INTRODUCTION GLYCOSIDES

1. INTRODUCTION

Glycosides, in general, are defined as the condensation products of sugars with a host of different varieties of organic hydroxy (occasionally thiol) compounds (invariably monohydrate in character), in such a manner that the hemiacetal entity of the carbohydrate must essentially take part in the condensation. It is, however, pertinent to state here that the polysaccharides are also encompassed in this broad-based overall definition of glycosides. The noncarbohydrate moiety is usually termed as aglycone (or aglycon), or a genin.

The rather older or trivial names of glycosides usually has a suffix ‘in’ and the names essentially included the source of the glycoside, for instance: strophanthidin from Strophanthus, digitoxin from Digitalis, barbaloin from Aloes, salicin from Salix, cantharidin from Cantharides, and prunasin from Prunus. However, the systematic names are invariably coined by replacing the “ose” suffix of the parent sugar with “oside”. The stereochemical anomeric prefix a or b and the configurational prefix (D- or L-) immediately precede the sugar nomenclature, and lastly the chemical name of the aglycone precedes the name of the sugar. It may be expatiated with the help of the following examples:

(a) Aloin (or Barbaloin): 10-Glucopyranosyl-1, 8-dihydroxy-3-(hydroxymethyl)-9 (10H)-anthracenone;

(b) Salicin: 2-(Hydroxymethyl) phenyl- -D-glucopyranoside;

(c) Amygdalin: D-Mandelonitrile–β-D-glycosido-6- β-D-glucoside;

(d) Digitoxin: 3-[0-2, 6-Dideoxy- β-D-ribo-hexopyranosyl -(1-> 4)-O-2, 6-dideoxy-β-D-ribohexopyranosyl (1->4), 2, 6-dideoxy-β-D-ribo hexopyranosyl) oxy]-14-hydroxycard- 20(22)-enolide.

Interestingly, the glycosides may be regarded as internal acetate. The two series of stereoisomeric glycosides are usually termed as β and β glycosides. Thus, taking into consideration the simple example of methyl D-glucosides, these a and b structures may be represented as shown below:

Figures 4.1 and 4.2 represent the open-chain structure, cyclic structure and boat configuration

of methyl–a-D-glucoside and methyl–β-D-glucoside respectively. In this particular instance the glycosidic likage is established by dehydration involving a hydroxy group of the aglycone portion (i.e., methyl alcohol) and the hydroxyl group on the hemiacetal carbon of the carbohydrate (in question), thereby ultimately resulting into the formation of an acetal type of structure.

α-Configuration If the —OCH₃ moiety (generalized as OR′) is in opposite steric sense as the—CH₂OH moiety on C-5 (for D-family sugars), the glycosidic structure is designated as α-configuration.

β-Configuration If the -OCH₃ moiety is in the same steric sense as the CH₂OH group on C-5, the glycosidic structure is designated as β-configuration.

It has been observed that the substantial quantum of naturally occurring glycosides essentially possess the stereo-configuration. However, this observation may be further expatiated with the help of the following typical examples of β-amygdalin and β-salicin:

Sailent Features (b-Amygdalin):

(i) Glycosidic linkage is b-because it is hydrolysed by emulsin (an enzyme),

(ii) The linking oxygen is on the same side of the plane of the ring as the CH₂OH moiety on C-5.

(iii) It contains several asymmetric C atoms i.e., chiral centres, and

(iv) It is optically active.

Salient Features (b-Salicin):

1. Hydrolysed by emulsin, hence it has b-configuration,

2. The linking oxygen is on the same side of the plane of the ring as the CH2OH moiety on C-5,

3. It has several chiral centres, and

4. It is optically active.

Glycosides, are found to exert a wide spectrum of therapeutic actions, both in modern medicines and in traditional medicaments, ranging from cardiotonic, analgesic, purgative, and anti-rheumatic, demulcent and host of other useful actions.

The Glycosidic Linkages The exact point of linkage between the carbohydrate (sugar) and non carbohydrate (aglycone) moieties is an ‘oxygen bridge’ that essentially connects the reducing group present in carbohydrate to either an alcoholic or a phenolic group present in the non carbohydrate.

Such glycosides are collectively termed as O-glycosides. However, if the ‘O’ is replaced by ‘S’ it is called S-glycosides; if replaced by ‘N’ is known as N-glycosides; and if replaced by ‘C’ is termed as C-glycosides.

These four types of glycosides shall be described briefly at this juncture with appropriate examples from the domain of medicinal plants.

Source: Pharmacognosy And Pharmacobiotechnology By Ashutosh Kar