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Paper 1/06429K

,5-Dimethyl-3-D-furan was prepared for the first time. Its recyclization into the corresponding thiophene and selenophene was investigated. The possibility of f3-C-protonation of the furan ring was confirmed by experimental and quantum chemical data.Many studies have been dedicated to answering the question of the. direction of protonation of furans in the reaction of acid hydrolysis, c~-C-Protonation of the furan ring was selected apriori by analogy with reactions of electrophilic substitution in [2]. Finnish researchers believe that hydrolysis of 2,5-dimethyl-and 2-methoxyfurans begins with/3-C-protonation of the ring [3, 4]. However, it was suggested in [5] that this hypothesis was based on incorrect determination of the structure of the products of hydrolysis of 2-methoxyfuran. The furan ions formed in the gas phase in conditions of mass spectrometry with chemical ionization (reagent gases: isobutane and methane) are described as products of t~-C-protonation in [6]. Stable furanium ions were obtained from tert-butylfurans in the liquid phase, and 2,5-dimethylfuran was completely resinified in the same conditions in [7].Acid-catalyzed isotope exchange is widely used for assessing the reactivity of heterocycles. In studying the rate of deuterium exchange of benzene, furan, thiophene, selenophene, and pyrrol, protonation of the/3-carbon atom of the furan ring was found. Only the mobility of the ~-H atoms could be measured in the deuterium exchange reaction, since hydrolytic splitting of the furan ring was simultaneously observed in the region of concentrations of H2SO 4 greater than 30 wt. %, where exchange of protons in/3 positions takes place with a sufficient rate [8].We continued the study of the mechanism of the reaction of recyclization of furans into thiophenes and selenophenes in conditions of acid catalysis. We investigated the possibility of/~-C-protonation as the ftrst stage of recyclization. 2,5-Dimethyl-3-D-furan (Ia) was investigated. Incorporation of a label in position 3 of 2,5-dimethylfuran (I) was conducted similar to the method described in [9] via mercury chloride (II) according to the following scheme:Recyclization of compound Ia into 2,5-dimethylthiophene (III) and 2,5-dimethylselenophene (IV) was conducted with hydrogen sulfide and hydrogen selenide in absolute ethanol saturated with dry hydrogen chloride, similar to the method described in [10]. The evolution of the reaction was monitored with the GLC data, and after incorporation and preservation of the label, it was monitored by PMR spectroscopy and mass spectrometry.

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