Oxidized xylan derivatives

Thermoplastic xylan derivatives have been prepared by in-hne modification with propylene oxide of the xylan present in the alkaline extract of barley husks [424,425]. Following peracetylation of the hydroxypropylated xylan in formamide solution yielded the water-insoluble acetoxypropyl xylan. The thermal properties of the derivative quahfy this material as a potential biodegradable and thermoplastic additive to melt-processed plastics. Xylan from oat spelts was oxidized to 2,3-dicarboxyhc derivatives in a two-step procedure using HI04/NaC102 as oxidants [426]. 

Other workers140 have also examined the products derived by hydrolysis of periodate-oxidized xylan. Both wheat straw and com cob xylan, after oxidation and hydrolysis, yield small amounts of L-arabinose and D-xylose. These sugars are obtained even after the xylans have been subjected to extended periods of oxidation. It is concluded that the D-xylose constituted branch points in the xylan. Likewise, the L-arabi-nose molecules must not have been terminal units in a xylan chain but must have been either interior units in the xylan molecule or have constituted an araban-like polysaccharide which is in combination or admixture with the xylan polysaccharide. 

Jain, Rajesh K., M. Sjdstedt, and W. G. Glasser. 2001. Thennoplastic xylan derivatives with propylene oxide. Cellulose 7(4), 319-336. 

It is apparent from the foregoing that a more specific method for the determination of xylan would be desirable. To this end, the determination of xylose, after acid hydrolysis of the polysaccharide material, has been attempted. Xylose may be oxidized to xylonic acid which can be precipitated with cadmium bromide as the double salt, but the precipitation is not quantitative. Xylose forms an insoluble, crystalline di-O-benzylidene dimethyl acetal which permits identification in the presence of other sugars, but the necessity for anhydrous reaction conditions precludes the adaptation of this method to ordinary analysis. AVise and Ratliff prepared this derivative of both d- and L-xylose, as well as analogous derivatives from other aromatic aldehydes, and concluded that, with either the di-O-benzylidene or the di-O-(p-isopropylbenzylidene) dimethyl acetal, an excellent, highly specific, qualitative test was available for d- or n-xylose. 

Dehydrogenation with iV-bromosuccinimide transforms the formazans of oxidized polysaccharides into their tetrazolium derivatives. In an alkaline medium, these colorless or cream-colored compounds readily revert to the bright-red formazans, as with the simple tetrazolium compounds. The formazans of oxidized, soluble polysaccharides (for example, dextran, or soluble starch) yield soluble tetrazolium derivatives, and those of oxidized, insoluble polysaccharides (for example, cellulose, or xylan) give insoluble compounds. 

The principal volatile products formed on pyrolytic decomposition of D-xylan, xylotriose, xylobiose, and methyl /S-D-xylopyranoside were 3-hydroxy-2-penteno-1,5-lactone and 2-furaldehyde. These products appear to be formed in independent reactions, the lactone being derived from non-reducing D-xylopyranosyl residues. Oxidation of ethyl 4,6-di-0-acetyl-2,3-dideoxy-a-D-eryt/iro-hex-2-eno-... 

Table 5. Chemical shift assignments for the C-NMR spectra of xylan and its bromine-oxidized derivative (25).

---