1,6-Anhydro saccharides

Glycals can be transformed into 1,6-anhydro sugar derivatives by intramolecular cyclization in the presence of Lewis and Brpnsted acids, a reaction that has been termed the intramolecular Ferrier glycosylation.168 Sharma el al.169 showed that a montmorillonite clay-supported silver reagent can be an efficient catalyst for this transformation. The 1,6-anhydro-2,3-dehydro sugars obtained were then selectively dihydroxylated to furnish 1,6-anhydro saccharides. 

The reactions discussed in this Section are those involving the removal of the elements of water from osazones, and not merely the preparation of osazones from anhydro saccharides. There are, at present, three tjT>es of anhydro-osazone and, in all three types, the elements of water are eliminated from the hydroxyl group at C-3 and another group of the osazone, accompanied in some cases by inversion of configuration at C-3. 

Similarly, anhydro derivatives of monosaccharides and disaccharides readily react with 359 to give the respective carba-di- and -tri-saccharide. 

G. V. M. Sharma, K. C. V. Ramanaiah, and K. Krishnudu, Clay montmorillonite in carbohydrates use of claysil as an efficient heterogenous catalyst for the intramolecular Ferrier reactions leading to 1,6-anhydro rare saccharides, Tetrahedron Asymm., 5 (1994) 1905-1908. 

Figure 5. 22.63-MHz C-13-NMR spectra of (a) agarose (4a.) (b) neoagaro-oligo-saccharides ("lb, n = 3.9) produced by enzymic hydrolysis of 4a with p-agarase and (c) agaro-oligosaccharides ( 4c, n = 2.5) produced by enzymic hydrolysis of 4a with u-agarase. Spectra recorded in DgO solution at 33°C (3b,5c) or 93°C (5a). G = D-galactose, A = 3,6-anhydro-h-galactose.

Gadella A, Defaye J (1991) Selective halogenation at primary postions of cyclomaltooligo-saccharides and a synthesis of per-3,6-anhydro cyclomaltooligosaccharides. Angew Chem Int Ed 30 78-80... 

Several anhydro derivatives has been isolated from the tars resulting from pyrolysis of parent ketohexoses (or ketohexose-containing saccharides) 2,5-anhydro-a-D-fructopyranose (62),261 264 -a-D-tagatopyranose,261 -a-D-psicopyranose,94,261 and -a-L-sorbopyranose261 (These compounds were previously named as 2,6-anhydrofuranoses, see Ref. 78.)... 

It should be realised that minor conformational isomers may be important reaction intermediates. For example, treatment of 6-0-tosyl-D-glucopyranose with base results in the formation of a 1,6-anhydro derivative. The starting material exists mainly in the 4Ci conformation. However, for reaction to occur the alternative 4 conformation has to be adopted (Scheme 1.2). The introduction of protecting groups may alter the preferred conformation of saccharides. 

A mimetic of a (1 2)-linked glycoside was devised by Ichikawa s group [32] using a 3-amino-2,6-anhydro-3-deoxy-heptonic acid building block to arrive at tetramer 23. A two-atom linker replacing the interglycosidic oxygen also characterizes this saccharide-peptide hybrid. A sulfated derivative of 23 blocked syncytium formation caused by HIV infection to CD4 cells at 50 pM concentration. 

Saccharides substituted on the C3 hydroxyl group form metasacchari-nates. Degradation of 3-0-raethyI-D-glucose, 3-0-methyl-D-fructose, and 3,6-anhydro-D-glucose " affords D-glucometasaccharinates (XV), 3-deoxy- D-gluconate, and 3-deoxy- D-mannonate (see Fig. 3). 

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