield (72%) of the homopropargylic azide was reached. Full insights are given about the factors that were essential for the success of the optimization process.

1,3a,6a-Triazapentalene derivatives as photo-induced cytotoxic small fluorescent dyes

1,3a,6a-Triazapentalene (TAP) is a compact fluorescent chromophore whose fluorescence properties vary greatly depending on the substituents on the TAP ring. This study investigated the photo-induced cytotoxicities of various TAP derivatives. Among the derivatives, 2-p-nitrophenyl-TAP showed significant cytotoxicity to HeLa cells under UV irradiation but no cytotoxicity without UV. In addition, the photo-induced cytotoxicity of 2-p-nitirophenyl-TAP was found to be cancer cell selective and effective against HeLa cells and HCT 116 cells. Under UV irradiation, 2-p-nitrophenyl-TAP generated reactive oxygen species (ROS) that induced an apoptosis and ferroptosis in cancer cells. Therefore, it was revealed that 2-p-nitrophenyl-TAP is the most compact dye that can generate ROS by photoirradiation.

Synthesis, Photophysical Properties, and Aromaticity of Pyrazine-Fused Tetrazapentalenes

Pyrazine-fused 1,2,6,6a-tetrazapentalenes (PyTeAP) are zwitterionic tricyclic compounds exhibiting an original pattern with four consecutive nitrogen atoms. They were obtained by a challenging cyclization through the formation of a N-N bond under thermolytic conditions. Ten derivatives were synthesized, and the original scaffold of PyTeAP was confirmed by single-crystal X-ray diffraction analysis of one derivative. Examination of their photophysical properties in solution revealed blue fluorescence with λ_em = 416-426 nm. Theoretical investigations of the aromaticity in these compounds through magnetic criteria evidenced the presence of a dominant 14-electron circuit at the periphery.

Pyridazino-1,3a,6a-Triazapentalenes as Versatile Fluorescent Probes: Impact of Their Post-Functionalization and Application for Cellular Imaging

Pyridazino-1,3a,6a-triazapentalenes (PyTAP) are compact fused 6/5/5 tricyclic scaffolds which exhibit promising fluorescent properties. Chemically stable, they can be post-functionalized using standard Pd-catalyzed cross-coupling chemistry. Several original PyTAP bearing additional unsaturated substituents in positions 2 and 8 were synthetized and their spectroscopic properties analyzed. They have been successfully tested as fluorescent probes for cellular imaging.

Bicyclic 1,3a,6a-Triazapentalene Chromophores: Synthesis, Spectroscopy and Their Use as Fluorescent Sensors and Probes

The 1,3a,6a-triazapentalene (TAP) is an aromatic heterocyclic fluorescent dye with interesting features such as its small size, large Stokes shift, solvatochromism, and emission wavelengths that are spread across the visible spectrum. TAPs have been synthesized via different synthetic strategies involving click−cyclization−aromatization domino reactions, gold-catalyzed cyclization of propargyl triazoles or triazolization of acetophenones. As a result, TAPs with diverse substitution patterns were obtained, showing varying fluorescence properties. Based on these properties, several TAPs have been selected and studied as fluorescent imaging probes in living cells and as sensors. This mini review provides an overview of the research on the bicyclic TAPs and does not comment on the literature about benzo or otherwise fused systems. The synthetic methodologies for the preparation of TAPs, the substituent effects on the fluorescence properties, and the behavior of the TAP core as an element of biological imaging probes and sensors are discussed.

Ring-Degenerate Rearrangement Resulting from the Azo Coupling Reaction of a 3-Aryl-1,3a,6a-triazapentalene

The 3-(4-(methoxycarbonyl)phenyl)triazapentalene is a highly fluorescent small molecule which is readily accessible via a two-step synthesis. In the search for postfunctionalization methods, a radical CH-arylation with diazonium salts was attempted. However, azo coupling resulted in a ring-degenerate rearrangement toward a 2-aryl-4-azotriazapentalene, which was confirmed via crystallographic analysis. A mechanism involving the generation of a nitrilimine is proposed. In addition, reduction of the azo group led to cleavage of the triazapentalene core. The present results further demonstrate the sensitivity of the triazapentalene fluorophore.

Development of a 1,3a,6a-triazapentalene derivative as a compact and thiol-specific fluorescent labeling reagent

For the fluorescence imaging of biologically active small compounds, the development of compact fluorophores that do not perturb bioactivity is required. Here we report a compact derivative of fluorescent 1,3a,6a-triazapentalenes, 2-isobutenylcarbonyl-1,3a,6a-triazapentalene (TAP-VK1), as a fluorescent labeling reagent. The reaction of TAP-VK1 with various aliphatic thiols proceeds smoothly to afford the corresponding 1,4-adducts in high yields, and nucleophiles other than thiols do not react. After the addition of thiol groups in dichloromethane, the emission maximum of TAP-VK1 shifts to a shorter wavelength and the fluorescence intensity is substantially increased. The utility of TAP-VK1 as a compact fluorescent labeling reagent is clearly demonstrated by the labeling of Captopril, which is a small molecular drug for hypertension. The successful imaging of Captopril, one of the most compact drugs, in this study demonstrates the usefulness of compact fluorophores for mechanistic studies.