Reactions of MonosaccharidesOxidation - Reduction

OHOOH

OH

OH

OHH

OHOOH

OH

OH

OHH

[O]

Aldoses are "reducing" sugars, that is, they are oxidizedunder very mild conditions. (Only the aldehyde is oxidized.)

[H]

1) Identify the starting sugar as D- or L-.2) Draw Fisher projections for the products.3) Draw a Haworth projection of the beta pyrano anomer.

[O] = Ag+, NH3, H2Oor Br2 / H2O

[H] = ?

MonosaccharidesOxidation-Reduction Products

OHOOH

OH

OH

OHH

R

S

R

R

OHHHOHOHH

H OH

CHO

CH2OH

OHHHOHOHH

H OH

CO2H

CH2OH

Ag+

+ Ag

NaBH4

Sorbitol

Redox Reactions of Monosaccharides

Oxidation

Ketones and alcohols cannot be oxidized by Br2

In a basic solution, ketoses are converted into aldoses

A strong oxidizing agent such as HNO3 can oxidize thealdehyde and the alcohol groups

Osazone Formation

Aldoses and ketoses react with three equivalents ofphenylhydrazine

The C-2 epimers of aldoses form identical osazonesReaction of Ketoses with Phenylhydrazine

The carbon chain of an aldose can be increased by onecarbon in a Kiliani–Fischer synthesis

The Ruff degradation shortens an aldose chain by one carbon

Preparation of the Calcium D-Gluconate for theRuff Degradation

MonosaccharidesD-glucose beta anomer

OHHHOHOHH

H OH

CHO

CH2OH

OOH

OH

OHOH

OH H

O

OHHOCH2OH

OH OHO

all groups are equatorialor 1,2 - trans to each other

Cyclic Structure of MonosaccharidesHemiacetal Formation

anomer anomer

The specific rotation of pure α-D-glucose or β-D-glucosechanges over time to reach an equilibrium (mutarotation)

• If an aldose can form a five- or six-membered ring, it will exist predominantly as a cyclic hemiacetal

Note …

• A sugar with an aldehyde, a ketone, a hemiacetal, or a hemiacetal group is a reducing sugar

Monosaccharidesester formation

OHHHOHOHH

H OH

CHO

CH2OH

OCH3CO2

O2CCH3

O2CCH3O2CH3

O2CCH3

O2CCH3CH3CO2CH2

O2CCH3CH3CO2CH3CO2

O

O

O O

OCH3H3CO+ alphaanomer

Acylation of Monosaccharides

Alkylation of the OH GroupsReactions of Monosaccharides

Glycoside (acetal) formation

OHOOH

OH

OH

OHH

OHOH

OH

OOH

OCH3

OHOH

OHOCH2CH OCH3

OH

CH3OH

H+

CH3OHH+ H+

Formation of Glycosides

The acetal (or ketal) of a sugar is called a glycoside

Mechanism of Glycoside Formation

The formation of a glycoside favors the α-glucoside product: the anomeric effect

Reactions of MonosaccharidesAcetonide (acetal) formation

OOH

OH

OHOH

OH

cis

trans

+ CH3 CH3

OHCl

OO

OH

OO

O

CH3

CH3

CH3 CH3

Formation of an N-Glycoside

Disaccharides

Composed of two monosaccharide subunits hooked together by an acetal linkage

In α-maltose, the OH group bonded to the anomeric carbon is axial

Maltose is a reducing sugar

In cellobiose, the two subunits are hooked together by aβ-1,4’-glycosidic linkage

Cellobiose is a reducing sugar

In lactose, the two different subunits are joined by aβ-1,4’-glycosidic linkage

Lactose is a reducing sugar

The most common disaccharide is sucrose

Sucrose is not a reducing sugar

Polysaccharides

Amylose is a component of starch

Amylopectin is another polysaccharide component of starch that has a branched structure

An example of a naturally occurring product derived from carbohydrates Monosaccharides

Identification: 1H NMR couplings

Examples1,2-disubstituted cyclohexanes

Dihedral angles

Coupling Constants Using Coupling Values

• A mixture of anomers of D-glucose wasseparated and analyzed by nmr.

• The coupling constants of the epimericprotons were 3.5 Hz for anomer X and 8.6Hz for anomer Y.

• Identify the alpha and beta anomer.

Possible Exam Questions• Given a Haworth structure: a) draw the Fisher structure of

the open straight chain, identify it as: eg. aldose, pentose,reducing, etc. b) draw a Haworth structure for the oppositeanomer of the given structure, c) draw the most stable chairconformer, is it the α- or β-?

• Identify an isomer: D- or L- from a structure. How manypossible stereoisomers could the structure possibly have?

• Give an example of mutarotation.• Identify/name structures: saccharides, glycosides,

glycosamine, etc.

Dihedral Angles

The larger coupling constant corresponds to thelarger dihedral angle. Therefore X is alpha.

Olestra, Fats & Lipids http://www.cspinet.org/olestra/

O

OOOCCH3

CH3COO

CH3COO

CH3COO O

OOCH3OOCCH3

OOCCH3

CH3COO

Simplesse

( )H

H

HN

NN

O

O

O

CH3

CH2OH

CH2CH2CO2H

Olestra

)

))

(

((

O

O

OR Can Be:

O

OOHHO

HO

HO O

OHOH

OH

HO