Furfural furan

On a commercial scale, furan is obtained from 2-formylfuran (furfural, furan-2-carbaldehyde) (see Section 6.2.7) by gas-phase decarbonylation, but in the laboratory, furans can be formed by the cyclodehydration of 1,4-dicarbonyl compounds. Heating in boiling benzene with a trace of /7-toluenesulfonic acid as a catalyst in a Dean-Stark apparatus is often effective (Scheme 6.30a). 

Furfural (furan-2-carbaldehyde Scheme 1) arises from the decomposition of sugars and is a commercially important raw material used in furfural-phenol resins and as a synthetic intermediate (see CHEC 1.15). 

Another approach is through the series of compounds furfural, furane, cyclote-tramethylene oxide, 1,4-dichlorobutane, and adiponitrile, as illustrated below. The furfural is obtained from oat hulls and corn cobs. 

Two classic furan syntheses were described in Chapter 4 an adaptation of the Paal-Knorr method (section 4.2.1) and the Feist-Benary synthesis (section 4.2.3), which starts with an alpha-haloketone and a beta-ketoester. Other ring-closing processes have been devised and are outlined elsewhere. It is of practical current importance to start syntheses with the readily available furfural (furan-2-carbaldehyde) or fiiran itself. The reason for this is that large-scale commercial syntheses of these compounds have been developed, and thus they are readily available and inexpensive. 

Furaldehyde. See Furfural 2-Furaldehyde, 5-methyl-. See Methylfurfural Furale. See Furfural Furan... 

Furfural Furan, furanone, methanol, other C1-C4 oxygenates (e.g., formaldehyde, hydroxyacetaldehyde, acetone, acetol, lactones, etc.), C5 and Cg-anhydro-sugars, humic substances Intermediate commodity chemical, solvent, resins, adhesives, food flavoring agent, precursor for specialty chemicak... 

However, it was originally prepared (over a century ago) from pentoses (five-carbon carbohydrates see Chapter 11), which, on sulfuric acid treatment, produce furfural (furan 2-carboxaldehyde) (Scheme 8.75), and which, in turn, can be easily oxidized to the corresponding carboxylic acid (2-furoic acid). The acid (2-furoic add) is then readily decarboxylated (i.e., loss of CO2) to furan (oxa-2,4-cyclopentadiene) and the resulting product is catalytically reduced with hydrogen (H2) (Scheme 8.102) over a Ni catalyst to yield the desired five-membered heterocyclic compound (oxalane, oxacyclopentane,THF). 

Interestingly, as shown in Scheme 8.103, if furfural (furan 2-carboxalehyde) is reduced with hydrogen (H2) in the presence of a Ni catalyst, the corresponding primary alcohol, tetrahydrofurfuryl alcohol, results. Then, if the that primary alcohol is heated to 350°C in the presence of an alumina (aluminum oxide, AI2O3) catalyst, rearrangement and dehydration occur to yield dihydropyran (oxahex-2-ene). 

Scheme 8.102. The formation of furfural (furan 2-carboxaldehyde) from pentoses (typical examples, D-ribose and its proton tautomer D-ribulose, are shown) on treatment with sulfuric acid (Scheme 8.75).The conversion to furan 2-carboxylic acid,furan (oxa-2,4-cyclopentadiene), and tetrahydrofuran (THF, oxacyclopentane, oxolane) follows.

Furan-2-carboxaldehyde see furfural Furan-2-carboxylic acid 78 Furan-2-methanthiol 78 Furazan see 1,2,5-oxadiazole Furazan oxide see furoxane Furfural 77 Furocumarin 325 Furoxan 253... 

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