V. G. Kharchenko

Oxygen-containing Heterocyclic Compounds from 1,5-Diketones. (Review)

Literature and characteristic experimental data correlated in the review concern the intramolecular cyclization by various means of substituted 1,5-diketones of the acyclic, semi-, and bicyclic series, proceeding to the formation of oxygen-containing heterocycles, such as pyrans, pyrylium salts, di- and tetrahydrofurans, and their condensed analogs, aroylfurans etc.

Mechanism of recyclization of furans to thiophenes and selenophenes under acid-catalysis conditions. 2. Kinetic investigations of the reaction of 2,5-dialkylfurans with hydrogen sulfide and hydrogen selenide in an anhydrous medium

The effect of the concentration of the acidic component on the rates of recyclization of 2,5-dialkylfurans to thiophenes and selenophenes was studied. On the basis of the direct correlation of log k on (Ho)I and the isotope effect of the solvent it was established that the reaction proceeds via a mechanism of specific acid catalysis. It was shown that the rates of the reactions with the participation of hydrogen sulfide and hydrogen selenide coincide at equal acidities and temperatures. A scheme for the mechanism of the reaction that includes a step involving the formation of the doubly protonated form of the substrate is proposed.

Study of the mechanism of recyclization of furans into thiophenes and selenophenes in conditions of acid catalysis. 5. Direction of protonation of furans

Abstract2,5-Dimethyl-3-D-furan was prepared for the first time. Its recyclization into the corresponding thiophene and selenophene was investigated. The possibility of β-C-protonation of the furan ring was confirmed by experimental and quantum chemical data.

Study of the mechanism of recyclization of furans into thiophenes and selenophenes in conditions of acid catalysis. 6. Experiments with labeled atoms. Quantum chemical calculations of intermediates of recyclization and hydrolysis

The reaction of 2-methyl-5-R-furans (R=Me, Bu, 2,2-pentyl-2-methylpentyl) with H218O was investigated. Furans and the corresponding 2,5-alkanediones containing the18O isotope were obtained. The general characteristics of the recyclization and isotope exchange reactions were established. A kinetic study of recyclization of 2,5-dimethylfuran with16O and18O into 2,5-dimethylthiophene was conducted. A reverse kinetic isotope effect was found. The schemes of the mechanisms of these processes were refined based on the experimental data and quantum-chemical calculations of the recyclization and hydrolysis intermediates.

Oxidation reactions of 4H-chalcogenopyrans

We have studied the oxidation of 4H-chalcogenopyrans and chalcogenopyrylium salts by lead tetraacetate. We have established that the major reaction products on oxidation of 2,6-diphenyl-4H-thiopyran and selenopyran are 2,6-diphenyl-4H-thio(seleno)pyran-4-ones, while 2-benzoyl-5-phenyl(seleno)thiophenes are formed as impurities. The major product on oxidation of 2,6-diphenyl-4H-pyran is 2-benzoyl-5-phenylfuran. We have shown that 2,4,6-triphenyl-4H-pyran does not undergo oxidation by hydrogen peroxide in neutral medium, in contrast to its S and Se analogs. This supports the hypothesis that the heteroatom participates in the oxidation process.

Tetrahydrochromenylium and -Thiochromenylium Salts from Triketones of the 2-(3-Oxopropyl)cyclohexane-1,3-dione Series

We have worked out the optimal conditions for synthesis of 2,4-diaryl-5-oxo-5,6,7,8-tetrahydrochromenylium and -thiochromenylium salts based on triketones of the 2-(1,3-diaryl-3-oxopropyl)cyclohexane-1,3-dione series. For the first time, along with 5-oxo-substituted salts, we have obtained 5-thioxo-5,6,7,8-tetrahydrothiochromenylium salts. A necessary condition for the formation of the latter on treatment with acids and S-nucleophiles is the presence of electron-donor groups on the aryl substituents of the indicated triketones.

Photochemical oxidation of 4h-seleno(thio)pyrans

Oxidation of solutions of 4H-thiopyrans and 4H-selenopyrans to the corresponding heteroaromatic cation bromide occurs on irradiation with ultraviolet light in the presence of CBr4. Reaction takes place by a free radical chain mechanism. The presence of CHBr3 and C2Br6 in the reaction mixture was detected chromatographically.

Study of the mechanism of recyclization of furans into thiophenes and selenophenes in conditions of acid catalysis. 4. study of the effect of the solvent and direction of reactions of hydrolysis and recyclization of 2,5-dimethylfuran

The kinetics of hydrolysis of 2,5-dimethylfuran and its recyclization into the corresponding thiophene were investigated in water-alcohol medium W R I and 50% ethyl alcohol in the presence of HCl. It was found that the rates of these reactions are a function of both the initial concentration of the acid component and the concentration of water in the alcohol. The rate of hydrolysis is a function of the dilution of the alcohol to a greater degree than the rate of recyclization.

Mechanism of recyclization of furans to thiophenes and selenophenes under acid catalysis. 1. Kinetic studies of the reaction of 2,5-dialkylfurans with hydrogen sulfide in the presence of hydrochloric acid

The transformation of 2,5-dialkylfurans to thiophenes by reaction with hydrogen sulfide in the presence of hydrochloric acid was studied. The reaction was found to be first order with respect to the furan; the rate of consumption of the furan did not change with increasing length of one of the alkyl substituents. Recyclization in the presence of acid proceeds in two independent directions: through the formation of an intermediate dicarbonyl compound, and by direct conversion of the furan to a thiophene. Kinetic data showed that the reaction occurs mainly by the second route.

Conversion of di(furyl-2)alkyl and di(furyl-2)arylmethanes to the thiophene analogs

On reaction with hydrogen sulfide in strongly acidic media, difurylalkyl-(aryl)methanes are converted to furylthienylalkyl(aryl)- or dithienylalkyl-(aryl)methanes, depending on the conditions.