| Abstract
| - A 2-O-benzyl-3,5-O-benzylidene-α-d-thioarabinofuranoside was obtained by reaction of the correspondingdiol with α,α-dibromotoluene under basic conditions. On activation with 1-benzenesulfinyl piperidine,or diphenyl sulfoxide, and trifluoromethanesulfonic anhydride in dichloromethane at −55 °C, reactionwith glycosyl acceptors affords anomeric mixtures with little or no selectivity. The analogous 2-O-benzyl-3,5-O-(di-tert-butylsilylene)-α-d-thioarabinofuranoside also showed no significant selectivity under the1-benzenesulfinyl piperidine or diphenyl sulfoxide conditions. With N-iodosuccinimide and silvertrifluoromethanesulfonate the silylene acetal showed moderate to high β-selectivity, independent of theconfiguration of the starting thioglycoside. High β-selectivity was also obtained with a 2-O-benzyl-3,5-O-(di-tert-butylsilylene)-α-arabinofuranosyl sulfoxide donor on activation with trifluoromethanesulfonicanhydride. The high β-selectivities obtained by the N-iodosuccinimide/silver trifluoromethanesulfonateand sulfoxide methods are consistent with a common intermediate, most likely to be the oxacarbeniumion. The poor selectivity observed on activation of the thioglycosides with the 1-benzenesulfinyl piperidine,or diphenyl sulfoxide, and trifluoromethanesulfonic anhydride methods appears to be the result of theformation of a complex mixture of glycosyl donors, as determined by low-temperature NMR work.
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