Thioglycoses synthesis

The most utilized synthons for the synthesis of thiooligosaccharides have been the per-acetylated 1-thioglycoses they have to be selectively deacetylated and... 

Several methods were described for the selective de-S-acetylation of 0-acetyl protected 1-thioglycoses. Sodium methoxide in methanol at low temperature (below -20 °C) was known to afford mainly the de-S-acetylated compound [16] or exclusively this compound when the reaction was quenched at low temperature by adding H-l- resin [17]. Demercuration of tetra-O-acetyl-l-phenylmercury(II)-thio- -D-glucopyranose (12) (Scheme 4) obtained by treatment of (8e) with phenylmercury(II)acetate afforded a convenient synthesis of tetra-0-acetyl-l-thio-/3-D-glucose (8a) [18]. This sequence applied to the a-anomer (10a) (Scheme 3) led to the expected de-S-acetylated compound (10b) [19]. Chemoselective deprotection of thioacetate at the anomeric position of peracetylated 1-thioglycoses was also achieved in good yield by action of cysteamine in acetonitrile or hydrazinium acetate in DMF [20,11]. 

The branched cyclodextrins (CDs, 17 a, 17 b) and their analogues with D-galactosyl and a-D-mannosyl residues (17c, 17d) have also been prepared under mild conditions by the approach depicted in Scheme 6 [24,25]. Selective in situ S-deacetylation and activation was obtained by treatment of peracetylated 1-thioglycoses (10a, 8e, 8g) by cysteamine in the presence of diAioerythritol in HMPA [26]. This method was very efficient for ffie synthesis of branched CDs (17a) (80%), (17b) (60%), and (17c) (85%) when the acceptor molecule (15b) bearing primary iodide was used. However, peracetylated 1-thioa-D-mannose (8f) failed as a donor under these conditions, but tetra-O-acetyl-l-thio-a-mannose (8 b) afforded the expected CD (17d) in high yield (83%). 

The most widely used synthons for the synthesis of thiooligosaccharides are the 1-thioglycoses and their synthesis has been widely studied. Approaches for the synthesis of 1,2-tra -thioglycoses include nucleophilic sulfur substitutions at the anomeric center of the glycosyl halides with either pscwdo-thiourea derivatives or other thionucleophiles [9, 10]. More recently, phase-transfer catalysis [11] and mild Lewis acid catalysed glycosidation of peracetylated sugars with thioacetic acid [12] have been applied. 

The first reported synthesis of peracetylated 1-thio-a-D-glucopyranose started from the Brigl s anhydride [9] but traditionally, 1,2-cw-thioglycoses have been generated from the 1,2-tranj-glycosyl halides by reaction of alkyl or benzyl xanthates in acetone [13], potassium thioacetate in hexamethylphosphoramide (HMPA) [14], or the tetrabutylammonium salt of thioacetic acid in toluene [15] (Scheme 1). A recent alternative is the peroxide-induced addition of thioacetic acid to hydroxy-glucal derivatives [16]. 

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