Glucoside

Glucoside, in chemistry, the generic name of an extensive group of substances characterized by the property of yielding a sugar, more commonly glucose, when hydrolysed by purely chemical means, or decomposed by a ferment or enzyme. The name was originally given to vegetable 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 commonest 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.  
  
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The simplest glucosides are the alkyl esters which E. Fischer (Ber., 28, pp. 1151, 3081) obtained by acting with hydrochloric acid on alcoholic glucose solutions. A better method of preparation is due to E. F. Armstrong and S. L. Courtauld (Proc. Phys. Soc., 1905, July 1), who dissolve solid anhydrous glucose in methyl alcohol containing hydrochloric acid. A mixture of α\- and β-glucose result, which are then etherified, and if the solution be neutralized before the β-form isomerizes and the solvent removed, a mixture of the α\- and β-methyl ethers is obtained. These may be separated by the action of suitable ferments. Fischer found that these ethers did not reduce Fehling’s solution, neither did they combine with phenyl hydrazine at 100°; they appear to be stereo-isomeric γ-oxidic compounds of the formulae I., II.: The difference between the α\- and β-forms is best shown by the selective action of enzymes. Fischer found that maltase, an enzyme occurring in yeast cells, hydrolysed α-glucosides but not the β; while emulsin, an enzyme occurring in bitter almonds, hydrolyses the β but not the α. The ethers of non-fermentable sugars are themselves non-fermentable. By acting with these enzymes on the natural glucosides, it is found that the majority are of the β-form; e.g. emulsin hydrolyses salicin, helicin, aesculin, coniferin, syringin, &c.

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 by Umney, who framed four groups: (1) 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. J. J. L. van Rijn (Die Glykoside, 1900) follows 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. Only the more important compounds will be noticed, the reader being referred to van Rijn (loc. cit.) and to Beilstein’s Handbuch der organischen Chemie for further details.

1\. Ethylene Derivatives.—These are generally mustard oils, and are characterized by a burning taste; their principal occurrence is in mustard and Tropaeolum seeds. Sinigrin or the potassium salt of myronic acid, C10H16NS2KO9·H2O, occurs in black pepper and in horse-radish root. Hydrolysis with baryta, or decomposition by the ferment myrosin, gives glucose, allyl mustard oil and potassium bisulphate. Sinalbin, C30H42N2S2O15, occurs in white pepper; it decomposes to the mustard oil HO·C6H4·CH2·NCS, glucose and sinapin, a compound of choline and sinapinic acid. Jalapin or scammonin, C34H56O16, occurs in scammony; it hydrolyses to glucose and jalapinolic acid. The formulae of sinigrin, sinalbin, sinapin and jalapinolic acid are:—

2\. Benzene Derivatives.—These are generally oxy and oxyaldehydic compounds. Arbutin, C12H16O7, 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,” C13H18O7, occurs in the willow. The enzymes ptyalin and emulsin convert it into glucose and saligenin, ortho-oxybenzylalcohol, HO·C6H4·CH2OH. Oxidation gives the aldehyde helicin. Populin, C20H22O8, which occurs in the leaves and bark of Populus tremula, is benzoyl salicin.

3\. Styrolene Derivatives.—This group contains a benzene and also an ethylene group, being derived from styrolene C6H5·CH:CH2. Coniferin, C16H22O8, occurs in the cambium of coniferous woods. Emulsin converts it into glucose and coniferyl alcohol, while oxidation gives glycovanillin, which yields with emulsin glucose and vanillin (see Eugenol and Vanilla). Syringin, which occurs in the bark of Syringa vulgaris, is methoxyconiferin. Phloridzin, C21H24O10, occurs in the root-bark of various fruit trees; it hydrolyses to glucose and phloretin, which is the phloroglucin ester of para-oxyhydratropic acid. It is related to the pentosides naringin, C21H26O11, which hydrolyses to rhamnose and naringenin, the phloroglucin ester of para-oxycinnamic acid, and hesperidin, C50H60O22(?), which hydrolyses to rhamnose and hesperetin, C16H14O6, the phloroglucin ester of meta-oxy-para-methoxycinnamic acid or isoferulic acid, C10H10O4. We may here include various coumarin and benzo-γ-pyrone derivatives. Aesculin, C15H16O9, occurring in horse-chestnut, and daphnin, occurring in Daphne alpina, are isomeric; the former hydrolyses to glucose and aesculetin (4·5-dioxycoumarin), the latter to glucose and daphnetin (3·4-dioxycoumarin). Fraxin, C16H18O10, occurring in Fraxinus excelsior, and with aesculin in horse-chestnut, hydrolyses to glucose and fraxetin, the monomethyl ester of a trioxycoumarin. Flavone or benzo-γ-pyrone derivatives are very numerous; in many cases they (or the non-sugar part of the molecule) are vegetable dyestuffs. Quercitrin, C21H22O12, is a yellow dyestuff found in Quercus tinctoria; it hydrolyses to rhamnose and quercetin, a dioxy-β-phenyl-trioxybenzo-γ-pyrone. Rhamnetin, a splitting product of the glucosides of Rhamnus, is monomethyl quercetin; fisetin, from Rhus cotinus, is monoxyquercetin; chrysin is phenyl-dioxybenzo-γ-pyrone. Saponarin, a glucoside found in Saponaria officinalis, is a related compound. Strophanthin is the name given to three different compounds, two obtained from Strophanthus Kombe and one from S. hispidus.

4\. 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 (q.v.). 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 (q.v.). Mention may also be made of indican, the glucoside of the indigo plant; this is hydrolysed by the indigo ferment, indimulsin, to indoxyl and indiglucin.

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