On the use of 3,5-O-benzylidene and 3,5-O-(di-tert-butylsilylene)-2-O-benzylarabinothiofuranosides and their sulfoxides as glycosyl donors for the synthesis of beta-arabinofuranosides: importance of the activation method.
A 2-O-benzyl-3,5-O-benzylidene-alpha-d-thioarabinofuranoside was obtained by reaction of the corresponding diol with alpha,alpha-dibromotoluene under basic conditions. On activation with 1-benzenesulfinyl piperidine, or diphenyl sulfoxide, and trifluoromethanesulfonic anhydride in dichloromethane at -55 degrees C, reaction with glycosyl acceptors affords anomeric mixtures with little or no selectivity. The analogous 2-O-benzyl-3,5-O-(di-tert-butylsilylene)-alpha-d-thioarabinofuranoside also showed no significant selectivity under the 1-benzenesulfinyl piperidine or diphenyl sulfoxide conditions. With N-iodosuccinimide and silver trifluoromethanesulfonate the silylene acetal showed moderate to high beta-selectivity, independent of the configuration of the starting thioglycoside. High beta-selectivity was also obtained with a 2-O-benzyl-3,5-O-(di-tert-butylsilylene)-alpha-arabinofuranosyl sulfoxide donor on activation with trifluoromethanesulfonic anhydride. The high beta-selectivities obtained by the N-iodosuccinimide/silver trifluoromethanesulfonate and sulfoxide methods are consistent with a common intermediate, most likely to be the oxacarbenium ion. 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 the formation of a complex mixture of glycosyl donors, as determined by low-temperature NMR work.
Crich
David
D
Department of Chemistry, University of Illinois at Chicago, 845 West Taylor Street, Chicago, IL 60607-7061, USA. dcrich@uic.edu
Pedersen
Christian Marcus
CM
Bowers
Albert A
AA
Wink
Donald J
DJ
D001089:Arabinose / Q000031:analogs & derivatives* / Q000138:chemical synthesis*; D001597:Benzylidene Compounds / Q000138:chemical synthesis*; D002240:Carbohydrate Sequence; D018360:Crystallography, X-Ray; D006031:Glycosylation; D007202:Indicators and Reagents; D009682:Magnetic Resonance Spectroscopy; D008958:Models, Molecular; D008968:Molecular Conformation; D013454:Sulfoxides / Q000138:chemical synthesis*
D001597:Benzylidene Compounds; D007202:Indicators and Reagents; D013454:Sulfoxides; D001089:Arabinose