a) Draw D-allose, the C3 epimer of glucose.

b) Draw D-talose, the C2 epimer of D-galactose.

c) Draw D-idose, the C3 epimer of D-talose. Now compare your answers with Figure 23-3.

d) Draw the C4 “epimer” of D-xylose. Notice that this “epimer” is actually an L-series sugar, and we have seen its enantiomer. Give the correct name for this L-series sugar.

a. Epimers are stereoisomers which differ in configuration at any one stereogenic or chiral center. C3 epimer of glucose means that configuration differs at carbon-3 of glucose which gives rise to different stereoisomer that is, D-allose. Position of hydroxyl group in glucose at carbon-3 is on the left, whereas position of hydroxyl group at carbon-3 of D-allose is on the right.

b. Similarly, D-talose is C2 epimer of D-galactose and differs in configuration at carbon-2. Position of hydroxyl group in D-galactose is on the right at carbon-2 whereas in D-talose, it is on the left. D-idose is C3 epimer of D-talose and differs in configuration at carbon-3.

c. Position of hydroxyl group is on the left at carbon-3 in D-talose. whereas it is on the right in D-idose.

d. C4 epimer of D-xylose is L-arabinose. Change in configuration at carbon-4 of D-xylose with respect to hydroxyl group changes the sugar series from “D” to “L” as the change occurs at bottom chiral center which as per convention, leads to formation of enantiomer. The D/L system names molecules by relating them to glyceraldehyde molecule.