Hydroxy glycals

A conceptually new direct oxidative glycosylation with glycal donors, employing a reagent combination of triflic anhydride and diphenyl sulfoxide, has recently been reported by Gin [83], This new 3-glycosylation method works very well with hindered hydroxy nucleophiles, including sterically shielded carbohydrate hydroxy systems, and can be run on large scales. 

On interruption of the reaction at the minimum point, only a low yield of the final product, 5-(methoxymethyl)-2-furaldehyde, could be isolated as the semicarbazone, but a crystalline phenylosazone (m.p. 120.5-121.5°, [< ]d —9° in chloroform) was obtained. It was not isolable at the end of the reaction. No osazone was obtainable from the hydroxy-glycal without prior, acid treatment. Analyses of the osazone indicated that it was a mono-O-methyl derivative of the parent CgHgCh. This is the same formula that was found by Bergmann and Zervas (see above ) for the parent of the phenylosazone obtained by direct treatment of 2,3,4,6-tetra-0-acetyl-(2-hydroxy-D-glucal) with phenylhydrazine. Whereas Bergmann and Zervas were unable to acetylate the compound further, Wolfrom, Wallace and Metcalf introduced an O-acetyl group into their mono-O-... 

Binkley, W. W., and Wolfrom, M. L., Composition of Cane Juice and Cane Final Molasses, VIII, 291-314 Blair, Mary Grace, The 2-Hydroxy-glycals, IX, 97-129... 

This delightfully simple methodology is generally applicable to hydroxy-glycal esters and has provided acylated ketoximes of type 25 in large variety acylated pyranoid ketoximes, featuring such useful properties as high tendency for crystallization, ease of isolation and stability (29). 

By the cycloaddition of dibenzyl diazenedicarboxylate and furanoid and pyranoid glycals it is possible to diastereoselectively introduce an amino function at the C-2 position of a carbohydrate29-34. A single diastereomer is produced from furanoid glycals, where the cycloaddition takes place trans to the C-3 hydroxy substituent protected as the silyl ether, e.g., formation of 14-1630. The more reactive bis(2,2,2-trichloroethyl) diazenedicarboxylate can be employed even with less reactive 3-acetyl glycals31. 

In the absence of substituents at C-3, satisfactory diastercoselcctivity is obtained if the glycal has a rigid structure imparted by the protection of the C-4 and C-6 hydroxy groups as the benzylidene ketal, e.g., 2629. 

Glycals, oxo reaction with, 66,86 —, 2-hydroxy-, oxo reaction with, 92 Glycenoses, 234... 

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