O-Silyl Glycosides

Finally, the activation at the anomeric position through the Mitsunobu reaetion will be considered. Besides the 1-hydroxy sugars, the 1-O-silylated glycosides used as glycosyl donors will also be included in this section. 

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]. 

Pyranose-derived glycals are also useful as substrates in C-glycosidations with pyrimidinyl mercuric acetates. As shown in Scheme 4.1.8, Farr, eta/.,7 carried out such reactions utilizing 3-O-silyl protected glycals. Palladium acetate was used as the catalyst and no salt additives were used. Compared to the results presented in the previous four schemes, the illustrated coupling proceeded cleanly with the formation of an 84% yield of a p-C-glycoside. 

Silylation, for instance, providing 1-0-trimethylsilyl glycosides from 1-O-unprotected sugars in an anomerically pure form, has been shown by Tietze and coworkers to be useful in phenyl glycoside and l,r-diacetal (see Section 1.2.S) synthesis with 0-silylated acceptors and trimethyisilyl triflate as catalyst. The diastereoselectivity was dependent on O-protection. 

Full details have been published on the [4+2]cyclo-addition of dibenzyl azodicarboxylate to O-silylated glycals, and the conversion of the adducts to 1,2-trans-related 2-amino-2-deoxy-glycosides by reaction with alcohol in the presence of an acid catalyst (c Vol.22, p.97). By using bis(trichloroethyl) azodicarboi late, this route has now been extended for use with acetylated... 

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