Regioselective aectylations of sialic acid α‐ketosides

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 Zemplén de‐ O ‐acetylation. Using trimethyl orthoacetate as acetylating agents 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 Zemplén 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 21 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 21 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 .