The synthesis and reactions of unsaturated sugars

The action of sodium iodide in acetone on methyl 4- O -benzyl-3-deoxy-3-iodo-2- O -tosyl-β- l -xylopyranoside ( 1 ) is described. At room temperature, methyl 4- O -benzyl-2,3-dideoxy-β- l - glycero -pent-2-enoside ( 2 ) and methyl 2- O -benzyl-3,4-dideoxy-β- l - glycero -pent-e-enoside ( 2a ) were formed. At lower temperatures (0–5°), the action of sodium iodide in acetone on 1 produces only the expected 2,3-alkene 2 , and a mechanism, based on conformational analysis, is suggested to account for this fact. The action of sodium iodide in acetone on methyl 4- O -benzyl-3-deoxy-3-iodo-2- O -tosyl-6- O -trityl-α- d -glucopyranoside [ 12 , made from the iodohydrin 9 obtained by the action of methylmagnesium iodide on methyl 2,3-anhydro-4- O -benzyl-6- O -trityl-α- d -allopyranoside ( 8 )] is also described. The structure of the iodohydrin 9 was established by mild, acid hydrolysis, followed by sequential hydrogenolysis, to yield methyl 3-deoxy-α- d - ribo -hexopyranoside. Treatment of 12 with sodium iodide in refluxing acetone yielded a crystalline alkene, which has an n.m.r. spectrum consistent with the structure of methyl 4- O -benzyl-2,3-dideoxy-6- O -trityl-α- d - erythro -hex-2-enoside ( 13 ). This structure was further confirmed by cis hydroxylation of 13 with neutral potassium permanganate, followed by catalytic hydrogenolysis, to give methyl α- d -mannopyranoside.