,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.

It was found for the first time that the acid hydrolysis of 2,5-dimethylfuran in water-ethanol solutions has self-oscillating character. Oscillations in the concentrations of the two products 2,5-hexanedione and an unidentified compound X were detected by GLC. The ranges of hydrochloric acid and ethanol concentrations in which these oscillations appear were determined. It is suggested that the formation of compound X results from tautomeric transformations of 2,5-hexanedione under the reaction conditions. Quantum-chemical calculations showed that the possible tautomers are formed with equal probability.During the development of a method for the production of thiophenes and selenophenes by the recyclization of furans in water-ethanol media under the conditions of acid catalysis it was shown that the reaction takes place in two directions, one of which includes a stage involving the formation of a diketone, i.e., hydrolysis. Kinetic investigations of the hydrolysis of 2,5-dimethylfuran (1) in 50% ethanol under the influence of HCl revealed the formation of 2,5-hexanedione (2) and an unidentified compound X. During the reaction the ratio of the concentrations of the products varied with some degree of periodicity, i.e., as it accumulated one product changed into the other, and these transformations were self-oscillating in nature [1,2].Study of the behavior of furan compounds in aqueous and water-alcohol media under the conditions of acid catalysis is of great theoretical and practical interest in so far as oscillating chemical reactions take place in various types of processes that determine the vital activity of living organisms [3], while furan derivatives are physiologically active substances.In the present work in order to determine the boundary conditions for the appearance of the abovementioned transformations the kinetics of the hydrolysis of dimethylfuran 1 in 50% aqueous ethanol with various hydrochloric acid concentrations (1.5, 2.0, 3.0, 3.5 N) were investigated by GLC. The results of the experiments are presented in Fig. 1. As seen, the fluctuations in the contents of the dione 2 and compound X in the reaction medium are observed at HCl concentrations (c) of 1.5-2.0 M. At c = 3 M both products are formed, but the ratio of their amounts in the reaction medium does not change with time.The experiments in 60 and 80% ethanol at HCl concentrations of 2.0 and 2.5 N (the dimethylfuran is poorly soluble in the 40% alcohol) showed that in 80% alcohol at the indicated acid concentrations only the hexanedione 2 is formed, while in 60% ethanol the concentrations of compound 2 and X oscillate in both cases (Fig. 2). __________________________________________________________________________________________

The reaction of 2-methyl-5-R-furans (R = Me, Bu, 2, Kinetic studies of recyclization of 2,5-dialkylfuram into thiopbenes and selenophenes in conditions of acid catalysis in ethanol and water-ethanol media were conducted previously [2][3][4][5]. It was found that in recyclization in the presence of a competing nucleophile --water --the processes can take place in two parallel directions: through intermediate formation of dicarbonyl compounds, and as a result of direct transformation of furans into isologs [2, 4]. The subsequent study of the kinetics of hydrolysis and recyclization of different alkyl-substituted furans showed that the rates of these reactions are differently dependent on the position and structure of the substituents and the concentration of the acid component and water [4, 5].The results of studying the mechanism of the hydrolysis and recyclization reactions with labeled atoms are reported here.The reaction of 2,5-dialkylfurans (I-liD with H2180 was investigated in the conditions of the recyclization reaction. The composition of the reaction mixture was determined by GLC. The incorporation of the "label" was monitored with the mass spectrometric data. It was shown that substitution of the 160 atom of the furan ring by the 180 isotope takes place together with the hydrolysis reaction (see Fig. 1). As a result, the products of isotope exchange of furans (IV-VI) and diketones (VII-IX) were obtained.

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173ChemInform Abstract The thiopyrrolidone (I) reacts with chloroacetic acid (IIa) and its methyl ester (IIb) to give the S-alkylation products (III) or the bicyclic compound (IV) depending on the reaction conditions. (IV) is coupled with the aldehyde (V), producing the benzylidene compound (VI). (IR-, 1H-NMR-data, mechanism, quantum chemical calculations).