Talopyranose pentaacetate

The last then-unknown carba-sugar, carba-) -DL-talopyranose pentaacetate (83), was synthesized from 36 as follows. Epoxidation of 36 with sodium methoxide gave, at the early stage of the reaction, the 2,3-epoxide... 

The first McCasland s pseudo-sugar, pseudo-a-DL-talopyranose (P) was synthesized from 4-acetoxy-2,3-dihydroxy-5-oxo-cyclohexanecarboxylic acid (6) as follows [13]. Reduction of the oxo acid 6 with sodium borohydride and subsequent esterification with methanol and trifluoroacetic acid, followed by acetylation gave methyl (1, 2, 3, 4/5)-2, 3, 4, 5-tetraacetoxycyclohexanecarboxylate (7). Hydrogenation of 7 with lithium aluminium hydride and successive acetylation yielded pseudo-a-DL-talopyranose pentaacetate (8). Hydrolysis of 8 in ethanolic hydrochloric acid gave pseudo-a-DL-talopyranose 9 in 23% overall yield from 6 [1] (Scheme 6). 

Deacetylation of 80 in methanolic sodium methoxide and successive O-isopropyli-denation with 2,2-dimethoxypropane gave 2-0-acetyl-3,4-O-isopropylidene-l,6-an-hydro-pseudo-p-DL-galactopyranose (81), after acetylation. Removal of the acetyl group of 81, followed by oxidation with ruthenium tetroxide and sodium metaperiodate afforded the 2-oxo derivative (82). Catalytic hydrogenation of 82 under the presence of platinum catalyst and acetolysis in a mixture of acetic acid, acetic anhydride and sulfuric acid gave pseudo-P-DL-talopyranose pentaacetate (83) [25] (Scheme 17). 

The acyloxonium rearrangement provides a good preparative access to D-talose from D-galactose. Thus, tetra-0-acetyl-/3-D-galactopy-ranosyl chloride (90) is treated with antimony pentachloride to give the mixture of salts 91 and 92, which is hydrolyzed and the mixture of products acetylated. Penta-O-acetyl-a-D-talopyranose, which is present as the pure a-D anomer, crystallizes out from the mixture, whereas the D-galactose pentaacetate does not. The latter exists as a difficultly crystallizable, a,/3 mixture, because hydrolysis of 91 and acetylation of the product leads to an anomeric mixture. 

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