Arabinose acidic conditions

L-arabinose molecules combine together in the furanose form to produce an araban. Since enzymes capable of hydrolysing pectic acid to n-galacturonic acid are known to be present in certain plant juices, and since the comparatively low temperature and slightly acid condition of plant materials would tend to favor the transformation of arabinose into the furanose form, the occurrence of the furanose structure in the polysaccharide derived from arabinose is not altogether unexpected. 

Although small proportions of other products are formed when D-xylose is exposed to rather high acid concentrations, arabinose, lyxose, and ribose form considerably more of alternative products (generally reductic acid) than of 2-furaldehyde under these conditions. Reductic acid (2,3-dihydroxy-2-cyclopenten-l-one, 47) has been detected as a product after acid exposure of D-xylose or its major dehydration product, 2-furalde-hyde. Further work performed with D-[l- C]xylose and [a- C]2-fural-dehyde showed that reductic acid having identical label distribution was obtained from both starting materials. This indicated that a common primary source was involved, probably 2-furaldehyde, as it is readily formed from D-xylose under acidic conditions. 

As D-xylose and L-arabinose are 4-epimers, both, as well as mixtures of them (as provided by hemiceUulose), can be used to prepare enantiomerically pure synthetic intermediates the center C(4) of which is deoxy or sp -hybridized (alkenes, ketones). For instance, treatment of pure o-xylose with acetone under acidic conditions, followed by selective C(5) benzoylation, benzylation of the 3-alcoholic moiety, hydrolysis of the benzoate, and iodination furnishes the 5-iodo derivative... 

Insoluble dietary fiber (40 g L- ) Wheat bran Sunzymes (0.4%, w/w) Degree of polymerization of 2-7 and the ratio of arabinose to jg lose of 0.27, and XOS was strongly resistant to lower acidic conditions... 

As an extension of their methodology, Armstrong and coworkers have synthesized 2-acetamido-2-deoxyhexose via the use of tethered internal nucleophiles. Thus, theU-4CR of aldehyde 9k, derived from D-arabinose, p-methoxybenzylamine (6i), acetic acid (10b), and Im, provided enamide 127 in a high yield as a 3.4 1 mixture of inseparable diastereomers at the a-carbon. In the presence of mild acidic conditions (HCI generated from AcCl), cleavage of the isopropylidene PG and protonation of the enamide followed by miinchnone 128 formation takes place. Internal nucleophilic attack by the secondary alcohol (attack by the primary alcohol is unfavorable as it forms a seven-membered ring) forms the expected six-membered 2-acetamido-2-deoxy-D-mannono-5-lactone 129 (Scheme 7.38) [56]. 

Aldono-1,5-lactones and free aldonic acids react with alcohols in the presence of hydrogen chloride to give the corresponding alkyl aldonates (84). The reaction is slower with 1,4-lactones. Because esterification takes place very slowly in the absence of an acidic catalyst, aldonic acids and their lactones may be recrystallized from boiling alcohols without appreciable esterification (85). However, in some instances, alkyl esters are formed under these conditions. For example, essentially pure ethyl L-mannonate was isolated (6.4% yield) from the mother liquors of crystallization L-man-nono-1,4-lactone, obtained by Kiliani synthesis from L-arabinose (86). Similarly, repeated recrystallization from ethanol of crude 2,3,4,6-tetra-O-acetyl-D-glucono- 1,5-lactone afforded the corresponding ethyl gluconate derivative (87). 

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