Aldohexoses pyranose ring

Like the 5-amino aldopentoses, the 5-amino aldohexoses have a pronounced tendency to form the pyranose ring in alkaline solution. In acid solution, three molecules of water are eliminated per molecule, to give the corresponding derivative of 3-pyridinol. 5-Amino-5-deoxyaldohexopyranoses are, however, distinctly more stable, as the Amadori rearrangement and pyridine formation occur at pH 5.7—6.2. With the pentose analogs, these reactions begin at pH 7—8. Because of the reactive a-amino alcohol arrangement at C-1, the 5-amino-5-... 

Cyclization forms the more stable ring size in a given molecule. The most common monosaccharides, the aldohexoses like glucose, typically form a pyranose ring, so our discussion begins with forming a cyclic hemiacetal from D-glucose. 

The most stable conformation of the pyranose ring of most D-aldohexoses places the largest group, CH2OH, in the equatorial position. An exception to this is the aldohexose D-idose. Draw the two possible chair conformations of either the a or p anomer of D-idose. Explain why the more stable conformation has the CH2OH group in the axial position. 

Aldohexoses exist in solutions mainly in six-membered pyranose ring forms, since these forms are thermodynamically more stable than furanose ring forms. 

For the pyranosides of the D-aldohexoses and higher-carbon sugars of the D-series, carbon atom 6 is written as projecting above the pyranose ring when written as above for the corresponding derivatives of the L-series, carbon atom 6 lies below the pyranose ring (see discussion earlier in this chapter). 

Aldohexoses can form cyclic hemiacetals that exhibit a pyranose ring. 

The orientation of the model described above results in a clockwise numbering of the ring atoms. Groups that appear to the right of the modified Fischer projection appear below the plane of the ring those on the left appear above. In the common Haworth representation of the pyranose form of D-aldohexoses, C-6 is above the plane. 

In the 4-acetamido-4-deoxyaldopentoses and 4-acetamido-4-deoxy-aldohexoses, the pyranose form and a nitrogen-containing, five-membered ring are in competition. Moreover, as the acetamido group is less reactive than the hydroxyl group, the more sterically favored pyranose form is assumed this always occurs. 

For an aldohexose such as glucose, the C-1 aldehyde in the open-chain form of glucose reacts with the C-5 hydroxyl group to form an intramolecular hemiacetal. The resulting cyclic hemiacetal, a six-membered ring, is called pyranose because of its similarity to pyran (Figure 11.4). Similarly, a ketone can react with an alcohol to form a hemiketal. 

The persubstituted derivatives having two, or three, different substituents also serve mainly for identification purposes. Suitable selection of conditions needed for cleavage of the 1,6-anhydride bond permits transformation of most of the persubstituted 1,6-anhydrohexo-pyranoses into 2,3,4-tri-O-substituted derivatives of aldohexoses (see Sect. IV,1). Ring-opening polymerization of ethers and esters has been discussed in the previous Section. 

Like aldopentoses, aldohexoses such as D-glucose are capable of forming two fura-nose forms (a and (3) and two pyranose forms (a and (3). The Haworth representations of the pyranose forms of D-glucose are constructed as shown in Figure 25.4 each has a CH2OH group as a substituent on the six-membered ring. 

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