Acetals-Hemiacetals
R
R
O •• + R"OH
R"OH
C
Cyclic Forms of Carbohydrates:
Furanose & Pyranose
••
R"O
••
••
C
••
R'
O
H
••
R'
Product is a hemiacetal.
hemiacetal.
Carbohydrates Form Cyclic Hemiacetals
Cyclic Hemiacetals
R
R
C
1
OH
C
O
CH
O
3
4
H
2
H
3
4
O
OH
1
O
4
OH
CH2OH
H
H
1
3
OH
OH
2
OH
stereochemistry is maintained during cyclic
hemiacetal formation
H
2
H
equilibrium lies far to the right
cyclic hemiacetals that have 5-membered rings
are called furanose forms
D-Erythrose
HH
1
CH2OH
D-Erythrose
CH
O
4
3
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
6-membered..
1
OH
2
2
3
4
D-Erythrose
D-Erythrose
1
2
1
4
turn 90°
3
2
3
4
move O into
position by rotating
about bond
between carbon-3
and carbon-4
D-Erythrose
3
1
4
2
1
4
2
3
3
2
1
4
3
2
close ring by
hemiacetal formation
between OH at C-4
and carbonyl group
D-Erythrose
D-Erythrose
1
2
3
D-Erythrose
1
4
1
4
1
4
3
2
H
H
CH
2
3
4
anomeric carbon
O
OH
HH
O
4
OH
CH2OH
H
3
OH
H
OH
1
2
H
OH
stereochemistry is variable at anomeric carbon;
two diastereomers are formed
The structures of cyclic sugars are best represented by
the Haworth projections
D-Erythrose
HH
O
4
H
3
H
1
2
OH
HH
H
O
4
OH
H
OH
α-D -Erythrofuranose
H
OH
1
3
2
OH
H
OH
β-D-Erythrofuranose
Haworth projections allow us to see the relative
orientation of the OH groups in the ring
D-Ribose
Question
1
CH
CH
Which structure is the Haworth formula of α-Derythrose?
erythrose?
A)
B)
O
H
2
OH
H
3
OH
H
4
OH
5 CH OH
2
C)
furanose ring formation involves OH group at C-4
D)
D-Ribose
1
CH
CH
D-Ribose
O
H
2
H
3
OH
H
4
OH
OH
5 CH OH
2
4
HO
5
5
CH2OH
H
H
H
3
HOCH2 OH
1
CH
2
OH
OH
need C(3)-C(4) bond rotation to put OH in proper
orientation to close 5-membered ring
O
H
4
H
3
OH
H
2
OH
CH
5
CH2OH
H
H
H
1
O
4
HO
3
1
2
OH
OH
CH
O
D-Ribose
D-Ribose
5
H
4
H
3
H
HOCH2 OH
1
CH
O
H
4
H
2
OH
5
5
HOCH2 OH
OH
3
H
1
CH
O
HOCH2
H O H
2
OH
4
H
OH
3
2
OH
OH
OH
1
H
β-D-Ribofuranose
CH2OH group becomes a substituent on ring
CH2OH group becomes a substituent on ring
Question
Assign the corresponding D,L and α and β
stereochemical descriptors to the structure
shown.
A) α - D
B) β - D
C) α - L
D) β - L
Cyclic Forms of Carbohydrates:
Pyranose
D-Ribose
Carbohydrates Form Cyclic Hemiacetals
1
CH
1
O
5
2
3
4
5
O
OH
1
H
4
3
2
CH2OH
cyclic hemiacetals that have 6-membered rings
are called pyranose forms
CH
O
H
2
OH
H
3
OH
H
4
OH
5
CH2OH
pyranose ring formation involves OH group at C-5
D-Ribose
1
CH
O
H
2
H
3
OH
H
4
OH
5
D-Ribose
5
CH2OH
H
H
H
OH
4
HO
3
CH
5
CH2OH
H
H
H
O
4
HO
2
OH
CH2OH
1
3
OH
1
CH
O
2
OH
OH
pyranose ring formation involves OH group at C-5
pyranose ring formation involves OH group at C-5
D-Ribose
D-Ribose
H
H
4
HO
5
H
H
O
OH
H
1
H
2
3
OH
4
HO
OH
H
5
CH2OH
H
H
H
3
1
CH
O
2
OH
OH
H
4
HO
H
5
H
H
O
OH
H
1
H
2
3
OH
1
O
CH
H
HO
3
H
HO
3
H
4
OH
H
4
OH
H
5
6
H
O
1
H
OH
2
3
OH
OH
D-Glucose
2
H
HO
5
H
H
α-D-Ribopyranose
D-Glucose
CH
4
OH
β-D-Ribopyranose
1
H
OH
CH2OH
pyranose ring formation involves OH group at C-5
O
H
2
OH
H
OH
5
6
OH
H
H
4
HO
6
CH2OH
5
OH
OH
3
H
CH
1
2
H
OH
CH2OH
pyranose ring formation involves OH group at C-5
O
D-Glucose
D-Glucose
6
H
H
OH
OH
4
CH
H
3
HO
6
CH2OH
5
1
2
H
O
3
HO
OH
need C(4)-C(5) bond rotation to put OH in proper
orientation to close 6-membered ring
CH
1
2
H
HO
3
1
OH
need C(4)-C(5) bond rotation to put OH in proper
orientation to close 6-membered
6-membered ring
6
6
HOCH2
HOCH2
4
5
H
OH
3
H
OH
1
2
H
6
HO
2
H
3
HO
CH
H
HOCH2
H
O
OH
OH
4
D-Glucose
6
CH
O
CH2OH
5
OH
D-Glucose
HOCH2 OH
H 5
H
4
OH
H
6
H
H
HOCH2 OH
H 5
H
4
OH
H
O
OH
H
1
2
H
OH
β-D-Glucopyranose
Question
Assign the corresponding D,L and α and β
stereochemical descriptors to Glucopyranose
(shown).
A) α -L-Glucopyranose
B) β -L-Glucopyranose
C) α-D-Glucopyranose
D) β-D-Glucopyranose
H
4
HO
5
H
OH
3
H
O
H
2
H
H
1
4
OH
HO
OH
α-D-Glucopyranose
5
H
OH
O
OH
H
1
2
3
H
H
OH
β-D-Glucopyranose
D-Glucose
6
HOCH2
H
4
HO
5
H
OH
3
H
O
OH
H
1
2
H
OH
β-D-Glucopyranose
pyranose forms of carbohydrates adopt chair
conformations
O
D-Glucose
D-Glucose
6
H 6
HOCH2 H
4
HO
HO
5
HOCH2
O
2
3
H
H
H
OH
1
4
OH
HO
5
H
OH
OH
H
1
2
3
H
H
O
H
HOCH2 H
HO
HO
H
OH
H
β-D-Glucopyranose
H
H
HOCH2 H
O
OH
1
D)
OH
1
H
OH
α-D-Glucopyranose
D-ribose
CH
Which is the most stable conformation of
β -D-galactopyranose?
-D-galactopyranose?
C)
H
O
OH group at anomeric carbon is axial
in α-D-glucopyranose
Question
B)
H
H
β-D-Glucopyranose
all substituents are equatorial in β -D -glucopyranose
A)
HO
HO
OH
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.
D-ribose
D-ribose
76% of the D-ribose is a mixture of the α and βpyranose forms, with the β-form predominating
H
H
H
H
O
HO
OH
OH
H
β-D-Ribopyranose (56%)
HOCH2
H O H
H
O
HO
H
H
OH
H
The α and β-furanose forms comprise 24% of
the mixture.
H
H
OH
OH
1
H
OH
α-D-Ribopyranose (20%)
H
OH
H
OH
OH
β-D-Ribofuranose (18%)
HOCH2
H O H
H
H
OH
OH
OH
α-D -Ribofuranose (6%)
Question
Which conformation of D-ribose is present in
greater amounts in aqueous solution?
A)
Mutarotation
B)
C)
D)
Mutarotation of D-Glucose
Mutarotation
H
HOCH2 H
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 α or β -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.
HO
HO
H
HO
HO
H
H
H
HOCH2 H
O
OH
1
OH
HO
HO
H
H
β-D-Glucopyranose
Initial: [α
[α ]D +18.7°
H
1
OH
HO
HO
H
H
β-D-Glucopyranose
Initial: [α
[α ]D +18.7°
H
O
OH
1
H
OH
α-D-Glucopyranose
Initial: [α
[α ]D +112.2°
Mutarotation of D-Glucose
H
HOCH2 H
O
OH
1
H
OH
α-D-Glucopyranose
Initial: [α
[α ]D +112.2°
Final: [α
[α]D +52.5°
O
OH
H
Mutarotation of D-Glucose
H
HOCH2 H
H
HOCH2 H
HO
HO
H
H
H
HOCH2 H
O
OH
1
OH
HO
HO
H
H
β-D-Glucopyranose
Initial: [α
[α ]D +18.7°
H
O
OH
1
H
OH
α-D-Glucopyranose
Initial: [α
[α ]D +112.2°
Final: [α
[α]D +52.5°
β-D-Glucose Is More Stable
Mutarotation of D-Glucose
H
HOCH2 H
HO
HO
H
H
H
HOCH2 H
O
OH
1
OH
HO
HO
H
H
β-D-Glucopyranose
H
O
OH
1
H
OH
α-D-Glucopyranose
Explanation: After being dissolved in water, the
α and β forms slowly interconvert via the openchain form. An equilibrium state is reached that
contains 64% β and 36% α.
β-D-glucose is the predominant form at equilibrium
Saccharides/ Enzymes:
Question
Glucose & Insulin
When α-D -glucopyranose is dissolved in water
its optical rotation changes from [α
[α]D
+112.2o to +52.5o . What is this phenomenon
called?
A) Anomeric effect
B) Epimerization
C) Mutarotation
D) Tautomerization
Saccharides, Protein binding, Taste Receptors & Sweetness
•Sweet
•Sour
•Bitter
•Salty
•Umami
Sweetness factor = 1.0
Sweetness factor = 0.74