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Carbohydrates 375

It was fortunate that the arbitary assignment was later
confirmed by X-ray crystallography. Otherwise, all of
the configurations assigned to carbohydrates during the
60 years that followed Fischer’s assignment would have
been in error, causing confusion and chaos in the chemical
literature.
FIGURE 2 Models and formula of D-Erythrose. The two models Once the absolute configuration at C-2 of the D-(+) iso-
on the left are rotamers of the compound on the right.
mer of glyceraldehyde had been established, it could be
related to the configuration of the corresponding center in
any aldose that is obtained from this isomer by ascending
with the carbonyl group at (or nearest to) the top and the
the series or that yields this isomer by repeated descending
last carbon atom in the chain (i.e., the one farthest from
degradations (Fig. 4). It should be stated at this point that,
the carbonyl group) at the bottom. (2) Each carbon atom
because C-2 in glyceraldehyde corresponds to the chiral
is rotated around its vertical axis until all of the vertical
center farthest from the carbonyl group in any aldose or ke-
(C C) bonds in the chain lie below an imaginary curved
tose, it is possible to group monosaccharides into two fam-
plane such as that of a rolled piece of paper, and all of
ilies: one related to D-(+)-glyceraldehyde and designated
the horizontal bonds (parallel to the x axis) lie above the
by the prefix D, and one related to L-(−)-glyceraldehyde
plane of the paper. The curved plane is then flattened, and
the projection of the molecule is represented as viewed and designated by the prefix L. Monosaccharides of the D
family all have the (R) configuration at the chiral center
(see Fig. 2).
farthest from the carbonyl (the OH attached to this chiral
carbon is to the right in a Fischer projection). Conversely,
a. Relative and absolute configuration. The rela-
monosaccharides of the L family have the (S) configura-
tive configuration of D-glucose was established by Emil
tion at this center (the OH group is to left).
Fischer in 1891 and constituted at the time a monumental
achievement, for which he earned a Nobel prize. Nowa-
b. Configuration of the acyclic form of aldoses.
days, the determination of the absolute configuration of
Fischer used an ingenious method to determine whether
a monosaccharide offers no difficulty, because the con-
in an aldose molecule, the configurations of the chiral cen-
figurations of a large number of related compounds are
ters that are equidistant from the center (e.g., C-2 and C-4
available. The unknown is simply converted to a com-
in a pentose) had the same sign. He determined whether
pound of known configuration by means of reactions that
conversion of the two groups situated at the “top” and
do not affect the configuration at the chiral center(s).
the “bottom” of an aldose molecule (CHO and CH 2 OH)
Since D-glucose is an aldohexose, it must possess four
4
chiral carbon atoms and can exist in 2 = 16 stereoiso- into the same type of group, for example, a carboxylic
group, rendered the product achiral (afforded a meso com-
mers (see Table I). In order to determine which of these
pound). If so, he could conclude that a plane of symmetry
isomers is actually D-glucose, a reference compound of
was created during the conversion (oxidation) and that the
known absolute configuration is needed, which can be
configuration of the chiral centers situated at equal dis-
prepared from, or converted to, D-glucose. Because the
tances from this plane of symmetry was identical. He also
first determination of the absolute configuration of an or-
reasoned that, if the dicarboxyclic acid possessed an axis
ganic compound had to await the advent of X-ray crystal-
of symmetry, it could be obtained from only one aldose,
lographic techniques developed in the middle of the twen-
whereas if it lacked an axis, it could be obtained from two
tieth century, such a reference compound did not exist in
different aldoses, an aldose having the aldehyde group at
Fischer’s time. This is why he was able only to propose
the top and the primary hydroxyl group at the bottom, and
a relative configuration for the monosaccharides known
another aldose having the aldehyde group at the bottom
in his time. He chose, as the reference compound, the
and the primary hydroxyl group at the top. Using this rea-
dextrorotatory form of glyceraldehyde, now designated D-
soning. Fischer was able to determine the configuration of
(+)-glyceraldehyde, and arbitarily assumed that the OH
all the aldoses known in his time.
attached to C-2 in this compound is to the right when
represented by a Fischer projection formula (see Fig. 3).
c. Cyclic structures and anomeric configuration.
Soon after the configuration of the acyclic form of glu-
cose and the other aldoses had been established, it be-
came apparent that these structures could not represent
the major components of the equilibrium mixture. Thus,
the IR spectrum of D-glucose does not exhibit a strong
1
−1
FIGURE 3 Fischer projection formula of D-glycer-aldehyde. carbonyl band at 1700 cm , and its H NMR spectrum