Carbohydrate derivatives containing the carbon-lithium and carbon-tin bonds

The monosaccharide derivatives containing the C-Sn and C-Li bonds are reviewed. Both types of these organometallics are useful intermediates in the synthesis of variety of optically pure compounds such as higher carbon sugars, C-disaccharides, carbocyclic derivatives, etc. Organometallics containing the C-Sn bond are usually stable and can be isolated in the pure form, but their reactivity is low. The tin moiety in such compounds can be replaced by the lithium atom, leading to derivatives with the C-Li bond. The latter are highly reactive but unstable. However, the lithium atom in such unstable organometallics can be replaced with tin moiety, thus providing stable stannyl intermediates, which can be easily purified. Such a facile mutual exchangeability of the metal (Li --> Sn and vice versa) makes these sugar organometallics particularly attractive during stereocontrolled syntheses. In this review carbohydrate derivatives containing the metal atom (tin or lithium) placed at the sp(3) carbon atom as well as at sp(2) and sp centers (at various positions of the sugar molecule) are reviewed. In addition, selected derivatives in which the metal is connected directly to the anomeric center are described. General methods of the synthesis of these important organometallic intermediates are evaluated. Application of lithiated or stannylated monosaccharide derivatives in the stereoselective syntheses is emphasized. The mechanistic aspects of these processes are discussed.