Silylated glycosyl acceptor

O-glycosidation between O-silylated glycosyl acceptors and several 1-substituted sugars, glycopyranosyl or furanosyl silyl ethers [79a], fluorides [80], phosphates [81], and trichloroacetimidates [82a] has been examined to develop other efficient and stereoselective procedures catalyzed by silicon Lewis acids. The trichloroacetimidate glycosyl donors are fairly reliable and were used for the synthesis of eanelioside GD by Ogawa (Fig. l)[82b]. 

A representative example of this strategy is depicted146 in Scheme 21. The glycosyl donor 107 was glycosylidated with glycosyl acceptor 108 in the presence of triethyl-silyl (TES) triflate to produce disaccharide 109 in 89% yield. The acceptor 108 can be also converted into a glycosyl donor by simple acylation, to afford pentenyl donor 110... 

Support-derived trisaccharide building block 79 was used in a block synthesis of dodecasaccharide 82 (Scheme 13 p. 23). Reaction of support-bound glycosyl acceptor 78 with 79 furnished hexasaccharide 81. Removal of a silyl protecting group and coupling with trisaccharide donor 79 was repeated twice, followed by photolytic cleavage of the linker to furnish dodecasaccharide 82 in 10% yield from 73 [64]. 

Glycosyl acceptors III.3 and III.4, used in the preparation of the )8( 1,3)-disac-charides, were obtained from methyl 2-azido-2-deoxy-)8-D-galactopyranoside [67]. Selective 3,6-di-O-silylation followed by treatment with benzyl bromide led to the installation of the benzyl ether at C4. Desilylation occurred in situ under the ben-zylation reaction conditions to produce III.15 and subsequent 6-O-acetylation of III.15 with 1-acetylimidazole produced III.3 in 78% yield (Scheme 25). The preparation of III.4, carried out as reported earlier by Jacquinet and Sinay [67], was obtained from III.16 by regioselective ring opening of the isopropylidene to unmask the 3-OH. 

Bols has also investigated the use of silyl acetals in intramolecular 0-glycosylation [92]. In early work, he showed that a-glucosides could be readily prepared stereospe-cifically by attaching the glycosyl acceptor through a dimethylsilyl acetal to the 2-posi-... 

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