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17
molecule flips over to the corresponding
1
C
4
chair form. This happens in
alginate, where the M (D-mannuronic acid) residues are in the
4
C
1
(only OH(2)
is axial)), but epimerisation at C5 moves the heavy carboxylate group (-COO
-
)
to the axial position, which is too unstable. The G (L-guluronic acid) residues
therefore flip to the more stable
1
C
4
chair form:
1.1.11. A
strategy
for determining
the
ring
form:
a) Compare the Haworth formula of the unknown sugar with that of D-glucose
b) Write the
4
C
1
form of
β
-D-glucose – all heavy groups are equatorial
c) Try to add the substituents of the unknown sugar to the glucose formula.
Substituents that are different in the Haworth formulae are positioned in the
axial positions
d) Judge the situation: Will the number of axial groups decrease by flipping to
1
C
4
?
Example:
First, draw
β
-D-glucose in its
stable
4
C
1
form:
O
OH
OH
OH
O
-
O
O
OH
HO
O
-
O
OH
HO
M (
4
C
1
): Stable
O
OH
HO
O
-
O
OH
HO
G (
4
C
1
): Unstable
G (
1
C
4
): Stable
C5-epimerization
1
2
3
4
6
5
O
CH
2
OH
OH
OH
OH
HO
O
CH
3
OH
O
HN
HO
O
CH
3
NO
2
Unknown (
α−
L-crazyose)
(2-nitro-3-acetamido-6-deoxy-L-galactose
β−
D-glucose