Fundamentals of Medicinal Chemistry

'Straight chain' form

H 2 O

H

H OH

2 O

H

O

CH 2 OH

O

CH 2 OH

H

HO

OH

H

H

OH

H

HO

OH

H

H

OH

O

H

C

H

OH

O

CH 2 OH

H

HO

OH

H

H

OH

α-D-glucose

[α]

298

D = +18.7

, water

β-D-glucose

[α]

298

D = +122.2

, water

Figure 1.13 The mutarotation of glucose anomers. The specific rotation of the aqueous equilib-

rium mixture isþ 52 

All monosaccharides have a number of stereogenic centres. The configurations

of these centres may be indicated by the use of the R/S nomenclature system.

However, the historic system where the configurations of all the chiral centres are

indicated by the stem name of the monosaccharide (Figure 1.14) is generally

preferred. In addition, monosaccharides are also classified as D or L according

to the configuration of their pentultimate CHOH group. In the D form

this hydroxy groupprojects on the rightof the carbon chain towards the observer

whilst in the L form itprojects on the leftof the carbon chain towards the

observer when the molecule is viewed with the unsaturated group at the top.

These configurations are usually represented, on paper, by modified Fischer

projections (Figure 1.14) with the unsaturated group drawn at the top of the

chain. The D and L forms of a monosaccharide have mirror image structures,

that is, are enantiomers (Figure 1.10).

HO H

OH

H OH

H

CO

OH

OH

OH

H

H

H

CO

D-Ribo

L-Ribo

HO

HO

H

H

CO

OH

HO

H

H

CO

HO

OH

H

H

CO

OH

OH

H

H

CO

D-Erythro D-Threo

CO

HO

HO H

H

H

HO

L-Erythro L-Threo L-Xylo L-Arabino

D-Xylo

H OH

CO

HO H

HO H

D-Arabino

HO H

H OH

H OH

CO

H

CO

OH

HO H

HO H

Figure 1.14 Examples of the stem names used to indicate the configurations of the chiral centres

in monosaccharides. The system is based on the relative positions of adjacent hydroxy groups with

the carbonyl group being used as a reference point for the hydroxy groups. L configurations are

mirror images of the corresponding D configurations.

CARBOHYDRATES 13