Pyranoses

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As pyranoses are lactols of monosaccharides referred to a six-membered ring of five carbon - and an oxygen atom as well as at one of the carbon atoms that bind the ring-forming oxygen, a hydroxy group contain; it is therefore a hemiacetal (or hemicetal ). In certain monosaccharides, the ring is formed by hemicetal or hemiacetal formation between a carbonyl group and an OH group, for example in glucose ( grape sugar ).

Tetrahydropyrans (Pyrans)
Surname Tetrahydropyran α- D - (+) - glucopyranose
Structural formula Tetrahydropyran α-glucopyranose
comment Tetrahydropyran ring ( blue ) Tetrahydropyran ring marked in blue

The six-ring shape is more stable than the five-ring, as the ring tension is the lowest here . In aqueous solution, all monosaccharides with at least five carbon atoms are in equilibrium between open-chain and cyclic forms (α-form and β-form, depending on the position of the glycosidic hydroxyl group) and are subject to mutarotation . Higher molecular carbohydrates ( di- , oligo- and polysaccharides ) are also built up from ring-shaped monomers.

The name pyranose is derived from the heterocyclic molecule pyran , which also consists of a six-membered ring with an oxygen bridge, but has two C = C double bonds . The analogy to tetrahydropyran fits more precisely , see illustration.

Different isomers can arise during ring formation . An example of a pyranose form is α- D -glucopyranose, an aldohexose in ring form.

Nomenclature of pyranoses

Chair conformation of a pyranose with the tetrahydropyran ring marked in blue and the numbering of the carbon atoms beginning with the anomeric carbon atom 1 using the example of α- D - (+) - glucopyranose.

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

α-Pyranose and β- Pyranose of selected D -aldoses
Alpha + beta D-Allose (PYRANOSE) V.3.png
D -Allose (middle) and the two forms of pyranose. The equilibrium composition of the aqueous D-allose solution contains 18 mass% α-pyranose and 70 mass% β-pyranose as well as 5 mass% α-furanose and 7 mass% β-furanose.
Alpha + beta D-Altrose (PYRANOSE) V.3.png
D -Altrose (middle) as well as its two pyranose forms. The equilibrium composition of the aqueous D-altrose solution contains 20 mass% α-furanose and 13 mass% β-furanose as well as 27 mass% α-pyranose and 40 mass% β-pyranose.
Alpha + beta D-glucose (PYRANOSE) V.3.png
D -glucose (middle) as well as its two pyranose forms. The equilibrium composition consists of 36 mass% α-pyranose and 64 mass% β-pyranose. The furanose content is <1 mass%. <
Alpha + beta D-Mannose (PYRANOSE) V.3.png
D -Mannose (middle) and its two forms of pyranose. The equilibrium composition consists of 67 mass% α-pyranose and 33 mass% β-pyranose. The furanose content is <1 mass%.
Alpha + beta D-Gulose (PYRANOSE) V.3.png
D -Gulose (middle) as well as its two forms of pyranose. The equilibrium composition consists of 22 mass% α-pyranose and 78 mass% β-pyranose. Both forms of furanose together are <1% by mass in the equilibrium composition of the aqueous D-gulose solution.
Alpha + beta D-Idose (PYRANOSE) V.3.png
D -Idose (middle) as well as their two pyranose forms. The equilibrium composition of the aqueous D-idose solution contains 16 mass% α-furanose and 16 mass% β-furanose as well as 31 mass% α-pyranose and 37 mass% β-pyranose.
Alpha + beta D-galactose (PYRANOSE) V.3.png
D- galactose (middle) and its two forms of pyranose. The equilibrium composition of the aqueous D-galactose solution consists of 27 mass% α-pyranose and 73 mass% β-pyranose.
Alpha + beta D-Talose (PYRANOSE) V.3.png
D -Talose (middle) as well as their two pyranose forms. The equilibrium composition of the aqueous D-talose solution contains 20 mass% α-furanose and 11 mass% β-furanose as well as 40 mass% α-pyranose and 29 mass% β-pyranose.

See also

Individual evidence

  1. ^ A b Siegfried Hauptmann : Organic Chemistry , 2nd Edition, VEB Deutscher Verlag für Grundstoffindindustrie, Leipzig, 1985, ISBN 3-342-00280-8 , pp. 638–641.
  2. Furhop, JH; Endisch, C .: Molecular and Supramolecular Chemistry of Natural Products and Their Model Compounds . CRC Press, 2000, ISBN 0-8247-8201-1 .
  3. a b c d e f g h Eberhard Breitmaier, Günther Jung: Organische Chemie , 7th edition, Thieme Verlag, 2012, pp. 864–865, ISBN 978-3-13-541507-9 .