hemiacetal or hemiketals occurs due to intramolecular nucleophilic addition
reaction between a –OH and a C À À
À À O group. Many monosaccharides exist in
an equilibrium between open chain and cyclic forms.
C H
R
OR
OH
C R'
R
OR
OH
Hemiacetal
Hemiketal
Hemiacetals and hemiketals have the important structural feature of –OH
and –OR attached to the same carbon as shown above. Through cyclization,
sugars attain a pyranose and/or a furanose form.
OH
O
H
H
H
O
H
OH
H
OH
H
CH 2 OH
OH
CH 2 OH
OH
OH
OH
O
H
O
OH
O
H
O
H O
H
OH
O
O
D-Glucose
..
..
β-D-Glucopyranose
Cyclization of glucose
Pyran
Furan
Hemiacetal
CH 2 OH
O
H
O
H
OH
H
OH
H
CH 2 OH
O
OH
OH
CH 2 OH
OH
CH 2 OH
O
OH
OH
O
H
O
H
CH 2 OH
O
OH
OH
CH 2 OH
OH
CH 2 OH
D-Fructose
..
..
Pyranose form of fructose
Cyclization of fructose
..
..
Furanose form of fructose
Hemiketal
Hemiketal
a
b
a
b
Cyclization produces a new chiral centre at C-1 in the cyclic form. This
carbon is called the anomeric carbon. At the anomeric carbon, the –OH
group can project upwards (b configuration) or downwards (a configuration).
OH
O
H
H
H
O
H
OH
H
OH
H
CH 2 OH
O
OH
O
H
O
H O
H
OH
O
O
H
O
H O
H
OH
OH
D-Glucose
Four chiral carbon atoms (∗)
β Anomer (β configuration)
α Anomer (α configuration)
1
1
New chiral centre at C-1 (anomeric carbon)
*
*
*
*
*
*
*
*
*
*
Five chiral carbon atoms (∗)
in the pyranose forms
306
CH6 NATURAL PRODUCT CHEMISTRY