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

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.