6-Deoxy sugars, reductive fragmentation

Crich and Yao have exploited a homolytic substitution at sulfur to trigger a radical cascade that includes a loss of carbon monoxide and a radical fragmentation of a 4,6-0-benzylidene moiety to give esters such as 113 after a final diastereoselective reduction (Scheme 32) [107]. This impressive outcome opens a direct avenue to 6-D-rhamnopyranosides and other 6-deoxy sugars. 

Scheme 36 Applications of the reductive fragmentation sequence for the synthesis of 6-deoxy sugars...

One of the most synthetically useful procedures of the preparation of highly advanced synthons is undoubtedly the Vasella reaction [34], It is based on the reductive fragmentation of 6-deoxy-6-halogenopyranosides with zinc in ethanol and provides the open-chain unsaturated sugars in a 60-80% yield (Figure 10.14) n-butyllithium may induce the same process. 

Analysis of a hydrolyzate of the lipopolysaccharide from Pasteurella pseudotuberculosis Group IIA, by g.l.c. of the derived alditol acetates, revealed an unknown component. Mass spectrometry of the alditol acetate obtained after reduction with sodium borodeuteride showed a fragmentation mode compatible with the alditol acetate from a 6-deoxy-heptose. The sugar was subsequently identified as 6-deoxy-D-manno-heptose. ... 

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