Question:

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 differs in configuration at any one stereogenic or chiral center. C3 epimer of glucose means 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 at left whereas position of hydroxyl group at carbon-3 of D-allose is at 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 at right at carbon-2 whereas in D-talose, it is at left. D-idose is C3 epimer of D-talose and differs in configuration at carbon-3.

The structure of D-talose

(c) Position of hydroxyl group is at left at carbon-3 in D-talose whereas it is at 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 occurred 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.