pyranóses

Pyranoses are cyclic hemiacetal forms of monosaccharides that contain a six-membered ring. This ring consists of five carbon atoms and one oxygen atom. The term "pyranose" is derived from pyran, a six-membered heterocyclic organic compound containing one oxygen atom. In sugars, the pyranose ring is formed by the intramolecular reaction between the hydroxyl group on the carbon atom farthest from the anomeric carbon and the aldehyde or ketone group of the same monosaccharide. This cyclization results in the formation of a hemiacetal linkage.

The structure of a pyranose can be represented in different ways, including Haworth projections and chair conformations. Haworth projections show the ring as a planar hexagon, with substituents drawn above or below the plane. Chair conformations provide a more accurate representation of the three-dimensional structure, showing the atoms in a staggered arrangement similar to that of cyclohexane. The carbon atoms in the pyranose ring are numbered starting from the anomeric carbon, which is the carbon atom involved in the hemiacetal linkage. The anomeric carbon has two different substituents: a hydroxyl group and an oxygen atom that is part of the ring.

The hydroxyl group on the anomeric carbon can be in one of two positions, leading to two stereoisomers called anomers: alpha (α) and beta (β). In the α-anomer, the hydroxyl group is typically oriented downwards in a Haworth projection, while in the β-anomer, it is oriented upwards. The equilibrium between the open-chain form and the cyclic pyranose form of a monosaccharide involves a process called mutarotation, where the anomers interconvert. Common examples of pyranoses include glucose (glucopyranose) and fructose (fructopyranose). Many common disaccharides and polysaccharides are formed by linking monosaccharide units through glycosidic bonds, which often involve pyranose rings.