Various partially acetylated sialic acid a-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 a-ketoside benzyl ester derivatives. The acetylation with dimethylacetamide dimethy1 acetal yielded 9-O-acetylated, 8-O-acetylated, 4,9-di-0-acetylated, 8,9-di-O-acetylated, and 4,8,9-tri-O-acetylated sialic acid a-ketoside benzyl ester derivatives. The partial Zemplen de-O-acetylation permitted the synthesis of 7,8,9-tri-O-acetylated sialic acid a-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 a-thioketoside 2k. The N-acetyl-8,9-di-O-acetylneuraminic acid a-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-0-and the 7,9-di-O-acetylated derivatives 21 and 2 m in a molar ratio of approximately 1:l. The 7,9-di-O-acetylated derivative 2 m could be separated and was completely stable in methanol. Using the isopropylidene protecting group we could also prepare the 4-O-acetylated sialic acid a-thioketoside 20. Treatment of the 9-O-acetylated derivative 2k with fluorescein isothiocyanate gave the fluorescent derivative 5 .Sialic acids are, as constituents of sialoglycoproteins and gangliosides, widely distributed in the In addition to N-acetylneuraminic acid, a great number of substituted neuraminic acid derivatives occur bearing e.g. acetyl groups at the hydroxyl The acetyl ester groups can be located in the positions 4, 7, 8, or 9 in various combinations. These glycoconjugates containing O-acetylated sialic acids are involved in many biological functions [4]. Especially the O-acetylation is responsible for different biological processes. An acetyl ester group in position 4 blocks the action of bacterial sialidases completely [5]. Acetylation in the glycerol side chain diminishes the sialidase actionL6] and influences in this way the sialic acid catabolism.There are few synthetic methods for obtaining selectively acetylated sialic acid derivatives. First, Haverkamp et al. realized the synthesis of 9-O-acetylated, 4,9-di-O-acetylated and 4,8,9-tri-O-acetylated sialic acid derivatives by using Nacetylimidazole as acetylating agentr71. The selective 9-0-acetylation with trimethyl orthoacetate was described by Ogura et al. [8,91. Also Hasegawa et a1.[l01 synthesized successfully 9-0-and 4,9-di-O-acetylated sialic acid derivatives by use of acetyl chloride/pyridine at -40°C. Recently, a further selective 9-O-acetylation method using dimethylacetamide dimethyl acetal was described byIn addition, Ogura, Hasegawa and coworkers synthesized 4-O-acetylated, 7-O-acetylated, 7,8-di-O-acetylated, 7,9-di-O-acetylate...
