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Regioselective Acetylations of Sialic Acid α-Ketosides
Reinhard, Bernd,Faillard, Hans
, p. 193 - 204 (2007/10/02)
Various partially acetylated sialic acid α-ketosides were obtained without expensive protecting group techniques by using trimethyl orthoacetate or dimethylacetamide dimethyl acetal as acetylating agents or by performing a partial Zemplen de-O-acetylation.Using trimethyl orthoacetate as acetylating agent, we synthesized 9-O-acetylated, 8-O-acetylated, and 8,9-di-O-acetylated sialic acid α-ketoside benzyl ester derivatives.The acetylation with dimethylacetamide dimethyl acetal yielded 9-O-acetylated, 8-O-acetylated, 4,9-di-O-acetylated, 8,9-di-O-acetylated, and 4,8,9-tri-O-acetylated sialic acid α-ketoside benzyl ester derivatives.The partial Zemplen de-O-acetylation permitted the synthesis of 7,8,9-tri-O-acetylated sialic acid α-ketoside benzyl ester derivatives.After catalytic hydrogenation the 8-O-acetylated derivatives 2e and 3d were not stable and were completely converted into the 9-O-acetylated sialic acid α-thioketoside 2k.The N-acetyl-8,9-di-O-acetylneuraminic acid α-aminophenylthioketoside 2l showed a migration of the acetyl group from position 8 to position 7 in polar solvents, yielding a mixture of the 8,9-di-O- and the 7,9-di-O-acetylated derivatives 2l and 2m in a molar ratio of approximately 1:1.The 7,9-di-O-acetylated derivative 2m could be separated and was completely stable in methanol.Using the isopropylidene protecting group we could also prepare the 4-O-acetylated sialic acid α-thioketoside 2o.Treatment of the 9-O-acetylated derivative 2k with fluorescein isothiocyanate gave the fluorescent derivative 5. - Key Words: Sialic acids / Thioketosides
