Xylose selective hydrolysis

Acid hydrolysis, as described, gives a largely uncontrolled degradation. Stepwise degradation of oligosaccharides can often be achieved by the selective hydrolysis of their osazones on an acid-type ion-exchange resin. This procedure was used by Howard to confirm the structure proposed for a D-xylose trisaccharide. 

Selective removal of one isopropylidene group from a diacetal may be achieved by a variety of procedures, most of them involving protic or Lewis acids.100 Particularly common is the hydrolysis of the acetal engaging of the primary position of di-O-isopropylidene derivatives. Bhaskar et al,101 studied the selective deprotection of di-O-isopropylidene acetals derived from D-glucose, D-xylose, and D-mannose, using acid zeolites and montmorillonite K-10. When 102 was submitted to acid hydrolysis in aqueous methanol, the best yields (85—96%) for the monoacetal 105 were obtained when H-beta and HZSM-5 zeolites were employed as catalysts (Scheme 24, Table IV). HY zeolite proved to be ineffective, whereas the yield obtained for the montmorillonite K-10-catalyzed reaction was low (22%). The zeolites found most effective were then used for the hydrolysis of the diacetal 103 and 104, providing excellent yields for the desired corresponding monoacetals 106 and 107. 

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... 

L-Arabinose reacts differently than o-xylose with acetone and gives a pyrano-side instead of a furanoside. Thus an alternative route to 18 was sought that can be applied both to o-xylose and L-arabinose [59]. It starts with the selective silylation of HO-C(5) [60], then acetonide formation protects alcohols moieties at C(l) and C(2). Subsequent benzylation of HO-C(3), hydrolysis of the silyl ether, and iodination provides 18 from o-xylose and 19 from L-arabinose (Scheme 7). Zinc reduction of 19 generates enal 20, but not the reduction of 18 [61]. Thus 18 and 19 are converted first into their methyl furanosides 22. The latter are reduced with Zn into... 

Synthesis from o-xylose Clavalanine (2) was synthesized from D-xylose by conversion to 1,2-0-isopropylidene-D-xylofuranose (98), which was selectively acetylated followed by treatment with l,T-thiocarbonyldimidazole (TCDl) to furnish 99 in 85% overall yield (Scheme 11). Radical reduction of 99 followed by acid hydrolysis of the... 

Tetroses and Pentoses - 4-0- -Butyldimethylsilyl-2,3-0-isopropylidene-L-threose (1) has been prepared in seven efficient steps from o-xylose. 3,4-0-Isopropylidene-D-eythrulose (4) has been synthesized from the known tetritol derivative 2 by primary protection as the silyl ether 3, followed by Dess-Martin oxidation and desilylation. Compound 2 was derived from D-isoascorbic acid (see Vol. 22, p. 178, refs. 9,10). In a similar reaction sequence, the enantiomer 5 has been obtained from L-ascorbic acid. The dehomologation of several di-0-isopropylidenehexofuranoses e.g., 6- 7) has been carried out in two steps without intermediate purification, by successive treatment with periodic acid in ethyl acetate, followed by sodium borohydride in ethanol. Selective reduction of 3-deoxy-D-g/jcero-pentos-2-ulose (8) to 3-deoxy-D-g/> cero-pent-2-ose (9) has been achieved enzymically with aldose reductase and NADPH." 4-Isopropyl-2-oxazolin-5-one (10) is a masked formaldehyde equivalent that is easily converted to an anion and demasked by mild acid hydrolysis. One of the three examples of its use in the synthesis of monosaccharides is shown in Scheme 1. ... 

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