Abstract
The peracetylated methyl ester 1 of N‐acetylneuraminic acid was transformed into the oxazoline derivative 2, which was hydrogenated with Pd/C/H~2~ to give the 4‐deoxy‐Neu5Ac2en derivative 3. Acid cleavage of the oxazoline 2 by trifluoroacetic acid (THF) gave the 4‐epi‐Neu5Ac2en derivative 4a, which was saponified to the known 4‐epi‐Neu5Ac2en 4b. Treatment of 4a with triphenylphosphane/diethylazodicarboxylate (DEAD)/HN~3~ in toluene gave the 4‐azido‐4‐deoxy‐Neu5Ac2en derivative 5a as well as the 4‐epi‐azido‐4‐deoxy‐Neu5Ac2en derivative 6a. Both epimers could be saponified yielding the free dehydrosialic acid derivatives 5b and 6b. Furthermore, 4‐formamido‐4‐deoxy‐Neu5Ac2en 7b was prepared via the derivative 7a, which was formed from 5a by reaction with triphenylphosphane/formyl acetate. In addition, the free dehydrosialic acid derivatives 5b and 6b were transformed into the stable PN ylides 8 and 10, but only 8 could be hydrolyzed to the triethylammonium salt of 4‐amino‐4‐deoxy‐Neu5Ac2en 9a, which was converted into the zwitter ionic compound 9b. Finally 4‐acetamido‐4‐deoxy‐Neu5Ac2en 11e was prepared from 4‐acetamido‐4‐deoxy‐N‐acetylneuraminic acid 10 via the derivatives 11a and 11b by well‐known methods. The compounds 4b, 5b, 6b, 7b, 9b, and 11c were investigated as inhibitors of Vibrio cholerae sialidase, and their K~i~ values were determined.