Download Pharmacognosy-I (Part-5) - Home

Carbohydrates
 Carbohydrates
[CX(H2O)Y] are usually
defined as polyhydroxy aldehydes and
ketones or substances that hydrolyze to yield
polyhydroxy aldehydes and ketones.
 Simple carbohydrates are known as sugars
or saccharides (Latin saccharum, sugar) and
the ending of the names of most sugars is –
ose. For example:
Glucose (for the principle sugar in blood)
2. Fructose (for a sugar in fruits and honey)
3. Sucrose (for ordinary table sugar)
4. Maltose (for malt sugar)
1.
Classification of carbohydrates
 Monosaccharide
(C6H12O6)
it is a simple carbohydrate, one that one attempted
hydrolysis is not cleaved to smaller carbohydrates.
e.g. glucose
 Disaccharide on hydrolysis is cleaved to two
monosaccharide, which may be the same or
different. e.g. sucrose
 Oligosaccharide (oligos is a Greek word that
means “few”) yields 3 10 monosaccharide unit on
hydrolysis. e.g. raffinose
 Polysaccharides
are hydrolyzed to more
than 10 monosaccharide units. Cellulose is a
polysaccharide molecule that gives
thousands of glucose molecules when
completely hydrolyzed.
Monosaccharides

Over 200 different monosaccharides are known.
 Monosaccharides
are classified according to:
The number of carbon atoms present in the
molecule.
2. Whether they contain an aldehyde or ketone group.
 A monosaccharide containing three carbon atoms
is called a triose; one containing four carbon atoms
is called a tetrose.
 A monosaccharide containing an aldehyde group is
called an aldose; one containing a keto group is
called a ketose.
1.
O
CH2OH
O
CH2OH
CH
C O
CH
C O
CHOH
CHOH
CHOH
CHOH
(CHOH)n
(CHOH)n
CH2OH
CH2OH
An aldose
A ketose
CH2OH
An aldotetrose
C4
CH2OH
A ketopentose
C5
D and L Designations of
Monosaccharides

The simplest monosaccharide is
glyceraldehyde, which contains a
stereocenter. Therefore, it exist in
two enantiomeric forms.
 In 1906, (+)-glyceraldehyde is
designated D-(+)- glyceraldehyde
and ()- glyceraldehyde is
designated L-()- glyceraldehyde.
 These two compounds serve as
configurational standards for all
monosaccharides.
O
H
C
(R) C
H
OH
CH2OH
(+)-Glyceraldehyde
O
HO
C
(S) C
H
H
CH2OH
(-)-Glyceraldehyde

A monosaccharide whose highest numbered
stereocenter (the penultimate carbon) has the
same configuration as D-(+)- glyceraldehyde is
designated as a D sugar; one whose highest
numbered stereocenter has the same configuration
as L-()- glyceraldehyde is designated as an L
sugar.

D and L designations are not related to the optical
rotations of the sugars to which they are applied.

One may encounter other sugars that are D-(+)- or
D-()- and ones that are L-(+)- or L-()-.
Structural formulas for
Monosaccharides

Fisher projection
 Haworth formulas
  anomer or  anomer
CHO
H OH
H
H
OH
HO
HO
HO
H
O
HO
H
H
OH
O
HO
HO
H
OH
H
H
OH
OH
H
OH
H
OH
H
indicates  or 
CH2OH

Not all carbohydrates exist in equilibrium
with six-membered hemiacetal rings, in
several instances the ring is five
membered.

If the monosaccharide ring is six
membered, the compound is called a
pyranose (e.g. -D-glucopyranose ); if
the ring is five membered, the
compound is designated as a furanose.
(e.g. -D-ribofuranose).
O
Pyran
O
Furan
Mutarotation

The spontaneous change that takes place in the
optical rotation of  and  anomers of a sugar when
they are dissolved in water. The optical rotations of
the sugars change until they reach the same value.
 the explanation for this mutarotation lies in the
existence of an equilibrium between the open-chain
form of D-(+)-glucose and the  and  forms of the
cyclic hemiacetals.
 the concentration of open-chain D-(+)-glucose in
solution at equilibrium is very small.
OH
R
C
OR'
OR'
HCl(g)
R
C
OR'
R' OH
R''
Hemiacetal (R'' may be H)
R''
An acetal (R'' may be H)
 Hemiacetal: a functional group, consisting of a
carbon atom bonded to an alkoxy group and to a
hydroxyl group. Hemiacetals are synthesized by
adding one molar equivalent of an alcohol to an
aldehyde or a ketone.
H2O
Glycoside Formation

Carbohydrate acetals, generally, are called
glycosides, and an acetal of glucose is called a
glucoside.
 The methyl D-glucosides have been shown to have
six-membered ring, so they are properly named
methyl -D-glucopyranoside and -Dglucopyranoside.
 Glycosides are stable in basic solutions because
they are acetals.
 In acidic solutions, glycosides undergo hydrolysis to
produce a sugar and an alcohol (aglycone).
Sugars that contain nitrogen
1. Glycosylamines
A sugar in which an amino
group replaces the
anomeric OH group.
H OH
H
O
HO
NH2
HO
H
Adenosine
is an example
of a glycosylamine that is
also called a nucleoside.
H
OH
H
 -D-Glucopyranosylamine
2. Amino sugars
A sugar in which an amino
group replaces a
nonanomeric OH group.
e.g. D-glucosamine.
H OH
H
O
HO
D-glucosamine
can be
obtained by hydrolysis of
chitin, a polysaccharide
found in the shells of
lobsters and crabs and in
the external skeletons of
insects and spiders.
OH
HO
H
NH2
H
 -D-Glucosamine
H
Other Reactions of Monosaccharides
1. Enolization, Tautomerization, and Isomerization
•
Dissolving monosaccharides in aqueous base
causes them to undergo enolizations and a series
of keto-enol tautomerizations that lead to
isomerizations.
OH
RC
CR2
Enol
O
RC
O
CR2
RC
CR2
Enolate ion

Enols are in equilibrium with an isomeric aldehyde or
ketone, but are normally much less stable than aldehydes
and ketones.

Enolate ion is the conjugate base of an enol. Enolate ions
are stabilized by electron delocalization.

Tautomerizm is a process by which two isomers are
interconverted by a movement of an atom or a group.
Enolization is a form of tautomerism.
2. Formation of Ethers
H OH
H OH
H O
HO
HO
OH
H
H
OH
H O
HO
HO
H
OCH3
H
O
OCH3
H3C
OSO3CH3
Mthyl glucoside
H OH
H O
HO
HO
H
H

OCH3
OCH3
The hydroxyl groups of monosaccharides are more acidic than
those of ordinary alcohols because the monosaccharide contains
so many electronegative oxygen atoms, all of which exert electronwithdrawing inductive effects on nearby hydroxyl groups.
 In aqueous NaOH, the hydroxyl groups are converted to alkoxide
ions and each of these in turn, reacts with dimethyl sulfate to yield a
methyl ether.
3. Conversion to Esters
O
H OH
O
H O
HO
HO
H
OH
H
H OCCH3
(CH3CO)2O
Pyridine, 0°C
H O
H3CCO
H3CCO
H
OH
O
H
OCCH3
O

OCCH3
O
Treating a monosaccharide with excess acetic
anhydride and a weak base (such as pyridine or
sodium acetate) converts all of the hydroxyl groups,
including the anomeric hydroxyl, to ester groups.
4. Oxidation Reactions of Monosaccharides
A. Fehling’s solution or Benedict’s reagent

A characteristic property of an aldehyde function is its
sensitivity to oxidation.
 Carbohydrates that give positive tests with Fehling’s or
Benedict’s reagents are termed Reducing Sugars.
 Ketoses are also reducing sugars, since under the
conditions of the test, ketoses equilibrate with aldoses by
way of enediol intermediates, and the aldoses are oxidized
by the reagents.
O
O
R CH
2Cu2+
5HO
Aldehyde From copper(II) Hydroxide
sulfate
ion
R CO
Carboxylate
anion
Cu2O
3H2O
Copper(I)
oxide
Water
B. Bromine Water: the synthesis of aldonic acids
CHO
(CHOH)n
CH2OH
Aldose

CO2H
Br2
H2O
(CHOH)n
CH2OH
Aldonic acid
Bromine water is a general reagent that selectively
oxidizes the CHO group to a CO2H group.
C. Nitric Acid Oxidation: Aldaric Acids
CHO
(CHOH)n
CH2OH
Aldose

CO2H
HNO3
(CHOH)n
CO2H
Aldaric acid
Dilute nitric acid – a stronger oxidizing agent than
bromine water – oxidizes both the –CHO group and
the terminal –CH2OH group of an aldose to –CO2H
group. These dicarboxylic acids are known as
aldaric acids.
D. Periodate oxidations: oxidative cleavage of
polyhydroxy compounds

Compounds that have hydroxyl groups on adjacent
atoms undergo oxidative cleavage when they are
treated with aqueous periodic acid (HIO4).
 In these periodate oxidations that for every CC
bond broken, a CO bond is formed at each
carbon.
O
C
OH
HIO4
C
OH
2
C
HIO4
H2O

When three or more –CHOH groups are
contiguous, the internal ones are obtained as
formic acid. For example, glycerol
O
H
H
H
C
C
(formaldehyde)
C
H
OH
OH
H
O
2 IO4
(formic acid)
C
H
C
OH
H
H
OH
O
Glycerol
(formaldehyde)
C
H
H

Oxidative cleavage also takes place when an –OH
group is adjacent to the carbonyl group of an
aldehyde or ketone (but not that of an acid or an
ester). For example, glyceraldehyde
O
O
C
H
C
(formic acid)
C
H
H
OH
OH
O
2 IO4
(formic acid)
C
H
C
OH
H
H
OH
O
Glyceraldehyde
(formaldehyde)
C
H
H

Periodic acid does not cleave compounds in which
the hydroxyl groups are separated by an
intervening –CH2 – group, nor those in which a
hydroxyl group is adjacent to an ether or acetal
function.
H2C
OH
IO4
CH2
H2C
OH
H2C
OCH3
HC
H2C
OH
R
IO4
no cleavage
no cleavage
5. Reduction of Monosaccharides: Alditols
CHO
(CHOH)n

CH2OH
NaBH4
(CHOH)n
CH2OH
CH2OH
Aldose
Alditol
Aldoses (and ketoses) can be reduced with sodium
borohydride to compounds called alditols. For
example, D-glucitol (or D-sorbitol)
Disaccharides
 Disaccharides
are carbohydrates that yield
two monosaccharide molecules on hydrolysis.
e.g. sucrose, lactose, maltose
 Structurally,
disaccharides are glycosides in
which the alkoxy group attached to the
anomeric carbon is derived from a second
sugar molecule.
H OH
Sucrose
6
4
5
H
O
HO
HO
H
3
1 H
2
OH
H
2
H
O
H
linkage
OH
5
O
3
Glucosidic

1
HOH2C
6
CH2OH
4
HO
H
Fructosidic
linkage
Ordinary table sugar (C12H22O11)
 Acid hydrolysis yields D-glucose and D-fructose.
 Sucrose is a nonreducing sugar; it gives negative
tests with Fehling’s solution because neither the
glucose nor the fructose portion of sucrose has a
hemiacetal group (both carbonyl groups are
present as full acetals (i.e. as glycosides).
Polysaccharides

Polysaccharides, also known as glycans, consist of
monosaccharides joined together by glycosidic
linkages.
 Polysaccharides that are polymers of a single
monosaccharide are called homopolysaccharides;
those made up of more than one type of
monosaccharide are called heteropolysaccharides.
 Homopolysaccharides are also classified on the
basis of their monosaccharide units. A
homopolysaccharide consisting of glucose
monomeric unit is called a glucan, one consisting of
galactose units is a galactan, and so on.
Polysaccharides
 Three
important polysaccharides, all of which
are glucans, are starch, glycogen, and
cellulose.
 Starch
is the principle food reserve of plant.
 Glycogen functions as a carbohydrate reserve
for animals.
 Cellulose serves as structural material in
plants.
Carbohydrate Antibiotics
 One
of the important
discoveries in
carbohydrate chemistry
was the isolation of the
carbohydrate antibiotic
called streptomycin.
NH
NH
HN
H2N
NH
OH
 Streptomycin
O
CHO
glycosidic linkage is
nearly always  .
L-Streptose
H3C
HO
HO
O
O
R
 The
Streptidine
OH OH
O
is made
up of three unusual
components.
NH2
R'
2-Deoxy-2-methylamino-L-glucopyranose
HO
Streptomycin
R = NHCH3
R' = CH2OH