Furanose reducing sugars

The popular, but erroneous, concept of an aqueous solution of a reducing sugar is of one containing large, and comparable, proportions of the two pyranoses, and only small proportions of the furanoses, but the composition may, in fact, vary within wide limits. At one extreme (for example, glucose), the furanoses can be barely detected, and, at the... 

It is reasonable to assume that reducing sugars which exist in the pyranose or furanose form must revert to their open-chain aldehydo or keto forms before any significant C—H ionization occurs. This reversion is brought about by ionization of the acidic hemiacetal hydroxyl group, followed by ring opening. Subsequent carbanion formation at the carbon... 

Reeves78 has noted that reducing sugars can interfere with the direct titration of true cis-diols with lead tetraacetate in acetic acid, possibly because they can usually form furanose rings. 

Figure 2.19 Conformations of representative pyranose and furanose forms of common reducing sugars.

The cyclic hemiacetal (or hemiketal) can react with an alcohol to form an acetal (or ketal), called a glycoside. If the name pyranose or furanose is used, the acetal is called a pyranoside or a furanoside. The bond between the anomeric carbon and the alkoxy oxygen is called a glycosidic bond. The preference for the axial position by certain substituents bonded to the anomeric carbon is called the anomeric effect. If a sugar has an aldehyde, ketone, hemiacetal, or hemiketal group, it is a reducing sugar. 

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