Like aldoses, ketoses also exist in a cyclic structure. But, ketoses has their keto group in the second Carbon. Rotation of the fifth carbon, in fructose chain, brings the C5-hydroxyl group and the C2 keto groups nearer to form hemiacetal containing a five-membered ring.
- The five-membered hemiacetal structure of fructose can be drawn as a flat pentagon like structure. Five membered cyclic structure of ketoses (like fructose) is akin to furan and so ketose cyclic structure is referred to as furanose structure.
- By convention, the five-membered furanose ring is drawn with the ring oxygen positioned above with the anomeric carbon atom in the right.
- The hemiketal carbon atom (C-1) is a new stereogenic centre and is referred to as anomeric carbon.
- Depending upon the orientation of the OH groups at this anomeric carbon, the hemiketal can be of two forms (anomers), as α- fructose and β- fructose.
- A structure with a hydroxyl group above the plane is designated as β-form and if below the plane as α-form. Both cyclic hemiketals (anomers) and the open-chain form are in equilibrium in solution.
Fructose
It is an important ketohexose. The molecular formula of fructose is C6H12O6 and contains a ketonic functional group at carbon number 2 and has six carbon atoms in a straight chain. The ring member of fructose is in analogy to the compound Furan and is named furanose. The cyclic structure of fructose is exhibited below:
