a solution of the molecule rotates plane-polarized light either to the right or left. An
improvement upon this designation is the D/L method for determining relative config-
uration. The D/L system uses the stereoisomers of glyceraldehyde as configurational
standards: (+)-glyceraldehyde is designated as having the D configuration, and
(−)-glyceraldehyde is designated as having the L configuration. Other molecules are then
compared to these two glyceraldehyde standards and are designated as either D or L
(relative to glyceraldehydes), depending on which isomer they superimpose upon best.
The R/S system is yet another improvement, since it determines absolute configuration.
In this system, the four groups of atoms attached to a central chiral carbon are priori-
tized in accord with the Cahn–Ingold–Prelog sequence rules; the lowest priority group
is directed towards the “rear” of the molecule and a curved arrow unites the other
groups from highest to lowest priority. If the arrow is clockwise, the configuration is R;
if the arrow is counterclockwise, the configuration is S.
The nomenclature and structure of monosaccharides is additionally complicated by
further stereochemical issues arising from the fact that monosaccharides may exist in
either “open-chain” or “cyclic” forms. In water solution, a monosaccharide can undergo
an intramolecular reaction to produce cyclic hemiacetals: either five-membered ring
hemiaceteals (called furanoses) or six-membered ring hemiacetals (called pyranoses)
can exist. An extensive body of experimental evidence reveals that one of the most
common monosaccharides, D-glucose, exists as an equilibrium between an open-chain
structure and two cyclic forms. The two cyclic forms are hemiacetals produced by an
intramolecular reaction of the –OH group at the C5 atom with the aldehyde group. This
cyclization creates a new stereocentre at the C1 position; the two cyclic forms are
diastereomers that differ in the configuration of C1. Diastereomers of this type are
calledanomers, and the hemiacetal carbon atom is referred to as the anomeric carbon
atom. Each D-glucose anomer is designated as either an α-anomeror a β-anomer,
depending upon the location of the –OH group on the C1 atom.
8.1.5.3 Types of Carbohydrates
There are many different monosaccharides and oligosaccharides. Glucose(also called
dextrose) is the most important monosaccharide; it is sometimes called blood sugar
because it is the principal monosaccharide of human blood. Fructose(also called levu-
lose) is the sweet-tasting sugar found in honey and fruit. Riboseanddeoxyriboseare
monosaccharides that form part of the polymeric backbone of nucleic acids. Maltose,a
disaccharide used in baby foods, is composed of two D-glucose building blocks.
Lactose, a naturally occurring disaccharide found in mammalian milk, is composed of
two different monosaccharides, D-glucose and D-galactose. Sucrose (common table
sugar) is also composed of two different monosaccharides, D-glucose and D-fructose.
Glycogenis a polysaccharide, found primarily in liver and muscles, that is used as a
storehouse for glucose in animal systems. Structures of some common carbohydrates
are given in figure 8.4.
Carbohydrates are relevant in medicinal chemistry. The outdated notion that carbo-
hydrates serve only as energy sources in animals and structural materials in plants is no
longer tenable. Complex carbohydrates are biochemically important. Carbohydrates, when
joined via glycosidic linkages to either proteins or lipids (to produce glycoproteinsor
476 MEDICINAL CHEMISTRY