fructopyranose

Fructopyranose is the six‑membered ring form (a pyranose) of the monosaccharide fructose, a ketohexose. It is produced when the carbonyl group at C2 reacts intramolecularly with the hydroxyl group at C6 to form a cyclic hemiketal, yielding a six-membered ring that includes an oxygen atom. Fructopyranose is one of several possible cyclic forms of fructose; the other major form is fructofuranose, the five‑membered ring.

In fructopyranose, the anomeric carbon is C2. Two anomeric forms can exist: alpha-D-fructopyranose and beta-D-fructopyranose, differing in the orientation of the hydroxyl group at C2. The pyranose ring generally adopts a chair conformation in solution, with substituents arranged to minimize steric strain. The term “fructopyranose” applies to both α and β anomers of the D or L enantiomers, though natural fructose is D- and mainly encountered in its furanose form.

Occurrence and significance: In aqueous solution, fructose exists in equilibrium among multiple forms, with fructofuranose typically present in higher proportions than fructopyranose. Fructopyranose forms are detectable but constitute a relatively small fraction. Interconversion occurs rapidly with the open-chain form and other cyclic isomers, and the distribution is influenced by solvent, temperature, and pH.

See also: Fructose, Fructofuranose, Mutarotation, Anomeric carbon.