Methyl -uronate

It is noteworthy that, while methyl tetra-0-acetyl-j8-D-glucopyranoside reacts at the anomeric center (see Section 11,9), the corresponding methyl uronate derivative (13) undergoes selective bromination at C-5, to give30... 

Methylated uronic acids are usually separated from methylated aldoses by virtue of the insolubility of their barium salts in chloroform. Recently, 4-0-methyl-D-glucuronic acid has been separated from a mixture of non-acidic reducing sugars by absorption on a weakly basic... 

When 5,6-anhydro-l,2-0-isopropylidene-a-I>-glucofuianose (67) in ether was allowed to react with carbon monoxide (12 atmospheres, at room temperature) in the presence of sodium cobalt tetracarbonyl for 3 days, the stoichiometric amount of carbon monoxide was absorbed. The mixture was cooled to —5 , and subsequent treatment with methanol and iodine by the procedure of Heck and Breslow resulted in the formation (in 80 % yield) of the methyl uronate (70) and, in a yield of about 10%, the 6-deoxy-hexos-5-ulose (69). Reduction of the methyl uronate (70) and of the dialdose derivative (68) with lithium aluminum hydride yielded identical sugars. 

A related strategy starting from a uronic acid was reported to construct the core stmcture of zaragozic acid [122,123]. The key point was the formation of the C4-C5 bond using aldol condensation. As shown in Scheme 11.40, methyl uronate 168 was first treated with KHMDS at —100°C, and then treated with a premixed solution of cerium chloride and (l )-glyceraldehyde acetonide to give the expected aldol as a mixture of diastereoisomers in a yield of 65 to 70%. 

Two-caibon chain-extension by condensation of a-D-i o-pentodialdo-l,4-fiininose derivative 24 with the anion formed from the acetyKron complex 27 gave a 2 1 mixtufe of products 25. Decompl catk>n with MBS in methanol furnished the separable methyl uronates 26." Two methods for three-carbon elongation, both u g thiaa>le based reagents, are illustrated in Scheme 4. As the M ttig route was complementary, in stereochemical terms, to the enolate route and sdective reduction of ketones 2S and 29 was readily adueved by choice of appropriate reaction conditions, ail four stereoisomers 30 were available."... 

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