11.4 Cyclic Structure of Monosaccharides: Hemiacetal Formation

Aldoses contain an aldehyde group with several alcohol groups. The cyclic forms of D-glucose are six-membered hemiacetals formed by an intramolecular reaction of the –OH group at C-5 with the aldehyde group.

Cyclic structures of monosaccharides are named according to their five- or six-membered rings (Scheme 4-5). A six -membered cyclic hemiacetal is called a pyranose , derived from the name of the six-membered cyclic ether pyran . A five-membered cyclic hemiacetal is called a furanose , derived from the name of the five -membered cyclic ether furan . For example, the six-membered ring of glucose is called glucopyranose and the five-membered ring of fructose is called fructofuranose.