Diose
A diose is a monosaccharide containing two carbon atoms. Because the general chemical formula of an unmodified monosaccharide is (C·H2O)n, where n is three or greater, it does not meet the formal definition of a monosaccharide. However, since it does fit the formula (C·H2O)n, it is sometimes thought of as the most basic sugar.
The large number of sugars prepared synthetically, some of which have not yet been found in nature, together with the natural sugars are subdivided into groups. We distinguish, in the first place, between the more simple sugars called monosaccharides and compound sugars called polysaccharides. The latter may be regarded as formed from two or more molecules of the former with elimination of water, and, as a matter of fact, the simpler sugars may be formed from them by hydrolysis.
The monosaccharides again are divided into subclasses governed by the number of carbon atoms in the molecule. Thus we have a diose (glycol aldehyde, or glycolose, HC(O)-CH2OH) which is the simplest possible sugar, and trioses, tetroses, pentoses, hexoses, heptoses, etc.
There is only one possible diose, glycolaldehyde (2-hydroxyethanal), which is an aldodiose (a ketodiose is not possible since there are only two carbons).
...
Wikipedia