According to my textbook (Class 12 NCERT Chemistry):

Fructose also exists in two cyclic forms which are obtained by the addition of —OH at C5 to the C = O group. The ring, thus formed is a five membered ring and is named as furanose with analogy to the compound furan.

I understood why the pyranose structure for glucose is more stable than the furanose structure ( The pyranose form has the perfect 60° dihedral angle between two non-ring atoms which minimises this type of strain Source: Stability of furanose vs. pyranose form of glucose?). But I do not understand why the same cannot be applied to the fructose structure.

So, to summarize : Why is fructose present in furanose form and not in pyranose form?


You are correct that the same reasoning can be applied to fructose, and, as with glucose, the beta-pyranose form is most stable.

It is common in biochemistry textbooks, however, to draw fructose as a furanose. The reason for this is that fructose 6-phosphate is a common metabolite, and the phosphate group prevents the 6'-hydroxyl group from participating in ring closure. As a result, the free 5'-hydroxyl attacks the carbonyl, and a furanose is formed. Similarly, fructan biopolymers often contain 1,6 linkages that also prevent the pyranose formation.


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