Furanose form

D-fructose, C HijOo. Crystallizes in large needles m.p. 102-104 C. The most eommon ketose sugar. Combined with glucose it occurs as sucrose and rafftnose mixed with glucose it is present in fruit juices, honey and other products inulin and levan are built of fructose residues only. In natural products it is always in the furanose form, but it crystallizes in the pyranose form. It is very soluble in... 

Aldoses incorporate two functional groups C=0 and OH which are capable of react mg with each other We saw m Section 17 8 that nucleophilic addition of an alcohol function to a carbonyl group gives a hemiacetal When the hydroxyl and carbonyl groups are part of the same molecule a cyclic hemiacetal results as illustrated m Figure 25 3 Cyclic hemiacetal formation is most common when the ring that results is five or SIX membered Five membered cyclic hemiacetals of carbohydrates are called furanose forms SIX membered ones are called pyranose forms The nng carbon that is derived... 

The two stereoisomeric furanose forms of d erythrose are named a d erythro furanose and p d erythrofuranose The prefixes a and p describe fhe relative configu ration of fhe anomeric carbon The configurafion of fhe anomeric carbon is compared wifh fhaf of fhe highesf numbered chiralify cenfer m fhe molecule—fhe one fhaf defer mines whefher fhe carbohydrafe is d or l Chemisfs use a simplified informal version of fhe lUPAC rules for assigning a and p fhaf holds for carbohydrates up fo and mclud mg hexoses... 

Generating Haworth formulas to show stereochemistry m furanose forms of higher aldoses is slightly more complicated and requires an additional operation Furanose forms of D ribose are frequently encountered building blocks m biologically important organic molecules They result from hemiacetal formation between the aldehyde group and the C 4 hydroxyl... 

As viewed m the drawing a 120° counterclockwise rotation of C 4 places its hydroxyl group m the proper position At the same time this rotation moves the CH2OH group to a position such that it will become a substituent that is up on the five membered ring The hydrogen at C 4 then will be down m the furanose form... 

Because six membered rings are normally less strained than five membered ones pyranose forms are usually present m greater amounts than furanose forms at equilib rium and the concentration of the open chain form is quite small The distribution of carbohydrates among their various hemiacetal forms has been examined by using H and NMR spectroscopy In aqueous solution for example d ribose is found to contain the various a and p furanose and pyranose forms m the amounts shown m Figure 25 5 The concentration of the open chain form at equilibrium is too small to measure directly Nevertheless it occupies a central position m that mterconversions of a and p anomers and furanose and pyranose forms take place by way of the open chain form as an inter mediate As will be seen later certain chemical reactions also proceed by way of the open chain form... 

Ketoses like aldoses exist mainly as cyclic hemiacetals In the case of d ribulose furanose forms result from addition of the C 5 hydroxyl to the carbonyl group... 

Most carbohydrates exist as cyclic hemiacetals Those with five membered rings are called furanose forms those with six membered rings are called pyranose forms... 

Furanose form (Section 25 6) Five membered nng ansing via cyclic hemiacetal formation between the carbonyl group and a hydroxyl group of a carbohydrate... 

Cyclic herniacetal formation is most common when the ring that results is five- or six-rnernbered. Five-rnernbered cyclic herniacetals of carbohydrates are called furanose forms six-rnernbered ones are called pyranose forms. The ring carbon that is derived... 

The two stereoisomeric furanose forms of D-erythrose ae naned a-D-erythro-furanose and p-D-erythrofuranose. The prefixes a and p describe the relative configuration of the anorneric cabon. The configuration of the anorneric cabon is cornpaed with that of the highest numbered chirality center in the molecule—the one that determines whether the cabohydrate is d or l. Chemists use a simplified, informal version of the lUPAC rules for assigning a and p that holds for ca bohydrates up to and including hexoses. 

Write Haworth formulas corresponding to the furanose forms... 

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. 

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