Glycoside a glycoside

From the view of chemistry, the C-12 position of dihydroartemisin is similar to the C-1 position of carbohydrates. So, these C-12 derivatives may be divided into three types 0-glycosides, A-glycosides, and C-glycosides using a similar term in carbohydrate chemistry. 

A-glycoside a glycoside with a nitrogen instead of an oxygen at the glycosidic linkage. 

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. 

Glycosidic thiol groups can be introduced into glycosyl bromides by successive reactions with thiourea and aqueous sodium disulfite (D. Horton, 1963 M. Cemy, 1961, 1963). Such thiols are excellent nucleophiles in weakly basic media and add to electrophilic double bonds, e.g., of maleic esters, to give Michael adducts in high yields. Several chiral amphiphiles have thus been prepared without any need for chromatography (J.-H. Fuhrhop, 1986 A). 

Mannosides are difficult to obtain since here a 2-O-acyl group blocks the -position. 2-O-Benzyl-a-mannosyl bromides give, however, high yields of pure -glycosides with a heterogeneous silver silicate catalyst preventing anomerization and SnI reaction of the bromide H. Paulsen, 1981 B, Q. 

The reaction conditions applied are usually heating the amine with a slight excess of aldehyde and a considerable.excess of 2d-30hydrochloric acid at 100 °C for a few hours, but much milder ( physiological ) conditions can be used with good success. Diols, olefinic double bonds, enol ethers, and glycosidic bonds survive a Pictet-Spengler reaction very well, since phenol and indole systems are much more reactive than any of these acid sensitive functional groups (W.M. Whaley, 1951 J.E.D. Barton, 1965 A.R. Battersby, 1969). 

Glycosides are a large and very important class of carbohydrate derivatives character ized by the replacement of the anomeric hydroxyl group by some other substituent Gly cosides are termed O glycosides N glycosides S glycosides and so on according to the atom attached to the anomeric carbon... 

Linamann (an O glycoside obtained from manioc a type of yam widely distnbuted in southeast Asia)... 

Adenosine (an N glycoside also known as a nucleoside adenosine is one of the fundamental molecules of biochemistry)... 

The preparalion of glycosides m Ihe laboratory is carried oul by simply allowing a carbohydrate to read wilh an alcohol m Ihe presence of an acid calalysl... 

A point to be emphasized about glycoside formation is that despite the presence of a number of other hydroxyl groups m the carbohydrate only the anomenc hydroxyl group IS replaced This is because a carbocation at the anomenc position is stabilized by the nng oxygen and is the only one capable of being formed under the reaction conditions... 

Disacchandes are carbohydrates that yield two monosaccharide molecules on hydroly SIS Structurally disaccharides are glycosides m which the alkoxy group attached to the anomeric carbon is derived from a second sugar molecule... 

Maltose obtained by the hydrolysis of starch and cellobiose by the hydrolysis of cellulose are isomenc disaccharides In both maltose and cellobiose two d glucopyra nose units are joined by a glycosidic bond between C 1 of one unit and C 4 of the other The two are diastereomers differing only m the stereochemistry at the anomeric carbon of the glycoside bond maltose is an a glycoside cellobiose is a (3 glycoside... 

The stereochemistry and points of connection of glycosidic bonds are commonly designated by symbols such as a(l 4) for maltose and (3(1 4) for cellobiose a and (3 designate the stereochemistry at the anomeric position the numerals specify the ring car bons involved... 

FIGURE 25 6 Molecu lar models of the disaccha rides maltose and cellobiose Two D glucopyranose units are connected by a glycoside linkage between C 1 and C 4 The glycosidic bond has the a orientation in maltose and IS p in cellobiose Mai tose and cellobiose are diastereomers... 

Both maltose and cellobiose have a free anomeric hydroxyl group that is not involved in a glycoside bond The configuration at the free anomeric center is variable and may be either a or (3 Indeed two stereoisomeric forms of maltose have been iso lated one has its anomeric hydroxyl group m an equatorial orientation the other has an axial anomeric hydroxyl... 

Lactose is a disacchande constituting 2-6% of milk and is known as milk sugar It differs from maltose and cellobiose m that only one of its monosaccharide units is D glucose The other monosaccharide unit the one that contributes its anomeric carbon to the glycoside bond is d galactose Like cellobiose lactose is a (3 glycoside... 

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