Chapter 18, Problem 18.42P

Interpretation Introduction

(a)

Interpretation:

The Haworth projections of $\text{α-D-ribofuranose}$ and $\text{β-D-ribofuranose}$ are to be drawn

Concept introduction:

α-D Ribofuranose and β-D Ribofuranose are stereoisomers that differ only at one asymmetric carbon called anomeric carbon and are called anomers.

Haworth projection of the cyclic structure of a ribose (a pentose) shows the cyclic structure as planar, with the substituents pointing straight up or down. The ring oxygen is shown directly opposite the observer, with the anomeric carbon to its immediate right.

Interpretation Introduction

(b)

Interpretation:

The mechanism of the formation of $\text{α-D-ribofuranose}$ and $\text{β-D-ribofuranose}$ from the acyclic form of the monosaccharide is to be drawn.

Concept introduction:

$\text{α-D-ribofuranose}$ and $\text{β-D-ribofuranose}$ are both produced from the same molecule, the acyclic form of $\text{D-ribose}$ via nucleophilic addition followed by proton transfers. The aldehyde group of the acyclic form is planar and can be attacked from both sides of the plane. Whether the nucleophilic attack by the oxygen of the C4 OH group is from above or below the plane (in the Haworth projection) determines whether the $\text{α}$-anomer or the $\text{β}$-anomer is formed.