136
chapter 9
Simple Carbohydrates
C H 2OH
c=o
— O H
HO— c — H
4I
H— C — O H
5I
h — <ji—
o h
c h 2o h
D -G Iucose
A lin ear re p re sen ta tio n of o -g lu c o s e
C H 2O H
%
O H
l z
O H
a -D -G lu c o p y ra n o se
C H ,O H
( o h
: 1
O H
H /?
—
Q H
H
O H
A
L / H
Ip n
*?/s>
p -D -G lu co p y ran o se
i o \ T
1/
u
c
F I G U R E 9 - 4
I 3
Ï2
Haworth projection formulas of anomers of D-glucopyranose. The thick
H
O H
line of the structure projects out toward the observer, and the upper edge
A m odified re p re se n ta tio n of th e
D -glucose m o lecu le, sh o w in g th e
form ation of th e h e m ia ce ta l linkage
(thin line) projects behind the plane of the paper.
A n o m eric c arb o n
6C H 2O H
6 C H 2O H
1
si
n
* 4 ________
n
M
VH
Jr
r°H
$
\L°
»
< r
V3
I 3
1
1
1
H
O H
1
H
O H
a - D -G lu co p y ran o se
p -o -G lu co p y ran o se
a - a n d p -A n o m ers of d -g lu co p y ran o se
FIGURE 9-3
Formation of
a -
and /i-anomers from D-glucose.
the
anomeric carbon,
when the carbon of the carbonyl
group reacts with the C
5
hydroxyl group. The two pos-
sible stereoisomers resulting from the cyclization are
called a- and /1-anomers (Figure 9-3). Aldohexoses in
their cyclic forms have five asymmetrical centers and
therefore 32 stereoisomers. In other words, each of the
16 isomers that belong to the D or L series has two
anomeric forms. The systematic names for these two
anomers are a-D-glucopyranose and /3-D-glucopyranose.
Three-dimensional
representations
of ring
structures
are frequently shown as Haworth projection formulas
(Figure 9-4), in which the lower edge of the ring is pre-
sented as a thick line, to indicate that this part of the struc-
ture projects out toward the observer, and the upper edge
as a thin line that projects behind the plane of the paper.
Carbon atoms of the ring are not explicitly shown but
occur at junctions of lines representing bonds. Sometimes
the hydrogen atoms are also omitted and are presumed
to exist wherever a bond line ends without a specified
group.
The pyranose ring is not planar, being similar to that of
cyclohexane. The bond angles in cyclohexane are similar
to those between the bonds of a tetrahedral carbon (i.e.,
109°). In the pyranose ring, all bond angles are similar to
those of cyclohexane, including the hemiacetal C-O-C
bond angle, which is 1110. Most pyranoses occur in the
chair conformation, in which most of the substituents can
assume equatorial positions (i.e., lie approximately in the
same plane as the ring) instead of axial positions (i.e.,
lie approximately vertically above or below the plane of
the ring). In the equatorial positions, the bulky substituent
groups (-OH, -CH
2
OH) can more easily be accommo-
dated than in the axial positions, and the preferred con-
formation is usually the chair conformation (Figure 9-5).
FIGURE 9-5
Conformational formulas for the boat and chair forms of pyranose.
