Glucose ring form

Fig. 3. Equilibiium mistuie of D-glucose forms in solution. Pyranose ring forms predominate.

In sucrose, fructose is present as the P anomer. Now, one of these sugars has acted as an alcohol to make a bond to the other sugar. We can look at this in two ways. Either frnctose acts as an alcohol to react with the hemiacetal glucose to form an acetal, or alternatively, glucose is the alcohol that reacts with the hemiketal fructose to form a ketal. In sucrose, the pyranose ring is an acetal, whilst the fnranose ring is a ketal. This all seems rather... 

A distinction must be made between septanosides that lack (6-unsubstituted) or contain (6-substituted) an exocyclic hydroxymethyl group at C-6. In the same way that D-glucose can form a septanose ring by connecting the 6-hydroxyl group to the anomeric center as in Fig. 1, all of the common aldohexoses can adopt a septanose... 

Fig. 1. Higher-order sugars L-g/yceroD-manno-Heptose (1, open-chain and 2, a-pyranose) and A -acetylneurami ilic acid (3, a-pyranose form). Structures 4-8 depict D-glucose (4) as its open-chain Fischer projection, and as the a-oxetose (5), a-furanose (6), a-pyranose (7), and a-septanose (8) ring forms.

The protonated amino group, however, has no nucleophilic activity, and does not form a hemiacetal an amino sugar can, therefore, be obtained, as a salt, in an otherwise unfavorable form. For example, 6-amino-6-deoxy-L-sorbose can be isolated as the hydrochloride of a furanose form (22) (presumably a). In alkaline solution, immediate ring-expansion to the pyranose form (23) occurs127 this reaction can be reversed under strongly acidic conditions. 4-Amino-4,6-dideoxy-D-glucose hydrochloride forms a 35 65 mixture of the a- and / -pyranose forms in solution.128... 

Figure 4-1 Natural abundance 13C-NMR spectrum of honey showing content of a and (3 pyranose ring forms of glucose and both pyranose and furanose ring forms of fructose. Open chain fructose, with a peak at 214 ppm, was present in a trace amount. From Prince et al.6...

It may be assumed that in this book sugars are always d unless they are specifically designated as l. When linkages remain uncertain abbreviated formulas are given. Because the glucose ring in lactose is free to open to an aldehyde and to equilibrate (in solution) with other ring forms, the name lactose does not imply... 

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