GLYCOSIDES

GLYCOSIDES,
a group of carbohydrate-containing substances formed during the condensation reaction of cyclic mono- and oligosaccharides with alcohols, phenols, thiols and amines, widely present in living organisms, especially in plants.
Many glycosides that have no natural analogues have also been synthesized. Glycosides are characterized by the ability to hydrolyze (i.e., split in a reaction with water) with the formation of one or more sugar residues and a non-carbohydrate substance, the so-called aglycone. Hydrolysis is carried out in warm water in the presence of specific enzymes or by boiling with dilute acids. Some types of glycosides are also hydrolyzed when heated with dilute alkali solutions. Also on topic:
ORGANIC CHEMISTRY

The term "glycoside" comes from the Greek. "glikos" meaning "sweet". This class is sometimes mistakenly called glucosides, but glucosides are only those glycosides whose hydrolysis releases only glucose (dextroglucose, or dextrose) as the only sugar component. The names of natural glycosides have the suffix -in, and the root is derived from the scientific or folk name of the plant or plant product in which the glycoside was first discovered: for example, gitagin from Agrostemma githago

(cockle), hederin from
Hedera helix
(ivy).

Saponin glycosides

(saponins) are a class of substances that, like soap, form foam when their aqueous solutions are shaken. Hence their name: “sapo” means “soap” in Latin. As a rule, saponins are amorphous, soluble in water and alcohol, neutral substances with an irritating, pungent taste. When hydrolyzed, they produce aglycones (sapogenins) with a fairly large molecular weight and relatively high amounts of sugars. Saponins are widely distributed in the plant world, especially among plants of the Rosaceae and Carnation families (soapwort of the genus Saponaria

).

Saponins act on the body in a characteristic way: 1) when they get on the nasal mucosa, they cause sneezing; 2) cause the formation of hematomas (hemolysis); 3) are a deadly poison for fish and lower animals; 4) significantly reduce the surface tension in liquids that serve as solvents. Saponins and saponin-containing materials are widely used in pharmacy, medicine and technology. They are used as detergents, especially for silk and other valuable fabrics, as poisons for fish and insects, and in fire extinguishers (to stabilize foam). Examples of saponins are digitonin from foxglove, sarsaponin from sarsaparilla (smilax officinalis or smilax chinensis), and trillin from trillium (crow's eye, a plant in the lily family).

GLYCOSIDES

GLYCOSIDES (from the Greek glykys-sweet and eidos-kind), compounds, in which the remainder is cyclic. forms of mono- or oligosaccharide (glycosyl, or carbohydrate, residue) associated with another org. residue (aglycone) through a heteroatom; resp. distinguish O-, N-, S-glycosides, etc. The connection between the glycosyl residue and the aglycone is called. glycosidic.

When glycosides are formed, a new asymmetric structure appears. glycosidic center. Its configuration is designated depending on whether it coincides or not with the configuration of the carbon atom of the monosaccharide, which determines whether the latter belongs to the D- or L-series. For example, in isomeric methyl-O-glucopyranosides, the configuration is reflected by f-loy I, configuration by f-loy II:

Based on the size of the carbohydrate residue cycle, glycosides are divided into furanosides (5-membered), pyranosides (6-membered) and septanosides. (7-); according to the number of monosaccharide residues in the carbohydrate part of the molecule - into monoosides, biosides, triosides and oligosides (respectively derivatives of mono-, di-, tri- and oligosaccharides).

The addition of a glycosyl residue to an aglycone (glycosylation) increases the hydrophilicity of the compound, which plays an important role in metabolism. Mn. carbohydrate residues, especially oligosaccharides, perform specific functions. markers of cell surfaces and biopolymers that determine their recognition by other cells.

O-glycosides in the broad sense of the word include not only glycosides with non-carbohydrate aglycones, but also internal. sugar anhydrides (internal glycosides), oligo- and polysaccharides. O-Glycosides are low-volatile crystalline. or amorphous substances. Glycosides of lower alcohols are easily soluble. in water, alcohols, not sol. in low-polar org. r-retailers. P-rity of glycosides with complex aglycones means. least determined by chemical features of the latter: conn. with polar aglycones (eg polyol glycosides) sol. in water, conn. with large hydrophobic aglycones, insoluble. in water and low-polarity media. Oligosides with large low-polar aglycones (for example, saponins) are characterized by foaming properties.

Glycosides have no chemical properties. The properties of reducing sugars due to the carbonyl group are not subject to mutarotation. They are easily acylated by anhydrides and halogenated anhydrides in pyridine to form esters, alkylated by typical alkylating agents in strongly alkaline media, and form cyclic compounds. acetals and ketals, upon condensation with carbonyl compounds, are oxidized by periodates with cleavage of C-C bonds, and undergo acid hydrolysis, alcoholysis, and formolysis with cleavage of the glycosidic bond. Hydrolysis rate in Naib. degree depends on the size of the cycle: furanosides are hydrolyzed two orders of magnitude faster than pyranosides. Mechanism of hydrolysis m. the following is presented. scheme (the ~ sign means that the glycoside molecule can have or configuration):

The rate of enzymatic hydrolysis, which is carried out under the action of glycoside hydrolases, depends on the structure of the aglycone.

Glycosides with aliphatic and alicyclic. aglycones are resistant to the action of alkali solutions, with aromatic. and some heterocyclic. aglycones are unstable. Thus, the alkaline cleavage of D-glucopyranosides with aromatic. aglycone gives 1,6-anhydroglucose (levoglucosan), which is a preparative method for the synthesis of the latter.

Methods for the synthesis of glycosides are based on nucleophiles. substitution at the glycosidic center of reducing sugars and their derivatives. Acidic alcoholysis of sugars in excess alcohol leads to a mixture of four isomeric glycosides (ipyranosides, ifuranosides), where pyranosides predominate in equilibrium. The specific composition of the mixture depends on the configuration of the carbohydrate. For the stereo- and regioselective synthesis of glycosides with a specific configuration of the glycosidic center and ring size, glycosylation of aglycons with carbohydrate derivatives with an activated glycosidic center and fully protected alcohol hydroxyls is used.

The most commonly used glycosylation agents are: acylglycosyl halides (for example, form III), benzylglycosyl halides, 5-membered cyclic. orthoesters (eg IV), pyruvonitrile derivatives (eg V), oxazoline derivatives (eg VI).

For the synthesis of aryl-O-glycosides, the fusion of complete acetates of reducing sugars with phenols in the presence is also used. strong acid catalysts (Helferich method). Nature O-glycosides are isolated by Ch. arr. from plants.

To nature O-glycosides include saponins, cardiac glycosides, flavonoid coenzymes (eg, rutin), glycolipids, glycoproteins, and certain antibiotics.

Chem. The properties of N-glycosides strongly depend on the nature of the substituents at the N atom. Aliphatich. and aromatic N-Glycosides containing a hydrogen atom at N undergo mutarotation (like reducing sugars) due to tautomeric transformation into the form of Schiff bases, for example:

They are easily subject to acid and alkaline hydrolysis.

N-Glycosylamides, incl. N-glycosylureas, and compounds, in which the glycosidic atom N is included in the amide structure, are not prone to mutarotation and are similar in resistance to hydrolysis to O-glycosides. Alifatich. and aromatic glycosides, in which the glycosidic atom N has a fairly high basicity, undergo rearrangement into 1-amino-1-deoxyketoses (Amadori rearrangement).

Alifatich. and aromatic N-glycosides are prepared by condensation of reducing sugars with amines; N-glycosylamides and glycopeptides - by reduction of glycosylazides from the last. N-acylation; nucleosides and their structural analogues - N-glycosylation of nitrogen-containing heterocyclics. compounds with acylglycosyl halides and their analogues.

To nature N-glycosides include nucleosides, nucleotides, nucleic acids, glycoproteins, certain mixed biopolymers, in which a glycosidic bond connects carbohydrate and peptide chains through the amide N atom of the asparagine residue. Many N-glycosides and structural analogs of nucleosides, being antimetabolites of nucleosides, exhibit high physiol. activity and are used in quality of lek. drugs, eg. fluorafur, cytarabine.

S-glycosides are characterized by specific properties associated with the possibility of oxidation-reduction. r-tions at the S atom. These include hydrogenolysis over Raney nickel, oxidation. cleavage by halogens, oxidation to sulfones, formation of glycosyl halides from S-glycosides. The hydrolysis of S-glycosides, in contrast to the hydrolysis of O-glycosides, can occur under mild conditions in the presence of. salts of Hg and Cd, which allows for selective solution without affecting the O-glycosidic bonds of the molecule. S-Glycosides, similarly aromatic. O-glycosides are prepared by condensation of alkali metal thiolates with acylglycosyl halides. Specific methods - condensation of acylglycosyl halides with thiourea with the last. hydrolysis of thiuronium salt and partial hydrolysis of sugar dialkyldithioacetals, catalyzed by Hg salts.

=== Use. literature for the article “GLYCOSIDES” : Carbohydrate Chemistry, M., 1967; Bochkov A.F., Afanasyev V.A., Zaikov G.E., Formation and cleavage of glycosidic bonds, M., 1978; Bochkov A.F., Zaikov G.E., Chemistry of the O-glycosidic bond: formation and cleavage. Oxf.-NY, 1979. A.F. Bochkov.

“GLYCOSIDES” page was prepared based on materials from the chemical encyclopedia.

Prevalence.

Glycosides are found in the bark, fruits, roots, tubers, flowers and other parts of plants. Sometimes one plant contains several different glycosides. They are formed where biosynthesis is actively taking place, for example in leaves and green stems, and are transported in dissolved form to places of accumulation - roots and seeds. Most plant pigments are glycosides. Many tannins are also glycosides. It was initially assumed that glycosides are formed only in plants, but it is now known that they can also arise in the body of animals during the digestion process, when some substances harmful to the body combine with glucuronic acid (which is related to glucose and plays the same role as glucose in plant glycosides) are excreted in the urine.

Basics of treatment with cardiac glycosides

Considering certain features and difficulties in selecting an individual dose and regimen, as well as a wide range of side effects and interactions with other drugs, cardiac glycoside preparations are prescribed only by an experienced doctor.

Digitalis preparations are well absorbed in the gastrointestinal tract when taken orally; they bind tightly to blood proteins, remain in the body for a long time and are slowly eliminated. This determines their use in chronic heart failure, when long-term, often lifelong, administration of cardiac glycosides is necessary.

Preparations of strophanthus and lily of the valley are poorly absorbed when taken orally, so they are used as injections intramuscularly or intravenously. This, among other things, ensures the rapid development of their pharmacological effect. In addition, cardiac glycosides of strophanthus and lily of the valley do not accumulate in the body and are quickly eliminated. That is why preparations of strophanthus and lily of the valley are used exclusively for acute heart failure.

Functions.

Of the several theories proposed to explain the role of glycosides in plant physiology, the following three are the most plausible. 1) In unripe fruits, glycosides, due to their bitter taste, serve to protect against being eaten by animals. As fruits ripen, the colorless bitter glycosides are broken down, releasing pigments that give the fruit its attractive color, aromatics that give it flavor, and sugars that make it sweet. All this attracts various animals, birds and insects, which leads to effective seed dispersal. 2) According to another theory, glycosides are a means of removing toxic substances by binding them and converting them into inert forms (detoxification). 3) The third theory states that glycosides are a form of storage of sugars as a nutritional reserve. Their breakdown is a quick way to provide the plant with sugars.

Identification and quantification.

Glycosides are recognized by identifying their breakdown products - sugars and aglycones. For this, conventional methods of separation and identification of organic compounds are used: various types of chromatography, mass spectrometry, nuclear magnetic resonance spectroscopy, etc. To quantify the content of glycosides in raw materials, free sugars are determined before and after hydrolysis: the increase in the amount of free sugars corresponds to the number of glycosidic bonds destroyed by hydrolysis. Knowing the composition of glycosides, it is possible to estimate their content in the sample. see also

STEROIDS; TANNINS.