苷－有机化学课件.pptVIP

25.13Glycosides Glycosides Glycosides have a substituent other than OH at the anomeric carbon. Usually the atom connected to the anomeric carbon is oxygen. Example Linamarin is an O-glycoside derived from D-glucose. Glycosides Glycosides have a substituent other than OH at the anomeric carbon. Usually the atom connected to the anomeric carbon is oxygen. Examples of glycosides in which the atom connected to the anomeric carbon is something other than oxygen include S-glycosides and N-glycosides. Example Adenosine is an N-glycoside derived from D-ribose Example Sinigrin is an S-glycoside derived from D-glucose. Glycosides O-Glycosides are mixed acetals. O-Glycosides are mixed acetals O-Glycosides are mixed acetals Preparation of Glycosides Glycosides of simple alcohols (such as methanol) are prepared by adding an acid catalyst (usually gaseous HCl) to a solution of a carbohydrate in the appropriate alcohol. Only the anomeric OH group is replaced. An equilibrium is established between the a and b-glycosides (thermodynamic control). The more stable stereoisomer predominates. Preparation of Glycosides Preparation of Glycosides Mechanism of Glycoside Formation carbocation is stabilized by lone-pair donation from oxygen of the ring Mechanism of Glycoside Formation Mechanism of Glycoside Formation 25.14Disaccharides Disaccharides Disaccharides are glycosides. The glycosidic linkage connects two monosaccharides. Two structurally related disaccharides are cellobiose and maltose. Both are derived from glucose. Maltose and Cellobiose Maltose is composed of two glucose units linked together by a glycosidic bond between C-1 of one glucose and C-4 of the other. The stereochemistry at the anomeric carbon of the glycosidic linkage is a. The glycosidic linkage is described as a(1,4) Maltose and Cellobiose Cellobiose is a stereoisomer of maltose. The only difference between the two is that cellobiose has a b(1,4) glycosidic bond while that of maltose is a(1,4). Maltose and Cello