Activations selenoglycosides

AgOTf has also been used to activate selenoglycosides preferentially over thioglycosides, which is advantageous for the con-stmction of larger oligosaccharides (eq 41). ... 

Disarmed as well as armed (see Section V and Scheme 9) phenyl selenoglycosides are reported to be activated by silver triflate in the presence of potassium or silver carbonate in the presence of armed ethyl thioglycoside acceptors, to produce ethyl thiodisaccharides in excellent yields. 

Such a synthesis is shown in Scheme 4.18a, in which a glycosyl bromide is coupled with the thioglycosyl acceptor to afford a thioglycosyl disaccharide. The glycosyl bromide was obtained from the corresponding 1-thioglycoside by treatment with Br2. It was shown [222-224] that phenyl selenoglycosides can be selectively activated in the... 

Selenoglycosides are attractive donor species owing to the wide range of reagents that can promote cation- and radical-cation-based processes for their activation and subsequent O-glycosylation (Scheme 4.102) [223,512],... 

Cation-Based Activation Pinto s works not only described Ag0Tf/K2C03 as the promoter of choice for the activation of phenyl selenoglycosides but also indicated... 

Scheme 4.109 Single-electron transfer activation of selenoglycosides.

Photochemical oxidation has been employed for the activation of O-glycosides [552], thioglycosides [553] and telluroglycosides [554], It is also effective for O-glycosylation with the phenyl selenoglycoside donors [555], although this approach has not been widely taken up for preparative use. 

Ley and coworkers reported the one-pot synthesis of tetrasaccharide 176 by combining selective activation with reactivity-based armed-disarmed glycosylation [113]. As shown in Scheme 6.35, the first chemoselective coupling was achieved via selective activation of the fluoride donor 173 over the selenoglycoside acceptor 174. The resulting disaccharide intermediate was more armed than the bifunctional... 

As expected, such a selective activation of perbenzylated ( armed ) phenyl selenoglycoside 163 was also possible in the presence of the benzy-lated thioglycoside acceptor 162. In this case, an //3 mixture (ratio 2.5 1) of disaccharide 166 was obtained in excellent yield (Table III). 

The selective activation of glycoside trichloroacetimidates over phenyl selenoglycosides was also demonstrated by glycosidation of phenyl seleno-glycoside acceptors 168 and 169 with glycosyl trichloroacetimidate 172. In the presence of catalytic amounts of triethylsilyl trifluoromethanesulfonate at -78°C, the disaccharide derivatives 170 and 171 were obtained in respectively 84 and 90% yields. 

Furthermore, selective activation of differently protected selenoglycosides was also possible with IDCP as promoter. Perbenzylated armed phenyl selenoglycoside 183 was selectively activated in the presence of disarmed acceptor 189 and an < //3 mixture of disaccharide 190 was obtained. With the less-reactive secondary hydroxyl group of 191, the yield of 192 was lower. 

Selenoglycosides can be selectively activated in the presence of thioglycosides. However, they can be glycosylated by glycosyl halides or trichloroacetimidates. These kinds of differences could be used in block syntheses of complex oligosaccharides [568,572]. 

Telluroglycosides are activated by the same type of promoters, including NBS, NIS and NIS-TMSOTf [564], and electrochemical activation has also been reported [573]. Telluroglycosides are even more reactive than selenoglycosides. In fact, they can be selectively activated in the presence of the latter [574]. 

Furuta, T, Takeuchi, K, Iwamura, M, Activation of selenoglycosides hy photoinduced electron transfer, Chem. Commun., 157-158, 1996. 

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