Thioglycosides as Acceptors

Nicolaou used glycosyl fluorides as donors and thioglycosides as acceptors, and the procedure took advantage of the ready conversion of thioglycosides into glycosyl fluorides with NBS and DAST [17]. The two-stage activation procedure is illustrated by the synthesis of a phytotoxic oligosaccharide (78), a member of rhynchosporosides [18] (Scheme 5.13). 

Hence, thioglycosides of sterically crowded mercaptans, as e.g. 1,1-dicyclo-hexylidenemethyl mercaptan, show lower affinity for thiophilic promoters and can be used as acceptors in condensations with ethyl thioglycosides as acceptors [20],... 

Thioglycosides can also be used as acceptors in glycosylation with tri-chloroacetimidates.18... 

Dehydrativeglycosylation of 1-hydroxy donors with Ph2S0/Tf20 in conjunction with thioglycoside acceptors opens the way for sequential double glycosylation, one-pot procedures for trisaccharide synthesis, as exemplified by the efficient one-pot synthesis of the a-Gal epitope and a hyaluronan trisaccharide [566]. This study also shows the potential of selenoglycoside as acceptors in dehydrative glycosylation (Scheme 4.112). 

The capacity of these thioglycosides as glycosyl donors was evaluated using various thiophilic promoters and three different acceptors, one primary (66) and two secondary (67 and 68), the latter relevant for the synthesis of H. influenzae type e structures. The yields obtained were extraordinary, both with the benzoylated and the benzylated donor (1.5 equiv.) and with all three acceptors quantitative or nearly quantitative yields of disaccharide products were observ-... 

A similar comparison was conducted for the synthesis of an a-(1 2)-mannan, but using a different, thioglycoside-derived MPEG derivative as acceptor (Scheme 35). Interestingly, while no problems were reported with the standard solution-phase approach, it was noted that appreciable amounts of an a-mannoside (190, n = 3) were formed at the level of the disaccharide in the MPEG-supported approach.42 A further variation on the PEG-supported approach to the a-(l >2)-mannan has also... 

Sequential Glycosylation Strategies A Focus on Thioglycosides as Donors and Acceptors... 

Here the glycosyl acceptor contains a group (bromide) that is incompatible with the use of a soft Lewis acid activator and so precludes the use of some alternative glycosylation donors, such as glycosyl bromides or thioglycosides. As a consequence of anchimeric assistance by the C-2 acetate group, the P-glucoside is the major product. 

Considering the observed differences in reactivities between ethyl and phenyl thioglycosyl donors, it was anticipated that selective activation could also be achieved when active p-A-acetamidophenyl thioglycosyl donors were used in the presence of latent p-nitrophenyl thioglycosides acting as acceptors. For this purpose, p-nitrophenyl 2,3,4-tri-0-benzoyl-l-thio- 3-D-galactopyranoside 17 was initially used as the acceptor (Scheme 7). 

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