Glycosyl fluorides treatment

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... 

In the first case, typical reactions are the formation of the methyl fi-D-glucoside from a-D-glucopyranosyl fluoride and of its 2-amino-2-deoxy derivative from 2-amino-2-deoxy-a-D-glucopyranosyl fluoride, on treatment with sodium methoxide in methanol. The products from the reaction of aqueous bases with the glycosyl fluorides depend on the concentration of alkali. At low concentrations, the normal hydrolysis products are formed. At higher concentrations of base, if the proper (trans) steric relation exists between C-6 and the fluorine atom at C-1, anhydro compounds are formed, as in (21) from (20). 

Clausen and Madsen [48] reported the transformation of NPG 78 into glycosyl fluoride 79 by treatment with NBS and (diethylamino)-sulfur trifluoride (DAST) (O Scheme 11c). 

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