section
9.1
Classification
135
Ç H O
C H O
— -2C — O H
H O » -2C — H
3!
3
!
C H 2O H
C H 2OH
(a)
C H O
C H O
-O H
H O -
CH2O H
CH2OH
ff»
C H O
1
C H O
HÇOH
1
H O C H
C H 2O H
1
C H 2OH
d
-G ly cerald eh y d e
1
-G ly cerald eh y d e
(c)
F IG U R E 9 -2
Stereoisomers of glyceraldehyde. (a) Perspective formulas showing
tetrahedral arrangement of the chiral carbon 2 with four different
substituents, (b) Projection formulas in which the horizontal substituents
project forward and the vertical substituents project backward from the
plane of the page, (c) A common method of representation.
monosaccharide with multiple asymmetrical centers is the
net result of contributions from the rotations of each opti-
cally active center. Thus, the prefix D or L provides no in-
formation with regard to optical rotation; it indicates only
the configuration around the asymmetrical carbon atom
located farthest from the carbonyl carbon.
The numbering system for monosaccharides depends on
the location of the carbonyl carbon (or potential carbonyl
carbon), which is assigned the lowest possible number.
For glucose (an aldohexose), carbon Q bears the carbonyl
group and the farthest asymmetrical carbon atom is C
5
(the
penultimate carbon), the configuration around which de-
termines the
D
and
L
series. For fructose (a ketohexose),
C 2,
bears the carbonyl group and
C 5
is the highest num-
bered asymmetrical carbon atom. Glucose and fructose
have identical configurations around
C 3
to Cg.
The structure of glucose, written in straight-chain form
(Figure 9-1), shows four asymmetrical centers. In general,
the total number of possible isomers with a compound of
n
asymmetrical centers is 2". Thus, for aldohexoses having
four asymmetrical centers, 16 isomers are possible,
8
of
which are mirror images of the other
8
(
enantiomers
).
These two groups constitute members of the
D
and L se-
ries of aldohexoses. Most of the physiologically important
isomers belong to the
D
series, although a few L-isomers
are also found. In later discussions, the designation of
D
and
L
is omitted, and it is assumed that a monosaccha-
ride belongs to the D series unless it is specifically desig-
nated an L-isomer. Of the D-series of aldohexoses, three
are physiologically important: D-glucose, D-galactose, and
D-mannose. Structurally, D-glucose and D-galactose dif-
fer only in the configuration around
C 4 ;
D-glucose and
D-mannose differ only in the configuration around C2.
Pairs of sugars (e.g., glucose and galactose, glucose and
mannose), which differ only in the configuration around
a single carbon, are known as
epimers.
D-Galactose and
D-mannose are
not
epimers, since they differ in configura-
tions around both C
2
and C
4
(Figure 9-1). D-Fructose, one
of eight
2
-ketohexoses, is the physiologically important
ketohexose. Monosaccharides with five or more carbons
occur predominantly in cyclic (ring) forms owing to a re-
action between the carbonyl group (aldehyde or ketone)
and an alcohol group:
R,— CCT
+HO—R,
H
I
R —Ç—O—R2
OH
A ld e h y d e
A lco h o l
H e m ia c e ta l
0
R ,— C — R 2 + H O — R3
K e to n e
A lco h o l
O R 3
I
R ,— C — R 2
I
OH
H em ik etal
Formation of the cyclic forms of monosaccharides is fa-
vored because these structures have lower energies than
the straight-chain forms. Cyclic forms of D-glucose are
formed by the hemiacetal linkage between the Ci aldehyde
group and the
C 4
or
C 5
alcohol group. If the ring structure
is formed between Ci and C
4
, the resulting five-membered
ring structure is named D-glucofuranose because it resem-
bles the compound furan:
HV /H
° r
o
H
H
If the ring structure is formed between C| and C
5
, the
resulting six-membered ring is named D-glucopyranose
because it resembles the compound pyran:
H C — O
HC/ /
/XCH
or
\
/ /
C - C
H2 H
f/
/
Aldohexoses exist in solutions mainly in six-membered
pyranose ring forms, since these forms are thermodynam-
ically more stable than furanose ring forms.
Cyclization of a monosaccharide results in the for-
mation of an additional asymmetrical center, known as
