Sugar synthesis Lewis acids

V0R8RUGGEN Nucleosidasynthesis Synthesis of nucleosides by condensation of sugars with siiyl heterocycles and Lewis acids such as SnCU or trimethylsityl Inflate 3. 

The synthetic value of this reaction has been enhanced by the development of a wide range of methodologies for glycal synthesis from 0-acylatcd glycosyl halides1 or from noncarbohydrate precursors. By use of the Lewis acid-catalyzed diene-aldehyde cyclocondensation reaction, glycals of natural and non-natural sugars can be obtained (Scheme 4).31... 

The transglycosylation method for the introduction of a sugar in a purine base is unique and often useful, especially for the synthesis of purine nucleosides of functionalized sugars that are difficult to prepare by conventional procedures, e.g. formation of 19. The transfer of the ribosyl moiety of acylated pyrimidine nucleosides to purines has been accomplished in the presence of a Lewis acid catalyst. ... 

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]. 

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