Glycosyl fluoride method

A Weak Lewis Acid Cleaves the C-F Bond. How Was the Glycosyl Fluoride Method Discovered 36... 

Various Promoters Employed in Glycosylation by the Glycosyl Fluoride Method 38... 

Scheme 2. The glycosyl fluoride method for glycoside formation (a) and its application (b) to the synthesis of a-cyclodextrin (6). (Ogawa, 1987) ...

In the Koenigs-Knorr method and in the Helferich or Zemplen modifications thereof, a glycosyl halide (bromide or chloride iodides can be produced in situ by the addition of tetraalkylammonium iodide) is allowed to react with a hydrox-ylic compound in the presence of a heavy-metal promoter such as silver oxide, carbonate, perchlorate, or mercuric bromide and/or oxide,19-21 or by silver triflu-oromethanesulfonate22 (AgOTf). Related to this is the use of glycosyl fluoride donors,23 which normally are prepared from thioglycosides.24... 

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... 

This Section deals with the methods of preparation of glycosyl fluorides generally (for the synthesis of glycosyl fluorides by addition reactions to glycals, see Section 11,6), and gives a number of examples from the types of glycosyl fluorides that have been prepared. 

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. 

Numerous methods to prepare glycosyl fluorides can involve nucleophilic fluori-nation reagents (e.g., HF, DAST) or electrophilic reagents (e.g., Selectfluor ) (cf. Chapter 2) The synthesis and reactivity of glycosyl fluorides have been... 

Carboxylates are stable to anhydrous hydrogen fluoride,30 but as described above, how ever, hemiacetal esters are readily cleaved and fluorinated by anhydrous hydrogen fluoride or 70% hydrogen fluoride/pyridine, this method has been widely applied in the synthesis of glycosyl fluorides from glycosyl esters for reviews see refs 29, 34, 277-279. 288, 289. Furthermore, p-toluene- or methanesulfonates (but not trifluoroacetates) of primary alcohols arc fluorinated by nucleophilic substitution using tctrabutylammonium hydrogen fluoride. This procedure is less suitable for secondary alcohols because of the considerable number of elimination products 306 for example, formation of 1 compared to 2.306... 

A method for the synthesis of glycosyl fluorides 11 starting from the glycosyl chlorides 10 with silver(I) tetrafluoroborate proceeds under very mild, homogeneous conditions.72,73 The ratio of isomers formed is dependent on the solvent used. The results are summarized in Table 14.72,73... 

The two-stage activation procedure for the synthesis of complex oligosaccharides developed in these laboratories [23] combines the chemistries of glycosyl fluorides and sulfides. It is, therefore, instructional to discuss first, the use of these two classes of glycosyl donors in oligosaccharide synthesis, before describing the two-stage activation method. 

Scheme 7 Methods for glycosyl fluorides using (a) thioglycosides [23] and (b) lactols [29,30], Thioglycosides as Glycosyl Donors...

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