Aldoses hemiacetals

Derivatives of aldose hemiacetals with the hydroxyl replaced by halogen or acetate have been described and, in many cases, isolated in the two forms demanded by stereochemical theory (14, 14 )>... 

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... 

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... 

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... 

The anomeric carbon of a furanose or pyranose form of a ketose bears both a hydroxyl group and a carbon substituent In the case of 2 ketoses this substituent is a CH2OH group As with aldoses the anomeric carbon of a cyclic hemiacetal is readily identifi able because it is bonded to two oxygens... 

Ketoses are characterized by the ending ulose m their name Most nat urally occurring ketoses have their carbonyl group located at C 2 Like aldoses ketoses cyclize to hemiacetals and exist as furanose or pyranose forms... 

Treatment of an aldose or ketose with NaBH4 reduces it to a polyalcohol called an alditol. The reduction occurs by reaction of the open-chain form present in the aldehyde/ketone hemiacetal equilibrium. Although only a small amount of the open-chain form is present at any given time, that small amount is reduced, more is produced by opening of the pyranose form, that additional amount is reduced, and so on, until the entire sample has undergone reaction. 

We should first consider the open-chain form of glucose 6-phosphate, rather than its pyranose hemiacetal form (see Section 12.2.1). The open-chain aldose has the requirements for enolization, namely a hydrogen a to the aldehyde carbonyl group. Enolization produces in this case an enediol, which can revert to a keto form in two ways, i.e. reforming... 

This enzyme [EC 5.1.3.3], also known as mutarotase, catalyzes the epimerization of the hemiacetal carbon atom of aldoses (thus, anomerization). Hence, a-D-glu-cose is reversibly converted to /3-D-glucose. Other sugars can act as substrates (e.g., L-arabinose, D-xylose, D-galac-tose, maltose, and lactose). 

Carbohydrates such as aldoses that undergo oxidation with metal ions are referred to as reducing sugars. Both copper(II) ions and silver ions are capable of oxidizing aldoses. Oxidation by copper(II) ions is the basis for Fehling s test and Benedict s test, whereas oxidation by silver ions is the key to Tollen s test. (Note These tests work for any sugar with a hemiacetal, but they don t work on acetals or ketals.)... 

In view of the stability of the cyclic (hemiacetal) form of the aldoses, it is reasonable to assume that the open-chain aldehydo structure, formed by elimination of the nitrile group, will be more stable if cyclization is inhibited (by acyloxy groups on carbon atoms 4 or 5) than if cyclization may readily occur (hydroxyl groups on carbon atoms 4 or 5). 

The reactivity of the 2,5-anhydrides of aldoses is determined by two essential structural features that do not exist in the sugars, namely, the presence of an oxolane ring and of a carbonyl group (most frequently, free) a to the ring-oxygen atom. These two characteristics make the 2,5-anhydroaldoses closer to tetrahydro-2-furaldehyde than to the aldoses, where only in exceptional cases is the carbonyl group not masked by the formation of an intramolecular, five- or six-membered, hemiacetal ring. 

These reactions are typical of the —CHO group. The Schiff and bisulfite reactions are reversible, and the equilibrium favors unreacted hemiacetal. Since osazone and Fehling reactions are irreversible, equilibrium is shifted to restore the low concentration (0.02%) of open-chain aldehyde as soon as some reaction occurs, and eventually all the aldose reacts. 

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