furanoses

Furanoses are five-membered ring forms of monosaccharides in which the ring consists of four carbon atoms and one oxygen. They arise when an aldose or a ketose cyclizes intramolecularly to form a cyclic hemiacetal (aldoses) or hemiketal (ketoses). The resulting five-membered rings are named after furan.

In aldopentoses, ring closure typically involves the terminal hydroxyl at C5 and the carbonyl at C1, producing ribofuranose–type rings. In ketopentoses, ring formation involves the C5 hydroxyl and the C2 carbonyl, yielding xylofuranose and related structures. The anomeric carbon, C1 for aldose-derived furanoses and C2 for ketose-derived furanoses, can adopt alpha or beta configurations, a feature known as anomerism. Furanoses, like other cyclic sugars, are in dynamic equilibrium with their open-chain forms.

Common furanoses include ribofuranose (from ribose), arabinofuranose, xylofuranose, and lyxofuranose. In biological systems, pentose sugars often adopt the furanose form in solution; in nucleic acids, the sugars are specifically beta-D-ribofuranose in RNA and beta-D-2′-deoxyribofuranose in DNA, linked to phosphate groups to form the backbone.

Furanoses are contrasted with pyranoses, which are six-membered ring forms of sugars. The term highlights the diversity of cyclic structures that monosaccharides can adopt and underpins the chemical behavior of carbohydrates in metabolism and nucleic acid chemistry.