Ring Structures of the Sugars

Two models for the mechanism of the isomerization have been proposed. The first model was originally proposed because of similarities between enzymes involved in the activation of C-H bonds next to a carbonyl function (Fig. la). Stereochemical considerations were used to suggest that the ring structure of the sugar is opened to... 

The ring-structure of the sugar moiety of the desoxyribo-sylpurines is still unknown by analogy with that of thymidine, and of the ribasyl-purines, it is probably furanose. 

The ring structure of the sugar of the purine ribonucleosides was shown by chemical methods to be furanoid by Levene and Tipson. These studies have been adequately reviewed by Tipson. The fact that 2-deoxy-inosine and 2-deoxyguanosine do not affect the conductivity of boric acid solutions indicates the absence of a m-1,2-glycol system and, coupled with the anhydronucleoside formation mentioned below, provides evidence that the purine 2-deoxyribonucleosides are also in the furanoid form. 

RING STRUCTURES OF THE SUGARS A. Necessity for Ring Structures... 

While the pyranose and furanose ring forms of the sugars dominate the carbohydrate literature, the uncommon septanose ring forms have long intrigued sugar chemists, with particularly notable contributions by Stevens in Australia. The chapter in this volume from Saha and Peczuh (Storrs, Connecticut) provides a comprehensive overview of the subject from both the synthetic and structural viewpoints and presents a detailed analysis of the conformational behavior of these ring forms. It may be noted... 

The stabilities of the hemiacetal ring-structures of the reacting sugar are of obvious importance. Their participation in mutarotation... 

The various types of carbohydrates considered in this chapter are shown in formulas I-XVIII. The two ring forms (II and III) of the aldoses are shown because careful oxidation can produce the lactones X and XI directly without breaking the oxygen bridge. In most cases, however, the products of oxidation have been isolated as salts of aldonic acids (IX) thus the nature of the initial oxidation product, either a lactone having the same ring structure as the sugar, or possibly the free... 

The 4,6-disubstituted deoxystreptamine aminoglycosides can be divided into kanamycins (Table 6) and gentamidns (Table 8), depending on the structure of the sugar moiety linked to the 6-po ition of the deoxystreptamine ring. 

For example, the ring systems of the sugar orthoesters 145-148 (Table 8.4) prepared from the sugar lactones 169-172 and 7 75 (Fig. 8.42) had the same structure. 

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