Furans derivatives

Furan is produced from furfural commercially by decarbonylation loss of carbon monoxide from furfural gives furan direcdy. Tetrahydrofuran (3) is the saturated analogue containing no double bonds. 

Furfural is derived from biomass by a process in which the hemiceUulose fraction is broken down into monomeric 5-carbon sugar units which then are dehydrated to form furfural. 

Tetrahydrofuran (3) is produced commercially from furfural by decarbonylation followed by hydrogenation it is also produced by several different methods from other raw materials. A complete discussion of tetrahydrofuran is found under Ethers. Polymers of tetrahydrofuran are covered under the general topic. Polyethers. Several other compounds containing the tetrahydrofuran ring, which are most readily produced from furfural, are discussed here. 

The furan nucleus is a cycHc, dienic ether with some aromaticity (2). It is the least aromatic of the common 5-membered heterocycles. A comparison of the aromaticity (3) of several of these compounds is shown below. 

The balance between aromatic and aUphatic reactivity is affected by the type of substituents on the ring. Furan functions as a diene in the Diels-Alder reaction. With maleic anhydride, furan readily forms 7-oxabicyclo [2.2.1]hept-5-ene-2,3-dicarboxyhc anhydride in excellent yield [5426-09-5] (4). 

Of the three isomeric furanoquinones (78-80), the vast majority of known quinones belongs to the p-quinonoid system (80). Representatives of the 5,6-dione system (81) are practically unknown. 

There are only few representatives of 79. When 2,3-diphenyl-7-hydroxy-benzofuran was oxidized with Fremy s salt, a mixture of the 6,7- and 4,7-dione was obtained (70MI2). Similarly, nitric acid oxidation of the dihydroxy precursor afforded the quinone 82 or the related 5,6-dione, corresponding to the system 81 (12LA384). 

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The reaction of the o-iodophenol 275 with an alkylallene affords the bcnzo-furan derivative 276[184], Similarly, the reactions of the 6-hydroxyallenes 277 and 279 with iodobenzene afford the tetrahydrofurans 278 and 280. Under a CO atmosphere, CO insertion takes place before the insertion of the allenyl bond, and a benzoyl group, rather than a phenyl group, attacks the allene carbon to give 280. Reaction of iodobenzene with 4,5-hexadienoic acid (281) affords the furanone derivative 282[185]. 

Diacetates of 1,4-butenediol derivatives are useful for double allylation to give cyclic compounds. l,4-Diacetoxy-2-butene (126) reacts with the cyclohexanone enamine 125 to give bicyclo[4.3.1]decenone (127) and vinylbicy-clo[3.2.1]octanone (128)[85,86]. The reaction of the 3-ketoglutarate 130 with cij-cyclopentene-3,5-diacetate (129) affords the furan derivative 131 [87]. The C- and 0-allylations of ambident lithium [(phenylsulfonyl)methylene]nitronate (132) with 129 give isoxazoline-2-oxide 133, which is converted into c -3-hydroxy-4-cyanocyclopentene (134)[S8]. Similarly, chiral m-3-amino-4-hyd-roxycyclopentene was prepared by the cyclization of yV-tosylcarbamate[89]. 

Alkynyl)oxiranes also react with carbon nucleophiles to afford furan derivatives. Furanes of different types are obtained depending on the structure of the substrates. 7-Methyl-2-ethynyloxirane (95) reacts with acetoacetate to give the furan 97 by the elimination of formaldehyde from the cyclized product 96. The hydroxy ester of the alkylidenefuran 98 and the corresponding lactone 99 are obtained by the reaction of i-methyl-2-(2-propynyI)oxirane[40, 42]. 

Since double bonds are no longer present, these compounds are more stable than the corresponding furan derivatives. Tetrahydrofurfuryl alcohol—ethylene oxide adducts [31692-85-0] are also usehil solvents for paint stripping formulations (136,141,143). 2-Methylfuran is a good solvent, but... 

Lubricating Oil Extraction. Aromatics are removed from lubricating oils to improve viscosity and chemical stabihty (see Lubrication and lubricants). The solvents used are furfural, phenol, and Hquid sulfur dioxide. The latter two solvents are undesirable owing to concerns over toxicity and the environment and most newer plants are adopting furfural processes (see Furan derivatives). A useful comparison of the various processes is available (219). 

Aqueous mineral acids react with BF to yield the hydrates of BF or the hydroxyfluoroboric acids, fluoroboric acid, or boric acid. Solution in aqueous alkali gives the soluble salts of the hydroxyfluoroboric acids, fluoroboric acids, or boric acid. Boron trifluoride, slightly soluble in many organic solvents including saturated hydrocarbons (qv), halogenated hydrocarbons, and aromatic compounds, easily polymerizes unsaturated compounds such as butylenes (qv), styrene (qv), or vinyl esters, as well as easily cleaved cycHc molecules such as tetrahydrofuran (see Furan derivatives). Other molecules containing electron-donating atoms such as O, S, N, P, etc, eg, alcohols, acids, amines, phosphines, and ethers, may dissolve BF to produce soluble adducts. 

Recently, many transformations of various heterocycles into pyridazines have been reported. From the synthetic point of view it appears that furan derivatives are the most valuable. 

Only the potentially 2,4-dihydroxy derivatives of furan and thiophene are known and these exist in the solid state and in polar solvents as the monoenols (82) (71T3839). However, in non-polar solvents the furan derivatives exist predominantly in the dioxo form (83). The 2,5-dioxo structure (84) is well established for X=0, NR, S and Se (71BSF3547) and there is no evidence for intervention of any enolic species. The formal tautomer (85) of succinimide has been prepared and is reasonably stable (62CI(L)1576). 

The addition of p-quinone to enamines normally produces furan derivatives, especially when the enamine possesses a 3 hydrogen (see Section III. A). 1,2 Cycloaddition is claimed to take place to give a cyclobutane derivative when p-quinone and an enamine with no jS hydrogens are allowed to react at low temperatures (51). However, little evidence is reported to verify this structural assignment, and the actual structure probably is a benzofuranol (52). Reaction of a dienamine (formed in situ) with p-quinone in the presence... 

For example, treatment of dione 12 with hydrochloric acid yielded furan 13, a key synthetic intermediate for the production of a variety of compounds that were recently investigated for anticancer activity. Related inquiries by members of the same research team identified furans derived from IS as potential treatments for RNA viruses. Furan IS was prepared by condensation of dione 14 with catalytic sulfuric acid in refluxing acetic anhydride. ... 

FtG. 2. Relative energy of the excited states of 2-methylfuraii and of some reactive intermediates. [Reprinted with permission from M. D Auria. Ab initio study on the photochemical isomerization of furan derivatives. J. Org. Chem. 65, 2494-2498 (2000)]. 

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