Aldoses hemiacetal formation

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... 

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. 

FIG. 1.—The formation of 1,5-lactones suggests that the starting aldoses exist in the 1,5-cyclic hemiacetal (pyranose) form. The difference between the rates of oxidation of the a and /3 anomers was taken as a proof that the ring is not planar. 

The formation of a hemiacetal ring gives rise to a new chiral center in the aldoses, namely, at the C-1 atom. This leads to two C-1 epimers, termed anomers. When the hydroxyl group at C-1 (glycosidic hydroxyl) in the Fischer projection formula is located on the same side as the hemiacetal ring, the anomer is termed the a form in the opposite case it is the j3 form (Fig. 2-5). 

The oxidation of the aldehyde or hemiacetal group of an aldose sugar with bromine water leads to the formation of a strongly acid solution, regardless of whether the oxidant be considered as hypobromous acid or as free bromine hydrobromic acid is formed in either case. Since the... 

It is obvious that, knowing the equilibrium constant (K), the droptime ( i), and a hypothetical diffusion current (id), it is possible to calculate the rate constant for formation of the depolarizer from the polaro-graphically inactive forms of substances (for example, the formation of non-hydrated glyceraldehyde, or acyclic forms of higher aldoses from their cyclic hemiacetal forms). 

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