In the new transition-state based sialyltransferase inhibitors, an amide group was placed at the corresponding C-2 position of CMP-sialic acid to mimic the geometry and charge distribution in the transition state, and simple aromatic or aliphatic rings were used instead of the sialic acid moiety. All synthetic compounds exhibited excellent α(2-6)-sialyltransferase inhibition, resulting in up to a 2600-fold higher affinity for the enzyme than CMP-Neu5Ac, suggesting that amide is a key element for simulating transition-state features.