Necessity for Ring Structures

RING STRUCTURES OF THE SUGARS A. Necessity for Ring Structures 

Soon after the formulation of glucose as a polyhydroxy aldehyde and of fructose as a polyhydroxy ketone, it became evident that the open-chain formulas would not account for all of the reactions of these sugars. Thus, the sugars give a negative test with the Schiff reagent (fuchsin and sulfu-rous acid) under the usual conditions of test although, under milder conditions, positive results are obtained 16). 

The aldehyde and ketone structures also do not account for the change of optical rotation which may be observed for the freshly prepared aqueous solutions of many sugars. This phenomenon, now called mutarotation, was observed by Dubrunfaut in 1846 for glucose solutions. 

When the hydroxyl groups of glucose are esterified by treatment with acetic anhydride and a catalyst, two isomeric pentaacetates are formed. Similarly, isomeric methyl glucosides are formed by treatment of glucose with methanol and hydrogen chloride. The existence of two glucosides 17) and two pentaacetates 18) cannot be predicted on the basis of the aldehyde formula, a conclusion stated by Fischer in the case of the methyl glucosides, which he discovered, and even earlier by Colley and Tollens. 

The isolation of crystalline isomers of the sugars provided additional evidence for the inadequacy of the aldehyde formulas. As early as 1856, two crystalline modifications of lactose were prepared by Erdmann 19), the forms which are now designated a- and iS-lactose he discovered their mutarotations to the common equilibrium rotation. Tanret 20) in 1895 reported the isolation of three forms of glucose which he described as a-, 

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