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25.13 Glycosides

25.13 Glycosides. Glycosides. Glycosides have a substituent other than OH at the anomeric carbon. Usually the atom connected to the anomeric carbon is oxygen. HOCH 2. O. HO. HO. OH. OH. HOCH 2. CH 3. O. HO. HO. O CC. N. OH. CH 3. Example.

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25.13 Glycosides

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  1. 25.13Glycosides

  2. Glycosides • Glycosides have a substituent other than OH at the anomeric carbon. • Usually the atom connected to the anomeric carbon is oxygen.

  3. HOCH2 O HO HO OH OH HOCH2 CH3 O HO HO OCC N OH CH3 Example • Linamarin is an O-glycoside derived from D-glucose. D-Glucose

  4. 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.

  5. NH2 N N OH HOCH2 HOCH2 N N O O H H H H H H H H OH OH OH OH Example • Adenosine is an N-glycoside derived from D-ribose D-Ribose Adenosine

  6. HOCH2 O HO HO OH OH HOCH2 NOSO3K O HO HO SCCH2CH CH2 OH Example • Sinigrin is an S-glycoside derived from D-glucose. D-Glucose

  7. Glycosides • O-Glycosides are mixed acetals.

  8. CH O OH O H CH2OH O-Glycosides are mixed acetals hemiacetal

  9. CH O OH O H CH2OH OR O H O-Glycosides are mixed acetals hemiacetal ROH acetal

  10. 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.

  11. CH O HOCH2 O HO OH H HO OCH3 H HO OH OH H OH H HOCH2 O HO CH2OH HO OH OCH3 Preparation of Glycosides CH3OH HCl + D-Glucose

  12. HOCH2 O HO HO OCH3 OH HOCH2 O HO HO OH OCH3 Preparation of Glycosides Methylb-D-glucopyranoside + Methyla-D-glucopyranoside(major product)

  13. HOCH2 •• O HO •• HO OH OH HOCH2 •• O HO •• HO + H OH Mechanism of Glycoside Formation • carbocation is stabilized by lone-pair donation from oxygen of the ring HCl

  14. HOCH2 HOCH2 •• CH3 O O HO •• HO HO O HO •• + OH H OH + O •• H H3C CH3 HOCH2 •• O O •• •• HO •• H HO + H OH Mechanism of Glycoside Formation +

  15. HOCH2 HOCH2 •• CH3 O O HO •• HO HO O HO •• + OH H OH + + O –H •• H H3C HOCH2 •• HOCH2 O •• HO •• O •• HO HO •• HO OCH3 OH •• OH OCH3 •• •• Mechanism of Glycoside Formation + +

  16. 25.14Disaccharides

  17. Disaccharides • Disaccharides are glycosides. • The glycosidic linkage connects two monosaccharides. • Two structurally related disaccharides are cellobiose and maltose. Both are derived from glucose.

  18. HOCH2 HOCH2 O O OH O HO OH OH HO HO Maltose and Cellobiose a • 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 1 4

  19. HOCH2 HOCH2 O O OH O HO 1 4 OH OH HO HO Maltose and Cellobiose b • 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 isa(1,4). Cellobiose

  20. Maltose and Cellobiose Maltose Cellobiose

  21. HOCH2 HOCH2 O O OH O HO 1 4 OH OH HO HO Cellobiose and Lactose b • Cellobiose and lactose are stereoisomeric disaccharides. • Both have b(1,4) glycosidic bonds. • The glycosidic bond unites two glucose units in cellobiose. It unites galactose and glucose in lactose. Cellobiose

  22. HOCH2 HOCH2 O O b OH O HO 1 4 OH OH HO HO Cellobiose and Lactose • Cellobiose and lactose are stereoisomeric disaccharides. • Both have b(1,4) glycosidic bonds. • The glycosidic bond unites two glucose units in cellobiose. It unites galactose and glucose in lactose. Lactose

  23. 25.15Polysaccharides

  24. Cellulose • Cellulose is a polysaccharide composed of several thousand D-glucose units joined by b(1,4)-glycosidic linkages. Thus, it can also be viewed as a repeating collection of cellobiose units.

  25. Cellulose Four glucose units of a cellulose chain.

  26. Starch • Starch is a mixture of amylose and amylopectin. • Amylose is a polysaccharide composed of 100 to several thousand D-glucose units joined by a(1,4)-glycosidic linkages.

  27. 25.16Cell-Surface Glycoproteins

  28. OH O HO CO2H CH3CNH O O OH CH2O NHCCH3 O O HO OH CH2OH Glycoproteins • Glycoproteins have a protein backbone to which carbohydrates are attached. One example is the cell-surface glycoprotein that is recognized by the influenza virus. PROTEIN

  29. Glycoproteins • A second example is the group of glycoproteins that define the various human blood groups.

  30. Human-Blood-Group Glycoproteins HO CH2OH O N-Acetylgalactosaminepolymer O R O PROTEIN O H3C O OH HO HO

  31. HO CH2OH O HO CH3CNH O HO CH2OH O HO OH = Type O: H R = Type A: R Type B: R =

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