Glycal procedures

Typical Procedure for the Preparation of Selenoglycosides from Glycals... 

Glycosyl cyanides have been thoroughly used as starting materials in the preparation of 1-formyl, and 1-cyano-glycals, mainly through base-induced elimination procedures (Scheme 10). 

Glycals, for example D-glucal (23), being vinyl ethers are susceptible to acid catalysed additions of alcohols which result in the formation of 2-deoxyglycosides, but elimination reactions accompany additions of this type 86) go such products are better prepared by way of halogen adducts or by the alkoxymercuration reaction (see below). On the other hand, the addition procedure can be more suitable than the alcoholysis... 

Other examples of furanoid and pyranoid glycals were independently prepared287 by using lithium in liquid ammonia, a procedure claimed to be more efficient and general. For instance, 2,3-O-isopro-pylidene-4-O-methyl-a-L-rhamnopyranosyl chloride (291) and 6-... 

For the synthesis of the D-C portion, two different concepts were followed either by modification of laminaribiose (166) [89] or by a stereospecific P, 1 ->3-glycosylation [20]. Laminaran is isolated from seaweeds or from Poria cocos Wolf, degraded by selective acetolysis, and the lower oligomers separated by preparative HPLC [90]. Following acetylation, the heptaacetyl laminaribiosyl bromide is prepared and transformed into the disaccharide glycal 167 by the classical approach in 93 % yield. The 2-deoxy-2-iodo-a-glycoside is formed by application of the NIS procedure after deprotection and subsequent 4,6-0-benzylidenation, the precursor 168 for the radical formation of the 6,6 -dibromo-6,6 -dideoxy derivative is at hand. This compound may be further reduced to methyl-3-0-(P-D-chinovosyl)-a-D-olivoside (169). 

The naturally occurring a,l ->3-linked oligosaccharides of, e.g., kijanimicin are synthesized either in a stepwise manner or following a one-pot procedure [109]. Starting with the 4-O-benzyl-L-digitoxal (228) [10], NIS is added in solution in a half-equimolar amount. Then the same aliquote of glycal 228 and NIS is added under the same conditions. Subsequently, addition of a half-molar equivalent of NIS and benzyl... 

Descotes et al. developed a stereoselective photocyclisation procedure of hydrox-alkyl glycosides to give spiro orthoesters [135]. By oxymercuration of the glycal 154... 

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