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25.13 Glycosides PowerPoint PPT Presentation


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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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25 13 glycosides l.jpg

25.13Glycosides


Glycosides l.jpg

Glycosides

  • Glycosides have a substituent other than OH at the anomeric carbon.

  • Usually the atom connected to the anomeric carbon is oxygen.


Example l.jpg

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


Glycosides4 l.jpg

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.


Example5 l.jpg

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


Example6 l.jpg

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


Glycosides7 l.jpg

Glycosides

  • O-Glycosides are mixed acetals.


O glycosides are mixed acetals l.jpg

CH

O

OH

O

H

CH2OH

O-Glycosides are mixed acetals

hemiacetal


O glycosides are mixed acetals9 l.jpg

CH

O

OH

O

H

CH2OH

OR

O

H

O-Glycosides are mixed acetals

hemiacetal

ROH

acetal


Preparation of glycosides l.jpg

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 glycosides11 l.jpg

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


Preparation of glycosides12 l.jpg

HOCH2

O

HO

HO

OCH3

OH

HOCH2

O

HO

HO

OH

OCH3

Preparation of Glycosides

Methylb-D-glucopyranoside

+

Methyla-D-glucopyranoside(major product)


Mechanism of glycoside formation l.jpg

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


Mechanism of glycoside formation14 l.jpg

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

+


Mechanism of glycoside formation15 l.jpg

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

+

+


25 14 disaccharides l.jpg

25.14Disaccharides


Disaccharides l.jpg

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 l.jpg

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


Maltose and cellobiose19 l.jpg

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


Maltose and cellobiose20 l.jpg

Maltose and Cellobiose

Maltose

Cellobiose


Cellobiose and lactose l.jpg

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


Cellobiose and lactose22 l.jpg

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


25 15 polysaccharides l.jpg

25.15Polysaccharides


Cellulose l.jpg

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.


Cellulose25 l.jpg

Cellulose

Four glucose units of a cellulose chain.


Starch l.jpg

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.


25 16 cell surface glycoproteins l.jpg

25.16Cell-Surface Glycoproteins


Glycoproteins l.jpg

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


Glycoproteins29 l.jpg

Glycoproteins

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


Human blood group glycoproteins l.jpg

Human-Blood-Group Glycoproteins

HO

CH2OH

O

N-Acetylgalactosaminepolymer

O

R

O

PROTEIN

O

H3C

O

OH

HO

HO


Slide31 l.jpg

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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