25.6
Cyclic Forms of Carbohydrates:
Furanose Forms
Recall from Section 17.8
R
R
C
O •• + R"OH
••
R'
••
R"O
••
C
R'
Product is a hemiacetal.
••
O
••
H
Cyclic Hemiacetals
R
R
C
O
OH
C
OH
O
Aldehydes and ketones that contain an OH
group elsewhere in the molecule can undergo
intramolecular hemiacetal formation.
The equilibrium favors the cyclic hemiacetal if
the ring is 5- or 6-membered.
Carbohydrates Form Cyclic Hemiacetals
1
CH
O
2
OH
O
4
1
3
3
4
2
H
CH2OH
equilibrium lies far to the right
cyclic hemiacetals that have 5-membered rings
are called furanose forms
D-Erythrose
1
H
CH
2
O
OH
HH
O
4
H
3
4
OH
CH2OH
H
H
1
3
OH
OH
2
OH
stereochemistry is maintained during cyclic
hemiacetal formation
H
D-Erythrose
1
2
3
4
D-Erythrose
1
2
3
4
1
4
turn 90°
3
2
D-Erythrose
move O into
position by rotating
about bond
between carbon-3
and carbon-4
1
4
3
2
D-Erythrose
1
4
3
2
1
4
3
2
D-Erythrose
1
4
3
2
close ring by
hemiacetal formation
between OH at C-4
and carbonyl group
D-Erythrose
1
4
3
2
1
4
3
2
D-Erythrose
1
H
CH
2
anomeric carbon
O
OH
HH
O
4
H
3
4
OH
CH2OH
H
3
OH
H
OH
1
2
H
OH
stereochemistry is variable at anomeric carbon;
two diastereomers are formed
D-Erythrose
HH
O
4
H
3
OH
H
H
1
2
OH
OH
a-D-Erythrofuranose
HH
O
4
H
3
OH
H
OH
1
2
H
OH
b-D-Erythrofuranose
D-Ribose
1
H
CH
O
2
OH
H
3
OH
H
4
OH
5 CH
2OH
furanose ring formation involves OH group at C-4
D-Ribose
1
H
CH
O
5
2
OH
H
H
3
OH
4
H
4
OH
HO 3
5 CH
2OH
CH2OH 1
H
H
CH
2
OH
OH
need C(3)-C(4) bond rotation to put OH in proper
orientation to close 5-membered ring
O
D-Ribose
5
5
HOCH2 OH
H
4
H
3
OH
H
2
OH
H
1
CH
O
4
CH2OH 1
H
H
CH
HO 3
2
OH
OH
O
D-Ribose
5
HOCH2 OH
H
4
H
3
OH
H
1
CH
O
2
OH
CH2OH group becomes a substituent on ring
D-Ribose
5
5
HOCH2 OH
H
4
H
3
OH
H
2
OH
1
CH
O
HOCH2
H
O H
4
H
3
OH
2
OH
1
H
OH
b-D-Ribofuranose
CH2OH group becomes a substituent on ring
25.7
Cyclic Forms of Carbohydrates:
Pyranose Forms
Carbohydrates Form Cyclic Hemiacetals
1
CH
O
5
2
3
4
5
OH
O
1
4
3
2
H
CH2OH
cyclic hemiacetals that have 6-membered rings
are called pyranose forms
D-Ribose
1
CH
O
2
OH
H
3
OH
H
4
OH
H
5
CH2OH
pyranose ring formation involves OH group at C-5
D-Ribose
1
H
CH
2
O
5
H
OH
H
3
OH
H
4
OH
4
CH2OH
H
H
HO 3
CH
O
2
OH
5
1
OH
CH2OH
pyranose ring formation involves OH group at C-5
D-Ribose
5
H
4
CH2OH
H
H
HO 3
1
CH
O
2
OH
OH
pyranose ring formation involves OH group at C-5
D-Ribose
H
H
4
HO
5
H
H
3
OH
5
O
OH
H
1
2
H
OH
b-D-Ribopyranose
H
4
CH2OH
H
H
HO 3
2
OH
OH
1
CH
O
D-Ribose
H
H
4
HO
5
H
H
3
OH
H
O
OH
H
1
2
OH
H
H
4
HO
5
H
H
3
OH
O
H
1
H
2
OH
OH
a-D-Ribopyranose
D-Glucose
1
CH
O
2
OH
HO
3
H
H
4
OH
H
H
5
6
OH
CH2OH
pyranose ring formation involves OH group at C-5
D-Glucose
1
CH
O
2
OH
HO
3
H
H
4
H
H
5
6
OH
OH
6
H
CH2OH
5
H
OH
4
OH
H
HO 3
CH
1
2
H
OH
CH2OH
pyranose ring formation involves OH group at C-5
O
D-Glucose
6
H
CH2OH
5
H
OH
4
OH
H
HO 3
CH
1
2
H
OH
need C(4)-C(5) bond rotation to put OH in proper
orientation to close 6-membered ring
O
D-Glucose
6
6
HOCH2 OH
H 5
H
4
OH
H
HO 3
CH
1
2
H
OH
O
H
CH2OH
5
H
OH
4
OH
H
HO 3
CH
1
2
H
OH
need C(4)-C(5) bond rotation to put OH in proper
orientation to close 6-membered ring
O
D-Glucose
6
6
HOCH2
HOCH2 OH
H 5
H
4
OH
H
HO
3
H
CH
1
2
H
O
4
OH
HO
5
H
OH
3
H
O
OH
H
1
2
H
OH
b-D-Glucopyranose
D-Glucose
6
6
HOCH2
HOCH2
H
4
HO
5
H
OH
3
H
O
H
2
H
H
1
4
OH
HO
OH
a-D-Glucopyranose
5
H
OH
3
H
O
OH
H
1
2
H
OH
b-D-Glucopyranose
D-Glucose
6
HOCH2
H
4
HO
5
H
OH
3
H
O
OH
H
1
2
H
OH
b-D-Glucopyranose
pyranose forms of carbohydrates adopt chair
conformations
D-Glucose
6
HOCH2
H 6
HOCH2 H
4
HO
HO
H
O
5
4
OH
2
3
H
H
1
OH
H
HO
5
H
OH
3
H
O
OH
H
1
2
H
OH
b-D-Glucopyranose
all substituents are equatorial in b-D-glucopyranose
D-Glucose
H
HOCH2 H
H
HOCH2 H
O
HO
HO
OH
O
HO
HO
H
1
H
H
1
OH
H
H
b-D-Glucopyranose
OH
H
OH
a-D-Glucopyranose
OH group at anomeric carbon is axial
in a-D-glucopyranose
Figure 25.5
CH
O
H
OH
H
OH
H
OH
CH2OH
Less than 1% of the open-chain form of D-ribose
is present at equilibrium in aqueous solution.
Figure 25.5
76% of the D-ribose is a mixture of the a and bpyranose forms, with the b-form predominating
H
H
H
H
O
HO
H
O
HO
H
OH
H
H
H
1
H
OH
OH
H
b-D-Ribopyranose (56%)
H
OH
OH
OH
a-D-Ribopyranose (20%)
Figure 25.5
The a and b-furanose forms comprise 24% of
the mixture.
HOCH2
H
O H
OH
H
H
OH
OH
b-D-Ribofuranose (18%)
HOCH2
H
O H
H
OH
H
OH
OH
a-D-Ribofuranose (6%)
25.8
Mutarotation
Mutarotation
Mutarotation is a term given to the change in
the observed optical rotation of a substance with
time.
Glucose, for example, can be obtained in
either its a or b-pyranose form. The two forms
have different physical properties such as
melting point and optical rotation.
When either form is dissolved in water, its
initial rotation changes with time. Eventually
both solutions have the same rotation.
Mutarotation of D-Glucose
H
HOCH2 H
H
HOCH2 H
O
HO
HO
OH
O
HO
HO
H
1
H
H
1
OH
H
H
b-D-Glucopyranose
Initial: [a]D +18.7°
H
OH
OH
a-D-Glucopyranose
Initial: [a]D +112.2°
Mutarotation of D-Glucose
H
HOCH2 H
H
HOCH2 H
O
HO
HO
OH
O
HO
HO
H
1
H
H
1
OH
H
H
H
b-D-Glucopyranose
Initial: [a]D +18.7°
OH
OH
a-D-Glucopyranose
Initial: [a]D +112.2°
Final: [a]D +52.5°
Mutarotation of D-Glucose
H
HOCH2 H
H
HOCH2 H
O
HO
HO
OH
O
HO
HO
H
1
H
H
1
OH
H
H
H
b-D-Glucopyranose
Initial: [a]D +18.7°
OH
OH
a-D-Glucopyranose
Initial: [a]D +112.2°
Final: [a]D +52.5°
Mutarotation of D-Glucose
H
HOCH2 H
H
HOCH2 H
O
HO
HO
OH
O
HO
HO
H
1
H
H
1
OH
H
H
b-D-Glucopyranose
H
OH
OH
a-D-Glucopyranose
Explanation: After being dissolved in water, the
a and b forms slowly interconvert via the openchain form. An equilibrium state is reached that
contains 64% b and 36% a.