Allylglycosides

Evans developed a new method for the synthesis of [(-C-allylglycosides, based on BusSnOTf-mediated ring-opening of glycal epoxides with allylstannanes as nucleophiles [81a], This methodology has been efficiently used in the (3-stereoselective introduction of the side chain (C44-C51) of spongistatin 2 (Scheme 8.43) [81b,c]. 

C-Allylglycosides are also obtained by reaction of 1 with glycal derivatives in the presence of TiCl4 (1 equiv.). In all cases, the principal C,-glycoside is the one produced by apparant axial attack at C, (with concomitant shift of the double bond to C2 C,).4... 

In the second example of the utility of glycosyl halides as substrates for the formation of C-arylglycosides, Allevi, et al.,89 demonstrated conditions utilizing silver triflate as the catalyst. This study, summarized in Scheme 2.4.13, proceeded in only 40% yield with total p selectivity. Thus, the developing trend, compared to that in the formation of C-allylglycosides is that C-arylglycosidations have a tendency to allow the formation of product mixtures highly enriched with p anomers. 

Besides monoallylethers of chiral alcohols, allylglycosides bearing a free alcohol functionality have been applied in asymmetric syntheses of enantiomerically pure cyclopropanemethanols with success." The cleavage of the chiral carbohydrate-derived auxiliary was achieved by treatment of the cycloadducts with a. Tf20, pyridine b. DMF/H2O, pyridine, whereas for the compounds derived from 57 a. Tf20, BU4NI and b. nBuLi were applied. 

Different approaches have been used for the hydrosilylation of allylglycosides with various H-functional polysiloxanes [71, 78, 85-87], Glucopyranosyl- and cellobiosyl-terminated oligodimethylsiloxanes with thioether or ether linkages have been prepared and tested as transdermal penetration enhancers [87], The monosaccharide-modified compounds exhibited a pronounced permeation acceleration for antipyrine, while the disaccharide ether siloxane had no enhancing activity. 

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