Rearrangement reactions 2,3-unsaturated glycosides

A different type of glycosylation reaction of glycals takes place with allyl rearrangement and leads to 2,3-unsaturated glycosides [615]. These glycosylations, often referred to as the Perrier rearrangement, are summarized in Table 4.17. 

Unsaturated glycosides, which are easily obtained by the reaction of glycals with nucleophiles [34], served as the starting materials for the first reported Claisen rearrangement in carbohydrate chemistry [35] by Perrier and Vetha-viyasar (Scheme 18). Heating vinyl ethers 88 a and 88b at 185 °C resulted in the... 

C-Glycosides are increasing in importance, and the synthesis of several 2,3-unsaturated C-pyranosides produced by allylic rearrangement reactions of glycals are referred to in Chapter 3. 

Besides nucleophilic substitutions, additions to the double bond of 1,2-unsaturated sugars (glycals) constitute an attractive route for the synthesis of glycosides. Glycals react in these reactions either by a 1,2-addition or by an allylic rearrangement. The following discusses these two reactions in turn. 

Treatment of 5,6-unsaturated sugar 382 (Scheme 11.79) with mercury(ll) salts was shown to induce what is called the second Ferrier rearrangement. The reaction proceeds via mercuration of the double bond and, in the presence of water, the alkyl glycoside 383 is then hydrolyzed to the aldehyde 384, which undergoes intramolecular aldol reaction to produce a (3-hydroxy cyclohexanone 385. This reaction produces only six-membered rings. An excellent review by Ferrier and Middleton was pubhshed some years ago [300]. 

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