Proof of Ring Structure

Subsequent to the proposal of the ring structures for the sugars and derivatives, acceptance by carbohydrate chemists 21) gradually took place. However, it was not until the period 1920 to 1930 that conclusive proof could be offered for the positions of the rings. Prior to this work, the rings usually were considered to be of the 1,4 type shown above in the Tollens formula, i.e., with the ring formation between carbons 1 and 4. This type of structure was based mainly on an analogy with the acid series for which it was known that y-hydroxy acids could be converted to inner esters (lactones) which have the 1,4- or 7-structure. 

Methods now are available for the unequivocal determination of the ring structures of the glycosidic derivatives of the sugars. The glycosides are made by condensing the sugars with alcohols in the presence of acids. (For a detailed discussion of the preparation of glycosides and of the details of the determination of the structures, see Chapter IV.) 

The possible structures for methyl a-D-glucoside are given in formulas (I) to (V). The brackets indicate the adjacent hydroxyl groups. 

The ordinary methyl a-D-glucoside consumes tw o moles of periodic acid, and no formaldehyde is produced. Hence, the structure must be that represented in (IV), which has a 1,5 oxygen bridge. 

The evidence given above and explained in more detail later (p. 212) confirmed in most instances the structures obtained by the earlier methyla-tion-oxidation studies. The periodic acid oxidation method is used widely because of its simplicity. As a result of the application of the methylation-oxidation technique and of the periodic acid method, it is knowm that the most common ring present in the glycosides is of the six-membered type connecting carbon atoms 1 and 5. However, rings formed between the 1- 

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