Pyranosyl halides

Fig. 14 Axial preference of pyranosyl halides for n-xylo and d-gluco configurations.

An interesting application of acetal elimination from sugars that is promoted by base is the formation of furanoid and pyranoid glycals from isopropylidene acetals of ftiranosyl and pyranosyl halides.285-287 Treatment of 2,3 5,6-di-0-isopropylidene-a-D-mannofuranosyl bromide (289) with a base gave the fiiranoid glycal 290. The yield was... 

Only a few poly-O-acetylglycofuranosyl halides have been prepared of these the best known is tetra-0-acetyl- 8-D-galactofuranosyl chloride. They are all much less stable than the corresponding pyranosyl halides. 

Another well-adapted method for removing the oxygen functionality at Cl uses a radical reduction of a protected pyranosyl halide (O Scheme 28) [211]. Formally, this is not a deoxygenation since the hydroxy group at Cl has already been replaced by a halide. However, gly-cosyl halides are easily available from aldoses by a one-pot procedure [212] and combined with the radical reduction this two-step route gives easy access to a number of 1,5-anhydroalditols. 1,4-Anhydroalditols, on the other hand, are more difficult to obtain by this method since the corresponding furanosyl halides require more steps for their preparation. The rad-... 

At the beginning of this article, some X-rav data of glyco-pyranosyl halides will be discussed that refer to the bond lengths at the anomeric center and which are of great importance for discussion of the anomeric effect. As is well known, acylated derivatives of 3-xylosyl halides exhibit rather interesting conformational effects because in solution all of them adopt the unusual tetraaxial conformation predominantly or totally (1—6). 

Subsequent studies are more directly related to pyranose sugars. An important family of derivatives is known, the pyranosyl halides, in which the alcohol hydroxyls are acylated (generally acetylated) and the hemiacetal hydroxyl is... 

Several pyranosyl halides have been studied as acetylated or benzoylated derivatives. In both the fluorides and chlorides, the equatorial carbon-halogen bonds are shorter than the axial bonds, with the data for fluorides illustrated in Fig. 14. The C - F bond-lengths in both derivatives of -D-xylo-pyranosyl fluoride are significantly shorter than the 143.2 pm found for the non-anomeric C-F bond in l,3,4-tri-C -acetyl-2-deoxy-2-fluoro-D-xylo-pyranose. There is also a variation of 4.4 pm in the C-1 - 0-5 bond-lengths. 

Furanoid and pyranoid glycals. The classical method for preparation of glycals is reduction of pyranosyl halides with zinc and acetic acid. A more recent method is lithium-ammonia reduction of furanosyl and pyranosyl halides with a blocking acetonide group. ... 

Activated zlnc/silver-graphite, used in tetrahydrofuran, efficiently converts acetal-protected and acetylated glycofuranosyl and glyco-pyranosyl halides into glycals - at least on the modest scales (2.2 mmol ) reported. The method has particular potential value for preparing furanosyl glycal derivatives. ... 

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