Unsaturated carbonhydrates. Part VIII. Intramolecular allylic isomerisations of 1-deoxyald-1-enopyranose (2-hydroxyglycal) esters

Abstract

On heating in inert solvents 1-deoxyald-1-enopyranose esters rearrange completely and stereospecifically to isomeric 3-deoxyald-2-enopyranose esters which have the same configuration at C-1 as was present at C-3 in the original compounds. An S_Ni′ mechanism is envisaged, and the reaction affords excellent means for obtaining thermodynamically unstable anomers in some instances. Alternatively, the isomerisations can be brought about at room temperature by the use of small amounts of boron trifluoride, but under these conditions anomerisation rates exceed those of the allylic rearrangements and equilibrated mixtures of α- and β-products are obtained.