A-Benzyl glycosides

A final example of the use of carbohydrate-derived dienophiles in Diels-Alder reactions involves the role of the carbohydrate as a chiral auxiliary. Shing and coworkers have described the Diels-Alder reaction of arabinose-derived dienophile 31 with butadiene under Lewis acid catalyzed conditions (Scheme 5). A mixture of cycloadducts 32 with R and S configurations at the cyclohexane ring stereocenter was obtained in 68% yield. The ratio was 73 27 in favor of the / -product when a benzyl glycoside was used lower selectivities were observed with the corresponding methyl glycoside (77). 

The a- and P-glycopyranosyl phosphates of 3-deoxy-D-ma no-2-octulosonic acid have been prepared from Kdo as inhibitors of 3-deoxy-D-/nowio-2-octuloso-nate-8-phosphate synthase, which occurs in Gram-negative bacteria. iV-Acetyl-neuraminic add has been 0-alkylated at C-4 by selective alkylation of an 8,9-0-isopropylidene derivative, and treatment of the a-benzyl glycoside of N-acetylneuraminic acid with iluoroacetyl imidazole has afforded a mixture of 4,5-and 4,9-di-0-(fluoroacetyl) derivatives. A 4-acetamido-4-deoxy-4-epi-derivative of -acetylneuraminic acid has been prepared using standard techniques. ... 

Control experiments do not provide evidence for oxidation of the secondary alcohol groups in the glycoside or for degradation of the ligand backbone. A similar regioselectivity was also observed in a benzyl alcohol/1-phenylethanol model system that showed no proof for the oxidation of the secondary alcohol by formation of acetophenone (18, 23,26). 

As a reducing-end building block for coupling with the phosphorylated oxazoline the previously used 3-0-palmitoyl benzyl glycoside appeared suitable if the end product was... 

In general, the anomeric centre is the first position to be protected during a series of protecting group manipulations. The anomeric centre can simply be protected as an alkyl, allyl or benzyl glycoside. These glycosides... 

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