With a view to development of novel sialidase inhibitors, mimetics of the natural inhibitor Neu5Ac2en have been prepared in which a phosphonate group replaces the sialic acid glycerol side chain. Different hex-4-en derivatives adopt half-chair conformations that place the glycosyl phosphonate in an equatorial position. For the a-L-threo-hex-4-en derivative this conformation is equivalent to that of Neu5Ac2en, and opposite to that seen for alkyl O-glycosides with the same overall stereochemistry.