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A glucoside is a glycoside that is derived from glucose. Glucosides are common in plants, but rare in animals. Glucose is produced when a glucoside is hydrolysed by purely chemical means, or decomposed by fermentation or enzymes.
The name was originally given to plant products of this nature, in which the other part of the molecule was, in the greater number of cases, an aromatic aldehydic or phenolic compound (exceptions are sinigrin and jalapin or scammonin). It has now been extended to include synthetic ethers, such as those obtained by acting on alcoholic glucose solutions with hydrochloric acid, and also the polysaccharoses, e.g. cane sugar, which appear to be ethers also. Although glucose is the most common sugar present in glucosides, many are known which yield rhamnose or iso-dulcite; these may be termed pentosides. Much attention has been given to the non-sugar parts of the molecules; the constitutions of many have been determined, and the compounds synthesized; and in some cases the preparation of the synthetic glucoside effected.
The simplest glucosides are the alkyl esters which have been obtained by reacting hydrochloric acid on alcoholic glucose solutions. A better method of preparation is to dissolve solid anhydrous glucose in methanol containing hydrochloric acid. A mixture of 12- and 13-glucose results, which are then etherified, and if the solution is neutralized before the 13-form isomerizes and the solvent removed, a mixture of the 12- and 13-methyl ethers is obtained. These may be separated by the action of suitable fermentation.
Classification of the glucosides is a matter of some difficulty. One based on the chemical constitution of the non-glucose part of the molecules has been proposed that frames four groups: (I) ethylene derivatives, (2) benzene derivatives, (3) styrolene derivatives, (4) anthracene derivatives. A group may also be made to include the cyanogenetic glucosides, i.e. those containing prussic acid. Other classifications follow a botanical classification, which has several advantages; in particular, plants of allied genera contain similar compounds. In this article the chemical classification will be followed, and only the more important compounds will be discussed here.
Ethylene Derivatives
These are generally mustard oils, which are characterized by a burning taste; their principal occurrence is in mustard and Tropaeolum seeds. Sinigrin, or the potassium salt of inyronic acid occurs in black pepper and in horseradish root. Hydrolysis with baryta, or decomposition by the ferment myrosin, gives glucose, allyl mustard oil and potassium hydrogen sulfate. Sinalbin occurs in white pepper; it decomposes to the mustard oil, glucose and sinapin, a compound of choline and sinapinic acid. Jalapin or scammonin occurs in scammony; it hydrolyses to glucose and jalapinolic acid.
Benzene Derivatives
These are generally oxy and oxyaldehydic compounds. Arbutin, which occurs in bearberry along with methyl arbutin, hydrolyses to hydroquinone and glucose. Pharmacologically it acts as a urinary antiseptic and diuretic; the benzoyl derivative, cellotropin, has been used for tuberculosis. Salicin, also termed saligenin and glucose occurs in the willow. The enzymes ptyalin and emulsin convert it into glucose and saligenin, ortho-oxybenzylalcohol. Oxidation gives the aldehyde helicin. Populin, which occurs in the leaves and bark of Populus tremula, is benzoyl salicin.
Styrolene Derivatives
This group contains a benzene and also an ethylene group, being derived from styrolene. Coniferin, C16H22O8, occurs in the cambium of conifer wood. Emulsin converts it into glucose and coniferyl alcohol, while oxidation gives glycovanillin, which yields with emulsin glucose and vanillin. Syringin, which occurs in the bark of Syringe vulgaris, is a methoxyconiferin. Phloridzus occurs in the root-bark of various fruit trees; it hydrolyses to glucose and phloretin, which is the phloroglucin ester of paraoxyhydratropic acid. It is related to the pentosides naringin, C21HEOi1, which hydrolyses to rhamnose and naringenin, the phioroglucin ester of para-oxycinnamic acid, and hesperidin, which hydrolyses to rhamnose and hesperetin, the phloroglucin ester of meta-oxy-para-methoxycinnamic acid or isoferulic acid, C10H10O4.
- Aesculin (C21H24O13), occurring in horse-chestnut, and daphnin, occurring in Daphne alpine, are isomeric; the former hydrolyses to glucose and aesculetin (C9H6O4 — 6,7-dihydroxycoumarin), the latter to glucose and daphnetin (7,8-dihydroxycoumarin).
- Fraxin, occurring in Fraxinus excelsior, and with aesculin in horse-chestnut, hydrolyses to glucose and fraxetin, 7,8-dihydroxy-6-methoxycoumarin.
- Flavone or benzo-7-pyrone derivatives are very numerous; in many cases they (or the nonsugar part of the molecule) are vegetable dyestuffs.
- Quercitrin is a yellow dyestuff found in Quercus tinctoria; it hydrolyses to rhamnose and quercetin, a dioxy-~3-phenyl-trioxybenzoy-pyrone.
- Rhamnetin, a splitting product of the glucosides of Rhamnus, is monomethyl quercetin; fisetin, from Rhus cotiiius, is monoxyquercetin; chrysin is phenyl-dioxybenzo--y-pyrone.
- Saponarin a glucoside found in Saponaria officinalis, is a related compound.
- Strophanthin is the name given to two different compounds, g-strophanthin (= ouabain) obtained from Strophanthus gratus and k-strophanthin from Stroph. kombé.
Anthracene Derivatives
These are generally substituted anthraquinones; many have medicinal applications, being used as purgatives, while one, ruberythric acid, yields the valuable dyestuff madder, the base of which is alizarin. Chrysophanic acid, a dioxymethylanthraquinone, occurs in rhubarb, which also contains emodin, a trioxymethylanthraquinone; this substance occurs in combination with rhamnose in Frangula bark.
The most important cyanogenetic glucoside is amygdalin, which occurs in bitter almonds. The enzyme maltase decomposes it into glucose and mandelic nitrile glucoside; the latter is broken down by emulsin into glucose, benzaldehyde and prussic acid. Emulsin also decomposes amygdalin directly into these compounds without the intermediate formation of mandelic nitrile glucoside.
Several other glucosides of this nature have been isolated. The saponins are a group of substances characterized by forming a lather with water; they occur in soap-bark. Mention may also be made of indican, the glucoside of the indigo plant; this is hydrolysed by the indigo ferment, indimulsiri, to indoxyl and indiglucin.
References
"Glucoside"