Glycosyl thioacetates

Chemoselective cleavage of anomeric thioacetates to the thioaldose in the presence of O-acetyl groups is possible under various conditions, e. g., methanolic sodium methoxide at low temperatures [121], phenylmercury acetate followed by demercuration [122], 2-aminoethanethi-ol [123], or hydrazinium acetate [124], and these methods have therefore often been used in the synthesis of thiooligosaccharides (O Sect. 3.2.1). As mentioned, glycosyl thioacetates can also be prepared from the 2-acetoxyglycal derivative (see Scheme 24). 

Glycosyl halides (7a-e) were stereoselectively transformed into l,2-tra s-thio-glycoses by i) (8a-d, 8j) a two-step procedure via the pseudothiourea derivatives [9,10a] the substitution of halide by thiourea is mostly a S l-type reaction since acetylated 1-thio-a-D-mannose (8b) was obtained from acetobromoman-nose (7b) [9cj ii) (8e-i) using thiolates in protic and aprotic solvents [10], or under phase transfer catalysis conditions [11]. Another approach involved the reaction of thioacetic acid with 1,2-trans-per-O-acetylated glycoses catalyzed with zirconium chloride [12]. The 1,2-trans-peracetylated 1-thioglycoses (8e-h) were obtained in high yield. No anomerized products could be detected in these reactions (Fig. 1). 

The first syntheses of 1,2-cfs-thioglycoses (a-D-gluco- and )3-D-manno- derivatives) have been achieved by the reaction in acetone of alkyl or benzyl xanthate or potassium thioacetate with the corresponding l,2-tra s-glycosyl halides [13]. More recently, tetra-O-acetyl-l-S-acetyl-l-thio-a-D-glucopyranose (10a) (Scheme 3) has been obtained i) by reaction of -acetochloroglucose (9 a) with either potassium thioacetate in HMPA or the tetrabutylammonium salt of thio-acetic acid in toluene [14] ii) by peroxide-induced addition of thioacetic acid to the pseudo-glucal (11) [15]. 

S. Suzuki, K. Matsumoto, K. Kawamura, S. Suga, and J.-I. Yoshida, Generation of alkoxycarbenium ion pools from thioacetals and applications in glycosylation chemistry, Org. Lett., 6 (2004) 3755-3758. 

This species can react with a sugar alcohol to form a disaccharide. The ready availability and stability of 75 and this selective and mild glycosylation procedure render this method synthetically attractive. Anodic oxidation of diaryl di-thioacetals results in S-S bond formation as well as C-S bond cleavage as shown inEq. (33) [176,177] ... 

Deacetalization and dethioacetalization. Dimethyl acetals, 4-p-methoxy-phenyl-l,3-dioxolanes are cleaved withDDQ in the presence of water. For deprotection of thioacetals under mild conditions photochemical assistance seems advantageous. The same reaction principle of deacetalization can be extended to ether exchange. Thus alcohol protection is possible by mixing with 2,2-dimethoxypropane in the presence of DDQ, and proximal diols are converted to acetonides. Replacement of anomeric arylmethoxyl groups by this method complements other glycosylation procedures. 

Suzuki S, Matsumoto K, Kawamura K et al (2004) Generation of alkoxycarbeniinn ion pools from thioacetals and applications to glycosylation chemistry. Org Lett 6 3755-3758... 

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