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Five membered rings

Monosaccharides can also exist as five-membered rings, as shown in figure 1.

Ribose and fructose can form 5 membered rings. The ribose ring is made of carbons 1 to 4 and oxygen, with the oxygen between carbon 1 and carbon 4. One O H group is attached to each of the carbons from carbons 1 to 3, and also carbon 5. In ribose, the O H group at carbon 1 and C H 2 O H group at carbon 4 both point up. The O H groups at carbons 2 and 3 point down. The fructose ring is made of carbons 2 to 5 and oxygen, with the oxygen between carbon 2 and carbon 5. One O H group is attached to each of the carbons from carbons 1 to 4, and also carbon 6. In fructose, the O H group at carbon 3, and the C H 2 O H groups at carbons 2 and 5, point up. The O H groups at carbons 2 and 4 point down.

Figure 1: Fructose and ribose also form rings, although they form five-membered rings as opposed to the six-membered ring of glucose.

Referred from:

Article
Ring-straight chain isomerism