Other Glycosylation Methods

Several different approaches that do not fall into the above categories have also been developed and used to varying degrees. Selected examples of this very diverse area are shown below to illustrate that the formation of glycosidic bonds can be approached by various, less conventional routes. 

Finally, it should be mentioned that enzymatic syntheses of glycosides and oligosaccharides developed rapidly, having provided an additional tool for the preparation of these compounds. As further discussion of this subject is beyond the scope of this chapter, the reader is referred to overviews given elsewhere [637-640] as starting points for further reading. 

SCHEME 4.62 Glycoside synthesis by reductive ring opening. 

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Due to the biological roles of glycolipids, many papers have been devoted to their syntheses over the last ten years. The coupling of a fully protected carbohydrate donor to a lipid acceptor requires efficient and highly stereoselective glycosylation methods because lipid derivatives often have low reactivity. A few examples of glycosphingolipids syntheses will be discussed below as well as multistep preparations of other amphiphilic carbohydrates designed as biochemical mimetics, surfactants or liquid crystals. 

Several other glycosidation methods can also be considered as proceeding by remote activation. For example, treatment of 0-benzyl glycosyl trichloroacetimidates with BFj-EtjO [2 see also Chap. 11], or 0-benzyl pentenyl glycosides with NBS [19 see also Chap. 13] leads to activation at an atom not directly attached to the anomeric center. 

Glycosyl fluorides are much more stable and easier to handle than other glycosyl halides. Today they are important building blocks in carbohydrate synthesis. The best method for their preparation is treating either fully or partially protected sugars with the HF-Py complex resulting in 1-fluoro derivatives with an usually predominant a configuration (equation 16)30. 

Several comprehensive reviews have been devoted to the use of 0-glycosyl trichloroacetimidates in glycosylation reactions. In this overview, so far the basic principles of the glycosylation methods, the strengths of the trichloroacetimidate method, differences to other methods, and methodological variations have been discussed. In this chapter recent applications of this method in complex glycoconjugate synthesis will be highlighted. 

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