Oxidation of furans

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. ... 

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 ... 

The Clauson-Kaas pyrrole synthesis was adapted to a soluble polyglycerol (PG) support <060L403>. Electrochemical oxidation of furan 33 in the presence of methanol followed by hydrogenation gave 2,5-dimethoxytetrahydrofuran 34. Cyclocondensation with primary arylamines gave A-arylpyrroles 35. Removal from the PG support was then accomplished by treatment of 35 with LiOH which gave 2-pyrrolepropanoic acids 36. 

Furan can be used as a source for malealdehyde. McKervey and coworkers found that oxidation of furan with dimethyldioxirane results in malealdehyde, which can be trapped in situ with a variety of Wittig reagents to generate dienes and polyenes (equation 174)295. The same sequence with substituted furans gives ketodienealdehydes and ketodieneesters295. 

FIGURE 8.13 Oxidation of furan-containing drugs can lead to areactive Michael acceptor. 

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

The electrochemical oxidation of furans has been exploited since 1952 [170]. The usual electrolyte is ammonium bromide in methanol, at - 5 °C, using an undivided cell with either platinum or graphite as anode and a nickel or stainless steel... 

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]. 

Bromine or electrolytic oxidation of furan in alcoholic solution gives the corresponding... 

Oxidation of 2,5-dialkylfurans with pyridinium chlorochromate results in high yields of a,(3-unsaturated y-dicarbonyl compounds (Scheme 25) (82S245, 80T661). Similar results are obtained by peracid oxidation of furans (see review (90MI206-01)). The acid most frequently used is MCPBA. It is assumed that the first step involves epoxidation as shown in Scheme 25 (83CL1771). 

The above-described problem demonstrates the first enantioselective synthesis of dysidiolide, a C25 isoprenoid antimitotic agent. The central transformations are the sulfenylation-dehydrosulfenylation sequence to prepare an a,/5-tnone, the biomimetic cationic 1,2-rearrangement to form stereoselectively the bicyclic scaffold, vinyl cuprate displacement of an iodide furnishing the C-l side chain and the photochemical oxidation of furan to generate the j hydroxy-butenolide functionality. 

Oxidation of furans to enediones.3 Oxidation of 2,5-dialkylfurans to cis-tn-iiones can be effected with MMPP rapidly at 25° in C2HsOH/H20 in 90-99% yield. m-Chloroperbenzoic acid is less efficient for this oxidation. 

The oxidation of furans has often been used to express the latent functionality present within this heterocyclic framework, and it results particularly attractive for the synthesis of the widespread butenolides [8c,56]. Dye-sensitized photo-oxygenation of furfural, furoic acid and a- or a,a -unsubstituted furans leads directly to butenolides [8c,56,60d,63]. The presence of a silyl group in a-position enhances the rate of endoperoxide formation and controls the regiochemical outcome of the second-step leading to the related butenolide via a rapid intramolecular silatropic shift [64]. An application of this reaction is shown in Sch. 39 as a step in the synthesis of spongianolide A, an antitumoral natural sestertepenoid [65],... 

Photo-oxidation of furan was used as the final step to realize the natural product (+)-cacospongionolide B possessing antimicrobial and cytotoxic activities <02JA11584>. 

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). 

Oxidation of furan and its derivatives to give maleic anhydride has long been known.1 Recently methods and the choice of suitable catalysts have been refined, and this results in increased yields. Thus, Costa Novella347 oxidized furfural to maleic anhydride in 72% yield, with a Mo/V catalyst on A1203. Similar work is described by Hillers and Tarvida.348 349... 

While oxidation of furan yields ill-defined polymers with severely interrupted Jt-conjugation, oxidation of 2,2, 5, 2"-terfuran 1 ( = 3) does produce fully jt-conjugated polyfuran films. These films, when doped with tri-fluoromethanesulfonate ions, exhibit conductivities of up to 2 x lO Scm and a gap of 2.35 eV between the... 

The useful synthetic utilities of photosensitized oxidation of furans were demonstrated. A notable example was the oxidation of the trisubstituted furan 29 shown in Scheme 16 to the (Z)-7-keto-o,/ -unsaturated ester intermediate 30, a crucial step for the construction of the ABC ring system of the complex heptacyclic marine alkaloid norzoanthamine <2004SCI495>. 

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. ... 

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