Glycosyl fluorides sugars

Glycosyl fluorides have also been prepared by treatment of per-O-acyl or partially 0-acylated sugars with hydrogen fluoride [liquid HF (for example, see Refs. 38 and 39) or HF in acetic acid or dichloromethane], as exemplified by 2,3,4-tri-O-benzyl-a-D-xylopyranosyl (18), a-D-glucopyranosyl (19), tetra-O-pivaloyl-a-D-glucopyranosyl (20), and 2,3,5-tri-O-acetyl-D-xylofur-anosyl fluorides (21) (see Table 1). Frequently, HF treatment - leads to... 

Several simple deoxyfluoro sugars and glycosyl fluorides were utilized " as probes for hydrogen bonding in the glycogen phosphorylase-o-glucose complex. 

The transformation of 1-hydroxy sugars to the corresponding glycosyl fluorides can be achieved by a mixture of Selectfluor 3 (Figure 2.2) and methyl sulfide presumably through a fluorosulfonium ion 9, which then reacts with the anomeric... 

The classical method for introducing the fluorine atom to the anomeric center is to treat per-O-acetylated sugars with hydrogen fluoride [15], using which a-glycosyl fluorides can be prepared stereoselectively, however, the procedure is incompatible with any acid-sensitive functionalities present in the molecule. In addition, hydrogen fluoride is highly corrosive, which makes this method unattractive to many... 

Figure 2.4 Synthesis of glycosyl fluorides from 1,2-anhydro sugars and glycal derivatives.

The search for new methods of synthesis of halogenated carbohydrates continues to be an active area of investigation. The compounds are of utility as synthetic intermediates, and many of them are of intrinsic value in biochemistry and pharmacology. In the present Chapter methods for the synthesis of deoxyfluoro sugars and glycosyl fluorides are discussed. 

Glycosyl fluorides are excellent precursors for glycosylations. Lewis acids promote the defluorination leaving a stabilized carbenium species which is able to react with nucleophiles, e.g. N-nucleophiles. The configuration at the newly formed C-N bond is not only influenced by the sugar configuration but also by the utilized Lewis acid. This means that this is not a clear SNl-type reaction SN2-type pathways also have to be considered.15 18... 

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. 

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