Disaccharides acetolysis

The highly polymerised compound cellulose is broken down into its octa-acetyl disaccharide cellobiose by acetolysis (Franchimont, Skraup, Ost). The bioses belong to the important group of glucosides. 

Systematic studies of the acetolysis of disaccharides have been performed.57-803 Lindberg81,82 showed that anomerization and acetolysis of acetylated glycosides is retarded by the presence of electronegative groups in the aglycon. A mechanism involving an acyclic intermediate (20) was proposed for these reactions (see Scheme 2). 

In early work [165] on the synthesis of the pentasaccharide (236), the azide (237) was condensed with (238) [an intermediate in the preparation of (237)]in the presence of silver perchlorate and polyvinylpyridine to give the a-linked disaccharide (239) in 60 % yield and this on acetolysis gave the disaccharide (240) which contains the potential terminal disaccharide unit of the Forssman antigen. Compound (240) was converted into the glycosyl bromide with titanium(IV) bromide under carefully controlled conditions [182] and condensed with l,6-anhydro-2,4-di-0-benzyl-D-galacto-pyranose in the presence of silver carbonate to give the potential terminal trisaccharide (241) of the Forssman antigen. 

For the synthesis of the D-C portion, two different concepts were followed either by modification of laminaribiose (166) [89] or by a stereospecific P, 1 ->3-glycosylation [20]. Laminaran is isolated from seaweeds or from Poria cocos Wolf, degraded by selective acetolysis, and the lower oligomers separated by preparative HPLC [90]. Following acetylation, the heptaacetyl laminaribiosyl bromide is prepared and transformed into the disaccharide glycal 167 by the classical approach in 93 % yield. The 2-deoxy-2-iodo-a-glycoside is formed by application of the NIS procedure after deprotection and subsequent 4,6-0-benzylidenation, the precursor 168 for the radical formation of the 6,6 -dibromo-6,6 -dideoxy derivative is at hand. This compound may be further reduced to methyl-3-0-(P-D-chinovosyl)-a-D-olivoside (169). 

Acetylation. Rapid peracetylation of carbohydrates can be effected in the presence of iodine. With more iodine and longer reaction times the selective acetolysis of primary benzyl ethers is achieved. Perbenzyl ethers of mono- and disaccharides undergo group exchange (OBn —> OAc) at the primary carbon atoms on treatment with ACjO-HOAc-ZnClj. 

Hydrolysis by methanolic hydrochloric acid leading to methyl glycosides would be less destructive but, in the worst case, a monosaccharide could be found in the product of methanolysis in four different chemical forms, the four methyl glycosides. Acetolysis, that is degradation by a mixture of acetic anhydride and sulfuric acid, transforms cellulose into the octoacetate of a disaccharide /3-D-Glcp-(l—>4)-Glc, but this is a preparative reaction rather than an analytical method. Acetolysis is sometimes recommended as an additional step in analytical work. 

Anhydro rings of the 1,3,6, and 3,4 types have all been opened by acetolysis. The first class of linkage can be broken specifically in the presence of a disaccharide (1— )-linkage. - As mentioned previously (see p. 16), it is also possible to cleave a 1,6-anhydro ring in the presence of benzyl ether groups by using very mild conditions. ... 

A more subtle specificity of acetolysis was suggested by Smith and Srivastava. From the results of work on the glucomannan of lies mannan, it was suggested that the linkage between two n-mannose residues is split more readily than the linkage between D-glucose and n-mannose residues. No work has since been done on simple systems to test this hypothesis. Experiments on a series of disaccharides could usefully extend our knowledge of acetolysis. 

In order to minimize such possible side-reactions as anomerization of glycosides and acetolysis of acetal structures, alternative techniques of acetylation have been studied. Whereas basic catalysis leads to severe complications in certain nitro sugar derivatives (see Section IV,2a and c pp. 124, 129), boron trifluoride etherate has proved to be an excellent agent for satisfactory acetylation of glycosides, O-benzyl-idenated glycosides, and disaccharides that contain a deoxynitro function.148,149... 

The insoluble polysaccharide was characterized by examination of the soluble, " G-labeled oligosaccharides isolated by a variety of treatments, including enzymic hydrolysis, partial hydrolysis with acid, or acetolysis. From the polymer containing D-mannose- C, several oligosaccharides were isolated having various ratios of D-mannose to D-glucose, from 1 1 to 3 or 4 1. One of these oligosaccharides was a disaccharide tentatively identified as an 0-/3-D-man-... 

Fujimoto and coworkers determined that the order of stabilities to acid hydrolysis of D-glucopyranosidic linkages is reversed under the conditions of acetolysis this may be due to steric factors affecting the acetylium ion. In consequence, acetolytic, degradative procedures have enabled secondary-linked disaccharide fragments to be obtained from a large number of native dextrans. The o-D-linked disaccharides obtained by acetolysis and deacetylation of a cross-section of dextrans are listed in Table II. The results may be seen to complement data from periodate... 

Reducing disaccharide. Isol. from the partial acetolysis of xanthan gum from Xanthomonas campestris. 

Cellulose is a carbohydrate, the structure of which may be deduced as follows. The molecular formula of cellulose is (C6HioOs)n, where n is a few thousand (see later). Hydrolysis of cellulose by boiling with concentrated hydrochloric acid yields D-glucose (II) in 95-96% yield. Thus cellulose is a polyanhydroglucose. When cellulose is subjected to acetolysis (i.e., simultaneous acetylation and hydrolysis) by treatment with a mixture of acetic anhydride and concentrated sulphuric acid, cellobiose octa-acetate is formed. Thus the structure of cellulose is based on the cellobiose unit. Cellobiose is known to be the disaccharide, 4-0-j3-D-glucopyranosyl-D-glucopyranose (III). Finally, very careful acetolysis of cellulose produces a cellotriose, a ceUotetraose and a cellopentaose and in each of these all the 1,4-links have been shown to be j3-links (from calculations of the... 

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