Carbohydrates furanoses

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... 

Aldoses exist almost exclusively as their cyclic hemiacetals very little of the open chain form is present at equilibrium To understand their structures and chemical reac tions we need to be able to translate Fischer projections of carbohydrates into their cyclic hemiacetal forms Consider first cyclic hemiacetal formation m d erythrose To visualize furanose nng formation more clearly redraw the Fischer projection m a form more suited to cyclization being careful to maintain the stereochemistry at each chirality center... 

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... 

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... 

Sfrucfurally O glycosides are mixed acefals fhaf involve fhe anomeric posifion of furanose and pyranose forms of carbohydrates Recall fhe sequence of mfermediafes m acefal formalion (Secfion 17 8)... 

Derivatives of aldoses in which the terminal aldehyde function is oxidized to a car boxylic acid are called aldonic acids Aldonic acids are named by replacing the ose ending of the aldose by omc acid Oxidation of aldoses with bromine is the most com monly used method for the preparation of aldonic acids and involves the furanose or pyranose form of the carbohydrate... 

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

A particular carbohydrate can mterconvert between furanose and pyra nose forms and between the a and (3 configuration of each form The change from one form to an equilibrium mixture of all the possible hemi acetals causes a change m optical rotation called mutarotation... 

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... 

Haworth formulas (Section 25.6) Planar representations of furanose and pyranose forms of carbohydrates. 

---