Pyran

In chemistry, a pyran is a six membered heterocyclic ring consisting of five carbon atoms and one oxygen atom and containing two double bonds. The molecular formula is C5H6O. There are two isomers of pyran that differ by the location of the double bonds. In 2H-pyran, the saturated carbon is at position 2, whereas in 4H-pyran, the saturated carbon is at position 4.

Although the pyrans themselves have little significance in chemistry, a variety of their derivatives are important biological molecules. 4H-pyrane easily disproportionates to the corresponding dihydropyrane and the pyrylium ion which is easily hydrolyzed in aqueous medium.

The term pyran is also often applied to the saturated ring analog which is more properly referred to as tetrahydropyran. In this context, the monosaccharides containing a six-membered ring system are known as pyranoses. Hence, the six-membered ring form of glucose is known as D-glucopyranose.