Anhydro sugars, anomeric

In the second approach to 1,6-anhydro sugars, anomeric esters, particularly glycosyl acetates, are activated by Lewis acids and displaced by labile ethers at 0-6. Rao has reported an efficient synthesis of tri-0-acetyl-l,6-anhydro-/3-D-glucopyranose by treatment of the trityl ether (4) with titanium chloride (O Scheme 3) [20]. 

The same procedure was also applied to the synthesis of a-C-galactosyl compounds. Similarly, reductive ring opening of 1,2-anhydro sugar 93 with titanocene(m) chloride produces an anomeric radical 97 that can be trapped... 

The synthesis of other anhydro sugar derivatives is usually accomplished by an internal displacement of a leaving group with inversion, as has already been emphasized for those anhydrides in which the anhydride bridge engages the anomeric position. Three such compounds that have been used in polymerizations deserve specific mention 5,6-anhydro-l,2-0-isopropylidene-a-D-glucofuranose,12 13 its 3-methyl ether,11 and 3,5-anhydro-l,2-0-isopropylidene-a-D-xylo-furanose,14 all of which were synthesized by p-toluenesulfonylation of the primary hydroxyl group of the parent, isopropylidene derivative, followed by treatment with base. 

II. Anhydro Sugars Involving the Anomeric Carbon Atom in the... 

Acid or Lewis-acid catalyzed self-condensation reactions of 1,2-anhydro sugars were studied for the synthesis of polysaccharides [438]. Polysaccharides of relatively low molecular weights (< 10,000) were obtained, and the anomeric configurations could be varied depending on the promoter and temperature [439]. 

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