NeuAc synthetase route . The relative rates of these C9-modified CMP-NeuAc derivatives shown in Scheme 30 were compared against the natural donor CMP-NeuAc in sialyltransferase assays that utilized enzymes from different sources with their appropriate natural acceptors. The rat liver a-2,6-sialyltransferase tolerated a wide range of functional groups without significant decreases in the relative rates. Synthetically useful relative rates were observed for most of the CMP-NeuAc analogs with porcine sialyltransferase and rat liver a-2,3-sialyltransferase. The exception was the 9-amino analog, which was a poor substrate for both enzymes. Overall, these assays demonstrated that a wide variety of modifications at the C9 position are tolerated by these sialyltransferases.
The relative rates of sialylation for the C5-modified CMP-NeuAc derivatives shown in Scheme 31 were also determined . All the C5-modified sialyl donors were accepted by the transferase enzymes, but the variants containing free amines were poor substrates for all the sialyltransferases examined. Generally, higher relative
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