Acetolysis glycosides

Glycosidic linkages may also be cleaved in other solvents, such as methanol. In this case, water is rigorously excluded, so that the elements of a molecule of methanol are added across the glycosidic linkage to afford the methyl glycosides. Methanolysis is usually catalyzed by the addition of dry hydrogen chloride to the methanol. Other solvents, such as acetic anhydride-acetic acid (acetolysis) and formic acid (formolysis), are occasionally used for special purposes. 

Formolysis and acetolysis are not common methods for cleavage of glycosidic linkages. They do have some unique applications, however. For instance, methylated polysaccharides are not generally soluble in hot water, and consequently, hydrolysis is best preceded by formolysis under these circumstances. For example, 5 mg of methylated polysaccharide is dissolved in 3 mL of 90% formic acid, and the solution is kept for 2 h at 100°. The formic acid is removed by evaporation at 40°. The residue is dissolved in 1 mL of 250 mM sulfuric acid and the solution is heated for 12 h at 100°, cooled, the acid neutralized with barium carbonate, the... 

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). 

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). 

One difficulty we encountered in the approach described above was liberation of the 3-carbon fragment from the auxiliary. The glycosidic linkage resisted acid hydrolysis and a rather severe acetolysis followed by basic peroxide oxidation to degrade the carbohydrate moiety had to be performed. A second approach to the use of glucose as an auxiliary is illustrated in scheme 11. 

C. J. Lawson and D. A. Rees, Carrageenans. Part VI. Reinvestigation of the acetolysis products of X-carrageenan, revision of the structure of a-l,3-galactotriose , and a further example of the reverse specificities of glycoside hydrolysis and acetolysis, J. Chem. Soc. C (1968) 1301-1304. 

Many rearrangement reactions are initiated by BF3 complexes in the presence of acetic anhydride, including the synthesis of functionalized and optically active pyrans from (+)-(/ ,R)-diethyl tartrate (Eq. 41) [74]. Other examples include the conversion of bicyclic into monocyclic [75] and tricyclic into bicyclic systems [76], the acetolysis of glycosides resulting in the formation of the fully acetylated acyclic derivative [77], and the unusual migration of nitrogen in the dienone-phenol rearrangement of an N-methoxy-/ -lactam [78]. 

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