Pyranose
In
Tetrahydropyran | |||
Name | Tetrahydropyran | α-D-(+)-Glucopyranose | |
structural formula | |||
Tetrahydropyrane ring highlighted blue | Tetrahydropyrane ring highlighted blue |
Formation
The pyranose ring is formed by the reaction of the
History
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.[3] It was determined that the pyranose ring is puckered, to allow all of the carbon atoms of the ring to have close to the ideal tetrahedral geometry.
Conformations
This puckering leads to a total of 38 distinct basic pyranose conformations: 2 chairs, 6 boats, 6 skew-boats, 12 half-chairs, and 12 envelopes.[4]
These conformers can interconvert with one another; however, each form may have very different relative energy, so a significant
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.
Nomenclature
To name conformations of pyranose, first the conformer is determined. The common conformers are similar to those found in cyclohexane, and these form the basis of the name. Common conformations are chair (C), boat (B), skew (S), half-chair (H) or envelope (E). The ring atoms are then numbered; the anomeric, or hemiacetal, carbon is always 1. Oxygen atoms in the structure are, in general, referred to by the carbon atom they are attached to in the acyclic form, and designated O. Then:
- Position the ring so that, if looking at the top face, the atoms are numbered clockwise.
- In the chair and skew conformations, the reference plane should be selected. In the chair conformation, the reference plane is chosen such that the lowest-numbered atom (usually C-1) is exoplanar. In the skew conformation, the plane contains three adjacent atoms and one other with the atom with the lowest possible number exoplanar.[6]
- Atoms above the plane are written before the conformer label, as a superscript
- Atoms below the plane are written following the conformer label, as a subscript[7]
NMR spectroscopy
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.
See also
References
- ^ ISBN 0-387-94951-8.
- .
- ^ ISBN 90-5702-315-6.
- PMID 16834195.
- PMID 17696342.
- ISBN 978-3-540-30429-6.
- ISBN 0-8247-8201-1.