A ring containing four carbon atoms and an oxygen atom. Such rings are formed by ketose sugars with five or more carbon atoms. When such sugars are in solution the ketone group on the second carbon of the sugar reacts with the hydroxyl group on the fifth carbon atom. Fructose existing in this form is termed fructofuranose. Aldose sugars can also form furanose rings. However aldoses also form *pyranose rings
, which are far more stable and so predominate in solutions of aldose sugars.