2.3-Dideoxy-4-0-vinyIhex-2-enopyranoside derivatives on heating to I 80" undergo the Claisen rearrangement to give 2,3,4-trideoxy-2-C-(formylmethyl)hex-3-enopyranoside isomers. Suprafacial [3.3] sigmatropic processes are involved, so that stereochemical integrity is maintained a t the allyiic centres. The reaction represents a novel means whereby branch points can be introduced into carbohydrates.
2,3-Unsaturated hexopyranoside derivatives with azido-, thiocyanato-, vinyloxy-, and (methy1thio)thiocarbonyloxy-groups at C-4 rearrange thermally to give 3,4-unsaturated isomers with azido-, isothiocyanato-, acetaldehydo-, and (methy1thio)carbonylthio-groups attached to C-2; the asymmetry of C-4 in the initial compounds is transmitted to C-2 during the isomerisations.1, %UNSATURATED cyclic carbohydrates, e.g. tri-0-acetyl-Dglucal (I) and its 2-acetoxy-derivative (11), in the presence of acid catalysts, can be converted into 2,3-unsaturated glycosyl compounds (111), e.g. glycosidesl and nucleosides,2 with high stereospecificity in reactions which offer a novel approach to the synthesis of saturated compounds of biochemical significance. In addition, for example, the 1,2unsaturated compound (11) can be caused to isomerise thermally to the 2,3-unsaturated ester (IV) in a reaction believed to proceed by the S,i' me~hanism.~ Such allylic isomerisations at other sites of carbohydrates are not well
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