Epimerization sugars

The transformation of sulfonic esters into anhydro sugars has been reviewed extensively. Reaction of 2-0-sulfonyl derivatives of free sugars with sodium methoxide has since received attention initial formation of a 1,2-anhydro sugar is followed by production of a methyl glycoside of the C-2 epimeric sugar, in which the hydroxyl group on C-2 is trans to the methoxyl group on C-1. 

All of these experiments strongly indicate that the alkaline hydrolysis of the sugar orthoacetates is a nucleophilic displacement reaction. Any one of the three carbon atoms of the five-membered ring may be attacked. Since the hydroxyl ion must approach the tetrahedron of the attacked carbon atom in the direction of the center of the face opposite the vertex occupied by the group to be displaced, it is evident that the reaction must be accompanied by the inversion of configuration of the attacked carbon atom. It is quite certain that the attack does not take place on carbon atom 2, because in such a case the reaction product would be the epimeric sugar derivative. Therefore, either carbon atom 1 or the central carbon atom of the orthoester group is attacked. In the first case, the reaction is as follows. 

Extension of the carbon chain of an aldose from the carbonyl by one unit at a time can be carried out fairly readily by the Kiliani reaction. A cyanhydrin is formed by addition of cyanide ion, followed by reduction and hydrolysis (in either order) historically, the sugar was unprotected, and the cyanohydrin was hydrolysed to the sugar lactone, and then reduced with sodium amalgam (Figure 1.4). Because a new asymmetric centre is formed, two epimeric sugars result (epimers are diastereomers that differ in the configuration of only one carbon). 

There are many different isomers (e.g., the epimeric sugars glucose and galactose) that make separation difficult. This problem is even nowadays... 

Pyrophosphate Uridyl Transferase. The over-all epimerase reaction is composed of three individual reactions. The sugar phosphates are not converted directly into each other, but are first incorporated into uridine nucleotides. Epimerization occurs in the hexose component of the UDP-sugar, which then is cleaved to yield the epimerized sugar phosphate. The synthesis of both UDPG and the corresponding galactose derivative, UDPGal, are catalyzed by enzymes called uridyl transferases. A pyrophosphate (PP) uridyl transferase has been studied in yeast and in several mammalian tissues. It catalyzes the reaction... 

When the hemiacetal hydroxyl of a 2-0-tosyl or 2-0-mesyl aldose is unsubstituted, titration with dilute alkali at room temperature eliminates the sulfonic ester grouping and provides the related epimeric sugar in good yield 6 a, 226a), The epimerization presumably proceeds through an intermediate 1,2-epoxide. 

The glycals, first reported by Fischer and Zach, were extensively investigated by Bergmann and Schotte. They are important intermediates for the interconversion of epimeric sugars and for the preparation of 2-deoxy-aldoses (Chapter II). 

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