Carbohydrates | Organic Chemistry 3

Carbohydrates are studied in this chapter: names and structures of carbohydrates, Fischer projection, conformations and cyclic forms of sugars, Haworth projection, oxidation and reduction of sugars, ester and ether formations, polysaccharides

Names and Structures of Carbohydrates

The simplest carbohydrates are sugars (polyhydroxy aldehydes and ketones)
Monosaccharides, also called simple sugars, are the simplest form of sugar and the most basic units of carbohydrates: they are aldehydes or ketones that contain at least 2 additional hydroxy groups
 

Nomenclature:

- Polyhydroxy aldehydes are aldoses; polyhydroxy ketones are ketoses
- Sugars are called according to their chain length: 3 carbons ⇒ trioses; 4 carbons ⇒ tetroses; 5 carbons ⇒ pentoses; etc.
- D and L configuration: sugars are classified as D when the highest-numbered stereocenter (= the one farthest from the carbonyl group) is R, L when it is S. Configuration D is the most common in nature
- 2 diastereomers that differ only at one stereocenter are called epimers

 

Monosaccharides:
 

 

Sugars:

Fischer Projection

The Fischer projection is used for open-chain carbohydrates. In a Fischer projection: horizontal lines correspond to atoms above the plane of the page
 

 

How to convert a skeletal structure to a Fischer projection:

 

Conversion of (1S,2R,3R)-2-amino-1-bromobutane-1,3-diol to a Fischer projection:

Conformations and Cyclic forms of Sugars

Hexoses and pentoses can take the form of 5- or 6-membered cyclic hemiacetals. The 5-membered ring is called furanose while the 6-membered ring is called pyranose. During the cyclization process, the carbonyl carbon turns into a new stereocenter called anomeric carbon. In the D-series sugar: if anomeric carbon is S ⇒ α anomer; if it is R ⇒ β anomer
 

 

Mechanism: hemiacetal formation

Haworth Projection

The Haworth projection is used for cyclic forms of carbohydrates. It represents the real 3D structure of sugar
 

 

How to convert a Fischer projection to a Haworth projection:

Groups on the right in the Fischer projection asre down in the Haworth projection:

Oxidation of Sugars

Oxidation to mono- or dicarboxylic acids:

Reduction of Sugars

Ester and Ether Formation

Ester formation:
 


Mechanism: addition-elimination

 

Ether formation:
 


Mechanism:

  1. Formation of alkoxides (RO-) by deprotonation of alcohols with Ag2O
  2. SN2 reaction between alkoxides and CH3I

Polysaccharides

Polysaccharides are polymers of monosaccharides. Nature is remarkably conservative in the construction of such macromolecules: the 3 most abundant natural polysaccharides (cellulose, starch and glycogen) are derived from the glucose
 

Cellulose is a polysaccharide:
 

 

Many sugars exist in nature in modified form or as simple appendage to other structures. Modified sugars may contain nitrogen: