Preparation of glycosyl fluorides

Classically, glycosyl fluorides were made by the actioD of a combination of hydrogen fluoride (HF) and pyridine [22] on an anomeric acetate. However, this rather harsh method is somewhat intolerant of acid-sensitive functionaUty in the molecule, and furthermore, HF is highly corrosive and methods involving its use have proved unattractive to many researchers. 

Formation of a glycosyl fluoride from a hemiacetal fluorination of 2,3 5,6-di-0-isopropylidene-a-D-mannofuranose by treatment with DAST [24]. 

Notes and discussion. This procedure is used to form a glycosyl fluoride from a hemiacetal and as such is the original and most straightforward use of DAST in the synthesis of glycosyl fluorides. The product is an anomeric mixture, but there appears to be some solvent dependence of the stereochemical outcome of the reaction, with more polar solvents giving a greater degree of selectivity. 

Round-bottomed flask (100 ml) with rubber septum and magnetic stirrer bar X 2 

Procedure. Equip a round bottomed flask with a magnetic stirrer bar and a rubber septum and flush with argon. A balloon of argon can be used to maintain the inert 

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The preparation of glycosyl fluorides is described next. Aiming to have a convenient glycosyl donor convertible into 1,2-CM-furanosides, Mukaiyama and coworkers prepared 2,3,5-tri-O-benzyl-yS-D-ribofuranosyl fluoride (36y3) by treatment of a protected D-ribofuranose (35) with 2-fluoro-1 -meth-ylpyridinium tosylate (FMPTs) the total yield was raised by anomerizing the simultaneously produced a-1-fluoride (36a, 7, p 66, 72.f 24 Hz) to 36 (7, F 63.5 Hz, 72,F very small ) by treatment with BF3-OEt2. 

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. 

Organofluorine Compounds and Fluorinating Agents. Part 18. Trifluoromethylzinc Bromide as a Reagent for the Preparation of Glycosyl Fluorides ... 

Fluoride-ion substitution of an acetyl group has, generally, been limited to the preparation of glycosyl fluorides [182] (Figure 3.32). 

Posner, G H, Haines, S R, A convenient, one-step, high-yield replacement of an anomeric hydroxyl group by a fluorine atom using DAST. Preparation of glycosyl fluorides, Tetrahedron Lett., 26, 5-8, 1985. 

Preparation and Chemical Glycosylation of Glycosyl Fluoride 3.1 Preparation of Glycosyl Fluoride... 

Preparation of glycosyl fluoride from glycosyl bromide... 

The preparation of glycosyl fluorides from other glycosyl halides is summarized in O Table 6. Glycosyl bromides were effectively reacted with metal fluorides such as AgF (O Scheme 11)... 

Lopez et al. [49] described the preparation of glycosyl fluorides 80, by reaction of NPGs with bis(pyridinium) iodonium (I) tetrafluoroborate (IPy2BF4) in the presence of tetrafluoroboric acid (O Scheme lid). The process was compatible with the presence of silyl and benzyl groups in the NPG. 

G. Home and W. Mackie, Preparation of glycosyl fluorides from phenyl 1-seleno- and phenyl 1-telluroglycosides, Tetrahedron Lett., 40 (1999) 8697-8700. 

A review on the synthesis of gew-difluoromethylene compounds has described carbohydrate (including nucleoside) examples. The synthesis of acetylated glycosyl fluorides directly from the unprotected methyl glycosides has been effected as the first step of a GLC analytical method. First the glycoside is treated with HF followed by the addition of acetic anhydride. Full details (cf. Vol. 25, p.l05) on the preparation of glycosyl fluorides by reaction of aryl 1-thioglycosides with 4-difluoroiodotoluene have been described. ... 

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