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Class of saccharide compounds structured as a 5 carbon, 1 oxygen ring

In organic chemistry, pyranose is a collective term for saccharides that have a chemical structure that includes a six membered ring consisting of five carbon atoms and one oxygen atom (a heterocycle). There may be other carbons external to the ring. The name derives from its similarity to the oxygen heterocycle pyran, but the pyranose ring does not have double bonds. A pyranose in which the anomeric |\sOH (hydroxyl group) at C(l) has been converted into an OR group is called a pyranoside. Tetrahydropyran Name Tetrahydropyran α D (+) Glucopyranose structural formula Tetrahydropyrane ring highlighted blue Tetrahydropyrane ring highlighted blue

The pyranose ring is formed by the reaction of the hydroxyl group on carbon 5 (C 5) of a sugar with the aldehyde at carbon 1. This forms an intramolecular hemiacetal. If reaction is between the C 4 hydroxyl and the aldehyde, a furanose is formed instead. The pyranose form is thermodynamically more stable than the furanose form, which can be seen by the distribution of these two cyclic forms in solution.

Hermann Emil Fischer won the Nobel Prize in Chemistry (1902) for his work in determining the structure of the D aldohexoses. A further refinement to the conformation of pyranose rings came when Sponsler and Dore (1926) realized that Sachse's mathematical treatment of six membered rings could be applied to their X ray structure of cellulose. These conformers can interconvert with one another; however, each form may have very different relative energy, so a significant barrier to interconversion may be present. The energy of these conformations can be calculated from quantum mechanics; an example of possible glucopyranose interconversions is given. The conformations of the pyranose ring are superficially similar to that of the cyclohexane ring. However, the specific nomenclature of pyranoses includes reference to the ring oxygen, and the presence of hydroxyls on the ring have distinct effects on its conformational preference. There are also conformational and stereochemical effects specific to the pyranose ring.

As shown by the relative structure energies in the diagram above, the chair structures are the most stable carbohydrate form. This relatively defined and stable conformation means that the hydrogen atoms of the pyranose ring are held at relatively constant angles to one another. Carbohydrate NMR takes advantage of these dihedral angles to determine the configuration of each of the hydroxyl groups around the ring.