Synthetic approaches to 1,2,4-triazolo[5,1-c][1,2,4]triazin-7-ones as basic heterocyclic structures of the antiviral drug Riamilovir ("Triazavirin®") active against SARS-CoV-2 (COVID-19).

Fragments of 1,2,4-triazolo[5,1-c][1,2,4]triazin-7-one are found in many compounds with various types of biological activities, including the antiviral drug Riamilovir (Triazavirin®), which shows activity against SARS-CoV-2 (COVID-19). Therefore, the development of convenient methods for the synthesis of new derivatives of 1,2,4-triazolo[5,1-c][1,2,4]triazin-7-one is always in demand. This review systematizes the information on the most common synthetic methods for constructing the 1,2,4-triazolo[5,1-c][1,2,4]triazin-7-one heterocyclic system.

Asymmetrically Functionalized 1,3-Di(2-pyridyl)benzenes: Synthesis and Photophysical Studies.

A convenient synthetic approach to asymmetrically functionalized 1,3-di(2-pyridyl)benzenes starting from 3-(3-bromophenyl)-1,2,4-triazines using sequential aza-Diels-Alder reactions and Stille cross-coupling is reported. Photophysical properties of the obtained compounds are studied.

Coumarin-pyridine push-pull fluorophores: Synthesis and photophysical studies.

A series of coumarin-pyridine-based push-pull fluorophores were prepared starting from 1,2,4-triazines by using direct C-H functionalization (SNH-reaction)-Diels-Alder-retro Diels-Alder domino reaction sequence. This efficient synthetic strategy allowed to obtain a series of 19 coumarin-pyridine fluorophores. Their photophysical properties were studied. While pyridine-substituted derivatives of 4-alkylcoumarins may be considered as alternative to coumarin dyes characterized by emission maxima mainly in a visible region with wavelengths of 402-415 nm, absorption in the UV range at 210-307 nm, and good photoluminescence quantum yields of 6-19%, all the derivatives of 4-phenylcoumarin did not exhibit any noticeable fluorescence. More detailed photophysical studies were carried out for two the most representative derivatives of 4-alkyl-coumarin-pyridines to demonstrate their positive solvatochromism, and the collected data were analyzed by using Lippert-Mataga equation, as well as Kosower and Dimroth/Reichardt scales. The obtained results demonstrate that the combining two chromophore systems, such as 2,5-diarylpyridine and coumarin ones, is promising in terms of improving the photophysical properties of the new coumarin-pyridine hybrid compounds.

5-Aryl-6-arylthio-2,2'-bipyridine and 6-Arylthio-2,5-diarylpyridine Fluorophores: Pot, Atom, Step Economic (PASE) Synthesis and Photophysical Studies.

A PASE (pot, step, atom, economic) synthetic approach to 5-aryl-6-arylthio-2,2'-bipyridine and 6-arylthio-2,5-diarylpyridine ligands/fluorophores has been reported via SNH in 6-aryl-5H-1,2,4-triazines/aza-Diels-Alder reaction sequence. In this article, the "1,2,4-triazine" methodology was successfully used for the synthesis of C6-thiophenol-substituted (2,2'-bi)pyridines as it is well known that thio-substituted (bi)pyridines and their aza-analogs are of wide practical interest. The photophysical properties of the obtained compounds are studied and compared with those reported earlier for 6-substituted 2,2'-bipyridines. The influence of the nature of substituents in the 6-arylthio(bi)pyridine core on the photophysical properties is discussed. It was observed that the new compounds exhibited promising photophysical properties and could be considered as potential push-pull fluorophores. In addition, they demonstrated greater Stokes shift values compared to the previously described 6-H, 6-arylamino and 6-pentafluoro-2,2'-bipyridines and higher fluorescence quantum yields values compare to pentafluorophenyl-substituted 2,2'-bipyridines. Depending on a nature of (bi)pyridine fluorophore LE (locally excited) and/or ICT (intramolecular charge transfer) state were prevailing in emission spectra.