Fundamentals of Medicinal Chemistry

Some monosaccharides may also be classified as beingepimers. Epimers are

compounds that have identical configurations except for one carbon atom. For

example,a-D-glucose anda-D-fructose are epimers. Epimers sometimes react

with the same reagent to form the same product. For example, botha-D-glucose

anda-D-fructose react with phenylhydrazine to form the same osazone.

H

HO H

HOH

H

H OH

CHO

CH 2 OH

PhNHNH 2

OH

HO H

C

HO

H OH

NNHPh

CH 2 OH

CH NNHPh

PhNHNH 2

OH

H

C

H

H OH

O

OH

CH 2 OH

CH 2 OH

D-Glucose Glucosazone D-Fructose

The chemical properties of monosaccharides are further complicated by the

fact that they can exhibit tautomerism in aqueous basic solutions (Figure 1.15).

This means that after a short time a basic aqueous solution of a monosaccharide

will also contain a mixture of monosaccharides that will exhibit their character-

istic chemical properties. For example, a solution of fructose will produce a

silver mirror when treated with an ammoniacal solution of silver nitrate (Tol-

len’s reagent). This is because under basic conditions fructose undergoes tauto-

merism to glucose, whose structure contains an aldehyde group, which reduces

Tollen’s reagent to metallic silver.

CHO

CH 2 OH

OH

CHOH

C CO HOC

HO

CH 2 OH

OH

HO

HO

H

H

H

H OH

CH 2 OH

H

HOH

OH H

OH

HO

H OH

H

H

H

CHO

CH 2 OH

OH

OH

HO H

H

H

CH 2 OH

CHOH

OH

OH

HO

OH

H

H

H

CH 2 OH

Various

other

ketoses

(10%)

Enolic form Mannose

(2.5%)

Fructose

(21%)

Glucose

(63.5%)

Enolic form

Figure 1.15 The tautomerism of glucose in a basic aqueous solution. The approximate concen-

trations of the isomers present at equilibrium are given in the brackets

1.4.2 The nomenclature of monosaccharides

Monosaccharides are normally known by their traditional trivial names. How-

ever, systematic names are in use. The systematic names of ‘straight chain’

14 BIOLOGICAL MOLECULES