The reaction of methyl 4,6-O-isopropylidene-2,3-O-thiocarbonyl-α-D-mannoside with methyl iodide : a synthesis of methyl α-Tyveloside (methyl 3,6-dideoxy-α-D-arabino-hexopyranoside)

Abstract

The reaction of methyl 4,6-O-isopropylidene-2,3-O-thiocarbonyl-α-D- mannoside with methyl iodide gives two major products, methyl 3,6- dideoxy-3,6-diiodo-2-O-(methylthio)carbonyl-α-D-altro-pyranoside and methyl 2,3-O-carbonyl-6-deoxy-6-iodo-α-D-mannopyranoside, both of which lack the 4,6-O-isopropylidene grouping but contain the 6-deoxy-6-iodo function, and another minor product. One of the major products, methyl 3,6-dideoxy-3,6-diiodo-2-O-(methylthio)carbonyl-α-D-altro-pyranoside, is converted into methyl α-tyveloside by sequential reduction and ester removal. The incorporation of iodine at C6 is seemingly due to hydrogen iodide, and may be suppressed in the presence of N,N,N,N- tetramethylnaphthalene-1,8-diamine or propylene oxide, or by the use of 4,6-O-benzylidene-2,3-O-thiocarbonyl-α-D-mannoside as substrate. Methyl iodide/propylene oxide is apparently an efficient combination for thiocarbonate (C=S) into carbonate conversion. The structure of the minor product was shown by single-crystal X-ray diffraction to be 1,6- anhydro-3-deoxy-3-iodo-2-O-(methylthio)carbonyl-β-D-altropyranose.