Photoinduced 6π-Electrocyclization of 1,3,5-hexatriene system containing allomaltol fragment: A convenient approach to polycondensed pyrrole derivatives

Exploring the molecular design principles for efficient diarylethene photoacid and photohydride generators based on the photochemical reaction mechanism

Photoacid generators (PAGs) and photohydride generators (PHGs) are specific photolabile protecting groups that release acid and hydride, respectively. Over the past decade, great efforts have been devoted to developing novel PAGs and PHGs with advanced efficiency, among which, two of the promising candidates are diarylethene (DAE)-based PAGs and PHGs, which release acids/hydrides during photochromic electrocyclization. The release quantum yield for PAGs is acceptable, while that of PHGs is only 4.2% even after molecular structure modification. In this work, time-resolved transient absorption spectroscopies with femtosecond and nanosecond resolutions along with DFT/TD-DFT calculations were utilized to unravel the detailed photochemical reaction mechanisms of DAE-based PAGs (1o) and PHGs (2o), respectively. The results show that the different photochemical mechanisms are the key that leads to distinctive release quantum yields between 1o and 2o. The factors affecting the release quantum yield are discussed in detail, and several key design principles are proposed to facilitate future rational design of DAE-based PAGs and PHGs.

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.

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.

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.