Dimethyl-tetrahydrofuran

Die kathodische Reduktion von Hcxandion-(2,5) an Quecksilber in 0,5 m Salzsaure fiihrt dagegen zu einem Substanzgemisch, das u. a. Hexandiol-(2,5) und 2,5-Dimethyl-tetrahydrofuran enthalt2. 

Phthalsaure-Derivate liefern in geringer Ausbeute die entsprechenden Phthalide (5-28% d.Th.)5. Bei der Elektrolyse von Hexandion-(2,5) tritt u. a. 2,5-Dimethyl-tetrahydrofuran auf6. 

In 20%iger Schwefelsaure wird Methyl-(4-oxo-pentyl)-amin an Blei-Kathoden reduk-tiv zu 1,2-Dimethyl-pyrrolidin (65% d.Th.) cyclisiert3. In verdiinnter Schwefelsaure fiihrt die Coelektrolyse von Aceton mit Allylalkohol zu 2,2-Dimethyl-tetrahydrofuran (95% d.Th.)4. 

When 2-methyl-2,5-pentaiiediot is treated with sulfuric acid, dehydration occurs and 2,2-dimethyl tetrahydrofuran is formed. Suggest a mechanism for this reaction. Which of the two oxygen atoms is most likely to be eliminated, and whv ... 

The importance of alkylperoxy radicals as intermediates had long been realized (see Sect. 2) and their subsequent reaction to yield the alkyl-hydroperoxide or decomposition products such as aldehydes and alcohols had been reasonably successful in describing the mechanism of the autocatalytic oxidation of alkanes. However, even though 0-heterocycles (which cannot be derived from intermediate aldehydes) had been found in the products of the oxidation of n-pentane as early as 1935 [66], the true extent of alkylperoxy radical isomerization reactions has been recognized only recently. Bailey and Norrish [67] first formulated the production of O-heterocycles in terms of alkylperoxy radical isomerization and subsequent cyclization in order to explain the formation of 2,5-dimethyl-tetrahydrofuran during the cool-flame oxidation of n-hexane. Their mechanism was a one-step process which involved direct elimination of OH. However, it is now generally formulated as shown in reactions (147) and(I67)... 

The correlation of these mechanisms with solvent polarity (Lewis basicity) is strongly supported by a study of the solvent effect (% E) on the EjZ product ratio of equation (95) at 25 °C dioxane (100), THF (100), THF +AlCl, (100), 2,5-dimethyl-tetrahydrofuran (55), EtjO (60), Et20 +AICI3 (60), O-Pr) (25) . The addition of a crown ether raised the yield in /-Pr20 to 70% E, presumably by facilitating dissociation and the ionic route a drop in reaction temperature to —25 °C lowered the yield in EtjO to 45% E, presumably by facilitating association and the aggregate route . ... 

Dubois P., Rigaud J. and Dekimpe J. (1976) Identification of 4,5-dimethyl tetrahydrofuran-2,3-dione in French flor sherry wine. Lebensm. Wiss. Technol. 9, 366-8. 

An instructive example for hydrogen abstraction by thiyl radicals, in this case from an ether, has been reported by Akhlaq et al. Experimental evidence is a racemisation of 2,5-dimethyl-tetrahydrofurane according to a reaction sequence which involves an intermediate planar radical site at the carbon a to the ether s oxygen. 

Methyl- and 2,5-dimethyl-tetrahydrofuran, when fluorinated over KC0F4 yield the fluorinated 2,5-dihydrofurans(l) and (2) as major products. Analogous reactions occur with sulphur heterocydes, tetrachlorothiophen yielding the 3-thiolen (3) with KC0F4, and thiophen itself yielding products... 

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