N> vi substrate specificity of the sialyltransferase. The ^-trimethylsilyl galactoside 139 is known to be a poor substrate for a-2,3-sialyltransferase; however, it is readily accepted by T. cruzi trans--sialidase. The sialylated oligosaccharide 138 was synthesized by the CMP-NeuAc regeneration system developed by Ichikawa and Wong . Then 139 underwent a transsialylation with 138 to provide the desired GM 4 precursor 141. The regeneration of 138 from the by-product 140 by sialyltransferase drives the equilibrium toward the product 141. This example demonstrates that trans-sialidases have some synthetic utility on their own but are much more general tools for oli-gosaccharide synthesis when coupled with sialyltransferases.
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