Synthesis of the methyl thioglycosides of 2-, 3-, and 4-deoxy-L-fucose.

Methyl thioglycoside derivatives of 2-, 3-, and 4-deoxy-L-fucopyranose have been prepared as glycosyl donors for the synthesis of sialyl Le(x) ganglioside analogues containing modified alpha-L-fucopyranose residues. Reductive dethioacylation of 2-(trimethylsilyl)ethyl 3,4-di-O-benzoyl-2-O-(phenoxy)thiocarbonyl-beta-L-fucopyranoside, prepared from L-fucose in eight steps, gave the 2-deoxy compound, which was transformed via selective removal of the 2-(trimethylsilyl)ethyl group, subsequent acetylation, and displacement of the 1-acetoxy group by a methylthio group, into methyl 3,4-di-O-benzoyl-2,6-dideoxy-1-thio- alpha,beta-L-lyxo-hexopyranoside (11). 2-(Trimethylsilyl)ethyl 2,4-di-O-benzoyl-3-O-(phenoxy)thiocarbonyl-beta- L-fucopyranoside, prepared from the unsubstituted glycoside in four steps, and 2-(trimethylsilyl)ethyl 2,3-di-O-benzoyl-4-O-(phenoxy)thiocarbonyl-beta-L-fucopyranoside, similarly prepared in two steps, were transformed via reduction of the (phenoxy)thiocarbonyloxy group, selective removal of the 2-(trimethylsilyl)ethyl group, O-acetylation, displacement of the 1-acetoxy group by a methylthio group as described for 11, and finally replacement of the benzoyl groups by benzyl groups, into the analogous, protected methyl 3- and 4-deoxy-1-thio-beta-L-fucopyranosides.