Glycosides the sugars

In steroidal glycosides, the sugar moiety is joined to the cyclopentanoperhy-drophenanthrene (steroid) nucleus. Steroidal glycosides include the cardeno-lides (cardiac glycosides) and saponins (sapogenic glycosides). 

Anthocyanins. Like the flavones. the anthocyanins are found throughout nature. This class uf polyphenolic compounds is responsible for the pink, red. violet, and blue colors found in plants. Like many other natural phenulic substances, anthocyanins occur in plants as glycosides the sugar-free anthocyanins are known as anihocyanidins. 

As in the case of glycosides, the sugar residue of a nucleoside is presumed to have either the furanose or the pyranose ring-structure. 

In preparing the various sugars used in the condensation reactions discussed above, a variety of sugar transformations has been carried out on glycosides. The sugar transformations performed on the purine nucleosides, themselves, will be discussed in this Section. 

Most flavan derivatives occur in plants as glycosides. The sugars are attached to the hydroxyls of the A ring and of the heterocycle, attachment to the hydroxyl of the C atom 3 occurring particularly frequently. 

D-galactose, C HiiOe. Crystallizes in the pyranose form m.p. 1I8-120 C (monohydrate), 165-5" C (anhydrous). An isomer of glucose which is fairly widely distributed in plants. It is a constituent of raffinose and slachyose, of hemicelluloses, of pectin, of gums and mucilages, and of some glycosides. In animals it forms half the lactose molecule and is the sugar found in the brain. Chemically it is very similar to glucose. It has the structure... 

D-glucose, dextrose, C Hi20 . The most common hexose sugar. It is present in many plants, and is the sugar of the blood. It is a constituent of starch, cellulose, glycogen, sucrose and many glycosides, from all of which it can be obtained by hydrolysis with acids or enzymes. 

In 1885, from a detailed study of juglone (52) it was proposed that its stmcture was 5-hydroxy-l,4-naphthoquinone (9). This stmcture was confirmed by oxidizing 1,5-dihydroxynaphthalene with potassium dichromate in sulfuric acid (53). Juglone occurs in walnuts as a glycoside of its reduced form, 1,4,5-trihydroxynaphthalene (54). Later it was deterrnined that the sugar is in the 4-position (10) (55). 

In general, the dyes occur as glycosides, the most common sugar being glucose. Some flavones contain more than one sugar. Their role as dyes in the plant is not definitely known a common suggestion is that they protect the plant from harm fill uv radiation. 

By far the majority of carbohydrate material in nature occurs in the form of polysaccharides. By our definition, polysaccharides include not only those substances composed only of glycosidically linked sugar residues but also molecules that contain polymeric saccharide structures linked via covalent bonds to amino acids, peptides, proteins, lipids, and other structures. 

Nucleosides are compounds formed when a base is linked to a sugar via a gly-cosidic bond (Figure 11.10). Glycosidic bonds by definition involve the carbonyl carbon atom of the sugar, which in cyclic structures is joined to the ring... 

Nucleosides are much more water-soluble than the free bases because of the hydrophilicity of the sugar moiety. Like glycosides (see Chapter 7), nucleosides are relatively stable in alkali. Pyrimidine nucleosides are also resistant to acid hydrolysis, but purine nucleosides are easily hydrolyzed in acid to yield the free base and pentose. 

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