Furaldehyde 5- -, formation

These observations of demethylation and 2-furaldehyde formation led ineluctably to the conclusion that lime-water had caused these methylated pentoses to form (by probable loss of 1 mole of methanol per mole) substances which were exceedingly easily converted, by further loss of 2 moles of methanol per mole, to 2-furaldehyde in acid solution. Accordingly, Lewis proposed the following scheme [shown with 2,3,4-tri-O-methyl-D-xylose (XIX) as the starting substance] for this sequence of reactions. 

Kinetic evidence from the DL-glycerose-1,3-dihydroxy-2-propanone isomerization has indicated that aldose-ketose isomerization and formation of a 3-deoxyosone proceed through a common intermediate. Ashmarin and coworkers and Petuely s observations, which indicate general acid and base catalysis of 2-furaldehyde formation, tend to support such a mechanism, since 2-furaldehyde and its derivatives may indeed be formed from 3-deoxyosones. 

The resinification of 2-furaldehyde promoted by acidic substances or by heat has been known to chemists since the end of last century, and attempts to explain the mechanism leading to the formation of black, insoluble resins have been published... 

In conclusion, the self-condensation of 2-furaldehyde promoted by heat occurs with the formation of di- and trifurylic intermediates. The functionality of the growing chain increases after each oligomerization step until gelation and precipitation of the resin occurs. Thus, the process is non-linear from the onset since the condensation product 4 possesses three sites for further attack, namely the free C-5 position and the two formyl groups. It is interestering to note that while the polycondensation of 2-furfuryl alcohol is essentially linear and cross-linking is due to side reactions, the thermal resinification of 2-furaldehyde is intrinsically non-linear and gel formation occurs at earlier conversions. 

The reaction mechanism postulated by Wolfrom, Schuetz and Cavalieri87 for the formation of 5-(hydroxymethyl)-2-furaldehyde from D-glucose involves the enol (XXXV) of 3-deoxy-D-glucosone as an intermediate an alternative pathway proposed by these same workers included the enol... 

With regard to the formation of levulinic acid by acid treatment of 5-(hydroxymethyl)-2-furaldehyde, Pummerer, Guyot and Birkofer92 pro-... 

Neher and Lewis177 obtained 2-furaldehyde from 2,3,4-tri-O-methyl-L-arabinose by heating with dilute acid after preliminary enolization with alkali. Isbell83 proposed a mechanism for this conversion similar to that for the conversion of tetra-0-methyl-(2-hydroxy-D-glucal) into 5-(hydroxymethyl)-2-furaldehyde XLIV was suggested as an intermediate. In Hurd and Isenhour s178 scheme for the formation of 2-furaldehyde from free pentose, the enol (XLV) of a 3-deoxypentosone was regarded as an inter-... 

The mechanism of pentose dehydration has been a matter of study for several years. The accepted pathway (see Scheme 1) to 2-furaldehyde from a pentose, in this case D-xylose (1), involves the reversible formation of a 1,2-enediol (2) followed by dehydration to the enolic form (3) of a 3-deoxypentosulose, which is further dehydrated to the 3,4-dideoxypent-3-enos-2-ulose (4) prior to cyclization to afford 2-furaldehyde 5. This mechanism, initially suggested by Isbell,has been substantiated by later work. This confirmation required incorporation of deuterium or tri-tium into the furaldehyde at various ring positions. However, when... 

A prior report proposed that 3-deoxyhexosuloses participate in the formation of 11 (see Scheme 7). The 3-deoxyhexosuloses were isolated from heated, acid reaction mixtures involving D-fructose and L-sorbose. It was suggested that 3-deoxyhexos-2-ulose (39) underwent reversible equilibrium with 38, its cis form, and was further dehydrated at C-3 and C-4, to form 3,4-dideoxyhex-3-enos-2-ulose (40). This intermediate cy-clized to the furaldehyde. Experiments involving treatment of both... 

Scheme 7.—Formation of 5-(Hydroxymethyl)-2-furaldehyde from o-Glucose.

Decarboxylation of L-ascorbic acid in acid solutions has been proposed to involve one of two possible mechanisms. One pathway required dehydration, decarboxylation, and formation of 2-furaldehyde. The second pathway involved a rearrangement to the 3-keto form followed by a... 

The formation of oxygen-containing heterocyclic compounds is also a consequence of the Maillard reaction. Amines and amino acids have a catalytic effect upon the formation of 2-furaldehyde (5), 5-(hydroxy-methyl)-2-furaldehyde (11),2-(2-hydroxyacetyl)furan (44),2 and 4-hy-droxy-5-methyl-3(2//)-furanone (111) (see Ref. 214). This catalytic effect can be observed with several other non-nitrogenous products, including maltol. The amino acid or amine catalysis has been attributed to the transient formation of enamines or immonium ions, or the 1,2-2,3 eno-lization of carbohydrates. Of interest is the detection of A -(2-furoyl-... 

An effort has also been made to determine the structure of products providing coloration in the Maillard reaction prior to melanoidin formation. The reaction between D-xylose and isopropylamine in dilute acetic acid produced 2-(2-furfurylidene)-4-hydroxy-5-methyl-3(2/f)-furanone (116). This highly chromophoric product can be produced by the combination of 2-furaldehyde and 4-hydroxy-5-methyl-3(2//)-furanone (111) in an aqueous solution containing isopropylammonium acetate. The reaction between o-xylose and glycine at pH 6, under reflux conditions, also pro-duces " 116. Other chromophoric analogs may be present, including 117,... 

---