Hemiacetals linkages

Monosaccharides normally exist as cyclic hemiacetals rather than as open-chain aldehydes or ketones. The hemiacetal linkage results from reaction of the carbonyl group with an —OH group three or four carbon atoms away. A... 

Pacsu4 5 has suggested a structure for starch involving a small number of non-cyclic hemiacetal linkages, the number being presumably sufficient to account for the number of endgroups determined by the methylation method. Halsall, Hirst and Jones6 have commented on this structure, however, and have shown it to be incompatible with the results of periodate-oxidation studies. In addition, these authors pointed out that it would be difficult to explain enzymic hydrolysis and dextrin formation on the basis of such a structure. 

The reaction starts of with a protonation - use the catalyst. Resist the urge to protonate the 4-hydroxyl, but go for the one at position 1 that has the added functionality of the hemiacetal linkage. It is going to be the more reactive one. Protonation is followed by loss of water as leaving group. The intermediate oxonium cation shown is actually a resonance form of the simpler carbocation now you can see the role of the adjacent oxygen. The reaction is completed by attack of the nucleophile, the 4-hydroxyl of another molecule. This is not special, but is merely another version of the hemiacetal synthesis done in part (a). 

Problem 22.38 Use shorthand formulas to show how osazone formation establishes the glucose unit of lactose to have the hemiacetal linkage (Problem 22.37). ... 

FIGURE 7-6 Formation of the two cyclic forms of D-glucose. Reaction between the aldehyde group at C-l and the hydroxyl group at C-5 forms a hemiacetal linkage, producing either of two stereoisomers, the a and fi anomers, which differ only in the stereochemistry around the hemiacetal carbon. The interconversion of a and fi anomers is called mutarotation. 

To unravel a pyranose form, locate the anomeric carbon and mentally convert the hemiacetal linkage to a carbonyl compound and a hydroxyl function. 

Monosaccharides normally exist as cyclic hemiacetals rather than as open-chain aldehydes or ketones. The hemiacetal linkage results from reaction of the carbenyl group witli an -OH group three or four carhou atoms away. A nvu-mrfn]b r cyclic hemiacetal is called a furanose, and a six-mcnibered cyclic hemiacetal is called a pyranose. Cycltzation leads to th formation of a new chirality center and production of two dia tereomerlc hemiacetaU, called a and p anomers. 

Since a hemiacetal is formed so easily from a carbonyl compound and alcohol, it is not surprising to find that carbohydrates (polyhydroxy derivatives of aldehydes and ketones) frequently exist as cyclic structures in which a hemiacetal linkage is formed intramolccularly. Furthermore, since hemiacetal formation is a reversible process, many carbohydrates exhibit the phenomenon of mutarotation. The liberation of the free aldehyde (V) from the internal hemiacetal of the sugar (IV) destroys the optical activity of the hemiacetal carbon atom (in this case carbon 1), and reformation results in the formation of an equilibrium mixture of two diastereoisomers. 

A modified representation of the D-glucose molecule, showing the formation of the hemiacetal linkage... 

Furthermore, the alkali-catalyzed, )3-elimination process can be used in the proof of the structure of glycopeptides, such as 79. A major type of carbohydrate-peptide linkage proved to be an O-glycosidic conjugation to L-threonine and L-serine, as in 80. The hemiacetal linkage in the -position of L-serine and L-threonine is readily split by alkali or by methoxide under mild conditions. The carbohydrate part is released, and an a,(3-unsaturated amino acid (enamine) is formed. This type of / -eliminative degradation was proved to occur with the synthetic, model compounds N- (2,4-dinitrophenyl) -3-0- (2,3,4,6-tetra-O-benzyl-a-D-gIuco-... 

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