Like aldoses, ketoses also exist in a cyclic structure. But, ketoses has its 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. Structure with 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.
