Pentose dehydration, mechanism

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... 

The analogous product from the pentose glycals and 2-deoxy-pentoses would be furfuryl alcohol, but, as it is unstable and is readily converted into levulinic acid under the conditions of formation, it is difficult to isolate. The spectral data52 and the fact that levulinic acid is the common product from 2-deoxy-D-erythro-pentose, D-ara-binal, and furfuryl alcohol19 substantiate the supposition that the mechanism is analogous to that just described. Other (unknown) products are formed in significant yield from the dehydration of 2-deoxy-D-ert/fhro-pentose.52,84 The mechanism of formation of levulinic acid is discussed in Section V (see p. 212). 

Kurata and Sakurai124 have investigated the mechanism of dehydration of L-ascorbic acid by examining the products from it and from L-xyZo-hexulosonic acid after treatment for 1 hour at 100° at pH 2.2, or with 5% sulfuric acid at 100°. In both experiments, 2-furaldehyde and 3-deoxy-L-threo-pentosulose (isolated as the phenylhydrazone) were the major products no pentose was detected. 

Furfural is easily produced from pentoses rich biomass by cyclodehydration. Various acidic reaction conditions have been applied [110, 111]. The precise order of the different dehydration steps is not known for certain. A plausible mechanism is depicted in Scheme 20 [112]. The worldwide production is currently about 300,000 tons per year. Thus, furfural is a low cost, polyfunctional substrate for the production of numerous bulk and fine chemicals. 

Professor Hurd s interests, although confined to organic chemistry, were most eclectic. His own graduate work had involved him in nitrogen chemistry, and he maintained a lively research interest in that field. He was also deeply interested in thermal transformations— his treatise on the pyrolysis of carbon compounds had been recently published—and in reaction mechanisms, such as that of the acid-catalyzed dehydration of pentoses. At that time, Hurd also taught the special, graduate course in... 

Model Systems. Model system studies have investigated the reaction of sulfiir sources and pentoses and have established that furfuryl mercaptan may be a significant product. The study of the cysteine/xylose model system under aqueous conditions at ISO C and pH 5 led Tressl to propose a mechanism for the production of furfuryl mercaptan via dehydration and reduction of the 3-deoxpentosone which is a known Maillaid reaction product (8). 

In strongly acidic solutions, ascorbic acid can decarboxylate and, like other sugars, dehydrate. In model experiments almost quantitative amounts of carbon dioxide and furan-2-carbaldehyde are produced. The reaction mechanism is shown in Figure 5.30. An important product is 3-deoxy-L-threo-pentos-2-ulose, also known by its trivial name as 3-deoxy-t-xylosone, which plays an important role in the MaiUard reaction of pentoses. 

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