The aldohexoses are a family of reducing monosaccharides that each contains a 6-carbon chain and an aldehyde at C-1. They may exist in a variety of forms in solution, most often as the cyclic hemiacetal pyranose and furanose forms as well as the familar acyclic form in which the aldehyde functional group is revealed. The Fischer depictions and rotatable structures below show the stereochemical relationships between the diastereomeric beta anomers of D-Glucose, D-Mannose, and D-Galactose in both their pyranose and furanose forms. Manipulate the models so that you are able to answer these questions:
In order to produce the pyranose form (6-membered ring, OH-5 in each case must attack the C-1 aldehyde to form the hemiacetal (shown above). It is also possible for OH-4 to attack C-1 and generate the furanose forms (5-membered ring). The models below show the consequences of this for D-Glucose, D-Mannose, and D-Galactose.
The pyranose forms for all three sugars are fairly obvious; the OH-5 group attacks the C-1 aldehyde and similar 6-membered rings are formed in each case. The differences in these structures are seen in the stereochemistry at C-2 or C-4 while the stereochemistry at C-5 is the same in each sugar. In the furanose form there is a change seen in the galactose case. Since the stereochemistry at C-4 is the opposite to that observed in glucose and mannose, the group at C-4 of the furanose ring will be inverted in relation to the other two structures.