The Base-Catalyzed Anomerization of Dinitrophenyl Glycosides. Evidence for a Novel Reaction Mechanism
Berven, L.A., David Dolphin, and Withers, S.G.
Can. J. Chem.
The mechanism of base-catalyzed anomerization per-O-acetylated 2,4-dinitrophenyl-β-D-glucopyranoside in DMSO has been investigated using a variety of techniques. A mechanism involving proton abstraction at C-1 was eliminated by the absence of proton exchange at that center and the measurement of a secondary deuterium kinetic isotope effect for the 1-deuterio substrate. A mechanism involving phenolate departure and recombination is rendered unlikely on the basis of remote substituent effects on the reaction rate and by the absence of any exchange of the Ph moiety with added phenolate. A mechanism involving nucleophilic aromatic substitution initiated by an attack of the dimethylsulfinyl anion to generate a glucosyl oxyanion intermediate that anomerizes and recombines with the reactive aryl intermediate is consistent with the observations. This mechanism is further supported by the observation of a purple Meisenheimer complex intermediate and by the observed exchange between the substrate containing a labeled sugar moiety and added unlabeled 2,3,4,6-tetra-O-acetyl-β-D-glucopyranose.