Chemistry Reference
In-Depth Information
conformers, in that the left-hand conformer of glucose
is termed
4
C
1
, and the right-hand one
1
C
4
.The'C'
indicates chair conformation, the superscript numeral
is the carbon atom that is above the plane of the
ring, and the subscript numeral is which carbon atom
is below the plane of the ring. For this description,
we consider the pyranose ring as originally planar,
distorted to a chair by pushing carbons 1 and 4 out
of the plane.
OH
OH
OH
4
OH
O
HO
HO
1
O
1
OH
4
HO
OH
OH
β
-
D
-glucose
4
C
1
conformer
1
C
4
conformer
At
first
glance,
the
preferred
conformation
for
we can easily follow the anomeric substituent. Since
α
L
-hexoses, e.g.
-
L
-rhamnose, appears different from
that of the
D
-hexoses. This is readily rational-
ized by considering the preferred conformation of
α
-
L
-glucose - the
α
-anomer is chosen simply because
α
-
L
-glucose is the enantiomer of
α
-
D
-glucose, we can
draw the mirror image representation, then rotate this
so that the heterocyclic oxygen comes to the required
position.
mirror image
rotate 180º
OH
OH
HO
O
O
HO
HO
HO
OH
OH
≡
O
HO
OH
HO
HO
HO
OH
HO
-
D
-glucose
preferred conformation
α
α
-
L
-glucose
α
-
L
-glucose
Note that you may also encounter another ver-
sion of the cyclic form referred to as the
Haworth
representation
(see Box 3.16). This shows the ring
as a planar system, and is commonly used in
biochemistry topics. However, we know that five-
membered and six-membered rings are certainly not
planar. The Haworth representation nicely reflects
the up - down relationships of the various substituent
groups, but is quite uninformative about the shape of
the molecule, and whether the substituents are equa-
torial or axial. In really bad cases, authors omit the
hydrogen atoms, giving an ambiguous structure - do
lines mean methyl or hydrogen? Haworth represen-
tations may be easier to draw, but you are strongly
encouraged to use the more informative conforma-
tional structures.
β
-
D
-glucose
β-
D
-ribose
CH
2
OH
OH
O
OH
OH
HOH
2
C
HO
OH
H
O
O
O
H
HO
HO
H
OH
H
OH
OH
H
H
HO
H
HO
OH
HO
OH
H
OH
H
Haworth
representation
conformational
representation
Haworth
representation
conformational
representation
CH
2
OH
OH
HOH
2
C
O
OH
often seen, but careless
versions that omit
hydrogens; don't think
the lines mean methyls!
O
OH
OH
HO
OH
OH
