Furans 2,5-dialkoxy

Furan, 3,4-diacetyl-2-(halomethyl)-5-methyl-synthesis, 4, 659 Furan, 2,5-dialkoxy-synthesis, 4, 654... 

Furan, 2,5-dialkoxy-2-( 1 -hydroxyalkyl)-2,5-dihydro-ring expansion, 1, 425 Furan, 2,5-dialkoxytetrahydro-pyrrole synthesis from, 4, 330 Furan, 2,4-dialkyl-synthesis, 4, 661, 685 Furan, 2,5-dialkyl-... 

The 2,5-dialkoxy-2,5-dihydrofurans can be obtained by electrolytic oxidation of furan in alcoholic ammonium bromide or by bromine oxidation of furan in the appropriate alcohol. ... 

As demonstrated in previous sections, the carbazole unit was introduced as a pendant group or as a chain member in major classes of EL polymers such as PPVs (95-105,141,177, 190) and PFs (62, 63, 242-245). A variety of 2,7-carbazole-derived polymers with different conjugated units, such as 2-alkoxy- and 2,5-dialkoxy-l,4-phenylene (549) and l,l -binaphtha-lene-6,6 -diyl (550 [658]), 2,5-pyridine (551), 2,7-fluorene (245 [345,346]), 2,5 -bithiophene (554 [345]), 5,8-quinoxaline (552), quinquethiophene-SjS -dioxide (450 [550]), 2,5-thiophene (553), 2,5-furan (555), and acetylene (556 [659]) were reported by Leclerc and coworkers... 

The most important derivatives of 2,5-dihydrofuran are the 2,5-dialkoxy and 2,5-diacetoxy compounds which are easily prepared from furans (Section 3.11.2.3). Their vast utility is as synthetic equivalents of malealdehyde and its derivatives (60MI31100). Suitable intermediates for Robinson-Schopf synthesis have been obtained in this way. Pyridazines are readily obtained (Scheme 121), and 3-pyridinols are available from 2-furylmethanamines (Scheme 122). This approach has been used in a synthesis of pyridoxine. The 2,5-diacetoxy... 

In this way furanes form 2,5-dialkoxy-2,5-dihydrofuranes in high yields on electrolysis in R 0H/NH4Br (Eq. (120) ) ... 

Bromine or electrolytic oxidation of furan in alcoholic solution gives the corresponding 2,5-dialkoxy-2,5-dihydrofuran 159 (R = Alk). Lead tetraacetate in acetic acid oxidation yields 2,5-diacetoxy-2,5-dihydrofuran 159 (R = Ac). 

Electrolytic oxidation of furan in alcoholic solution gives the corresponding 2,5-dialkoxy-2,5-dihydrofuran. 

Via Elimination of A kohol or W alter from Dihydrofurans Pyrolysis of 2,5-dialkoxy- or -diacetoxy-2,5-dihydrofurans results in a smooth reaction with the formation of 2-substituted furans.39 40... 

The acid fission of furans, to yield 1,4-dicarbonyl compounds, is not satisfactory from the preparative point of view. This fission reaction has recently been restudied.169, 287-292 It is preferable to use the route involving 2,5-dialkoxy-2,5-dihydrofurans, which on hydrogenation and acid hydrolysis give the desired 1,4-dicarbonyl compounds. 

Alkoxyfurans are best prepared from 2,5-dialkoxy-2,5-dihydro-furans, by acid-catalyzed alcohol eliminations40,242,243 ... 

Silyl enolates of a,a-dialkoxy ketones react with furans to form [3 4- 4] cycloaddition products in the presence of SbCls (Eq. 6) [17], Cycloaddition also occurs when 2,2-dialkoxycyclopropanecarboxylic esters and carbonyl compounds are treated with SbCls [18]. 

The hydrolysis (or alcoholysis) of furans involves nucleophilic addition of water (or an alcohol) to an initially formed cation, giving rise to open-chain 1,4-dicarbonyl-compounds or derivatives thereof. This is in effect the reverse of one of the general methods for the construction of furan rings (18.13.1.1). Suc-cindialdehyde cannot be obtained from furan itself, presumably because this dialdehyde is too reactive under conditions for hydrolysis, but some alkyl-furans can be converted into 1,4-dicarbonyl products quite efficiently, and this can be viewed as a good method for their synthesis, and of cyclopentenones derived from them. ° Other routes from furans to 1,4-dicarbonyl compounds are the hydrolysis of 2,5-dialkoxy-tetrahydro-furans (18.1.1.4) and by various oxidative procedures (18.2). 

The bromine/methanol (18.1.1.4) oxidation of furans to give 2,5-dialkoxy-2,5-dihydrofurans and the cycloaddition of singlet oxygen (18.7) are discussed elsewhere in this chapter. 2,5-Dialkoxy-2,5-dihydrofurans can also be obtained by electrochemical oxidation in alcohol solvents or conveniently by oxidation with magnesium monoperoxyphthalate in methanol. Reaction of furan with lead(IV) carboxylates produces... 

The electrochemical or bromine/methanol oxidations of furans to give 2,5-dialkoxy-... 

Bromine/potassium acetate 2,5-Dialkoxy-2,5-dihydrofuran from furan ring... 

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