2.5- Dimethoxy-2,5-dihydrofurans

Furan can be catalyticaHy oxidized in the vapor phase with oxygen-containing gases to maleic anhydride (93). Oxidation with bromine or in an electrochemical process using bromide ion gives 2,5-dimethoxy-2,5-dihydrofuran [332-77-4] (19) which is a cycHc acetal of maleic dialdehyde (94—96). 

Thiophenecarboxaldehyde [498-62-4] has been commercially available (35) via carbonylation of 2,5-dimethoxy-2,5-dihydrofuran, followed by treatment with hydrogen sulfide, which introduces the sulfur atom with loss of methanol, inducing aromaticity and producing 3-thiophenecarboxaldehyde directly. 

Pyridazine itself is best prepared (in about 60-67% yield) from 2,5-diacetoxy- or 2,5-dimethoxy-2,5-dihydrofuran (50ACS1233, 56JOC764) or by hydrodehalogenation of 3-chloro-... 

The conversion of furans by oxidative acetylation or methoxylation to 2,5-diacetoxy- or 2,5-dimethoxy-2,5-dihydrofurans respectively, and their subsequent hydrogenation to the corresponding tetrahydrofurans, provides a useful source of protected 1,4-dicarbonyl compounds capable of conversion inter alia into the other five-membered heterocycles [Pg.142]

Furan, reaction with bromine and methanol to yield 2,5-dimethoxy-2,5-dihydrofuran, 40, 29 Fuean, 2,5-DmYDRO-2,5-dim-ethoxy-, 40, 29... 

Paired electrosynthesis of phthalide and p-fert-butyl benzaldehyde (Putter and Hannebaum, 2000) and 2,5 Dimethoxy-2,5-dihydrofuran from furan (Steckhan et al, 2000)... 

Anodic oxidations of heteroaromatic cycles (furans, pyrroles, benzofurans) in the presence of methanol have been extensively studied [148-165]. The electromethoxyla-tion of differently substituted furans gives 2,5-dimethoxy-2,5-dihydrofurans in moderate to good yields (Scheme 96) [148-159, 166-170]. 

Technically important is the dimethoxylation of furan and its derivatives to form 2,5-dimethoxy-2,5-dihydrofuran (Eq. (13)) and the oxidation of aldoses to... 

The reversal of the well-known transformation of sugars into pyrans has been detailed as a method for assembling simple monosaccharides from simple furans (71T1973). A compound of the 2-furylcarbinol type was converted by the Br2/MeOH procedure into a mixture of the cis and trans isomers of the corresponding 2,5-dimethoxy-2,5-dihydrofuran derivative (129). Mild acid hydrolysis of (129) resulted in cleavage of the acetal bonds with formation of the dicarbonyl compound (130) which underwent immediate cyclization to 2,3-dideoxy-DL-alk-2-enopyranos-4-ulose (131 Scheme 29). 

Another general approach to sugars starts from (2-furyl)methanols (furfuryl alcohols, 22). Bromination at low temperature in methanol (Clauson-Kaas reaction) and subsequent mild acidic hydrolysis of 2,5-dimethoxy-2,5-dihydrofurans 23 obtained, leads to 6-hydroxy-2,3-dihydro-6//-pyran-3-ones 24 (Scheme 8) [40], The same dihydropyranones can be... 

Anodic addition to an electron-rich heteroaromatic compound is used to transform furan to 2,5-dimethoxy-2,5-dihydrofuran, a valuable synthetic intermediate. Again, an indirect electrochemical process occurs. The bromide ion as redox catalyst is electrochemically oxidized to give bromine, which then acts as chemical oxidant for furan [7] ... 

The oxidative reaction of furan with bromine in methanol solution or an electrochemical process using sodium bromide produces 2,5-dimethoxy-2,5-dihydrofuran (19), which is a cyclic acetal of maleic dialdehyde. The double bond in (19) can be easily hydrogenated to produce the corresponding succindialdehyde derivative. Both products find application in photography and as embalming materials, as well as other uses. 

BASF developed a process for the preparation of 2,5-dimethoxy-2,5-dihydrofuran from butene-1,4-diol37) ... 

Dimethoxytetrahydrofuran from Eastman Kodak Company was used without further purification. This material also can be prepared by catalytic hydrogenation2 of 2,5-dimethoxy-2,5-dihydrofuran.3... 

The direct electrochemical methoxylation of furan derivatives represents another technically relevant alkoxylation process. Anodic treatment of furan (14) in an undivided cell provides 2,5-dimethoxy-2,5-dihydrofuran (15). This particular product represents a twofold protected 1,4-dialdehyde and is commonly used as a C4 building block for the synthesis of N-heterocycles in life and material science. The industrial electroorganic processes employ graphite electrodes and sodium bromide which acts both as supporting electrolyte and mediator [60]. The same electrolysis of 14 can be carried out on BDD electrodes, but no mediator is required The conversion is performed with 8% furan in MeOH, 3% Bu4N+BF4, at 15 °C and 10 A/dm2. When 1.5 F/mol were applied, 15 is obtained in 75% yield with more or less quantitative current efficiency. Treatment with 2.3 F/mol is rendered by 84% chemical yield for 15 and a current efficiency of 84% [61, 62]. In contrast to the mediated process, furan is anodically oxidized in the initial step and subsequently methanol enters the scene (Scheme 7). 

The best yields, up to 99%, were obtained in hexafluoro-isopropanol but acetonitrile and methanol were also satisfactory. The role of the salt was to enhance the formation of the radical species involved bringing about a significant increase in reaction yield furan and 2,5-dimethylfuran in methanol were converted into 2,5-dimethoxy-2,5-dihydrofurans. The use of PhI(py+)22TfO in acetonitrile-water provided better results [130]. 

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