Pentosan synthesis

Pure xylan is not employed in industry. but crude xylan or pentosans are of industrial importance. Xylan has been proposed as a textile size but is not employed as yet for this purpose.130 Perhaps the largest use of pentosans is in their conversion to furfural, which has many applications and serves as the source of other furan derivatives. At the present time, large quantities of furfural are used in the extractive purification of petroleum products, and recently a large plant has been constructed to convert furfural by a series of reactions to adipic acid and hexamethylene-diamine, basic ingredients in the synthesis of nylon. In commercial furfural manufacture, rough ground corn cobs are subjected to steam distillation in the presence of hydrochloric acid. As mentioned above, direct preferential hydrolysis of the pentosan in cobs or other pentosan-bearing products could be used for the commercial manufacture of D-xylose. 

The two most important natural pentoses, 1 -arabinose and 1 -xylose, occur in nature as polymeric anhydrides, the so-called pentosans, viz. araban, the chief constituent of many vegetable gums (cherry gum, gum arabic, bran gum), and xylan, in wood. From these pentapolyoses there are produced by hydrolysis first the simple pentoses which are then converted by sufficiently strong acids into furfural. This aldehyde is thus also produced as a by-product in the saccharification of wood (cellulose) by dilute acids. Furfural, being a tertiary aldehyde, is very similar to benzaldehyde, and like the latter undergoes the acyloin reaction (furoin) and takes part in the Perkin synthesis. It also resembles benzaldehyde in its reaction with ammonia (p. 215). 

Tetrahydrofuran (THF), the cyclic monomer used for the synthesis of polytetramethyleneglycols (see Chapters 7.1-7.3) by cationic ring opening polymerisation, was produced in the earlier stages of this technology from furfurol. Furfurol results from the acid hydrolysis of pentosanes existing in many agricultural wastes (corn on the cob, straw and so on). 

Xylitol (Figure 17.2), a polyol starter for rigid polyether synthesis, having five hydroxyl groups, is produced by the hydrogenation of the same pentosans used for THF synthesis [8]. By propoxylation of xylitol excellent rigid polyether polyols (see Chapter 4.1) are obtained. 

Furaldehyde, an important intermediate in the synthesis of a number of natural products and pharmaceutical reagents, is prepared industrially by treating plant residues rich in pentoses (pentosan) with dilute sulphuric acid. Steam distillation is then used to obtain 2-fiiraldehyde as a pure substance for further functionalization. 

Besides stimulating the production and release of certain hydrolases, GA can also retard enzyme activity. Consistent with the observation that GA induces degradation of aleurone cell walls is the observation that GA inhibits the synthesis of pentosan (arabinose and xylose) components of the wall. Between 4 and 16 h after GA addition to isolated aleurone tissue there is a marked decline in pentosan biosynthesis, in part due to the reduced activity of a membrane-bound arabinosyl transferase, and perhaps also of xylosyl transferase [62]. Changes in such membrane-bound enzymes could reflect a shift in the role of membranes from the production of cell wall components to the production, or packaging of hydrolases. 

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