| Abstract
| - An N-acetyl-5-N,4-O-carbonyl-protected thiosialoside donor, the structure of which has been definedthrough X-ray crystallography, was prepared and tested in couplings to a wide range of acceptors. Thisdonor gives excellent yields and α-selectivities in linking with various primary alkyl and carbohydrateacceptors under the N-iodosuccinimide and trifluoromethanesulfonic acid in situ activation method at−40 °C in dichloromethane. The favorable affect of the oxazolidinone substructure for α-sialylation isillustrated by a comparison study with a N,N-diacetylsialyl donor, which exhibited inferior yields andα-selectivities. The sialylation selectivity is independent of the anomeric configuration of the donor, butis highly related to the reaction temperature under the NIS/TfOH activation method. In contrast to theNIS/TfOH method, the Ph2SO/Tf2O promotion gives β-selective couplings in dichloromethane. Theoxazolidinone of the N-acetyl-5-N,4-O-carbonyl protected sialosides, both α- and β-anomers, could becleaved cleanly by treatment with sodium methoxide under mild conditions without removal of theacetamide.
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