Glycosyl fluorides treatment with

Halide derivatives may be fluorides, chlorides, or bromides. Fluorides are best prepared by the reaction of hydroxy groups with (diethylamino)sulfur trilluoride ( DAST M. Sharma, 1977) or of glycosyl thioethers with DAST/NBS (K.C. Nicolaou, 1990 B). The other halides are usually only introduced at the glycosidic position, where treatment with hydrogen chloride... 

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... 

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. 

Despite the high utility of glycosyl fluorides as stand-alone glycosyl donors, there has been only one example of a direct dehydrative glycosylation whereby hemiacetal activation proceeds through a glycosyl fluoride intermediate. Hirooka and Koto have detailed the use of diethylaminosulfur trifluoride (DAST) for dehydrative glycosylations with hemiacetal donors (Scheme 3.16) [160]. Treatment of a mixture of hemiacetal 1 and alcohol acceptor (R OH) with DAST 108 (2 equiv) at 0°C provides the... 

Silver tetrafluoroborate in ether or toluene has also been used for the synthesis of glycosyl fluorides. Peracetylated 2-chloro-2-deoxy-D-gluco- and -mannopyranosyl fluorides have been prepared by treatment of the corresponding chlorides with the aforementioned reagent.50,51 Products of kinetic control were obtained when diethyl ether was used as the solvent, whereas products of thermodynamic control were obtained when toluene was used instead. Peracetylated... 

The cis-glycosyl fluorides of the peracetylated 2-deoxy-2-fluoro saccharides have been obtained by addition of trifluoro(fluoroxy)meth-ane to the corresponding peracetylated glycals, whereas the preparation of the other derivatives was achieved by treatment of a suitable precursor with hydrogen fluoride or silver fluoride in acetonitrile peracetylated 2-deoxy-2-fluoro-a-Drmannopyranosyl fluoride70,71 was prepared by equilibration of the /3 anomer in hydrogen fluoride. 

Treatment of thioglycosides with /V-bromosuccinimide (NBS) in the presence of diethylaminosulfur trifluoride (DAST) or HF-pyridine complex furnishes glycosyl fluorides.23 These can be activated with AgC104/SnCL in the presence of another... 

The manno thioglycoside 55 on treatment with DAST give the 2-deoxy-2-phenyl-thio-glucosyl fluoride as anomeric mixture 56. On glycosylation with the 6-hydroxy-glucoside 57 in dichloromethane and the presence of SnCl2, the ot,l- 6 disaccharide 58 is obtained exclusively (92%), which on reduction with Raney-Nickel yields the 2 -deoxy derivative 59. 

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