1. Ketohexose A ketose SUGAR with six carbon atoms , e . g . fructose.

2. The number of open chain stereoisomers for an aldose monosaccharide is larger by one than that of a ketose monosaccharide of the same length.

3. Glucose is an aldose (terminal carbonyl) that Cyclizes into a pyranose, fructose is a ketose (internal carbonyl) that Cyclizes into a furanose

4. Systems for converting aldose sugars to ketose sugars and separating and/or concentrating these sugars using differences in the sugars' abilities to bind to specific affinity ligands are described.

5. The present invention belongs to the field of organic synthesis, relates to a method for synthesizing a saponin, and in particular, to a method for constructing a glycosidic bond in a saponin, and comprises the following steps: directly reacting an excessive amount of aldose or ketose with aglycone in a suitable organic solvent at 30 °C to a reflux temperature and with the presence of an acid catalyst, to obtain a target product saponin.