Photoinduced 6π-Electrocyclization of a 1,3,5-Hexatriene System Containing an Allomaltol Fragment

Synthesis of 1,2,3-triazoles containing an allomaltol moiety from substituted pyrano[2,3-d]isoxazolones via base-promoted Boulton-Katritzky rearrangement

For the first time, the interaction of aroyl containing pyrano[2,3-d]isoxazolone derivatives with various hydrazines was studied. It was shown that the considered process includes formation of corresponding hydrazones followed by Boulton-Katritzky rearrangement. As a result, the general method for the synthesis of substituted 1,2,3-triazoles bearing an allomaltol fragment was elaborated. The suggested approach can be applied to various aromatic and heterocyclic hydrazines. At the same time for unsubstituted hydrazine the Boulton-Katritzky recyclization is not implemented. In this case the opening of the pyranone ring was observed leading to pyrazolylisoxazole derivatives. Both types of aforementioned structures were proved by X-ray analysis.

Anionic photochemical rearrangement of 3-hydroxypyran-4-ones bearing oxazol-2-one fragment

The photochemical behavior of in situ generated anions of 3-hydroxypyran-4-ones containing an oxazol-2-one moiety was studied. For the first time, it was demonstrated that blue LED light irradiation (450 nm) of substituted 3-hydroxypyran-4-ones in the presence of a base leads regiospecifically to the formation of isomeric 3-hydroxypyran-2-ones. Transformation of the starting 3-hydroxypyran-4-ones into the corresponding anions is necessary for the presented photoprocess. Based on the considered visible light induced rearrangement, a general method for the synthesis of 3-hydroxypyran-2-ones with an oxazol-2-one moiety was elaborated. The structure of one of the synthesized compounds was confirmed by X-ray diffraction.

1,1'-Carbonyldiimidazole-mediated transformation of allomaltol containing hydrazides into substituted 3-acetyltetronic acids

For the first time, the reaction of allomaltol containing hydrazides with 1,1'-carbonyldiimidazole (CDI) was studied. It was shown that under the considered conditions, 3-hydroxy-4-pyranone derivatives were transformed into 3-acetyltetronic acids bearing a pyrrolidin-2-one moiety. We have found that the key intermediates of the investigated process are substituted 6-oxa-1-azaspiro[4.5]dec-7-ene-2,9-diones. The structures of one final product and one intermediate were confirmed by X-ray analysis. The disclosed reaction was tested using a wide range of substituted allomaltols with various carboxamide units. It was demonstrated that in the case of hetaryl containing hydrazides and hydroxamic acids, the direction of the process is completely changed and cyclization into substituted pyrano[3,2-b]pyrans occurs.

Oxidative Ring-Opening Transformation of 5-Acyl-4-pyrones as an Approach for the Tunable Synthesis of Hydroxylated Pyrones and Furans

A selective and tunable approach for oxidation of 4-pyrones has been developed via ring-opening transformations leading to various hydroxylated oxaheterocycles. The first step of the strategy includes the base-catalyzed epoxidation of 5-acyl-4-pyrones in the presence of hydrogen peroxide for the effective synthesis of pyrone epoxides in high yields. The epoxides bearing the CO2Et group are reactive molecules that can undergo both pyrone and oxirane ring-opening via deformylation to produce hydroxylated 2-pyrones or 4-pyrones. The acid-promoted transformation led to 3-hydroxy-4-pyrones (24-76% yields), whereas the K2CO3-catalyzed ring-opening process of 2-carbethoxy-4-pyrone epoxides proceeded as an attack of alcohol at the C-3 position bearing the CO2Et group to give functionalized 6-acyl-5-hydroxy-2-pyrones (27-87% yields). The base-catalyzed reaction of 2-aryl-4-pyrone epoxides was followed by ring contraction and the dearoylation process to produce 3-hydroxyfuran-2-carbaldehydes in 42-80% yields. The transformation of 3-aroylchromone epoxides led to flavonols and 3-hydroxybenzofuran-2-carbaldehyde in the acidic and basic conditions, respectively. The prepared hydroxylated heterocycles demonstrated high reactivity for further transformations and low cytotoxicity and are promising fluorophores or UV filters.

Synthesis of substituted 8H-benzo[h]pyrano[2,3-f]quinazolin-8-ones via photochemical 6π-electrocyclization of pyrimidines containing an allomaltol fragment

For the first time, we elaborated a method for the synthesis of pyrimidines containing an allomaltol unit. The suggested approach is based on the reaction of 2-(1-(dimethylamino)-3-oxo-3-arylprop-1-en-2-yl)-3-hydroxy-6-methyl-4H-pyran-4-ones with cyanamide. The photochemical behavior of the obtained pyrimidines was investigated. It was shown that for the hydroxy derivatives the main pathway of phototransformation is a 6π-electrocyclization of the 1,3,5-hexatriene system and subsequent [1,9]-H sigmatropic shift leading to dihydrobenzo[h]pyrano[2,3-f]quinazolines. At the same time, for methylated analogues the photoreaction proceeds in two directions resulting in the formation of a mixture of the corresponding dihydrobenzo[h]pyrano[2,3-f]quinazolines and polyaromatic products. The obtained dihydro derivatives are stable compounds and do not undergo aromatization upon further UV irradiation. The structures of two of the dihydrobenzo[h]pyrano[2,3-f]quinazolines were confirmed by X-ray diffraction analysis. Based on the performed studies, a two-stage telescopic method for the synthesis of polyaromatic benzo[h]pyrano[2,3-f]quinazolines including the initial photocyclization of the starting pyrimidines and the final dehydration was proposed.

Photorelease Reaction Mechanism Study of a Diarylethene Caged Dual Functions Compound

Femtosecond transient absorption, nanosecond transient absorption, and nanosecond resonance Raman spectroscopy techniques coupled with density functional theory calculations were performed to unravel the photocyclization and photorelease mechanisms of a diarylethene based compound (1o) containing two caged groups (OMe and OAc). Since the ground state parallel (P) conformer of 1o with a large dipole moment is stable in DMSO, the fs-TA transformations observed for 1o in DMSO mainly have contributions from the P conformer, which undergoes an intersystem crossing to generate a corresponding triplet state species. In a less polar solvent like 1,4-dioxane, in addition to the P path behavior of 1o, an antiparallel (AP) conformer can also take place to give a photocyclization reaction from the Franck-Condon state and finally give a deprotection via this pathway. This work provides a deeper understanding for these reactions, which not only facilitate the applications of diarylethene compounds but also help for the future design of functionalized diarylethene derivatives for particular applications.

4a,7a-Dihydroxy-1-(2-hydroxyethyl)-5-methyl-2′,3′,4a,5′,6′,7a-hexahydrospiro[cyclopenta[b]pyridine-4,4′-pyran]-2,7(1H,3H)-dione

An environment-friendly photochemical approach to the synthesis of 4a,7a-dihydroxy-1-(2-hydroxyethyl)-5-methyl-2′,3′,4a,5′,6′,7a-hexahydrospiro[cyclopenta[b]pyridine-4,4′-pyran]-2,7(1H,3H)-dione from 2-(4-(3-hydroxy-6-methyl-4-oxo-4H-pyran-2-yl)tetrahydro-2H-pyran-4-yl)-N-(2-hydroxyethyl)acetamide was elaborated. The suggested method is based on the ESIPT-promoted contraction of 3-hydroxypyran-4-one fragment followed by intramolecular cyclization of generated in situ α-hydroxy-1,2-diketone intermediate. The distinctive feature of the presented protocol is the employment of water as a solvent for the considered photoreaction. The structure of the obtained photoproduct was confirmed by 1H, 13C-NMR, IR spectroscopy and high-resolution mass spectrometry.

A study of the photochemical behavior of terarylenes containing allomaltol and pyrazole fragments

The photochemical properties behavior of 3-hydroxy-4-pyranone containing terarylenes with a pyrazole bridge fragment were studied. It was shown that UV-induced 6π-electrocyclization of the 1,3,5-hexatriene system was not observed for the considered objects molecules. At the same time, the phototransformation of such systems proceeds exclusively in the direction of the contraction of the pyranone ring leading to unstable α-hydroxydiketones. For the first time the possibility of isolation of the resulting α-hydroxydiketones in pure form was demonstrated. Wherein, it was shown that relatively low stable α-hydroxydiketones can be trapped by reaction with 1,2-phenylenediamine. The general method for the preparation of the corresponding quinoxalines on the basis of the aforementioned condensation was implemented. It was demonstrated that the studied photoreaction does not depend on the type of pyrazole bridge. The structures of three of synthesized products were established by X-ray diffraction.

Recent synthetic methodologies for imidazo[1,5-a]pyridines and related heterocycles

Imidazo[1,5-a]pyridine is a significant structural component of a large number of agrochemicals and pharmaceuticals. The synthesis of imidazo[1,5-a]pyridine has been a subject of intense research for numerous decades. A large number of transformations are now available to conveniently access imidazo[1,5-a]pyridine from readily available starting materials. This review details the recent development in imidazo[1,5-a]pyridine construction involving cyclocondensation, cycloaddition, oxidative cyclization, and transannulation reactions.