25.13 Glycosides - PowerPoint PPT Presentation

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25.13 Glycosides

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HO. HO. HOCH2. CH2. NOSO3K. Glycosides. O-Glycosides are mixed acetals. ... HO. HO. HOCH2. Methyl. b-D-glucopyranoside. Methyl. a-D-glucopyranoside (major product) ... – PowerPoint PPT presentation

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Title: 25.13 Glycosides


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
Example
D-Glucose
  • Linamarin is an O-glycoside derived from
    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
Example
  • Adenosine is an N-glycoside derived from D-ribose

D-Ribose
Adenosine
6
Example
D-Glucose
  • Sinigrin is an S-glycoside derived from D-glucose.

7
Glycosides
  • O-Glycosides are mixed acetals.

8
O-Glycosides are mixed acetals
hemiacetal
9
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
Preparation of Glycosides
CH3OH
HCl

D-Glucose
12
Preparation of Glycosides
Methylb-D-glucopyranoside

Methyla-D-glucopyranoside(major product)
13
Mechanism of Glycoside Formation
HCl
  • carbocation is stabilized by lone-pair donation
    from oxygen of the ring

14
Mechanism of Glycoside Formation

15
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
Maltose and Cellobiose
a
Maltose
1
4
  • 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)

19
Maltose and Cellobiose
b
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).

20
Maltose and Cellobiose
Cellobiose
Maltose
21
Cellobiose and Lactose
b
Cellobiose
  • 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.

22
Cellobiose and Lactose
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.

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
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

Type O
R
H

Type A
R
Type B
R
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