Pyranose ring structure of glucose is due to hemiacetal formation between C _{1} and C _{5} carbon atoms. In general, a pyranose is any cyclic isomer that has a five carbon atoms and one oxygen atom in a ring of six atoms. If a hydroxyl at the 5 position of an aldohexose, such as glucose, forms a hemiacetal with the aldehyde (position 1), the resulting isomer is glucopyranose.