Reducing sugars furanose form

In fact, it has been found (52) that in unbuffered solution, at room temperature, authentic 2-deoxy ribose 5-phosphate reduces more than 4 molar equivalents of periodate, but. that there is no noticeable slowing down of the reaction rate after the reduction of the first molar equivalent. This may be owing to the fact that only the aldehydo form (76) of 2-deoxy ribose 5-phosphate has a free vicinal diol group as the acyclic 2-deoxy ribitol 5-phosphate reduces one molar equivalent of periodate quite fast (58), it is probable that the time-curve of periodate uptake by the phosphorylated sugar reflects the rate of formation of the aldehyde form from the furanose form. 

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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