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?


1 Answer 1


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.


Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.