1,2-Anhydro sugars, glycoside formation

Comprehensive reviews of the chemistry of both the glycals [105-107] and anhydro sugars [108,109] have appeared in the recent literature. This chapter will attempt to summarize what is known about their preparation and reactivity in the formation of the glycosidic linkage and highlight recent examples to emphasize practical and strategic considerations in the choice of glycosyl donors. 

An opposite reaction to the acid-catalyzed hydrolysis is the above-mentioned reversion. Acids can also catalyze the formation of anhydro sugars (see Section 2.3.4). Reversion tends to result in formation of (1—>6)-glycosidic bonds. The degradation of pentoses and uronic acids into furfural and of hexoses into hydroxymethylfurfural, levulinic, and formic acids are also important acid-catalyzed reactions, which, however, require concentrated acid and higher temperatures (Fig. 2-31). 

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. 

Acids catalyze the reversible reaction that results in the formation and hydrolysis of glycosides. Also possible are acid-catalyzed epimerization and dehydration reactions that result in the formation of furfural derivatives and anhydro sugars. This article deals only with the reaction which predominates in dilute solutions of both glycoside and acid at temperatures of 1(X)° or below, that is, hydrolysis. 

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