pyran

Pyran is a heterocyclic organic compound that forms a six-membered ring containing one oxygen atom and five carbon atoms. It serves as the parent structure for a large family of oxygen-containing heterocycles known as pyrans. The unsubstituted parent has the formula C5H6O and exists in two constitutional isomers, 2H-pyran and 4H-pyran, which differ in the positions of the double bonds and the hydrogen on the ring. The ring is unsaturated but not aromatic, and it serves as a core motif in many natural and synthetic compounds.

Substituted pyrans are widespread in chemistry and biology. They occur as standalone compounds and as cores of fused ring systems such as chromenes and benzopyrans. The six-membered pyran ring is also the basis for the pyranose forms of many carbohydrates, including glucose and galactose, which exist in cyclic forms in solution.

Preparation and reactivity: Pyrans are typically prepared by cyclization of suitable unsaturated precursors or by ring-closing reactions under acid or Lewis acid catalysis. They can participate in a range of reactions characteristic of alkenes and heterocycles, including electrophilic addition to their C=C bonds and ring-opening under acidic or basic conditions. Oxidation or dehydrogenation can convert pyrans into related oxygen-containing rings such as pyranones.

Significance: Although often encountered as a structural motif within larger molecules, the pyran ring is a fundamental building block in natural products, pharmaceuticals, and materials. In carbohydrate chemistry, the term pyran refers to the six-membered ring form of many sugars, known as pyranose.