Pyranose

As pyranoses lactols of monosaccharides are referred to, which include a six-membered ring of five carbon and an oxygen atom, as well as on one of the carbon atoms that bind the ring-forming oxygen a hydroxy group; it is therefore present a hemiacetal (or hemiketal ). The ring is created in certain monosaccharides by hemiketal or hemiacetal formation between a carbonyl group and a hydroxyl group, such as the glucose (dextrose ). The six-membered ring form is more stable than the five-membered ring, as the ring strain is here at its lowest. All monosaccharides having at least five carbon atoms are in equilibrium in aqueous solution between the open-chain and cyclic forms ( α - form and β - form, depending on the position of the glycosidic hydroxyl group ) before and subject to mutarotation. Higher molecular weight carbohydrates ( di-, oligo-and polysaccharides) are made up of circular monomers.

The name derives from the pyranose heterocyclic pyran molecule from which also consists of a six-membered ring with an oxygen bridge, however, has two C = C double bonds. More specifically, the analogy fits the tetrahydropyran, see picture.

In the ring- forming different isomers may arise. For example a pyranose is α -D-glucopyranose, an aldohexose ring shape.

Nomenclature of pyranoses

In order to assign a pyranose conformation, we analyzed - similar to cyclohexane - the possible conformations, such as a chair, boat, half-chair or twist shapes. The atoms of the six-membered ring ( tetrahydropyran ) are numbered in such a way that the anomeric carbon atom (original carbon atom of the carbonyl group of the aldose or ketose ) 1 is always assigned the position. The position of the hydroxyl groups is then obtained from the locants of the carbon atom to which it is bound in the acyclic form. Other conventions are described in the scientific literature.