Furans, anodic oxidation

Furfural — see Furan-2-oarbaldehyde, 532 Furfuryl acetate, o -(butoxycarbonyl)-anodic oxidation, 1, 424 Furfuryi acrylate polymerization, 1, 279 Furfuryl alcohol configuration, 4, 544 2-Furfuryl alcohol polyoondensation, 1, 278 reactions, 4, 70-71 Furfuryl alcohol, dihydro-pyran-4-one synthesis from, 3, 815 Furfuryl alcohol, tetrahydro-polymers, 1, 276 rearrangement, 3, 773 Furfuryl chloride reactions... 

Furan was dimethoxylated to give 2,5-dihydro-2,5-dimethoxyfuran, using electrogenerated bromine molecules generated from bromide salts in electrolyte solutions [71]. This reaction was characterized in classical electrochemical reactors such as pump cells, packed bipolar cells and solid polymer electrolyte cells. In the last type of reactor, no bromide salt or electrolyte was used rather, the furan was oxidized directly at the anode. H owever, high consumption of the order of 5-9 kWh kg (at 8-20 V cell voltage) was needed to reach a current efficiency of 75%. 

The manner in which radicals attack furans has been studied deeply and earlier misconceptions have been corrected, especially the idea that anodic oxidation in methanol involves methoxy radicals.248 Furan cation radi-... 

The anodic oxidation of furans has been studied from both synthetic and theoretical points.285 Because of the many possibilities for delocalization, tetraphenylfuran is a special case but it does allow certain basic processes to be monitored by cyclic voltammetry.286 In nitrobenzene two steps are seen ... 

In many furans the dicationic state is probably much less easily reached, and it is usual to regard anodic oxidations as ECEC sequences. Thus one electron is removed and cation radical 110 reacts with a nucleophile (methanol, for example). The radical center can lose another electron and then react with another methanol molecule giving the product (Scheme 56).287-289 Thus the net result is usually the same as in Scheme (55). 

Studies of cathodic reduction have been few. Amusingly, attempted anodic oxidation of the furyl ketone 123 actually resulted in cathodic reduction to the dimer 124 the corresponding ester was oxidized normally, however.301 Sometimes the dimethoxydihydrofurans formed in oxidation processes are reduced in a side reaction leading to the tetrahydrofuran derivatives.302 By using dimethylformamide as solvent instead of the protic solvents used formerly, a Czech group has demonstrated that the cathodic reductions of furans can produce fairly stable anion radicals having ESR spectra which agree well with theory.3023... 

On the other hand, the anodic oxidation of 1,3,5-cycloheptatrienes is one of the most powerful key tools for the preparation of a variety of non-benzenoid aromatic compounds such as tropylium salts, tropones, tropolones, 2H -cyclohcpta h furan-2-oncs and azulenes14. 

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]. 

A 2,5-dihydroxylation can be obtained by anodic oxidation of furan derivatives under suitable experimental conditions (Scheme 107) [149,196],... 

Finally, the intramolecular coupling reaction between an olefin and a pyrrole ring has been examined (Scheme 40). In this example, a 66% isolated yield of the six-membered ring product was obtained. A vinyl sulfide moiety was used as the olefin participant and the nitrogen protected as the pivaloyl amide in order to minimize the competition between substrate and product oxidation. Unlike the furan cyclizations, the anodic oxidation of the pyrrole-based substrate led mainly to the desired aromatic product without the need for subsequent treatment with acid. 

Anodic oxidation of fiirans in acetic acid leads to the 2,5-diacetoxy-2,5-dihydro-furan 58 [185, 186]which is readily converted to 2-acetoxyfiiran, This has proved a valuable intermediate for the synthesis of butenolides [187]. Reactions in moist acetonitrile yield the 2,5-dihydro-2,5-dihydroxyfurans which can be oxidised to the maleic anhydride 59 [188], Oxidation of furan-2-carboxylic acid in methanol and sulphuric acid is a route to the ester of a-ketoglutaric acid [189]. 

In connection with the anodic intramolecular coupling of 1-benzyltetrahydroiso-quinoline derivatives, the anodic oxidation of 4-(3,4-dimethoxybenzyl)-6,7-dimethoxy-isochromanone 15 has been studied rather extensively. The main product obtained from the isochromanone is a y-lactone, 7a,8-dihydro-3,10,ll-trimethoxy-2 H-phenanthro[9.8a, b]furan-2,7(5 H)-dione 16 13 ... 

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). 

Two sequential electrochemical annulations of an electron-rich 3,4-disubstituted furan ring were employed to generate the fused tricyclic ring system of arteannuins. The second reaction, as depicted below, represented the formation of a quaternary carbon center by a formal 6-endo cyclization under these anodic oxidation conditions <07OL4599>. 

The anodic oxidation of furans is one of the most extensively studied reactions because electrooxidation of furans in methanol yields 2,5-dimethoxy-2,S-dihydrofi] s (21 equation 41), which are useful starting materials in organic synthesis.44... 

A variety of cyclopentenone derivatives have been synthesized using the anodic oxidation of furans as a key step. ... 

The 7c-excessive character of furan facilitates single-electron oxidation however, the cation-radicals produced from simple furans are not, in general, persistent. The mediation of 2,5-dimetliylfuran cation-radical (1) has been proposed in anodic oxidations yielding methoxylated, cyanomethoxylated, and acyloxylated products.Although not specifically implicated, cation-radicals may be involved in very similar oxidative reactions. ... 

Otherwise, lithium amides of secondary amines undergo anodic dimerization to form hydrazines in moderate yields [12]. Hydrazines are also generated, if secondary amines are electrochemically oxidized in the presence of an alkali hydroxide [27,28]. This reaction is mainly effective if the coupling takes place intramolecularly to give cyclic hydrazine derivatives [28]. If lithium amides of secondary amines are anodically oxidized in tetrahydro-furan (THF) solution in the presence of the free amine, 2-aminotetrahydrofurans are formed in reasonable yields. In contrast, the respective aminomagnesium bromides only gives A, A -coupling products [29]. 

In the case of the enaminonitrile XVIIa, coupling through the phenyl ring is observed [127a]. Enamines related to the key intermediate in thiamine diphosphate-dependent enzymatic pathways lead to dimers in a one-electron oxidation via a thiazolium cation radical as intermediate [127b]. Anodic oxidation of enaminones, prepared from / -substituted acetophenones and A,A-dimethylformamide dimethylacetal, afford dimers and substituted furans in fair yield [127c]. 

SCHEME 28. Anodic oxidation of p-methoxyphenols in the presence of cyclic enol ethers and furan... 

The anodic oxidation of furans in methanol was also carried out without an intentionally added electrolyte (Figure 12.11) [38, 39]. 2,5-Dimethoxy-2,5-dihydro-furan was obtained in 98% yield. The anodic methoxylation and acetoxylation of various organic compounds was also achieved using this system. 

The synthesis of tropine and of its esters has been made practical because succinic dialdehyde has become easily available. Furane, now available commercially, gives on anodic oxidation in methanol with ammonium bromide as electrolyte good yields of 2,5-dimethoxy-2,5-2H-furane (37a). This mixed ketal of maleic dialdehyde could be hydrogenated readily and quantitatively over Raney nickel to 2,5-dimethoxy-4H-furane (37b). The latter as a mixed ketal of succinic dialdehyde undergoes acid hydrolysis easily. Optimum conditions for the condensation of this dialdehyde formed in situ to tropinone have been recorded (38) with yield up to 93% and at a higher rate than described earlier (18). 

Two model p-quinone methide ring systems of kendomycin were obtained by oxidation with 2,2-dimethyldioxirane (DMDO) and NalO, respectively. The demonstrated chemistry paves the way for the to synthesis of kendomycin <04OL3131>. Anodic oxidation of 2,3-dihydrobenzol )]furan derivatives was also utilized to synthesize 2-fluoro-and 2,3-difluoro-2,3-dihydrobenzo[fc]furan derivatives <04JOC5302>. 

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