2.5- Dihydrofuran

Quantum State Molecular Constants Reference Method 

The values of the fixed parameters are those of the normal species, see [93Lop], 

Centrifugal distortion constants fixed to the values of the parent isotopologue. Reference 

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The 7, i5-unsaturated alcohol 99 is cyclized to 2-vinyl-5-phenyltetrahydro-furan (100) by exo cyclization in aqueous alcohol[124]. On the other hand, the dihydropyran 101 is formed by endo cyclization from a 7, (5-unsaturated alcohol substituted by two methyl groups at the i5-position. The direction of elimination of /3-hydrogen to give either enol ethers or allylic ethers can be controlled by using DMSO as a solvent and utilized in the synthesis of the tetronomycin precursor 102[125], The oxidation of the optically active 3-alkene-l,2-diol 103 affords the 2,5-dihydrofuran 104 in high ee. It should be noted that /3-OH is eliminated rather than /3-H at the end of the reac-tion[126]. 

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). 

Treatment with acidic catalysts dehydrates i j -butenediol to 2,5-dihydrofuran [1708-29-8], C H O (100). Cupric (101) or mercuric (102) salts give 2,5-divinyl-l,4-dioxane [21485-51-8], presumably via 3-butene-l,2-diol. 

With various catalysts, butanediol adds carbon monoxide to form adipic acid. Heating with acidic catalysts dehydrates butanediol to tetrahydrofuran [109-99-9] C HgO (see Euran derivatives). With dehydrogenation catalysts, such as copper chromite, butanediol forms butyrolactone (133). With certain cobalt catalysts both dehydration and dehydrogenation occur, giving 2,3-dihydrofuran (134). 

Hexafluoro-2,5-dihydrofuran [24849-02-3] is distilled into sulfur trioxide [7446-11-9] at 25°C. Addition of trimethyl borate [121-43-7] initiates a reaction which upon heating and distillation leads to a 53% yield of difluoromaleic anhydride. Dichloromaleic anhydride [1122-17-4] can be prepared with 92% selectivity by oxidation of hexachloro-1,3-butadiene with SO in the presence of iodine-containing molecules (65). Passing vaporized... 

These precursors are prepared by reaction of fuming nitric acid in excess acetic anhydride at low temperatures with 2-furancarboxaldehyde [98-01-1] (furfural) or its diacetate (16) followed by treatment of an intermediate 2-acetoxy-2,5-dihydrofuran [63848-92-0] with pyridine (17). This process has been improved by the use of concentrated nitric acid (18,19), as well as catalytic amounts of phosphoms pentoxide, trichloride, and oxychloride (20), and sulfuric acid (21). Orthophosphoric acid, -toluenesulfonic acid, arsenic acid, boric acid, and stibonic acid, among others are useful additives for the nitration of furfural with acetyl nitrate. Hydrolysis of 5-nitro-2-furancarboxyaldehyde diacetate [92-55-7] with aqueous mineral acids provides the aldehyde which is suitable for use without additional purification. 

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 isomerization of vinyl- or ethynyl-oxiranes provides a frequently exploited source of dihydrofurans or furans, but analogous conversions of vinylaziridines have not been applied so often. While most of the examples in Scheme 87 entail cleavage of the carbon-heteroatom bond of the original heterocycle, the last two cases exemplify a growing number of such rearrangements in which initial carbon-carbon bond cleavage occurs. 

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]

Dihydrofuran (376) and 2,5-dihydrofuran (377) react with nitrile oxides to give furo[2,3-6 ]isoxazoles (378) and furo[3,4-rf]isoxazoles (379), respectively, as cycloadducts. The double bonds of furan, pyrrole and thiophene also react when the nitrile oxide is generated in situ. Thus furan and benzonitrile oxide gave (380), and with 2-methyl-2-oxazoline the cycloadduct (381) was obtained (71AG(E)810). These and related cycloadditions are discussed in Chapter 4.36. 

Ph2CHC02-2-tetrahydrofuranyl, 1% TsOH, CCI4, 20°, 30 min, 90-99% yield. The authors report that formation of the THF ether by reaction with 2-chlorotetrahydrofuran avoids a laborious proce ure that is required when dihydrofuran is used. In addition, the use of dihydrofuran to protect the 2 -OH of a nucleotide gives low yields (24-42%)." The tetrahydrofuranyl ester is reported to be a readily available, stable solid. A tetrahydrofuranyl ether can be cleaved in the presence of a THP ether. ... 

Hydroxy-3,3-dimethyl-y-butyrolactone (3-hydroxy-4,4-dimethyl-4,5-dihydrofuran-... 

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 2,5-dialkoxy-2,5-dihydrofurans are cyclic acetals of male-aldehyde and may be used to generate this substance in situ. Also, the 2,5-dialkoxy-2,5-dihydrofurans readily undergo hydrogenation to provide the cyclic acetals of succindialdehyde." ... 

Conflicting findings have been reported with thiols and in one case a hemiacetal was dehydrated to a dihydrofuran. A by-product found in varying amounts in the oxidation of secondary, cyclic alcohols is the corresponding thiomethoxymethyl ether. 

Treatment of 2,5 and 2,4-furandicarboxylic acids with sulfur tetrafluonde in an excess of hydrogen fluoride leads simultaneously to conversion of the carboxylic groups into trifluoromethyl groups and addition of two fluorine atoms to the furan ring to give highly fluorinated diastereoisomersof2,5-dihydrofuran [225,226,227] (equations 117 and 118)... 

When the reaction of perfluQro-3,4-dimethyl-3-hexene with methanol is performed in the presence of pyridine, a perfluorinated dihydrofuran is formed, probably by a process involving generation of an anionic oxygen atom by nucleophilic cleavage of a supposed intermediate ether [27] (equation 24)... 

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