Sugars, acetates, anomerization

Anomerization through the agency of a Lewis acid (electron acceptor) has found numerous applications for the preparation of the more stable form of a sugar acetate—usually a member of the a-D-series. Several of these applications are listed in Table III. Acetic anhydride, and mixtures of acetic anhydride and acetic acid, are the most commonly used solvents. Painter96 has shown that the rate of reaction is increased by addition of an inert solvent, carbon tetrachloride. Zinc chloride and sulfuric acid have been most frequently used as catalysts. Although these transformations have found frequent application, few theoretical studies have been made. 

It has been pointed out, with reference to Table II, that the strongly acidic conditions which are ordinarily employed to replace the Cl-acetoxy group of a sugar acetate by halogen have yielded only one of the two anomeric O-acetylglycosyl halides. In 1901, Fischer and Armstrong106 described diastereoisomers for the acetochloro and acetobromo derivatives of... 

Bonner has measured the rates of anomerization and the equilibrium constants for a number of sugar acetates (see Table XVI) and has suggested that the reactivity sequence supports a bimolecular mechanism. Actually, this sequence is very similar to that obtained for the solvolyses of poly-0-acetylglycosyl halides (see Table XIV), which are known to proceed by a unimolecular mechanism. Bonner s results would seem, therefore, to be more in accord with a unimolecular mechanism for the anomerization. 

This process is the main course for the acid-catalyzed anomerization of sugar acetates, methyl glycosides, and related compounds in non-aqueous solvents.104,244,245 260,263 In many instances, formation of the cyclic carbonium ion is assisted by participation of a neighboring acetyl group,104 but this process is not a necessary feature. Anomerization of tetra-O-acetyl-2-deoxy-a-D-arafomo-hexopyranose takes place through a cyclic carbonium ion as depicted.245[Pg.43]

Glycosides m which the anomeric carbon is part of an acetal function are not reducing sugars and do not give a positive test... 

The 3-deoxyhexoses were obtained crystalline after separation on a column of cellulose, a rather tedious operation recently separation of these sugars by fractional crystallization has been reported (41). From the major unsaturated ester 27, 3-deoxy-D-ribo- and -arabino-hexose were shown to have been produced respectively in the ratio 66 34, and from ester 28 in the ratio 24 76. Saturation of the enol grouping of the esters 27 and 28 gives, therefore, the 1,2-cis products preferentially. However, some hydrogenolysis of the anomeric acetate group accompanies simple saturation of the double bond and poses another separation problem. 

This reduction technique also applies to the benzaldehyde acetals of sugars with reduction of the benzaldehyde acetals taking place in preference to reduction of the anomeric acetal (Eq. 305).497... 

A number of conditions are available for the reduction of the anomeric acetals to the reduced sugars. Among these are the combinations Et3SiH/BF3-OEt2/ TFA483 and Et3SiH/TMSOTf,503,504 although some isomerization is found to... 

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