Exploration of the Oxazolidinthione Protecting System for the Synthesis of Sialic Acid Glycosides

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An N-acetyl oxazolidinthione-protected sialyl thioglycoside was synthesized and its use as a sialyl donor studied. The strongly electron-withdrawing nature of the oxazolidinthione moiety is such that activation could not be achieved at −78°C. Couplings were therefore conducted at the lowest convenient temperature (−50°C). Glycosides were formed in good yield but in two out of three cases studied selectivities were lower than those seen with the corresponding N-acetyl oxazolidinone-protected donor. The resulting N-acetyl oxazolidinthione-protected disaccharides were converted to the corresponding N-acetyl oxazolidinones by treatment with N-iodosuccinimide and triflic acid in the presence of water at 0°C.