Ribose from Fischer projection

Notice that the eclipsed conformation of D-ribose derived directly from the Fischer projection does not have its C-4 hydroxyl group properly oriented for furanose ring formation. We must redraw it in a conformation that permits the five-membered cyclic hemiacetal to form. This is accomplished by rotation about the C(3)—C(4) bond, taking care that the configuration at C-4 is not changed. 

Begin the problem by converting the Fischer projection of D-ribose to a perspective view. Remember that the horizontal lines of a Fischer projection represent bonds coming toward you, and the vertical lines are going away from you. 

We can also draw Fischer projections for the monosaccharides that typically have five or six carbon atoms with several chiral carbons. As discussed in the previous section, the —OH group on the chiral carbon farthest from the carbonyl group is used to determine the D or L stereoisomer. The following are the Fischer projections for the d and l stereoisomers of ribose, a five-carbon monosaccharide, and the d and l stereoisomers of glucose, a six-carbon monosaccharide. 

We have employed Fischer s projection formulas for our discourse on isomerism. In the transition from projection formula to the spatial model and back, it should be noted that the projection rule (cf. Chapt. 1-3) applies to each individual C atom Neighboring C atoms must be behind, while H and OH groups are in front of the plane of projection. For a pentose (n-ribose) the correct three-dimensional model is shown in Fig. 39. 

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