Synthetic Studies on Sialoglycoconjugates. 4: Synthesis of 5-Acetamido-3,5-Dideoxy-d-Galacto-2-Octulosonic Acid Derivatives and Analogs

Synthesis of chemically modified sialic acid-containing sialyl-LeX ganglioside analogues recognized by the selectin family

Sialyl Lewis X ganglioside analogues containing 5-acetamido-3,5-dideoxy-L-arabino-2-heptulopyranosylonic acid (C7-Neu5Ac), 5-acetamido-3,5-dideoxy-D-galacto-2-octulopyranosylonic acid (C8-Neu5Ac), and 5-acetamido-3,5-dideoxy-L-glycero-D-galacto-1-2-nonulopyranosylonic++ + acid (8-epi-Neu5Ac) in place of N-acetylneuraminic acid (Neu5Ac) have been synthesized. Glycosylation of 2-(trimethylsilyl)ethyl 6-O-benzoyl-beta-D-galactopyranoside with the phenyl or methyl 2-thioglycoside derivatives of the respective sialic acids, using N-iodosuccinimide (NIS)-trifluoromethanesulfonic acid as a promoter in acetonitrile, gave the three required 2-(trimethylsilyl)ethyl (2S)-sialyl-(2-->3)-beta-galactopyranosides. These were converted via O-benzoylation, selective transformation of the 2-(trimethylsilyl)ethyl group to acetyl, and introduction of the methylthio group with methylthiotrimethylsilane into the corresponding glycosyl donors. Glycosylation of 2-(trimethylsilyl)ethyl O-(2,3,4-tri-O-benzyl-alpha-L-fucopyranosyl)-(1-->3)-O-(2-acetamido-6-O- benzyl- 2-deoxy-beta-D-glucopyranosyl)-(1-->3)-2,4,6-tri-O-benzyl-beta-D- galactopyranoside with these donors in the presence of dimethyl(methylthio)sulfonium triflate (DMTST) afforded the expected beta-glycosides, which were converted into the corresponding alpha-trichloroacetimidates, and these, on coupling with (2S, 3R, 4E)-2-azido-3-O-benzoyl-4-octadecene-1,3-diol, gave the required beta-glycosides. Finally, these were transformed via selective reduction of the azide group, condensation with octadecanoic acid, O-deacylation, and de-esterification into the target compounds in good yields.

Synthesis of the methyl thioglycosides of deoxy-N-acetyl-neuraminic acids for use as glycosyl donors

Methyl 2-thioglycoside derivatives of 4-, 7-, 8-, and 9-deoxy-N-acetylneuraminic acids have been prepared as glycosyl donors for the synthesis of sialoglycoconjates. Reduction of a (phenoxy)thiocarbonyl group, selectively introduced at the 4 position of methyl [2-(trimethylsilyl)ethyl 5-acetamido-3,5-dideoxy-8,9-O-isopropylidene-D- glycero-alpha-D-galacto-2-nonulopyranosid]onate (1), gave the 4-deoxy compound, which was transformed via O-deisopropylidenation, acetylation, selective removal of the 2-(trimethylsilyl)ethyl group, subsequent acetylation, and displacement of the 2-acetoxy group by a methylthio group, into methyl (methyl 5-acetamido-7,8,9-tri-O-acetyl-3,4,5-trideoxy-2-thio-D-manno-2- nonulopyranosid)onate (17). Methyl [2-(trimethylsilyl)ethyl 5-acetamido-8,9-di-O-acetyl-4-O-benzoyl- 3,5,7-trideoxy-alpha-D-galacto-2-nonulopyranosid]onate, prepared from 1 in five steps, and methyl [2-(trimethylsilyl)ethyl 5-acetamido-4,7,9-tri-O-acetyl-3,5,8-trideoxy-alpha-D-galacto-2- nonulopyranosid]onate, prepared from 1 in six steps, were converted via selective removal of the 2-(trimethylsilyl)ethyl group, O-acetylation, and displacement of the 2-acetoxy group by a methylthio group as described for 17, into the corresponding methyl 7- and 8-deoxy-2-thioglycosides. Reductive dechlorination of methyl [2-(trimethylsilyl)ethyl 5-acetamido-4,7-di-O-benzoyl-9-chloro-3,5,9-trideoxy-D-glycero-alpha-D-g alacto- 2-nonulopyranosid]onate, prepared from methyl [2-(trimethylsilyl)ethyl 5-acetamido-3,5-dideoxy-D-glycero-alpha-D-galacto-2-nonulopyranosid++ +]onate by selective 9-O-tert-butyldimethylsilylation, benzoylation, removal of the 9-silyl group, and selective chlorination, gave a 9-deoxy compound. This was transformed, via O-debenzoylation, O-acetylation, selective removal of the 2(trimethylsilyl)ethyl group, 2-O-acetylation, 2-chlorination, displacement with potassium thioacetate, selective S-deacetylation, and S-methylation, into the methyl 2-thio-alpha-glycoside of 9-deoxy-N-acetylneuraminic acid.