744. The constitution of a wheat-straw hemicellulose

Abstract

A wheat-straw hemicellulose, containing uronic acid residues (ca. 3%), gave on hydrolysis xylose and arabinose in the ratio of 11 : 1. Hydrolysis of the methylated polysaccahride yielded 2 : 3 : 5-tri-O-methyl-~-arabinose, 2 : 3 : 4-tri-O-methyl-~-xylose, 2 : 3-di-O-methyl-~-xylose, %O-rnethyl-~-xylose, and 2 : 3 : 4-tri-U-methyl-~-glucuronic acid, together with some 3-Umethyl-2-0-(2 : 3 : 4-tri-~-methy~-~-g~ucuronosy~)-~-xy~ose. The structure of the polysaccharide is discussed in the light of these and other results.PREVIOUS structural investigations have indicated that a variety of closely related polysaccharides constitute wheat-straw hemicellulose. Adams and Bishop showed that the hemicellulose contains a backbone of 1 : 4-linked p-D-xylopyranose residues, to which are attached through position 3 a number of L-arabofuranose and D-glucopyruronic acid residues as single-unit side-chains. Earlier studies in these laboratories showed that it was possible to fractionate the hemicellulose and to isolate a xylan, devoid of arabinose residues, but still containing uronic acid residues as an integral part of the molecular structure. These investigations were carried out on hemicellulose fractions isolated by alkaline extraction from straw which had previously been treated with acidified sodium chlorite solution to remove lignin. In view of the possibility of degradation of polysaccharides during chlorite delignifi~ation,~ we have now examined the hemicellulose extracted directly with alkali from wheat straw rather than from the holocellulose.The hemicellulose was extracted from straw, previously extracted with benzene and methanol, by the procedure used in the extraction of beechwood hemicellulose A,5 the structure of which we have previously studied.6 The polysaccharide contained uronic acid residues (ca. 3%) and yielded on hydrolysis xylose and arabinose in the ratio of 11 : 1. Graded hydrolysis liberated arabinose preferentially, suggesting that this sugar occurred in the furanose form, but selective removal of these labile residues to yield a xylan devoid of arabinose residues could not be achieved without extensive degradation of the molecule, as appreciable quantities of xylose and xylose-containing oligosaccharides were released before all the arabinose residues had been removed. On more vigorous hydrolysis, a resistant acidic fraction was isolated ; reduction of the derived methyl ester methyl glycoside with potassium borohydride , followed by hydrolysis, yielded 4-O-methylglucose, glucose, and xylose, indicating the presence in the polysaccharide of residues of glucuronic acid (partly as the 4-methyl ether).The xylan was converted into its fully methylated derivative, hydrolysis of which yielded the following methylated sugars, characterised by the formation of crystalline derivatives : 2 : 3 : 5-tri-O-methyl-~-arabinose, 2 : 3 : 4-tri-O-methyl-~-xylose, 2 : 3-di-O-methyl-~-xylose, and 2-O-methyl-~-xylose (in the approximate ratio of 3 : 1 : 41 : 4). Small quantities of 3-O-methyl-~-xylose were also present ;...

Extraction, fractionation, and characterization of structural polysaccharides from wheat straw.

“…Aspinall and Meek (1956) also stated that the same sugars were present in wheat straw hemicellulosic fractions A and B. However, some authors have indicated that hemicellulosic fraction A of wheat straw contained only three neutral sugar residues (xylose, arabinose, and glucose) and glucuronic acids, whereas hemicellulosic fraction B possessed four neutral sugar residues (xylose, arabinose, glucose, and galactose) and glucuronic acids (Summerell and Burgess, 1989;Bishop, 1953;Aspinall and Meek, 1956;Dekker, 1979). This variation in findings may reflect the influence of factors such as soil, climate, and method of analysis on the sugar content of wheat straw hemicellulosic fractions.…”

Organic chemistry

“…21 Lithium aluminium hydride in the presence of aluminium chloride reduces unsymmetrical epoxides to the less highly substituted alcohol 22 and converts @-unsaturated ketones directly into hydrocarbons, -CO*C:C-+ -CH,*C:C-. 23 The latter reaction will clearly be used with compounds of this type as an alternative to Wolff -Kishner or Clemmensen reduction. The same reagent, in a similar reduction, yields a mixture of safrole and isosafrole from the allylic alcohol, 3 : 4-methylenedioxycinnamyl alcohol : 24…”

“…the diagnosis of meso-ionic structures in the tetrazole series; thus the meso-ionic compound 5-imino- ~t r u c t u r e s . ~~ On being heated, the ethoxycarbonyl compound (20) cyclises to a product (21), which can also be satisfactorily formulated as the betaine (22) .m cc-Arylazothioalkanoic acids (23) have been treated with acetic anhydride and a tertiary base, to give a series of very stable 3-aryl-4-oxo-lthia(SIV)-2 : 3-diazolines. 31 The unusual structure (24) proposed for these products involves a dsp2-hybridised sulphur atom ; 32 however, the alternative meso-ionic formulation (26) has not been convincingly disproved.…”

“…This is in contrast to the use of alkyl halides 70 where the competitive N-alkylation, though not usually extensive, is irreversible. The Michaeltype reaction is therefore more general and gives yields of monoalkylated products (23) in the range 40430% from cyclic and acyclic ketones and aldehydes. The structure of intermediate (22) requires the presence of at least one a-methylene group in the original carbonyl compound.…”

“…~~ Phenols.-The tautomerim of phenols has been reviewed.26 1 : 2 : 3 : 4Tetrahydroxybenzene is converted by acidification of an alkaline solution in absence of air into a diketonic form (19), the structure of which follows from its methylation and oxidation to 2 : 3-d.knetho~ybenzoquinone.~~ Phloroglucinol on treatment with potassium cyanide and sulphuric acid gives a red cyanohydrin (20) which with concentrated hydrochloric acid undergoes dehydration and hydrolysis to 3 : 5-dihydroxybenzoic acid.28 Spectroscopic data favour the trioxime structure (21) for the product obtained from phloroglucinol and hydro~ylamine.2~ tained from phloroglucinol by the action of heat or Phloroglucide, obacids, is shown by synthesis to be represented by formula(22) 30. The cyclohexadienone(23) has been obtained by alkylation of sodium 2 : 6-dimethylphenoxide with ally1 bromide in benzene.31 3 : 4 : 3' : 5'-Tetra-and 3 : 4 : 5 : 3' : 5'-penta-24 M. Pailer, L. Belohlav, and E. Sirnonitsch, Monntsh., 1958, 87, 249.…”