- Stereoselective α-sialylation with sialyl xanthate and phenylsulfenyl triflate as a promotor
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Reaction α- and β-xanthates 2 and 3 of sialic acid with glycosyl acceptors 5-8 in the presence phenylsulfenyl triflate (PST) as a promoter in a 2:1 mixture of CH3CN/CH2Cl2 at low temperature affords α-sialosides in good yield and stereoselectivity. PST is prepared-in. situ by reacting benzenesulfenyl chloride with silver triflate. Less reactive acceptors 5 and 6 give a higher α/β ratio than more reactive allylic alcohol 7 and primary alcohol 8; α-stereoselectivity is increased in a dilute solution. A possible mechanism of the reaction that involves intermediate α- and β-nitrilium cations 16 and 17 is discussed.
- Martichonok, Valeri,Whitesides, George M.
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- Stereoselective synthesis of 2-thioglycosides of N-acetylneuraminic acid
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Treatment of methyl 5-acetamido-2,4,7,8,9-penta-O-acetyl-3,5-dideoxy-β-D-glycero-D-galacto-2-nonulopyranosonate (1) severally with thiophenol, phenylmethanethiol, and ethanethiol in the presence of boron trifluoride etherate gave the corresponding 2-thio-β-glycosides 3-5.Treatment of methyl (5-acetamido-4,7,8,9-tetra-O-acetyl-3,5-dideoxy-β-D-glycero-D-galacto-2-nonulopyranosyl chloride)onate (2) severally with thiophenol in the presence of N,N-di-isopropylethylamine, O-ethyl S-potassium dithiocarbonate, and O-ethyl S-potassium thiocarbonate gave the corresponding 2-thio-α-glycosides, 7, 10, and 11, of which 10 was converted into the ethyl 2-thio-α-glycoside 9 on heating with N,N-dimethylformamide-sodium iodide.The phenyl 2-thioglycosides 3 and 7 were converted into the corresponding sulfones.
- Marra, Alberto,Sinay, Pierre
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