Scientists of the Zelinsky Institute of Organic Chemistry developed a photochemical method for the preparation of polycyclic compounds

Photochemical transformations have become a powerful tool in modern organic chemistry. The key step in such processes is the transition of the irradiated molecule to an excited state, resulting in a change in its reactivity, which leads to various chemical transformations. This approach opens up access to a wide range of organic compounds that are difficult to access by other methods.

Scientists of the Laboratory of Heterocyclic Compounds of the ZIOC continue to study the photochemical behavior of 3-hydroxypyran-4-one (allomaltol) derivatives. The key feature of this class of compounds is the intramolecular proton transfer in the excited state (ESIPT process). As a result of such a photoreaction, the pyranone ring is contracted and unstable α-hydroxy-1,2-diketones are formed. These intermediates can be trapped both with the use of external reagents and with the help of various functional groups that are part of the side chain of allomaltol. In one of their latest works, the authors proposed using a nitrogen-containing heterocyclic fragment to trap unstable α-hydroxy-1,2-diketones. They obtained derivatives of 3-hydroxypyran-4-ones containing a benzimidazole residue. It was shown that under the action of UV irradiation, the studied products undergo ESIPT-induced contraction of the pyranone ring and subsequent cyclization with the participation of the benzimidazole fragment. As a result of the reaction, benzo[4,5]imidazo[1,2-a]cyclopenta[e]pyridine derivatives are formed. The researchers also showed that the synthesized products existed as an interconvertible mixture of tautomers in solution and as a single tautomer in crystalline form. At the same time, despite the tautomerism observed in the solution, further chemical transformations of the mixture of photoproducts proceed regiospecifically and lead to the formation of derivatives of one or another isomeric form, the structure of which was unambiguously confirmed by X-ray diffraction analysis.

Source:

Andrey N. Komogortsev, Boris V. Lichitskii, Constantine V. Milyutin, Valeriya G. Melekhina Photochemical synthesis and ring–chain–ring tautomerism of benzo[4,5]imidazo[1,2-a]cyclopenta[e]pyridines // Org. Biomol. Chem., 2023, 21, 2720-2728. DOI: 10.1039/d3ob00273j.