STRUCTURAL TRANSFORMATIONS OF N-ACETYLNEURAMINIC ACID .25. SYNTHESIS OF METHYL-2-ALPHA-GLYCOSIDES OF 4-EPI-N-ACETYLNEURAMINIC, 7-EPI-N-ACETYLNEURAMINIC, 8-EPI-N-ACETYLNEURAMINIC, AND 7,8-BIS-EPI-N-ACETYLNEURAMINIC ACIDS

The alpha-methylketoside of N-acetylneuraminic acid methylester (4) is transformed via the deacetylated compound 5 into the 9,8-O-isopropylidenderivative 6 which could be oxidized regioselectivity by RuO4 to the corresponding 4-oxo-sialic acid analogue 7. Reduction with the borane-ammonia complex produces a 1:1 mixture of 6 and the desired alpha-methylketoside of 9,8-O-isopropyliden-4-epi-N-acetyl-neuraminic acid methylester (8). Removing of the isopropylidene group gives the alpha-methylketoside of 4-epi-N-acetylneuraminic acid alpha-methylketoside (10). On the other hand compound 5 was turned into the 4,8,9-tri-O-t-butyldimethylsilylderivative 11 a from which the corresponding 7-oxo-compound 12 by oxidation with RuO4 derives. The reduction of 12 with BH3 - NH3 yielded a 1:1 mixture of the starting material 11 a and the desired 7-epi-derivative 13 a which gives either via the purified peracetylated alpha-methylketosid of 7-epi-N-acetylneuraminic acid methylester (14) or a direct saponification the sodium salt of 7-epi-N-acetylneuraminic acid-alpha-methylketoside (15). Applying the Konigs-Knorr procedure to the peracetylated 8-epi-N-acetylneuraminic acid methylester (16) gives rise to the formation of a 1:1 mixture of the corresponding alpha- and beta-methylketosides 17 and 18 besides traces of the corresponding 2,3-dideoxy-2,3-dideohydro-sialic acid derivative 19. After chromatographic separation of 17 further saponification leads to the sodium salt of 8-epi-N-acetylneuraminic acid-alpha-methylketoside (20). In an analogous procedure the sodium salt of 7,8-di-epi-N-acetylneuraminic acid-alpha-methylketoside (25) was prepared starting from the peracetylated 7,8-di-epi-N-acetylneuraminic acid methylester (21), whereby a mixture of the alpha- and beta-methylketosides 22 and 23 was formed in a ratio 95:5 besides traces of the peracetylated 2,3-dideoxy-2,3-didehydrosialic acid methylester (24).