Sugars acetolysis

Acetolysis of LXXIV gives, as might be expected, a mixture of the acetates of n-mannose and D-idose and as the dianhydro sugar has a free hydroxyl on C2, it should prove to be a source of 2-substituted n-mannose or D-idose and of 2,4-derivatives of n-mannose and 2,3-derivatives of D-idose. 

Depolymerization of the permethylated carbohydrate is achieved by hydrolysis with acid. Under these conditions, the amino sugar residues are N-deacetylated, and the aminohexosidic linkages become resistant to hydrolysis. Stellner and coworkers29 showed that, when the acid degradation is conducted in 95% acetic acid, the amino sugar residues are also liberated, and can be analyzed by the methylation technique.29 Therefore, acetolysis followed by acid hydrolysis is now commonly used, as it allows the analysis both of hexose and hexosamine residues. 

The partially methylated monosaccharides obtained on depolymerization of the permethylated sample are preferably analyzed as acetates by g.l.c.-m.s., as shown by Bjomdal and coworkers.41,42 The neutral sugars and the amino sugars obtained in acetolysis-acid hydrolysis are reduced, and acetylated for the analysis, and the amino-hexitol and the neuraminic acid residues are acetylated after methanolysis. Identification with the aid of g.l.c.-m.s. has been described for all of the common components of protein- and lipid-linked glycans and oligosaccharides from animal cells, namely, the neutral sugars,41-43 hexitols,44 hexosamines,29,43,45,46 aminohexitols,31,32 and neuraminic acids.33,34,47... 

It is intended that the present Chapter shall be complementary to Haines s article in this Volume8 that deals with the selective deprotection of protected sugars. The reader interested in all aspects of the hydrolysis, the alcoholysis, the acetolysis, and the isomerization of cyclic acetals pf sugars should refer to Haines s article. A review has also been published concerning the formation and migration of cyclic acetals of carbohydrates.80... 

Mycosamine differs from most of the other amino sugars isolated from antibiotic substances in not being V-methylated.86 The tetraacetate XXVI of mycosamine (XXV) can be isolated by acetolysis of nystatin or of the antifungal antibiotic substance amphotericin B, and it yields the acetamido derivative XXVII on catalytic deacetylation. 

Since acid hydrolysis of methyl a-D-altroside leads principally to D-altrosan, it became necessary to resort to other devices in order to complete the transformation to the free sugar. The pioneer work of Robertson and his collaborators had established a clear route from D-glucose to many D-altrose derivatives. The final steps, as described by Richtmyer and Hudson, are the acetolysis of methyl o-o-altroside (XXXVIII), or more simply of its benzylidene derivative (XXXVII), followed by catalytic deacetylation of the pentaacetyl-a-D-altrose thus produced. In this way crystalline D-altrose becomes readily available. 

The rate of acetolysis of 1,6-anhydro sugars is dependent upon the electron-withdrawing ability of the protecting groups and, in particular, of the group at C-2. Thus, ester protecting groups lower the rate of acetolysis compared to their benzylated counterparts [41]. 

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