Mechanism of anomerization of cyclohexyl 2-deoxy-3,4,6-tri-O-methyl-2-(N-methylacetamido)-alpha- and beta-D-hexopyranosides under reductive-cleavage conditions.
The fully methylated cyclohexyl glycosides of 2-acetamido-2-deoxy-alpha- and beta-D-hexopyranoses having the gluco, manno, and galacto configurations were each subjected to reductive-cleavage conditions using one of three promoters, namely trimethylsilyl trifluoromethanesulfonate, a mixture of trimethylsilyl methanesulfonate and boron trifluoride etherate, or boron trifluoride etherate alone. As expected, the fully methylated 1,2-trans-linked acetamido sugar derivatives were rapidly converted to their respective oxazolinium ions with all three promoters. Surprisingly, however, the fully methylated 1,2-cis-linked acetamido sugar derivatives were also converted to their respective oxazolinium ions, albeit at a much slower rate. In the latter case, evidence was obtained for anomerization to the 1,2-trans-linked isomers under reductive-cleavage conditions. Since the anomerization was relatively slow at room temperature in dichloromethane, a modified procedure was developed in which the reaction was carried out at 70 degrees C in 1,2-dichloroethane. Using the modified procedure, all 1,2-cis- and 1,2-trans-linked acetamido sugar derivatives were rapidly converted into their respective oxazolinium ions and subsequent quenching of the reactions with anhydrous methanol gave the respective 1,2-trans-linked methyl glycoside derivatives in quantitative yield. The modified procedure is recommended for the total reductive cleavage of polysaccharides comprised of acetamido sugar residues.
Ahn
Y M
YM
Department of Chemistry, University of Minnesota, Minneapolis 55455, USA.
Gray
G R
GR
D000116:Acetylgalactosamine / Q000031:analogs & derivatives; D000117:Acetylglucosamine / Q000031:analogs & derivatives; D003510:Cyclohexanes / Q000138:chemical synthesis*; D006027:Glycosides / Q000138:chemical synthesis / Q000378:metabolism*; D006595:Hexosamines / Q000138:chemical synthesis / Q000737:chemistry / Q000378:metabolism*; D007700:Kinetics; D009682:Magnetic Resonance Spectroscopy; D008698:Mesylates / Q000378:metabolism / Q000494:pharmacology; D008968:Molecular Conformation; D010084:Oxidation-Reduction; D014297:Trimethylsilyl Compounds / Q000378:metabolism / Q000494:pharmacology
D003510:Cyclohexanes; D006027:Glycosides; D006595:Hexosamines; D008698:Mesylates; D014297:Trimethylsilyl Compounds; C053353:trimethylsilyl trifluoromethanesulfonate; D000116:Acetylgalactosamine; D000117:Acetylglucosamine; C002022:N-acetylmannosamine