Preparation of unsaturated carbohydrates. A facile synthesis of methyl 4,6-O-Benzylidene-D-hex-2-enopyranosides

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Abstract
A simple method was explored for the preparation of methyl 4,6-O-benzylidene-hex-2-enopyranosides. Methyl 2,3-anhydro-4,6-O-benzylidene-D-hexopyranosides were converted to diaxial iodohydrins using sodium iodide in acetone which contained sodium acetate and acetic acid. Treatment of the iodohydrins with either methane- or p-toluenesulfonyl chloride in refluxing pyridine yielded the 2,3-unsaturated derivative in excellent yield. The four isomers for methyl 4,6-O-benzylidene-D-hex-2-enopyranoside were thus prepared. The rotations of the anomeric pairs followed Hudson's rules of isorotation. Evidence is provided that, in the absence of strong destabilizing interactions in the product, the epoxide ring of one of the above-mentioned 2,3-anhydroglycosides is opened by lithium iodide in ether to form the iodohydrin as the lithium alkoxide in high yield. This observation provides an explanation for the reduction of methyl 2,3-anhydro-4,6-O-benzylidene-α-D-allopyranoside to 4,6-O-benzylidene-D-allal. Reaction of methyl 4,6-O-benzylidene-2-deoxy-2-iodo-α-D-idopyranoside with methyl lithium provided 4,6-O-benzylidene-D-gulal. The conformational properties of a number of the compounds prepared and of acetylated methyl pentopyranosides, as derived from their nuclear magnetic resonance parameters, are discussed with particular reference to the role that geminal coupling constants may play in the determination of configuration and conformational equilibria.

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