Silver triflate, glycosidations using

Paulsen and co-workers [156] have also prepared the blood group B (type 2) tetrasaccharide (324) from the lactosamine derivative (318). They attached the 2 -0-l-fucosyl residue using the fucosyl bromide (322) in the presence of mercury(II) bromide and molecular sieves and obtained (319) in 80% yield. Removal of the benzoyl group and glycosidation with 2,3,4,6-tetra-O-benzyl-oc-gaIactopyranosyl bromide in the presence of mercury(II) bromide and molecular sieves at 20 °C or with silver triflate — silver carbonate at —25 °C gave the 3 -0-a-D-galactosyl derivative (323) in 80% yield. In this paper Paulsen discusses in detail the conditions (reactivity of alcohol,... 

Condensation [274] of the bromide (412) with the unsaturated derivative (378) using silver triflate gave an a p yield ratio of 42 21% and condensation of (412) with (377) gave 26 8%. From these results the authors concluded that the 3-P-hy-droxyl group on the neuraminic acid halide prevents dehydrohalogenation and assists glycosidation. 

Friedel-Crafts alkylation has been used in an important synthesis of aryl C-glycosides, which are potent anti-tumor agents, from glycosyl fluorides (equation 99)65 661. The reaction takes place rapidly in dichloromethane, at room temperature using a novel zirconium complex and silver perchlorate combination catalyst. A similar alkylation has been performed by replacing the aromatic compound with either a silyl enol ether or an allylic compound using silver triflate as the catalyst662,663. 

As outlined in Figure 14, a more recent synthesis [55,74] of this disaccharide as the methyl glycoside (13, Fig. 6) made use of thioglycoside donor 33. This synthesis began with the preparation of alcohol 49 from 24a, through a sequence of silylation, benzoylation, and desilylation. Glycosylation promoted by A-iodosuc-cinimide and silver triflate afforded the disaccharide 50, which could be deprotected with sodium methoxide to provide 13. 

With the ability to make use of fluoroglycosides as C-glycosidation substrates, chloroglycosides have been comparably useful. As shown in Scheme 2.5.10, Allevi, et al.,89 condensed silylenol ethers with 2,3,4,6-tetra-Obenzyl-a-D-glucopyranosyl chloride. Silver triflate was utilized as the Lewis acid catalyst. In the examples studied, the yields exhibited were consistently greater than 80%. Furthermore, complete a selectivity was observed. 

Quite recently, it has been established that silver triflate can be used as a promoter for direct galactosylation reactions64. Contrary to expectations, many primary hydroxy groups are completely unreactive in glycosylation reactions, or give the desired glycosides... 

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