Carbohydrates | Organic Chemistry 3

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:

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:

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

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 to mono- or dicarboxylic acids:

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 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: