Gulose Fischer projection

Once the eight Fischer projections have been written they are named m order with the aid of the sentence All altruists gladly make gum m gallon tanks The words of the sentence stand for allose altrose glucose mannose gulose idose galactose talose... 

The aldohexoses are allose, altrose, glucose, mannose, gulose, idose, galactose, and talose. The mnemonic All altruists gladly make gum in gallon tanks is helpful in writing the conect Fischer projection for each one. 

Reaction of an aldose with NaBH produces a polyol (alditol). Because an alditol has the same functional group at both ends, two different aldoses can yield the same alditol. Here, L-gulose and D-glucose form the same alditol (rotate the Fischer projection of L-gulitol 180° to see the identity). 

If the two end groups of structure X are interchanged, the product looks strange indeed. Remember that we can rotate a Fischer projection by 180° when we do that, it becomes clear that the product is an unusual sugar of the L series (L-gulose). Structure X must be D-glucose. 

AUrose Glucose Mannose Gulose Fig. 4. Acyclic forms of D-aldoses in their Fischer projections. 

FIGURE 22.50 Oxidation wuth nitric acid forms an aldaric acid. Both the aldehyde end and the primary alcohol end are converted into the same group, a carboxylic acid. Because D-glucose and gulose give the same aldaric acid, they must differ by turning the Fischer projection end to end. 

FIGURE 22.51 IfD-gl ucose has structure A, then gulose has the structure shown. Oxidation of gulose gives the same aldaric acid as oxidation of D-glucose. Turning the Fischer projection of the oxidized product of gulose 180° shows you the two oxidized products are the same. 

The answer is obtained by rotating the Fischer projection shown by 180° and comparing it with that of o-gulose in Figure 24-1 It is L-gulose. 

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