2,3-Didehydro-2-deoxysialic acids structurally varied at C-5 and their behaviour towards the sialidase from Vibrio cholerae.

2,3-Didehydro-2-deoxy-N-trifluoroacetylneuraminic acid (5-trifluoroacetyl-Neu2en) (3) has been synthesised from Neu5Ac2en (1) by hydrazinolysis, to give Neu2en (2), followed by N-trifluoroacetylation. 2,3-Didehydro-2,3-dideoxy-D-glycero-D-galacto-2-nonulopyranoson ic acid (Kdn2en, 8) and 5-azido-2,3-didehydro-2,3,5-trideoxy-D-glycero-D-galacto-2-nonu lopyranosonic acid (5-azido-5-deoxy-Kdn2en, 9) have been prepared from the acetylated methyl esters of Kdn (4) and 5-azido-5-deoxy-Kdn (5) via Zemplén saponification. The behaviour of the above 2,3-didehydro-2-deoxysialic acids towards Vibrio cholerae sialidase has been investigated.