pyranosering

Pyranose ring, or pyranose, is a six-membered ring structure that most commonly forms in sugars when the carbonyl group of an aldose or a ketose reacts intramolecularly with a hydroxyl group. In aldohexoses such as glucose, cyclization typically occurs when the carbonyl at C1 is attacked by the hydroxyl on C5, producing a hemiacetal and a six-membered ring in which the ring oxygen comes from the C5 hydroxyl. Ketoses form pyranose rings in a similar fashion, with the carbonyl at C2 participating to give a six-membered ring.

The ring consists of five carbon atoms and one oxygen. The carbon at the anomeric position (C1 in aldose-derived pyranoses, C2 in ketose-derived pyranoses) becomes a stereocenter, yielding two anomeric forms: alpha and beta. The difference between these forms is the orientation of the substituent at the anomeric carbon relative to the ring. These forms are commonly depicted in Haworth projections and can interconvert in solution through mutarotation, a process influenced by solvent, temperature, and pH.

In solution, many sugars exist in the pyranose form, often with the beta form being more prevalent for aldoses such as glucose. Some sugars also exist in a five-membered furanose form, and an equilibrium can exist between pyranose and furanose forms depending on conditions. In disaccharides and polysaccharides, pyranose rings commonly participate in glycosidic bonds, as seen in maltose (two glucopyranose units) and other glucose-containing polymers. The pyranose form contrasts with the furanose form, which is five-membered, and the two forms together reflect the structural versatility of carbohydrates.