New 2-deoxy-2,3-dehydro-sialic acids and 2,7-anhydro-sialic acids, which are useful as sialidase inhibitors, and enzymatic methods for preparing them are disclosed. The methods include forming a reaction mixture comprising a glycoside acceptor, a sialic acid donor, and a sialyltransferase; maintaining the reaction mixture under conditions sufficient to form a sialoside; and contacting the sialoside with a Streptococcus pneumoniae sialidase to form the sialic acid product. Methods for the inhibition and sialidases and the treatment of cancer and infectious diseases are also disclosed.