2-butylfuran

Furan undergoes phenylation rather than diazo coupling on reaction with ben-zenediazonium salts, and thiophene similarly yields 2- or 2,5-diaryl derivatives rather than coupled products. However, 2,5-dimethylfuran and 2-/-butylfuran give coupled products with 2,4-dinitrobenzenediazonium ion (Scheme 26). 

Methylfuran 2-Ethylfuran 2-Butylfuran 2-Pentylfuran 2-Hexylfuran 2-Heptylfuran 2-Octylfutan Dimethylfuran 2-Ethyl-5-methylfuran ... 

A successful procedure for the formation of 2,5-di-t-butylfuran involves reaction of the parent heterocycle with f-butyl chloride in the presence of iron(III) chloride and iron(III) oxide. Iron(III) oxide acts as a hydrogen chloride scavenger and at the same time regenerates the catalyst. Concurrent polymerization normally deactivates the catalyst (82CI(L)603). 

Several groups have used the Paal-Knorr reaction to synthesize trialkylfurans. Ballini prepared a series of 3-alkyl-2,5-dimethylfurans (53) in 60-94% yield by treatment of the corresponding diones (52) with tosic acid. Weirsum produced the sterically congested tri-r-butylfuran 55 from dione 54, but was unable to use the same strategy to furnish the tetra-t-butyl derivative. ... 

In the photochemical isomerization of isoxazoles, we have evidence for the presence of the azirine as the intermediate of this reaction. The azirine is stable and it is the actual first photoproduct of the reaction, as in the reaction of r-butylfuran derivatives. The fact that it is able to interconvert both photochemically and thermally into the oxazole could be an accident. In the case of 3,5-diphenylisoxazole, the cleavage of the O—N bond should be nearly concerted with N—C4 bond formation (8IBCJ1293) nevertheless, the formation of the biradical intermediate cannot be excluded. The results of calculations are in agreement with the formation of the azirine [9911(50)1115]. The excited singlet state can convert into a Dewar isomer or into the triplet state. The conversion into the triplet state is favored, allowing the formation of the biradical intermediate. The same results [99H(50)1115] were obtained using as substrate 3-phenyl-5-methylisoxazole (68ACR353) and... 

Katritzky and his colleagues have used equilibrium proto nation to estimate the aromaticity of furan and similar heterocycles using 2,5-di-tert-butylfuran as substrate in sulfuric acid. In agreement with the foregoing discussion, the furan does not behave as a Hammett (i.e., oxygen) base but as a carbon base similar to azulene. On Reagan s Hc scale, the furan gave n = 1.22, correspond-... 

Toluene has been converted into benzyl chloride (67%) by hypochlorite under phase-transfer conditions, but the procedure does not appear to have been exploited generally, possibly because of competing arene epoxidation [16]. Oxidation of 2,5-di-f-butylfuran yields 2,2,7,7-tetramethyloct-4-en-3,6-dione (83%) [17]. 

Furans with bulky a-substituents such as 2-rerf-butylfuran [176] and 2-acetoxy-furan dimethyl acetal [177] show no effect of steric hindrance and are readily converted to the dihydro-dimethoxy product. 3,4-Substituted furans are also converted to dihydrodimethoxy derivatives [178, 179],... 

One of the major areas of study in the photochemistry of heterocycles is the photorearrangement of five-membered heteroaromatic systems. Various mechanisms have been proposed to account for these transformations.123 A ring contraction-ring expansion pathway, via an intermediate cyclopropene, appears to be involved in the rearrangement of derivatives of furan. The isolation of an intermediate (140) of this type together with an allene (141) has been effected on irradiation of 2,4-di-tert-butylfuran (142), as shown in Scheme 9.124 Similar transformations have been reported in perfluorotri-... 

When 2,5-fc rf-bu tylthiophene S-oxide (688) is photoirradiated, 2,5-tert-butylfuran (689) is produced almost quantitatively. When 2,4-tert-butylthiophene S-oxide is photoirradiated, 2,4-ferf-butylfuran is isolated. Here, however, also side products can be observed. 

The chloromercuric derivative of furan is also suitable for alkylation. When tert-butyl bromide is added to the solution of this compound in chloroform, an exothermic reaction takes place. Three products were isolated from the reaction mixture, with 2,5-di-ieri-butylfuran in moderate yield131 ... 

A mono-ieri-butylfuran can be isolated in small yield using isobutylene with BFS catalyst.131... 

In contrast, the dimer of dipivaloylketene 344 (R = R = R = t-Bu) reacts with car-bodiimides to give [2+2] cycloadducts 345. A mixed dimer obtained in the thermolysis of 4-pivaloyl- and 4-methoxycarbonyl-5-t-butylfuran-2,3-dione also undergoes the [2+2] cycloaddition with dialkylcarbodiimides. 

It is now 17 years since the first reports of the photolability of furan and its simple derivatives appeared. Since this time several accounts of the process which yields, for example, from 2,5-di-t-butylfuran (14), (15), and the 2,4-positional isomer, have been published. Yields are, however, generally very... 

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