Download Lecture 4 - Sugars, ring structures

Wood Chemistry
PSE 406
Lecture 4
More Sugars: Disaccharides and Rings
PSE 406 - Lecture 4
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Hemiacetal Formation
A hemiacetal is a compound having the C(OH)OR group that results
from the reaction of an aldehyde with one mole of an alcohol.
Sugar molecules contain an aldehyde group and lots of alcohol groups.
PSE 406 - Lecture 4
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Hemiacetal Formation in
Sugars (1)
CH2OH
H
C
C
H
OH
HO
CH2OH
O
..
H
H
C
C
C
H
OH
O
H
H
C
C
H O
..
OH
C
C
H
OH
H
C
C
H
OH
HO
CH2OH
HO
H
O
OH
C
H
CH2OH
H
H
HO
C
C
C
H
OH
O
H
C
C
H
H O
OH
C
OH
H
C
C
H
OH
H
Notes
PSE 406 - Lecture 4
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Hemiacetal Formation in
Sugars (2)
β-D-glucopyranose
α-D-glucopyranose
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Important Monosaccharide
Projections
CHO
H
HO
CH2OH
O
OH
H
H OH
H
O
HO
H
H
OH
OH
OH
CH2OH
D-glucose
Fisher
HO
H
H
OH
HO
H
OH
HO
OH
α-D-glucopyranose
Haworth
α-D-glucopyranose
Chair Configuration
Notes
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Conformation: The Chair
a
a=axial
e=equatorial
e
HO
CH2OH
O
HO
OH
Chair
OH
-d-glucopyranose
β-D-glucopyranose
The most stable form of a sugar in a six membered ring is in the chair formation.
The substituent groups on the molecule are either in an axial or equatorial position
depending on what gives the most stable molecule.
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Conformation: Other Forms
Skew Boat
Boat
We won’t be worrying about these forms.
PSE 406 - Lecture 4
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The Haworth Projection
 This is a very common
way that wood
chemists draw sugars
in the ring form.
 In this projection, the
thick bonds are
perpendicular to the
plane of the paper
CH 2OH
O
OH
OH
HO
OH
 Substituents are
parallel to the plane of
the paper and lie either
above or below the
plane of the molecule
PSE 406 - Lecture 4
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Formation of Anomers
 Ring closure creates new
chiral center at C1; new
pair C1 epimers termed
anomers. For D sugars,
the following is true:
» alpha form: OH below the
ring
» beta form: OH above the
ring
 For L sugars, the opposite
is true.
H OH
H O
HO
H
HO
H
OH
H
OH
a-D-glucopyranose
H OH
H O
HO
OH
HO
H
H
-D-glucopyranose
PSE 406 - Lecture 4
OH
H
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Nomenclature
H
O
H H
OH
HO
H
CH2OH O
OH
H
OH
-D-xylopyranose
H
OH
OH
H
OH
H
H
OH
H
OH
H
H
CH2OH O
O H
HO
H H
-D-xylofuranose
H
H H
OH
OH
H
H
OH
OH
a-D-xylofuranose
a-D-xylopyranose
PSE 406 - Lecture 4
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Mutarotation (1)
CH2OH
O
H H
OH
HO
CH2OH
OH
H H
OH
HO
H
H
H
OH
H
OH
H
O
H
H
H
C
OH
H
CH2OH
OH
H H
OH
HO
H
H
HO
CH(OH)2
H
H
OH
d-glucose (hydrate form)
CH2OH
OH
O
OH
H
H
OH
H
OH
a-d-glucofuranose
OH
O
H
OH
d-glucose (open chain)
CH2OH
HO
OH
a-d-glucopyranose
CH2OH
OH
H H
OH
HO
H
H
H
-d-glucopyranose
Notes
O
H
-d-glucofuranose
PSE 406 - Lecture 4
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Mutarotation (2)
 Note: the cyclic form must contain a hemi-form in
order to have a migratory hydrogen and therefore be
capable of ring-opening (that is, mutarotation).
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Fischer to Haworth
 In order to convert a Fischer projection of a sugar to a
Haworth project, certain rules must be followed in
order to maintain correct orientation of the molecule.
 Remember, the horizontal bonds in the Fischer
projection (solid wedges) are out of the plane and the
vertical wedges (broken wedges) are into the plane.
CH3
H
C OH
CH3
H
OH
C2H5
C2H5
PSE 406 - Lecture 4
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Fischer to Haworth
CHO
H
OH
H
1
OH
H
OH
H
OH
H
2
OH
H
OH
H
OH
H
3
OH
HO
4
H
HO
H
H
OH
CH2OH
HO
H
H
O
HOH2C
5
H
6
CH2OH
O
5
HO
4
1
3
OH
2
6
CH2OH
O
OH
OH
 Modify the Fischer formula so that all of the ring atoms lie along
a vertical line. When rotating substituents around a chiral
carbon, to keep the correct configuration, after a 90 rotation, the
opposite substituents must be switched.
 Proceed around the Haworth formula placing the groups on the
left above the hexagon and those on the right below the plane.
PSE 406 - Lecture 4
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Linkages Between Sugars
 In the tree, monosaccharides
are linked through
enzymatic processes.
 Linkage proceeds through a
dehydration process (loss of
H2O) (acetal formation).
 The bond between the
glucoses units in cellobiose
is a glycosidic bond. Once
linked, the glucose unit on
the left is no longer a
hemiacetal, it is now an
acetal.
OH
CH2OH
O
O
OH
OH
HO
OH
O
OH
CH2OH
Cellobiose
4-O-(-D-Glucopyranosyl)-D-Glucopyranose
 Cellobiose is disaccharide
produced from the partial
hydrolysis of cellulose. It is
not naturally found in wood.
 The squiggly bond on C1
above means it can be either
a or .
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Linkages Between Sugars
 The glycosidic linkage
between the glucose units is
between the  C1 of one
glucose to C4 of the other
glucose. So this bond is a
(14) linkage.
 The glucose molecules in the
drawing on the right are
opposite to make drawing
the (14) linkage easy.
OH
CH2OH
O
O
OH
OH
HO
OH
O
CH2OH
OH
Cellobiose
4-O-(-D-Glucopyranosyl)-D-Glucopyranose
C6
CH 2OH
C5 O
C4
HO
PSE 406 - Lecture 4
C1
OH
C3
C2 OH
OH
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More Disaccharides
 Neither of these sugars
are found in wood.
Sucrose occurs
naturally in sugarcane
and in sugar beets.
Maltose is formed
through the
degradation of starch.
CH2OH
CH2OH
O
O
HO
OH
O
HO
CH2OH
OH
OH
Sucrose
a-D-Glucopyranosyl--D-Fructofuranoside
CH2OH
CH2OH
O
O
OH
OH
OH
O
HO
OH
OH
Maltose
4-O-(a-D-Glucopyranosyl)-D-Glucopyranose
PSE 406 - Lecture 4
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