Thioglycosides protected, with glycosylation

Wong and coworkers showed that 5-azido sialyl donors protected with O-acetyl ester are useful for a-selective glycosylations of primary hydroxyls (Scheme 4.6e) [169]. It was proposed that the linear and electron-withdrawing nature of the C-5 azido moiety stabilizes the reactive axial acetonitrile adduct to allow the incoming nucleophile to approach the a-face in an SN2-like fashion. In addition, a chemose-lective glycosylation method has been developed for the synthesis of NeuAca-(2 —> 9) NeuAc as thioglycoside donor for use in the subsequent glycosylations [169]. 

Thioglycosides were the first example of an anomeric derivatization that serves the dual role of protection and activation. They were introduced in glycosylation reactions by Ferrier et al. in 1973, who used mercury(II) salts as activator.1 After the development of improved methods of their synthesis and activation, thioglycosides together with trichloroacetimidates are now the most commonly used glycosyl... 

Despite these restrictions, the acceptor-bound succinoyl MPEG28 was found to permit glycosylations with glycosyl trichloroacetimidates promoted by boron trifluoride, triflic anhydride, and trimethylsilyl or triethylsilyl triflates, with glycosyl halogenates promoted by silver triflate, and with thioglycosides promoted by the iodonium ion. It is compatible with long-term ester protection, with allylic,... 

Thioglycosides are also used as glycosyl donors [7]. In contrast with glycosyl bromides and trichloroacetimidates, thioglycosides are more stable under most protection and deprotection conditions. Unlike imidates and bromides, which have to be prepared just... 

Furthermore, these authors were also able to perform the -mannosylations with the thiogly-coside 35 under the action of PhSOTf [45] or 1-benzenesulfinyl piperidine (BSP)-Tf20 [46] (O Scheme 17). In the cases of both glycosyl sulfoxide and thioglycoside, protection of the 4- and 6-positions by a cyclic acetal is critical. The corresponding perbenzylated donors gave poor -selectivity [47]. 

Protected thioglycoside 15 was attached to the soluble polymer support via a DOX-linker 16 (Scheme 20.5) using the NIS/AgOTf protocol, followed by acidic hydrolysis of the acetal using acetic acid. Dissacharide 17 was coupled with glycosyl... 

It was found that the reactivity of glycosyl donors protected with dispoke, CDA, or BDA groups had a range in reactivity between the fully benzylated and fully benzoylated system, which implies that 1,2-diacetal protected thioglycosides might be regarded as semidisarmed substrates. 

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