Aryl thioglycosides

In contrast, cleavage of alkyl and also aryl thioglycosides with dimethyl(meth-ylthio)sulfonium tetrafluoroborate takes place with total inversion of configuration [9] (equation 12) The latter reagent is commercially available or easily prepared as a crystalline nonhygroscopic solid [10]. All the three above-mentioned reagents... 

Fully acetylated hexopyranoses react with thiols in the presence of Lewis acids, such as BF3-Et20.32,33 The reaction is faster with 1,2-trans acetates than with the corresponding 1,2-cij ones and 1,2-trans products predominate. Alkyl, alkenyl, and aryl thioglycosides are produced by this method. Variations on this method include the use of trimethylsilyl34 or stannyl derivatives35 of the thiols. 

This preparation, specifically of acylated aryl thioglycosides, is carried out by reaction of, for instance, a 2,3,4,6-tetra-O-acety 1-1 -thio-P-D-glucopyranose with a di-azonium salt, followed by thermal decomposition of the intermediate diazo product. 

G. Balavoine, A. Gref, J.-C. Fischer, and A. Lubineau, Anodic glycosylation from aryl thioglycosides, Tetrahedron Lett. 31 5761 (1990). 

A. Fiirstner, Synthesis and reductive elimination reactions of aryl thioglycosides, Liebigs Ann. Chem., 1 (1993) 1211-1217. 

In addition, a number of methods to synthesize thioglycosides direct from hemiacetals (especially perbenzylated) have been reported. Hence, treatment of fully protected hemiacetal monosaccharides (both furanoses and pyranoses) with a triaUcylphosphine and a diaryl disulfide gave high yields of the corresponding aryl thioglycosides, predominantly with converted stereochemistry at the anomeric center, indicating a Sn2 type of reaction (O Scheme 14) [82,83,84]. 

Deoxy functions in the sugar ring also increase the acid lability of the glycoside. Thus, was the methyl glycoside of daunosamine (2,3,6-trideoxy-3-amino-L-/yjt o-hexopyranose) efficiently transformed into the corresponding aryl thioglycoside by treatment with a thiol and BF3-etherate (O Scheme 23) [96]. The reaction was highly a-selective. 

Apart from direct arylation using aryl halides, aryl thioglycosides can be prepared most efficiently by treatment of the thioaldose with an aryidiazonium salt followed by thermal decomposition of the produced diazo derivative (O Scheme 46) [158]. The procedure is usually very... 

An indirect method, specifically for aryl thioglycosides, is the reaction... 

THIOGLYCOSIDES Table 4.2 Activators employed in glycosylations using alkyl/aryl thioglycosides 131 as glycosyl donors... 

Notes and discussion. This reaction can be utilized for the preparation of a-allyl, ethyl, phenyl and substituted aryl thioglycosides of sialic acid. NMR and NMR spectra of the crude reaction mixtures have confirmed that complete anomeric inversion occurs under the PTC condition. Adoption of tetrabutylammoniumhydro-gen sulfide as a PTC is critical for mild reaction conditions, otherwise, if triethylbenzylammonium chloride is employed, more harsh conditions are required. The use of ethyl acetate is advantageous over dichloromethane owing to the production of PhSCH2Cl and (PhS)2CH2 resulting from nucleophilic displacement of one or two chlorides of dichloromethane with thiophenol in the model experiment. 

The most common approach to the electrosynthesis of glycosides is that based on the direct anodic oxidation of suitable glycosyl donors, usually aryl thioglycosides. Nevertheless, some non-conventional methods have been proposed, e.g. the use of electrochemically generated activators or the activation of the glycosyl acceptors. 

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