O
H
OH
H
H
O
H
H
O
H
OH
H
CH 2 OH
O
O
H
OH
H
H
O
H
H
O
H
OH
H
O
O
H
D-Galactose
HNO 3
Galactaric acid, a meso compound
Plane of symmetry
6.3.7 Reactions of monosaccharides as alcohols
Ester formation
Monosaccharides contain a number of alcoholic hydroxyl groups, and thus
can react with acid anhydrides to yield corresponding esters. For example,
when glucose is treated with acetic anhydride and pyridine, it forms a
pentaacetate. The ester functions in glucopyranose pentaacetate undergo the
typical ester reactions.
O
O
H
O
H O
H
OH
OH
O
AcO
AcO
AcO
OAc
OAc
β-D-Glucopyranose
AC 2 O
β-D-glucopyranose pentaacetate
Pyridine
Monosaccharides also form phosphate esters with phosphoric acid. Monosaccharide phosphate esters are important molecules in biological system.
For example, in the DNA and RNA nucleotides, phosphate esters of
2-deoxyribose and ribose are present, respectively. Adenosine triphosphate
(ATP), the triphosphate ester at C-5 of ribose in adenosine, is found
extensively in living systems.
Ether formation
When methyl a-D-glucopyranoside (an acetal) is treated with dimethyl
sulphate in presence of aqueous sodium hydroxide, the methyl ethers of the
alcohol functions are formed. The methyl ethers formed from monosaccharides are stable in bases and dilute acids.
O
O
H
O
H
O
H
OH
OMe
O
OMe
MeO
MeO
MeO
OMe
Methyl-α-D-Glucopyranoside
(CH 3 ) 2 SO 4
NaOH, H 2 O
Methyl-2,3,4,6-tetra-O-methyl-α-D-Glucopyranoside
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CH6 NATURAL PRODUCT CHEMISTRY