Thioglycosides concept

Transfer of the Armed-Disarmed Concept to Thioglycoside Glycosylation... 

This concept can indeed be transferred to oligosaccharide syntheses using thioglycoside donors, as shown in Scheme 9.9192... 

Thus, it seems that the concept of anomeric electronic activation-deactivation at the anomeric center taking precedence over armed-disarmed in the remainder of the pyranose ring might have reasonable validity, but, in many instances, the difference in reactivity of the p-nitrophenyl thioglycoside versus the p-acetamidophenyl thioglycoside is not enough to make this work (Scheme 1l).94... 

An alternative could be steric activation-deactivation at the anomeric centers of the donor and acceptor. An example using this concept is shown in Scheme 12. In the electronic activation-deactivation concept, deactivated p-nitrophenyl thioglycosides can, after functioning as glycosyl acceptors, be transformed into... 

The armed-disarmed concept is tranferrable to having thioglycosides both as donors and acceptors91,92 (Scheme 9). Anomeric activation can, in favorable instances, also be used for this purpose93 (Scheme 10). 

The sulfoxide method has been applied to the concept [319,374] of intramolecular aglycone delivery for the formation of [1-mannosidcs by means of a silylene linker. In the original work, the acceptor and a thioglycoside donor were joined by means of a silylene group before the oxidation to the sulfoxide [141]. However, it was later found that the preformed sulfoxide was tolerated by the chemistry for the introduction of the linker [286,375]. The intramolecular aglycone delivery step was shown to function effectively for the transfer of the donor to the 2-, 3- and 6-position of glucopyr-anosides, as exemplified in Scheme 4.64. 

Another concept for the activation of thioglycosides is their reaction with soft sulfur electrophiles. The first reagents of this type were dimethyl(methylthio)sulfonium triflate (DMTST, 83) and the corresponding tetrafluoroborate (DMTSF, 84) (Figure 4.4) [86,183-185], which are powerful methylsulfenylating agents (E = SMe) (Scheme 4.15), and are still extensively used. 

In a different version of the same concept. Stork [67,68] and Bols [69-72] introduced silylene type tethering with glycosyl sulfoxide [67,68] or thioglycoside [69-72] donors. The silicon-tethered acceptor approach was also extended to the synthesis of a-D-gluco- [70,72] and a-D-galactopyranosyl linkages in oligosaccharides [70,72]. 

A number of other pathways have been developed, which include in situ utilization of 0-pentenyl glycosides117143 and selenoglycosides.137 The versatility of the two-step preactivation concept was further expanded by development of a one-pot procedure based on the preactivation of tolyl thioglycosides. This strategy is discussed18137 in Section VI. 1. 

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